ML14007A709
| ML14007A709 | |
| Person / Time | |
|---|---|
| Site: | Columbia  |
| Issue date: | 01/07/2014 |
| From: | Lyon F Division of Operating Reactor Licensing |
| To: | Clay Johnson Office of Nuclear Reactor Regulation |
| References | |
| G20130776, MF3031 | |
| Download: ML14007A709 (185) | |
Text
NRR-PMDAPEm Resource From: Lyon, Fred Sent: Tuesday, January 07, 2014 12:43 PM To: Charles K. Johnson Cc: Banic, Merrilee
Subject:
RE: 10 CFR 2.206 Pre-Meeting Call and Requested Letter (G20130776; MF3031)
Attachments: JLD-ISG-2013-01 Public Comments.pdf; JLD-ISG-2013-01.pdf; FSAR Table 2.4-1.doc Chuck, heres the quick answer. Ive also attached the FSAR table and ISG for your information. Thanks, Fred Energy Northwest performed a preliminary screening level analysis using the Peak Outflow without Attenuation Method, the Peak Outflow with Attenuation Method, and the Volume Method, from NRC interim staff guidance JLD-ISG-2013-01. Based on the Volume Method, the dams on the list were deemed potentially critical. That list is comprised of those dams in FSAR Table 2.4-1 and additional dams not listed in that table. All dams in the Columbia River Watershed were evaluated to these methods. Based on the Volume Method, the upstream dams listed in FSAR Table 2.4-1, along with the following upstream dams, were found to be potentially critical for the Columbia site:
Noxon Rapids Revelstoke Post Falls Middle Channel O Sullivan North Corra Linn The next step in a dam failure analysis would be to subject the potentially critical dams from the screening to an analytical Probable Maximum Flood (PMF) from their own contributing watershed. Determining if the potentially critical dams would fail due to an analytical PMF from their own contributing watershed would require additional dam information (Stage Storage and Discharge curves and other operating details) maintained by the USACE. Energy Northwest therefore requested that the USACE perform dam failure analyses for the above listed dams and the upstream dams addressed in FSAR Table 2.4-1.
Note that Mica, Keenleyside/Arrow, and Duncan (from FSAR Table 2.4-1), and Revelstoke and Corra Linn (from the above list), are Canadian dams.
Downstream Tributary Dams Downstream tributary dam assessments using the Volume Method, the Peak Outflow without Attenuation Method, and the Peak Outflow with Attenuation Method from JLD-ISG-2013-01 is in progress. The list of downstream dams that would be potentially critical for the Columbia site will be provided on completion of the assessment.
From: Lyon, Fred Sent: Tuesday, January 07, 2014 6:14 AM To: 'Charles K. Johnson' Cc: Banic, Merrilee
Subject:
RE: 10 CFR 2.206 Pre-Meeting Call and Requested Letter (G20130776; MF3031) 1
Ill see if I can find the criteria used for the dams selected to be analyzed for Columbia. Thanks, Fred From: Charles K. Johnson [1]
Sent: Monday, January 06, 2014 6:01 PM To: Lyon, Fred Cc: Banic, Merrilee
Subject:
Re: 10 CFR 2.206 Pre-Meeting Call and Requested Letter (G20130776; MF3031)
Hi Fred, There are a number of Canadian dams on this map that don't seem to be in the list you gave me. Thanks! -
Chuck http://en.wikipedia.org/wiki/File:Columbia_dams_map.png On Mon, Jan 6, 2014 at 2:56 PM, Charles K. Johnson <johnsonc20@gmail.com> wrote:
Hi Fred, Thank you for the documents and the explanation earlier today. I don't see the Mica Dam listed. Are none of the Canadian dams on the mainstem of the Columbia River being looked at for this study? Thank you!
Chuck On Mon, Jan 6, 2014 at 11:11 AM, Lyon, Fred <Fred.Lyon@nrc.gov> wrote:
Mr. Johnson, as you requested, attached is the EN letter to NRC requesting USACE assistance. It is publicly available.
Energy Northwest is requesting that the USACE perform dam failure analyses for all upstream dams listed in FSAR Table 2.4-1. Additionally, Energy Northwest is requesting that the USACE perform dam failure analyses for the following upstream dams, which are not addressed in FSAR Table 2.4-1:
Noxon Rapids Revelstoke Post Falls Middle Channel O Sullivan North Corra Linn In addition, I spoke with Merrilee regarding additional participants on the pre-meeting call with the Petition Review Board. Petitioners have had other participants on the phoncon, and the POC (thats 2
you) typically coordinates them, e.g., names, sequence, time allotment. The phoncon is limited to 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, with about 45-50 minutes total for the petitioners; the rest for NRC comments.
Ill need to know about how many participants you expect, so that I know the number of telephone lines to reserve. It was good to finally make contact with you.
Ive also been informed that I can provide you with the NRCs Determination of Immediate Safety Concerns, which Ive attached, regarding your 10/31/13 letter. It provides the NRCs reasoning for not immediately shutting down Columbia while your petition is considered. The determination was approved by the Deputy Director of the NRC Office of Nuclear Reactor Regulation, Jennifer Uhle, on December 23, 2013.
Thanks, Fred Lyon, NRR project manager for Columbia Generating Station, et al.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Charles K. Johnson 5031 SE Haig St.
Portland, OR 97206 (503) 777-2794 johnsonc20@gmail.com
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Charles K. Johnson 5031 SE Haig St.
Portland, OR 97206 (503) 777-2794 johnsonc20@gmail.com 3
Hearing Identifier: NRR_PMDA Email Number: 992 Mail Envelope Properties (Fred.Lyon@nrc.gov20140107124200)
Subject:
RE: 10 CFR 2.206 Pre-Meeting Call and Requested Letter (G20130776; MF3031)
Sent Date: 1/7/2014 12:42:30 PM Received Date: 1/7/2014 12:42:00 PM From: Lyon, Fred Created By: Fred.Lyon@nrc.gov Recipients:
"Banic, Merrilee" <Merrilee.Banic@nrc.gov>
Tracking Status: None "Charles K. Johnson" <johnsonc20@gmail.com>
Tracking Status: None Post Office:
Files Size Date & Time MESSAGE 5296 1/7/2014 12:42:00 PM JLD-ISG-2013-01 Public Comments.pdf 324430 JLD-ISG-2013-01.pdf 3089881 FSAR Table 2.4-1.doc 96318 Options Priority: Standard Return Notification: No Reply Requested: No Sensitivity: Normal Expiration Date:
Recipients Received:
NRC Responses to Public Comments Japan Lessons-Learned Project Directorate Interim Staff Guidance JLD-ISG-2013-01: Guidance for Assessment of Flooding Hazards Due to Dam Failure (Docket ID NRC-2013-0073)
ADAMS Accession No. ML13151A161 July 2013
I. Introduction This document presents the U.S. Nuclear Regulatory Commission (NRC) staffs responses to comments received on the Draft interim staff guidance (ISG) document, JLD-ISG-2013-01: Guidance for Assessment of Flooding Hazards Due to Dam Failure. The draft ISG was published in the Federal Register on April 25, 2013 (78 FR 24439). The public comment period closed on May 28, 2013; there were no late comments received.
Comment submissions on the draft document are available electronically at the NRCs Electronic Reading Room at http://www.nrc.gov/reading-rm/adams.html. From this page, the public can gain entry into the Agencywide Documents Access and Management System (ADAMS), which provides text and image files of NRC's public documents.
This comment resolution document is also available electronically at the NRCs Electronic Reading Room under ADAMS Accession No. ML13151A161.
The final ISG can be found in ADAMS at Accession No. ML13151A153.
II. Comment submissions The NRC responded to 105 comments received in six submissions. The NRC-designated identifier for each unique comment submission, the name of the submitter, the submitters affiliation (if any), and the ADAMS Accession No. is provided below.
Summary Table Name Affiliation ADAMS Accession No.
Karin M. Hollister Sargent & Lundy, LLC ML13150A154 Mark Moenssens Westinghouse Electric Company ML13149A007 James H. Riley Nuclear Energy Institute ML13193A302 J. W. Shea Tennessee Valley Authority ML13150A155 K. Canavan Electric Power Research Institute ML13157A265 Michael L. Conner U.S. Bureau of Reclamation ML13163A074 Page 2 of 66
III. Public Comments and NRC Response Table 1: Comments Comment No. Comment NRC Response
- 1. Location: Various Response:
[K. Hollister] Comment: Maximum normal pool elevation is defined as the elevation corresponding to the top of the active In the Terms and Definitions section, provide a definition for each storage.
reservoir/pool level discussed in the document. For example, Section 4.2.2.2 discusses the "maximum normal pool elevation," Section 5.6 The average reservoir level (average pool elevation) discusses "maximum normal operating ("full-pool") and average is the 50% exceedance duration pool level reservoir levels," and Section 6.2.2 discusses "normal pool elevation calculated using average daily water levels for the (invert of the highest outlet or spillway)" and "top of dam/maximum high period of record.
pool." full pool, normal pool, maximum high pool are Please include a definition for these levels and any others that are no longer used in the document.
included in the final version of the Dam Failure ISG.
Action:
- 1) definitions added to appropriate sections to the Terms and definitions section (under storage)
- 2) full pool is no longer used in the document
- 2. Location: General Response:
[M. Moenssens] Comment: For onsite water control structures such as dams, levees, impoundments, etc. (including seismic In general, it does not appear that there is a direct off ramp or reduced category I, but excluding tanks), failure due to path for the instance where a dam, levee, embankment, etc. is owner hydrologic or sunny-day mechanisms are to be controlled and licensed by the NRC as a seismic category I structure.
evaluated as part of the R2.1 Flooding Reevaluation.
These structures were originally qualified in the safety analysis report Methods acceptable to the staff for this purpose are submitted with the licensee's application and verified by the NRC safety described in this ISG. Seismic failure of such evaluation report. A direct statement(s) should be included in the ISG to Page 3 of 66
Comment No. Comment NRC Response clearly state what is required for seismic category I structures. structures falls under the scope of the R2.1 Seismic Currently, the ISG does not make any direction mention to the term Reevaluation.
seismic category I. Action:
Text added to Section 2.1 (scope) to clarify this position.
- 3. Location: Page 1, Section 1 Response:
[M. Moenssens] Comment: Cannot find this word in 2nd paragraph. Combination is used in 3rd paragraph, but is already followed by Add "of' after the word "combination" in the second paragraph of.
Action: No change to text
- 4. Location: Page 1, Section 1.1 Response:
[M. Moenssens]
Comment: This ISG supplements and clarifies other NRC guidance that discusses dam failure such as RG-Is not it more realistic to only utilize this ISG rather than referring back 1.59.
to Reg. 1.59 which is very much outdated?
Action: No change to text
- 5. Location: Page 2, Section 1.1 Response:
[M. Moenssens] Comment: In general, this ISG should be followed if there are differences in NRC guidance. However, licensees If there are differences on a certain issue between the different guidance, which guidance should be followed? The most stringent? are not required to follow NRC guidance.
Moreover, if the Licensee also owns a dam, does it need to be in As stated in section 1.1, this guidance should in no compliance with all regulations or only the nuclear ones? way supersede or be used in lieu of guidance developed by any agency that owns, operates or regulates the dam(s) of interest.
The purpose of this ISG is to provide guidance in estimating the consequences of potential dam Page 4 of 66
Comment No. Comment NRC Response failures in terms of flood hazards at the NPP.
Action: No change to text
- 6. Location: Page 4, Section 1.3.2 Response:
[M. Moenssens] Comment: Since the flood hazard reevaluation reports are to be submitted under Oath and Affirmation, it is Does NRC require a P.E engineer certification for breach analysis (but expected that the technical work be performed by not the screening and simplified analyses)? If so, does he/she need to competent professionals. However NRC does not have the P.E from the state where the dam is located?
have explicit requirements regarding licensure. Other state or federal agencies with jurisdiction for dam safety may have such requirements.
Action: No change to text
- 7. Location: Page 4, Section 1.3.2 Response:
[M. Moenssens]
Comment: The NOAA/NWS hydrometeorological reports Second paragraph, does this ISG recommend using the current NOAA (HMRs) are the most comprehensive information on extreme rainfall estimate available at this time.
hydrometeorological reports for dam failure analysis, some of which However, due to the age of these reports (e.g. HMR-date back to the late 1970's but are still the "current" report for a region? 51 was published in 1978), the licensee should exercise due diligence and examine the record of extreme storms in the region of interest to provide assurance that the HMR estimates are still valid.
Action:
Following text added to Section 1.3.2:
Existing estimates for design storms and floods (e.g.,
PMP and PMF) in the region of interest developed by federal, state or other agencies may be used.
However, some of these reports may be quite old Page 5 of 66
Comment No. Comment NRC Response (e.g. the NOAA/NWS Hydrometeorological Report 51 for the Eastern U.S. was published in 1978). The licensee should exercise due diligence and examine the record of extreme storms and floods in the region of interest to provide assurance that the existing estimates are still valid.
- 8. Location: Page 4, Section 1.3.2 Response:
[M. Moenssens]
Comment: Sediment transport should be considered in the analysis. Ignoring sediment deposition may result in If flood levels do not reach the site, does the licensee still need to underestimates of water level elevations.
evaluate the transport of sediments and debris?
Conversely, ignoring sediment erosion may mean that potentially dangerous scouring around structures is ignored.
If flood levels do not reach the site, then waterborne debris impacts would not need to be considered at the site. However, waterborne debris impacts on an upstream dam may still be germane.
Section 4.2.8 of the revised ISG provides more detail on estimating waterborne debris impacts.
Detailed guidance on sediment transport modeling is beyond the scope of this ISG, but Section 9.3 of the revised guide proved references. In many cases, simplified conservative estimates for erosion and sedimentation may be used in lieu of detailed analysis.
Action:
Section 9.3 of has been added to the revised guide.
Page 6 of 66
Comment No. Comment NRC Response
- 9. Location: Page 8, Section 1.4.2 Response:
[M. Moenssens]
Comment: Section 1.4 has been reorganized and this phrase is no longer used.
Add "is" after "it" to read "it is acceptable" Action:
No change to text.
- 10. Location: Page 10, Section 1.5.3 Response:
[M. Moenssens] Comment:
On the sixth line of the first bullet, "developi" is misspelled Action:
Spelling corrected.
- 11. Location: Page 11, Section 1.6 Response:
[M. Moenssens] Comment:
The section states that, "Details of dam breach modeling are discussed Action:
in ISG Section 7." This should be Section 8 Section cross reference corrected,
- 12. Location: Page 11, Section 1.6 Response:
[M. Moenssens] Comment:
The section states that, "Details of flood routing are discussed in ISG Action:
Section 9." This should be Section 10 Section cross reference corrected
- 13. Location: Page 11, Section 1.6 Response:
[M. Moenssens] Comment: In the revised ISG, operational failures and controlled releases are discussed in section 4.2.7 The section describes the organization of the guidance but does not describe Section 7, "Operational Failures and Controlled Releases." Action:
Page 7 of 66
Comment No. Comment NRC Response No changes to text in section 1.6
- 14. Location: Page 24, Section 3.2 Response:
[M. Moenssens] Comment: The figure 10 is correct. Inconsequential dams may be excluded before implementing the screening Do you include "all" dams (items 1.a and 2.b) or only "all" dams that are procedures discussed.
consequential (i.e., after screening)? The text contradicts what is on Figure 10. Action:
Clarification added to text for screening steps 1.a and 2.b and 3.b
- 15. Location: Pages 33,38,81; Sections 4.2.2.1,4.2.7,10 Response:
[M. Moenssens]
Comment: The need to address mud flows has been removed from the ISG.
Does NRC recommend the utilization of 2D / 3D modeling software package such as FLO-2D or Delft3D instead of HEC-RAS to account for Certain widely-used modeling software packages are sediment production and transport, mud flows, and debris transport?
mentioned in the ISG for illustrative purposes, but the NRC does not recommend specific software packages. In general, hydrologic and hydraulic simulation models accepted in standard engineering practice by Federal agencies and other authorities responsible for similar design considerations may be used.
Action:
Language similar to the preceding paragraph has been added to the revised ISG section 1.1 (Purpose)
- 16. Location: Page 37, Section 4.2.4 Response:
[M. Moenssens] Comment: There is an extra bullet.
Page 8 of 66
Comment No. Comment NRC Response There appears to be an extra bullet at the end of the Staff position in Action:
Section 4.2.4. Is this an extra bullet or was an additional staff position Extra bullet removed.
statement supposed to be located here?
- 17. Location: Page 38, Section 4.2.7 Response:
[M. Moenssens] Comment: The RUSLE method was developed to estimate erosion for agricultural applications. The database Can the licensee utilize the RUSLE method to identify the potential for used to develop the method was based on erosion in the watershed?
agricultural plot-scale sites with disturbed soils. In general, it would not be applicable to the large watersheds of interest in this ISG without significant modification. However, the requirement to consider mud flows has been removed from the ISG, so this comment is no longer relevant.
Action:
The section on mud flows has been removed.
- 18. Location: Page 40, Section 4.2.7.2 Response:
[M. Moenssens] Comment: Reference Updated The first paragraph after Table 3 references the 2005 version of Action:
ASCE/SEI 7-05. There is a more recent version, ASCE/SEI 7-10.
Reference to ASCE/SEI 7-05 has been replaced with ASCE/SEI 7-10.
- 19. Location: Page 40, Section 4.2.7.2 Response:
[M. Moenssens]
Comment: The USACE report shows that the methods are USACE ERDC/CRREL TR-02-2 evaluated several different methods of equivalent for a certain range of velocities and under certain assumptions regarding the stiffness of the estimating debris loading for logs. It concluded that "all three approaches can be derived from a single-degree-of-freedom model of debris and structure impacted. If the NASSTRA method (work energy method) or the AASHTO Page 9 of 66
Comment No. Comment NRC Response the collision and are equivalent." Therefore, it is recommended that the method (contact stiffness method) are used, the NAASTRA (Highway Bridge Design Specification) and AASHTO (LRFD licensee should justify that the results are equivalent Bridge Design Specifications) methods also be referenced in Staff or more conservative than the impulse momentum Position bullet 3. approach outlined in the ASCE standard.
Action:
No change to text.
- 20. Location: Page 43, Section 5 Response:
[M. Moenssens]
Comment: Cross reference is to section 5.6 There is a section cross reference that appears to have been lost at the Action:
end of the last sentence in the third paragraph. Added cross reference to section 5.6
- 21. Location: Page 43, Section 5.1.1 Response:
[M. Moenssens]
Comment: A site-specific analysis should be performed. The analysis may utilize the USGS 2008 seismic hazard Will NRC accept a regional PSHA study or another study from a tools, as discussed in ISG section 5.1. For the neighboring site or is it a "must" to conduct a site-specific PSHA as part purposes of JLD-ISG-2013-01, it is not necessary to of the dam failure analysis due to seismic event?
use the tools and methods being applied in the Recommendation 2.1 seismic reevaluations for NPP sites.
Action:
No change to text.
- 22. Location: Page 46, Section 5.2.1 Response:
[M. Moenssens] Comment:
In the last sentence of the 3rd paragraph, "downstream" is repeated. Action:
One of the instances should probably be upstream.
Page 10 of 66
Comment No. Comment NRC Response Repeated word deleted.
- 23. Location: Page 52, Section 5.4.1 Response:
[M. Moenssens] Comment: Should be UHS In staff position bullet 1, the term "UHRS" is used, but not previously Action:
defined. Should this term be UHS, as defined in Section 5.7.1.4? Text corrected.
- 24. Location: Page 52, Section 5.4.1 Response:
[M. Moenssens] Comment: Reference is to 5.3.1 Staff position bullet 1 states that, "...(based on the UHRS and Action:
accounting for site amplification) as described in Section 5.4.1." The Reference corrected.
reference to Section 5.4.1 should probably be changed to Section 5.7.1.4.
- 25. Location: Page 52, Section 5.4.1 Response:
[M. Moenssens] Comment:
In staff position bullet 2, the term "UHS" is used, but not previously Action:
defined. It is defined later in Section 5.7.1.4. UHS is now defined in Section 5.3.1
- 26. Location: Page 52, Section 5.4.1 Response:
[M. Moenssens] Comment: Repeat of previous comment The last sentence of Staff position bullet 1 states that, "...in light of the Action:
UHS developed in Section 5.4.1 including effects of..." The reference to No change Section 5.4.1 should probably be changed to Section 5.7.1.4.
- 27. Location: Page 67, Section 6.1.3 Response:
[M. Moenssens] Comment:
Page 11 of 66
Comment No. Comment NRC Response The text of the Levee section was removed based on past comments Action:
but the section header remains. It is recommended that a statement be Section header removed.
added under the header that Sunny Day Failure is not applicable to levees since they are not normally loaded.
- 28. Location: Page 68, Section 6.2 & 6.2.2 Response:
[M. Moenssens]
Comment:
On Page 68 there are two instances of where Section 7 is referenced. Action:
These should be references to Section 8. Cross reference updated.
- 29. Location: Page 79, Section 9 Response:
[M. Moenssens]
Comment:
The second word of the 3rd paragraph should be "from," not "form." Action:
Text corrected.
- 30. Location: Page 82, Section 10.1.2 Response:
[M. Moenssens] Comment: Text states that, in this case, a 2D model will better With respect to the last paragraph, there is a statement about the use of simulate flows in flat topography.
1D flood models in flat-lying topography. The paragraph does not Action:
directly state that 1D modeling should not be used in this case. 1D No change to text.
modeling tools are a poor choice of modeling tools for this scenario.
Low relief areas where distributary flow may occur should rely on 2D (at minimum) models to deal with the complexity of non-channelized flow.
There is a tremendous amount of academic research on this, and it isn't clear why 1 D models are still used in these areas. The ISG should take a firm position for the applicability of 1D versus 2D / 3D modeling.
Page 12 of 66
Comment No. Comment NRC Response
- 31. Location: Page 83, Section 10.2 Response:
[M. Moenssens]
Comment: The discussion in question is aimed at the efficacy of 1D vs. 2D modeling approaches and applies to any This section ambiguously references the models used in HEC-RAS. It is hydraulic modeling package.
recommended to state directly that HEC-RAS is appropriate when it is determined that a one-dimensional flow model is suitable. The NRC does not recommend specific modeling software packages.
Action:
No change to text.
32 Location: Page 100, ASCE (2005b) Response:
[M. Moenssens]
Comment: Reference updated.
The reference should be updated to ASCE/SEI 7-10 since this is the Action:
most up-to-date reference for the standard. Reference to ASCE/SEI 7-05 has been replaced with ASCE/SEI 7-10.
33 Location: General Response:
[J. Riley] Comment: On-site or off-site temporary structures can continue to be credited in the R2.1 flood hazard reevaluation if The ISG is not clear on how off-site temporary structures can be such credit has been evaluated and accepted by credited for flood protection NRC staff prior to the 50.54(f) information request Concern: (USNRC 2012). All other temporary structures, or Temporary off-site structures may already be in place for some plants. measures (including mitigation or compensatory measures), should not be credited in the flood Proposed Resolution: Not provided hazard reevaluation. Temporary structures or Understanding of Current Status: Not provided measures not credited in the hazard reevaluation may be proposed as interim actions and discussed in Page 13 of 66
Comment No. Comment NRC Response the appropriate section(s) of the hazard reevaluation response as described in the 50.54(f) information request letter (USNRC 2012).
Action:
The preceding text has been added to section 4.2.2 34 Location: Sec. 1 / p. 1 Response:
[J. Riley] Comment: Tanks are excluded.
Failures of water-storage or water-control structures (such as onsite Action:
cooling or auxiliary water reservoirs and onsite levees) that are located Tank exclusion added to Section 1.1 (Scope) at or above the grade of safety-related equipment are potential flooding mechanisms.
Concern:
List should specifically exclude tanks.
Note that the 50.54(f) letter only asks for external flood evaluations Proposed Resolution:
Specifically include tanks in the list.
Understanding of Current Status:
We need to develop additional guidance on the scope of the ISG as well as the flooding reevaluations in general.
35 Location: Sec 1.3.1, p 2 Response:
[J. Riley] Comment: This ISG is applicable to estimating flood hazards Many sites have owner-controlled levees, embankments, dams, cooling due to failure of all offsite and some onsite water ponds, etc. above power block grade that are licensed by the NRC as control structures and impoundments. For offsite structures, hydrologic, seismic and sunny-day failure Seismic Category I. These structures were evaluated as Seismic Page 14 of 66
Comment No. Comment NRC Response Category I in the licensing basis / safety analysis report and affirmed as mechanisms are within the scope of the R2.1 such by the NRC in a safety evaluation report. These structures are Flooding Hazard Reevaluation and this ISG. This typically controlled via operating procedures, preventative ISG provides guidance that is applicable to the maintenances, and surveillance tests. However, the Dam Failure ISG evaluation of onsite dams and levees, including dam-does not discuss an alternative, shortened assessment or screening or levee-like structures associated with onsite path specifically for these types of structures, nor does the ISG make reservoirs (e.g., earthen cooling reservoir any reference to the term Seismic Category I. Do Seismic Category I impoundments). Thus, while Section 2.4.4 of water retention structures qualify for an abbreviated screening process NUREG-0800 includes failure of all onsite water that credits their NRC approved design and operation? control or storage structures (e.g., levees, dikes, and Concern: any engineered water storage facilities that are located above site grade and may induce flooding at The ISG is not clear on how seismic category 1 structures are to be the site such as tanks and basins), this ISG provides evaluated for flooding effects. Allowing for the analysis of these guidance applicable to only a subset of those onsite structures during the Fukushima 50.54(f) letter seismic reevaluations structures. For example, even though the evaluation could lead to questions on the completeness of the Integrated of site flooding from structures such as concrete Assessment which may have been completed prior to the seismic cooling tower basins is within the scope of the NTTF reevaluation. Recommendation 2.1 flood hazard reevaluations, Proposed Resolution: Not provided provision of guidance to support evaluation of such structures is not within the scope of this ISG.
Understanding of Current Status: Not provided Moreover, evaluation of flooding from tanks is not within the scope of the NTTF Recommendation 2.1 flood hazard reevaluations and associated guidance is not provided in this ISG. Seismic failure of onsite structures may require input from the R2.1 Seismic Reevaluations.
Seismic failure of onsite structures falls within the scope of the R2.1 Seismic Reevaluations and is not discussed in this ISG.
Page 15 of 66
Comment No. Comment NRC Response Action:
Section 1.2 (Scope) has been updated to reflect the response.
36 Location: Sec. 1.3.2 / p. 4 Response:
[J. Riley] Comment: The current staff position is that hydrologic failure and seismic failure can be ruled out with appropriate 4th full paragraph of p. 4, last sentence: In lieu of a detailed analysis, justification. For dams that are not screened out one can simply assume that the dam fails under appropriate loading according to section 3 (i.e. the dam is potentially and move on to estimation of the consequences.
critical), this will require a detailed analysis. The Concern: detailed analysis can be from an existing study In lieu of a detailed analysis, does the licensee have any alternate performed by the dam owner, if they meet the intent options to justify that a dam (which is not screened-out according to of the ISG. However, a sunny-day failure cannot be Section 3) will not fail, rather than simply assuming dam failure? ruled out, even by detailed analysis, since there is no widely accepted methodology for estimating failure Proposed Resolution: probabilities on the order of 1e-6 per year.
Explain what is meant by a detailed analysis - analyze non-failure or A detailed analysis is generally one that takes into analyze how the failure would occur. account specific characteristics of the watershed and Clarify if there are any alternative options to simply assuming dam the dam(s) and does so in a manner that failure in lieu of a detailed analysis. For example, if a federal agency incorporates more of the physics than the screening can provide justification that the dams they own and operate will not fail approaches. We do not provide a precise definition under the scenarios described in this ISG, clarify if the licensee can rely for detailed analysis since the components of a on the assertion of a federal agency in lieu of a detailed analysis. detailed analysis will vary on a case-by-case basis.
Professional judgment is required.
Understanding of Current Status:
Studies by federal and state dam safety agencies, We understand that the details of sharing analysis results performed by that meet the intent of the ISG, can be used to other federal agencies is still being developed and that the intent of the support a conclusion that hydrologic or seismic ISG is to allow use of analyses prepared by other agencies as long as failure is not credible. However, existing (or new) the analysis meets the guidance in the ISG. studies cannot be used to rule out sunny-day Page 16 of 66
Comment No. Comment NRC Response failure.
Action:
No proposed changes to text.
37 Location: 1.3.2, p. 4 Response:
[J. Riley] Comment: The screening methods described in Section 3 are intended to be performed using publicly available Dam failure flood hazard estimation will require collecting data on the information (e.g. public NID fields). Detailed dam (s) to be analyzed (e.g., design documents, construction records, analyses will generally require the types of maintenance, and inspection program, planned modifications) information referenced in the comment.
Concern:
What can be done if records cannot be located? Are there any The type and amount of information required to reasonable assumptions that can be made? Are there a minimum set of support a detailed analysis will vary on a case-by-records needed.
case basis. In some cases conservative Note that the rigor of justification is going to be dependent on the assumptions may be used in lieu of data.
availability of information. Professional judgment is needed. If sufficient Proposed Resolution: information to support detailed analysis is not available, failure should be postulated and the If detailed historical information cannot be obtained, recent (last 5 consequences analyzed.
years) inspection reports and evaluations by the dam regulator can be used to determine if there are flaws or vulnerabilities that should be Action:
evaluated for dam failure risk. No text change.
Understanding of Current Status: Not provided 38 Location: 1.3.2, p. 4 Response:
[J. Riley] Comment: Analysis methods for flood-born debris are discussed in section 4.2.8.
Transport of sediment and debris by flood waters should be considered. The main concerns regarding sediment transport Page 17 of 66
Comment No. Comment NRC Response Concern: include: 1) impacts to predicted water surface Not clear what this statement is requiring and how to perform a elevations (e.g. sediment deposition will result in sediment and debris analysis beyond engineering judgment. Where is higher water levels for a given discharge); 2) scour at sediment a concern? What scale/type of debris is of concern? SSC structures; and 3) sediment accumulation in UHS impoundment.
The ISG leaves this evaluation up to the licensee and will probably result in large variation. Additional guidance on how to deal with debris However, detailed guidance on sediment transport and sediment in the dam break flood wave is needed. modeling is beyond the scope this ISG.
Proposed Resolution: Action:
Added section 9.3 which discusses general If an analysis is required and expected to be part of the report, this considerations for sediment transport modeling and statement would need to be expanded to further characterize when sediment and debris needs to be considered and the specific concerns provides references to the technical literature.
that need to be addressed. If the concern is to consider sources of large debris in the routing path that could be transported to the nuclear site, it should be stated as such.
Understanding of Current Status: Not provided 39 Location: Sec. 1.4.2, p. 7 Response:
[J. Riley] Comment: Due to the lack of widely accepted methods for estimating failure probabilities on the order of 1e-6 General comment: This section states that the probability target for per year, this section has been revised. The revised judging the likelihood of a particular failure mode/scenario (either from a approach for potentially critical/critical dams is as single hazard or appropriate combination) is 1x10-6 annual probability.
follows:
From the above statement it appears that dams which are safe for (1) demonstrate that dam is capable of passing floods with a probability of 10-6 per year need not to be checked for the PMF considering anticipated prevailing failure during PMF. conditions as described section 4 of the ISG Concern: (2) demonstrate that the dam is capable of If it can be demonstrated that a dam will not fail during a flood with withstanding seismic load combinations as described in section 5 of the ISG probability of 10-6 per year, can hydrologic dam failure be excluded Page 18 of 66
Comment No. Comment NRC Response without considering PMF? (3) evaluate the flood height and associated effects of sunny day failure Proposed Resolution:
More clarification is required to clarify that dams not failing for 10-6 Action:
flooding can be considered as safe and potential failure during PMF This section has been revised to reflect the approach does not need to be evaluated outlined above.
Understanding of Current Status:
We understand that the 10-6 criteria will be removed.
40 Location: 1.4.2, p. 7 Response:
[J. Riley] It is recognized that the text in the draft ISG is Comment:
confusing.
Last bullet - staff position states acceptable to use the 1x10-4 annual frequency ground motions, at spectral frequencies important to the dam, Action:
for seismic evaluation of dams, instead of 1x10-6, as discussed above. This section has been revised to eliminate reference However, appropriate engineering justification must be provided to to 1e-6 ground motion.
show that the dam has sufficient seismic margin. Otherwise the 1x10-6 ground motions should be used.
Concern:
- It is not clear how the 10-4 and 10-6 criteria should be used. If sufficient margin cannot be established with the 10-4 criteria, how could adequate justification be achieved with the 10-6 criteria when it is associated with a larger earthquake?
- What constitutes sufficient margin if a 10-4 seismic hazard analysis is performed verses a 10-6 seismic hazard analysis?
Proposed Resolution:
- Clarify how the two seismic criteria are to be used
- Provide guidance on what amount of margin is sufficient.
Page 19 of 66
Comment No. Comment NRC Response Understanding of Current Status:
We understand that the 10-6 criteria will be removed.
41 Location: Sec 1.4.2, p. 8 Response:
[J. Riley]
Comment: The discussion of margin is related to the 1e-6 criteria, which has been removed.
2nd bullet on p. 8, next to last sentence: However, appropriate engineering justification must be provided to show that the dam has Action:
sufficient seismic margin.
This section has been revised to reflect the approach Concern: outlined above (see response to comment 39).
No quantitative criteria for sufficient margin are provided.
Proposed Resolution:
The 10-4 annual frequency ground motion is comparable to GMRS.
Factor of safety in NRC regulatory guidance for liquefaction and slope stability for GMRS can be used to demonstrate sufficient margin.
Understanding of Current Status:
We understand that the 10-6 criteria will be removed.
42 Location: Sec. 1.4.2, p.8 Response:
[J. Riley] Comment: The 1e-6 criteria has been removed. However, staff considers the current state of practice insufficient to 2nd bullet on p. 8, last sentence: Otherwise 10-6 ground motions reliably estimate failure at probabilities on the order should be used.
of 1e-6. This informs the staff position to require that Concern: consequences of a sunny-day failure be analyzed.
The 10-6 ground motion criteria appears to be more conservative than Action:
NRC ISG-20, PRA based Seismic Margins Analysis where 1.67
- This section has been revised to reflect the approach Page 20 of 66
Comment No. Comment NRC Response GMRS is used as a screening criteria. outlined above.
Comment also applies to Sec 5.3.1, p. 48, 1st paragraph.
Proposed Resolution:
Otherwise 10-6 ground motions should be used. should be replaced by Otherwise dam seismic capacity greater than 1.67*(10-4 ground motions) should be demonstrated.
Understanding of Current Status:
We understand that the 10-6 criteria will be removed.
43 Location: Sec. 1.5.3, p. 10 Response:
[J. Riley] Comment: Still working on the MOA. However, the details on the content of the MOA is not the subject of this ISG Staff Position, 1st bullet: If a federally owned dam is identified as critical to the flooding reanalysis, the licensee should contact NRC Action:
promptly. NRC will act as the interface between these agencies and No change to text.
licensees. Memoranda of Agreement or other mechanisms are being developed to facilitate sharing of data (including necessary safeguards to protect sensitive information) between NRC and the appropriate federal agencies.
Concern:
- If information from a federal agency is considered classified, would this information be limited to the government agencies or would the licensee be involved?
Proposed Resolution:
Following the development of the Memoranda of Agreement, include in this ISG information regarding how to handle requests for information that may be considered classified by a federal agency.
Page 21 of 66
Comment No. Comment NRC Response Understanding of Current Status:
We understand that a Memorandum of Agreement is under development that will describe how information can be communicated and controlled.
45 Location: Sec. 1.5.3 / p. 10 Response:
[J. Riley] Comment: Still working on the MOA, but the details of the MOA are beyond the scope of this ISG.
Staff Position, 1st bullet: It is important to note that in many cases federal agencies that own or operate dams have a conducted detailed Action:
failure analysis. To the extent these analyses are applicable, they No Change to text.
should be used in the Recommendation 2.1 flooding reanalysis.
Concern:
Details of the agencys existing dam failure analyses may not be provided to the licensee or may be considered classified. If the full details of the agencys existing analyses are not available to the licensee, it may not be possible to determine that the analyses are applicable and meet the criteria for the Recommendation 2.1 flooding reanalysis.
Proposed Resolution:
Clarify whether the onus is on the licensee or the federal agency to determine that the existing dam failure analyses performed by federal agencies are applicable and meet the criteria for the Recommendation 2.1 flooding reanalysis, in the event that the details of these analyses are not provided to the licensee.
Understanding of Current Status:
We understand that a Memorandum of Agreement is under development that will describe how information can be communicated Page 22 of 66
Comment No. Comment NRC Response and controlled 46 Location: Response:
[J. Riley] Sec 1.5.3, p. 10 The actions discussed here are envisioned as taking place during the evaluation.
Comment:
Use of existing studies, as applicable, is envisioned.
Staff Position, 1st bullet: In the case of dams and levees owned or operated by U.S. federal agencies, the federal agency responsible Action:
(owner/operator) for the dam should be involved in any discussions, No change to text.
including possibly reviewing any analysis performed.
Concern:
It is unclear if this possible review is to occur as part of the evaluation or concurrently with NRC review. It is noted that the NRC-mandated schedule for evaluations may not permit such agencies to perform a review given their other commitments and responsibilities. This statement would appear to imply support for using previous analyses of upstream structures that have been reviewed and accepted by the federal owner/operators of such structures. FERC is a federal agency which does not own or operate dams, but directly regulates dam safety of licensed hydropower dams.
Proposed Resolution: Not provided Understanding of Current Status: Not provided 47 Location: Sec 1.5.3, p. 10 Response:
[J. Riley] Comment: The licensee should notify NRC if they encounter difficulties in obtaining information Staff Position, 3rd bullet: In most cases dams and levees will be owned and operated by private entities and regulated by a state agency. In this Action:
case, the licensee should interact directly with the owner and regulator. No change to text.
The licensee should notify NRC if they encounter difficulties in obtaining Page 23 of 66
Comment No. Comment NRC Response information. On a case-by-case basis, NRC may be able to provide some assistance in interfacing with state agencies.
Concern:
Based on experience, many dam owners consider dam safety-related information to be highly sensitive. Dissemination of information related to dam failure mechanisms, dam stability, and hydraulic capacity is likely to be restricted. FERC has a specific designation, CEII, (Critical Energy Infrastructure Information) that is applied to sensitive information, thereby, labeled as non-public. The NRC should consider proactively reaching out to state dam safety regulatory agencies to inform them of forthcoming information requests from plant owners and to emphasize the importance of this information to support these evaluations. There can be hundreds or even thousands of dams in the watershed upstream of a nuclear facility; therefore, direct interaction with each owner would/could be cost and time prohibitive.
Proposed Resolution: Not provided Understanding of Current Status: Not provided 48 Location: Sec. 2.2.3, p. 20 Response:
[J. Riley] Comment: Operational failures and controlled releases are of concern mainly in flooding scenarios.
Last bullet in list: Inability to warn in advance Action:
Concern:
Discussion of operational failures and controlled Unlike the other bullets in the list, this bullet seems more like a releases moved into section on hydrologic failures.
consequence of failure rather than a causative failure mechanism, except possibly in the case of a cascading failure sequence, which is discussed in the next section.
Proposed Resolution:
Page 24 of 66
Comment No. Comment NRC Response Suggest deleting bullet, or clarifying how it might apply as a failure mechanism.
It is understood that the failure mechanism is associated with the failure of upstream dams Understanding of Current Status:
We understand that the text will be modified to indicate the concern with upstream dams and to focus on failures that my result in inability to warn in advance.
49 Location: Sec 3.2, p. 23 Response:
[J. Riley] Comment: The 500-year flood was selected to conservatively account for antecedent conditions.
Why was 500-year flood data selected to be used for analyses rather than 100-year data? Action:
Concern: No change text.
Proposed Resolution: Not provided Understanding of Current Status: Not provided 50 Location: Sec. 3.2, p. 24 Response:
[J. Riley] Comment: Choice of modeling package is up to licensee. The NRC does not endorse specific modeling software Item 4: Hydrologic Model Method (see Figure 13): Use an available packages.
rainfall-runoff-routing software package (e.g. HEC-HMS) to assess dam failure scenarios. Action:
Concern: No change to text.
Can HEC-1 be used as the hydrological model method?
Proposed Resolution: Not provided Page 25 of 66
Comment No. Comment NRC Response Understanding of Current Status: Not provided 51 Location: Sec 3.2.1, p. 28 Response:
[J. Riley] Comment: The ISG states in the third paragraph that clustering of dams should make hydrologic sense.
2nd para. : Topographic information from LiDAR or a DEM at the location of the hypothetical dam is used to develop a stage-storage The point is that water volumes should be function for the hypothetical dam. This stage storage function is used to conserved, not heights of dams. If topographic determine the water surface elevation of the hypothetical dam. information is not used to develop a stage-storage curve for the hypothetical dam, the stage-storage Concern:
curve may be derived by summing the storage Grouping a large number of dams together would result in an curves of the individual dams. The height of the unrealistically large reservoir volume. Applying actual topographic hypothetical dam developed in this manner would be information to develop a stage-storage function for such a reservoir may equal to the height of the tallest individual dam with a result in very large water surface elevations and, thus, very large maximum storage equal to the summed storage of hydraulic head. The ISG should acknowledge (similar to the wording in the individual dams. The invert elevation of the the third paragraph) that the hypothetical dam should be representative hypothetical dam would be derived from the of the collective dam heights of the individual structures it represents, topographic information.
while simultaneously representing an appropriately conservative Breach models consistent with the screening level scenario through the application of a hypothetical collective storage analysis in this section should require only basic volume.
information (height of dam and perhaps reservoir In addition, selecting breach development parameters, such as breach volume). More detailed breach models would not be development time, require engineering judgment in consideration of the appropriate for the screening analysis.
fact that the dam in question is hypothetical and not an actual structure.
Action:
Proposed Resolution: Not provided Paragraph added to describe alternative approach:
Understanding of Current Status: Not provided As an alternative (if a DEM is not used to develop a stage-storage curve), the stage-storage curve for the hypothetical dam may be derived by summing the stage-storage curves of the individual dams. The height of the hypothetical dam developed in this Page 26 of 66
Comment No. Comment NRC Response manner will be equal to the height of the tallest actual dam. The actual elevation of dam would be derived from the DEM.
No change to text.
52 Location: Sec 4.2.2.3, p. 34 Response:
[J. Riley] Comment: With regard to crediting release capacity through appurtenances other than the spillway (e.g., outlets, Staff Position, 2nd bullet: at least one turbine should always be turbines), existing federal guidance is not consistent.
assumed to be down (e.g., for maintenance or other reasons) in For example, USACE engineering manual EM 1110-performing flood routings.
2-1603, Hydraulic Design of Spillways states that a Concern: powerhouse should not be considered as a reliable Dam operators typically perform their maintenance activities outside of discharge facility when considering the safe the flood season. Assumption that one unit is out of service is conveyance of the spillway. Conversely, FERC excessive. Engineering Guidelines for the Evaluation of Hydropower Projects states that those release
- Overly conservative assumption facilities which can be expected to operate reliably Proposed Resolution: under the assumed flood condition can be credited for flood routing. USBR best practice guidelines
- Assume all units are usable, use full power plant discharge capacity. (USBR 2011) suggest that at least one turbine
- In large river systems with multiple generating dams does each should always be assumed to be down (e.g. for generating dam have to consider one turbine out of service?. maintenance or other reasons) in performing flood routing.
Understanding of Current Status:
Staff Positions:
We understand that the document may be revised to allow for justification of turbine availability in large river systems with multiple
- Release capacity through appurtenances generating dams. other than the spillway (e.g., outlets, turbines) may be credited as part of the total available release capacity, with appropriate engineering justification that these Page 27 of 66
Comment No. Comment NRC Response appurtenances will be available and remain operational during a flood event. Access to the site during a flood event should be considered.
- The generators and transmission facilities to support the credited turbine(s) must be shown to be operational under concurrent flood and expected prevailing weather conditions if the turbines are credited as part of the total available release capacity.
Action:
ISG revised to include the text above, 53 Location: 4.2.2.3, p. 34 Response:
[J. Riley] Comment: The discussion of spillway blockage has been extended to provide additional guidance. Historical The potential for flood-borne debris to reduce spillway capacity should information and debris studies are proposed as the be considered.
best sources of information. Guidance for spillway Concern: capacity reduction is provided for dams with debris The criteria for considering potential debris blockage at a spillway are management.
not clear. If a spillway is gated with 40-foot wide gates, are there criteria Action:
for how much blockage should be considered or how the spillway capacity may be reduced by flood-borne debris? More detailed staff positions on spillway blockage have been added to this section:
- This statement needs a reference. Could not find the source
- The potential for flood-borne debris to Proposed Resolution: reduce spillway capacity should be If debris blockage is considered as a potential vulnerability of a spillway, considered. Historical information on debris clarify criteria regarding spillway capacity reduction. production in the watershed or similar watersheds should be used to assess the Page 28 of 66
Comment No. Comment NRC Response Understanding of Current Status: potential debris volumes.
We understand that this additional guidance is being developed.
- For dams that have debris management, a sensitivity study assuming a 5-10%
reduction in capacity should be performed.
Describe structures, equipment and procedures used to prevent spillway blockage by waterborne debris.
- For dams that lack debris management greater capacity reductions should be considered. The appropriate capacity reduction will vary on a case-by-case basis.
Justification for the reduction used should be provided (e.g., debris studies for the watershed or similar watersheds).
54 Location: 4.2.6, p 38 Response:
[J. Riley] Comment: Difficult to provide detailed guidance on gate failure due to wide variety of gate types. The concern here Staff Position: As written, the guidance is ambiguous as to the is that reasonable allowance should be made for evaluation(s) that should be conducted for gate failure. Further, it does potential failures. If a gate failure can be handled not address gate failure for multiple upstream dams.
(e.g., freeboard still adequate), good. If all gates Concern: available required to avoid overtopping (i.e.,
There are infinite permutations for failure of gates given the information everything needs to work perfectly), then there provided. should be some concern.
The second staff position is incomplete Fuse plugs are generally considered to be reliable, but there is some inherent uncertainty about the Proposed Resolution: exact depth and duration of overtopping needed to Clarify the guidance for gate failure. initiate breach. There is also uncertainty about the exact rate of breach development. Understanding Understanding of Current Status: the magnitude of these uncertainties is important Page 29 of 66
Comment No. Comment NRC Response We understand that this additional guidance is being developed. because delayed operation of the fuse plug to lead to failure of the dam.
Staff position:
- With regard to fuse plugs, one should show that flood routings are not sensitive to the depth and duration of overtopping needed to initiate breach so that delayed operation does not lead to failure of a main dam.
Action:
Text added for fuse plugs 55 Location: 4.2.7.1, p 38 Response:
[J. Riley] Comment: Mud flows removed.
Staff Position: Action:
The potential for basin to generate mud/debris flows should be Section modified to address debris and sediment.
considered.
Concern:
What is the significance and concern with mud/debris as it relates to dam failure analysis or impact to the reservoir? Are basin specific studies being recommended or required?
Proposed Resolution:
The purpose analyzing mud/debris needs to be described including the hazard/risk associated with mud flows.
Understanding of Current Status:
We understand that this section may be deleted or modified to address Page 30 of 66
Comment No. Comment NRC Response debris and sediment, not mud.
56 Location: 4.2.7.2, p 39 Response:
[J. Riley] Comment: Potential for waterborne debris impacts to damage embankment or key appurtenances should be Staff Position:
considered.
Impact loads on structures due to waterborne debris should be In the event that the dam fails, water borne debris considered. In general, methods outlines in the FEMA Coastal impacts should be considered for SSCs important to Construction Manual and average size/weight for objects specified in safety.
ASCE Standards are acceptable Action:
Concern:
No change to text.
What structures need to be evaluated for impact loads for the HRR versus the IA? Does this apply only to the dams and appurtenances? If this analysis is intended for the NPP site, discrete velocities will be required at each structure being evaluated. The debris sources along with the size and depth of the flood will determine the volume Proposed Resolution:
Clarify position on the conditions being used to generate the debris (PMF or dam failure, etc) and where impact loads must be evaluated. If IA assumes all flooded SSCs are lost, would debris dynamic load analysis would not be required, or is it only intended to determine if flood retaining structures survive the debris impacts?
Understanding of Current Status:
We understand that the following two staff positions will be added to address this item:
- Loads due to waterborne debris carried by flood waters should be considered with regard to impacts on the dam (i.e., gates and Page 31 of 66
Comment No. Comment NRC Response associated mechanical equipment, appurtenances, parapets, etc.).
- In the case of dam break flood waves, debris impacts to SSCs important to safety should be considered.
Note that we believe that the second of the above bullets should be changed as follows to provided additional clarification: loads due to debris impact should be determined.
57 Location: Sec 5.2.1, p. 46 Response:
[J. Riley] Comment: The intent is to describe the conventional wisdom about how arch dams may fail in a seismic event.
3rd para. : This type of cracking eventually leads to isolated blocks This information could be used to model a breach of within the dam that subsequently rotate and swing downstream or the dam, if needed.
downstream, releasing the reservoir.
Action:
Concern:
Text changed to:
Please reword this sentence to clarify the intent.
This type of cracking eventually leads to isolated Proposed Resolution: Not provided blocks within the dam. The isolated blocks may Understanding of Current Status: Not provided subsequently rotate (swing downstream or upstream), catastrophically failing the dam and releasing the reservoir.
58 Location: 5.2.4, p 48 Response:
[J. Riley] Comment: The modified approach to seismic failures states that the 500 year flood should be used when the dam Staff position for levee failure during a seismic event - assumption of fails under 1/2 of the 1e-4 seismic ground motion. So, starting water level is not indicated when dam failure is assumed w/o any seismic Concern: analysis, the 500-year flood condition should be Proposed Resolution: used.
Starting water level should be consistent with that assumed for a Levees are not designed to withstand significant Page 32 of 66
Comment No. Comment NRC Response seismic dam failure evaluation seismic loads, so the latter of the two cases in the Understanding of Current Status: Not provided preceding paragraph is more applicable. If the 500-year flood is in excess of the levee height, then the top of the levee is more appropriate.
Action:
Staff position modified to reflect the discussion above..
59 Location: Sec .5.6, p. 55 Response:
[J. Riley] Comment: Use maximum normal pool elevation (i.e. top of active storage pool). Other starting water surface Staff Position, 1st bullet: Dam failure due to an earthquake should be elevations may be used, with appropriate considered for both maximum normal operating (full pool) and average justification. Justification should be based on reservoir levels.
operating rules and operating history of the Concern: reservoir.
- The maximum full pool level generally corresponds to a 10%/year Use hydrodynamically consistent headwater/tailwater frequency. Thus, the joint event failure probability considering the relations for routing. But favorable maximum normal operating full pool level is conservative by an order of headwater/tailwater relations (e.g., enhancing magnitude. stability) should not be assumed in the seismic
- Head water/tail water relationship prescribed is not possible for capacity analysis multiple reservoirs being simulated in a continuous hydraulic model for Action:
cascading dam failures.
Staff positions modified to clarify water surface Proposed Resolution: elevation and headwater/tailwater positions.
- Suggested change: Dam analysis to show sufficient margin for 10-4 ground motions should consider median (or average) reservoir levels. Maximum operating full pool level (10 percentile) should be considered with 10-3 ground motions.
- Revise guidance for the head water/tail water relationship as applied to cascading dam failures Page 33 of 66
Comment No. Comment NRC Response Understanding of Current Status: Not provided 60a Location: Sec. 5.6, p. 55 Response:
[J. Riley] Comment: The understanding of current status is correct. ISG will revert to modified ANS-2.8 approach in which Given the hazard frequency target of 1x10-6 discussed in Section SSE is replaced by 1e-4 seismic hazard ground 1.4.2, the dam failure flood wave at the site should be combined with motion and OBE is replaced by half of the 1e-4 flows of a frequency that result in a combined annual probability of ground motion. If the dam fails under the 1e-4 1x10-6. For example, if the dam fails under a 10-4 ground motion, ground motion, it should also be checked for 1/2 of combine the dam break flood wave with a 100-year flood. If the dam the 1e-4 ground motion.
fails under a 10-3 ground motion, combine the dam break flood wave it with a 1000-year flood. If seismic failure is just assumed, starting water surface elevations corresponding to the 500-year Concern:
flood should be assumed.
- In the example, the combined event probability does not reasonably Action:
account for the fact that the 1000-year flood is a seasonal event and the maximum flood water level at the plant site for the 1000-year river flood Text modified to follow the modified ANS-2.8 is present for a limited part of the year only. The earthquake ground approach.
motion (and the resulting flood wave) and the 1000-year flood are independent events. Thus, the joint probability of occurrence of the combine event should consider the limited duration of the maximum flood level for a 1000-year flood.
- The combining of an earthquake and a flood by simply multiplying their annual probabilities of occurrence does not allow for the very small duration within a year for the earthquake to coincide with a longer but still only a fairly small fraction of a year for the duration of most floods.
- This paragraph is changed from previously expressed NRC positions as discuss in public meetings
- What combination should be applied if seismic failure is just assumed?
Proposed Resolution:
Page 34 of 66
Comment No. Comment NRC Response
- Suggested change: For example, if the dam fails under a 10-4 ground motion, combine the dam break flood wave with a 10-year flood. If the dam fails under a 10-3 ground motion, combine the dam break flood wave with a 100-year flood. This example assumes that the high flood level at the plant site for the 10-year and 100-year floods will last approximately 1-month (10% of one year) or less before receding.
- See methodology in: Event Combination Analysis for Design and Rehabilitation of U.S. Army Corps of Engineers Navigation Structures by Bruce R. Ellingwood, Contract Report ITL-95-2, July 1995, US Army Corps of Engineers, Waterways Experiment Station
- Use event combinations as previously described in public meetings: 1.
seismic hazard frequency target of 1x10-4 with 25 year flood, 2. 0.5 x seismic hazard frequency target of 1x10-4 with 500 year flood.
Understanding of Current Status:
We understand that the ANS 2.8 seismic and flooding event combinations (modified with 10-4 ground motion) will be used in the final version of the ISG. i.e.,
- 10-4 ground motion with 25 year flood (Alt 1),
- 1/2 of 10-4 ground motion with 1/2-PMF or 500 year flood, whichever is less (Alt 2) 60b Location: Sec. 6.1.3 / p. 67 Response:
[J. Riley] Comment: Sunny-day failure of levee is not very likely to result in flooding.
General comment: It is unclear whether the sunny day failure mechanism is applicable to levees, since levees are normally subject to Action:
water loading only during flooding events. Sunny-day failure of levees removed from ISG Concern:
Page 35 of 66
Comment No. Comment NRC Response It is recognized that levee failure should be assumed if the levee is overtopped. Levee failure at elevations less than overtopping should be investigated; however, it is debatable whether these conditions can be considered sunny day.
Proposed Resolution:
Suggest consideration be given to removing levees from the sunny day failure mechanism section, and adding the information about levee failures included here to the hydrologic failure mechanism, with additional information as needed.
Understanding of Current Status:
The guidance on levees was moved from this section but the heading for the 6.1.3 still needs to be deleted.
61 Location: 6.2, p 68 Response:
[J. Riley] Comment: Current staff position is that sunny-day failures of critical dams should be postulated and Sunny day failure may be excluded from further consideration if it can consequences analyzed.
be shown by the licensee that the probability of failure is 10-6 per year or less. The 10-6 value is chosen since there is not sufficient data to Action:
allow for accurate calculations of this event. Reasonable arguments ISG modified to remove probabilistic analysis for justifying the case for a lower failure probability include but are not sunny-day failures.
limited to a recurring dam inspection and monitoring program, expert assessments that the dam is in good condition, and detailed inspection reports.
Concern:
What methodology for estimating a probability of failure is 10-6 per year or less would be acceptable to the NRC for sunny-day failure including piping or internal erosion failures.
Page 36 of 66
Comment No. Comment NRC Response Proposed Resolution:
Understanding of Current Status:
We understand that a probabilistic approach to sunny day dam failure exclusion will not be included in the document. Sunny day failures will need to be considered for all critical dams assuming the dams withstand hydrologic event 62 Location: Sec. 6.2.1 / p. 68 Response:
[J. Riley] Comment: Current staff position is that sunny-day failures of critical dams should be postulated and Staff Position bullet: Reasonable arguments justifying the case for a consequences analyzed.
lower failure probability include but are not limited to Action:
Concern:
ISG modified to remove probabilistic analysis for It is unclear what lower failure probability means in this context. Does sunny-day failures.
it mean lower than 10-6 failure probability?
Proposed Resolution:
Additional description of how to apply probability to the sunny day failure mechanism and possible pathways to take credit for non-failure would be helpful.
Understanding of Current Status:
We understand that a probabilistic approach to sunny day dam failure exclusion will not be included in the document.
63 Location: Sec. 6.2.1 / p. 68 Response:
[J. Riley] Comment: Current staff position is that sunny-day failures of critical dams should be postulated and The Staff Position states that reasonable arguments for a lower than 10-consequences analyzed.
6 per year risk of sunny day failure can be made using the existence of Page 37 of 66
Comment No. Comment NRC Response recurring dam inspection, monitoring program, expert assessments that Action:
the dam is in good condition and detailed inspection reports. ISG modified to remove probabilistic analysis for Concern: sunny-day failures.
Federal agency dam owners generally have all of this information at hand. Utilities would have to request this data from the Federal agency dam owners.
Proposed Resolution:
Propose that the NRC ask the federal agency dam owners to agree via an MOU to provide this data to certify that their dams need not be analyzed in detail for a sunny day failure.
Understanding of Current Status: Not provided We understand that a probabilistic approach to sunny day dam failure exclusion will not be included in the document 64 Location: Sec 6.2.2 / p. 68 Response:
[J. Riley] Comment: In view of the uncertainties involved in estimating reservoir levels that might reasonably be expected to The Staff Position to use the maximum observed or maximum normal prevail at the time of failure, the default starting water pool elevation for the sunny day breach analysis is excessive.
surface elevation used in flood routings for Concern: evaluation of overtopping should be the maximum
- the maximum observed pool elevation may be a very extreme event normal pool elevation (i.e. top of active storage pool).
and not reflect sunny day conditions, which if considered in conjunction Other starting water surface elevations may be used, with runoff from a PMP could result in an unreasonable predicted with appropriate justification. Justification should be maximum pool elevation. Such an extreme historical event may have a based on operating rules and operating history of the very low frequency and short duration relative to historical operation reservoir. The operating history used should be of depending on the riverine system and the upstream watershed. sufficient length to support any conclusions drawn (e.g., 20 years or more). But consideration should be
- The implication of the term sunny day is that it occurs during non- given to possible instances where the operating Page 38 of 66
Comment No. Comment NRC Response flood conditions. Use of the maximum observed pool links it to the inflow history and/or rules have been influenced by of record for the dam. anomalous conditions such as drought.
Proposed Resolution: Action:
- The default starting water surface elevation used in flood routings for Preceding text used as staff position.
evaluation of overtopping or sunny day failure is the maximum normal pool elevation. Other starting water surface elevations may be used with appropriate justification.
Understanding of Current Status:
We understand that the text will be modified to read:
the default initial water level used in breach analysis and flood routings for evaluation of sunny-day failure should be the higher of the maximum observed pool elevation or the maximum normal pool elevation. Other water levels may be used with justification (e.g.,
records showing that water levels above max normal poll are infrequent and of short duration).
Note that it would be useful to describe the attributes of a justification of infrequent and short duration.
65 Location: Sec 8.1, p. 72 Response:
[J. Riley] Comment: The loading conditions on the remaining monoliths after one has failed will be significantly different than 2nd paragraph: However, by using a dam-breach flood prediction before failure (e.g. the monoliths on either side of the model and making several applications of the model wherein the breach width parameter representing the combined lengths of assumed failed failed section will be subject to hydrodynamic forces of the water flowing through the breach). The monoliths is varied in each application, the resulting reservoir water surface elevations can be used to indicate the extent of reduction of the stability of the dam under the modified loading condition is a point to consider.
loading pressures on the dam. Since the loading diminishes as the breach width increases, a limiting safe loading condition which would Action:
Page 39 of 66
Comment No. Comment NRC Response not cause further failure may be estimated. No change to text.
Concern:
The benefit of this process is unclear. The maximum loading condition during an overtopping event would be present at time zero for all monoliths. Since failure of a single monolith is assumed to be quite short (on the order of minutes), reductions in upstream water levels are likely to not be significant enough to reduce pressures on other monoliths. Sensitivity analyses incorporating peak downstream breach flows and water surface elevations should also be considered as appropriate approaches to estimating breach width.
Proposed Resolution: Not provided Understanding of Current Status: Not provided 66 Location: Sec 8.2.2, p 76 Response:
[J. Riley] Comment: Xu & Zhang state that their erodibility index is based on the classifications presented in the Briaud paper.
However, their paper does not provide clear criteria for selecting the However, Briaud states in his paper that the erodibility index.
classification system is meant as a preliminary Concern: design tool and that the error in his category Xu and Zhang (2009) do not provide detailed criteria for selecting the assignments could as much as plus or minus one erodibility index because they state that they used definitions in a paper classification level. In addition, Briaud does not by Briaud, which provides detailed definitions. provide information regarding the number of soil tests that provide the basis for the classification Proposed Resolution: Not provided system. It appears that the classification is based on Understanding of Current Status: samples tested in his EFA device at TAMU (e.g results from a single device/laboratory). Regardless We understand that the Xu and Zhang (2009) breach methodology of the level of experimental support for the Briaud alone is not recommended for the 2.1 hazard re-analysis and if used, classification system, Xu & Zhang also state that would have to be bench-marked against another approach. their erodibility index takes into account additional Page 40 of 66
Comment No. Comment NRC Response factors such as dam cross-sectional geometry, slope surface protection, and compaction method.
However, their paper does not provide any of this additional information for the dams examined in the study or provide insight into how these additional factors were used to determine their erodibility index.
Thus, there is a lack of objective criteria for assigning the Xu & Zhang erodibility index to new dams.
Action:
No change to text.
67 Location: Sec 8.2.2, p 76 Response:
[J. Riley] Comment: Definition of failure time in Xu & Zhang paper is the widely used definition. But Teton Dam example given In addition, anecdotal evidence suggests that their relation for failure in paper indicates that authors did not consistently time may be biased in favor of longer times (Wahl, 2013).
apply the definition. Definition of failure time must be Concern: internally consistent within the regression analysis Xu and Zhang define failure time differently than in other empirical and between the regression analysis and the breach parameter studies. This means that one must use their failure hydrologic/hydraulic model.
time estimates in a breach model (e.g. HEC-RAS) in a way that is Action:
consistent with their definition. It is not a fundamental deficiency or flaw Additional text added to explain issues with Xu &
in the method.
Zhang paper
- The difference in reported failure time is more appropriately
- 1) Inconsistent use of failure time definition characterized as a difference in how it is defined based on the starting
- 2) Lack of basis for erodilbility. Briauds work and ending point. Not sure that anecdotal evidence is appropriate for an focus is on measurement.
ISG document Inappropriate for use w/ HEC-RAS Proposed Resolution:
Remove the statement Page 41 of 66
Comment No. Comment NRC Response Understanding of Current Status:
We understand that the Xu and Zhang (2009) breach methodology alone is not recommended for the 2.1 hazard re-analysis and if used, would have to be bench-marked against another approach.
68 Location: Sec 8.2.2.1, p 77 Response:
[J. Riley]
Comment: Evaluating the applicability of the proposed resolution would require in-depth examination of the Uncertainty in Predicted Breach Parameters and Hydrographs case studies that were used to develop the Concern: regression equation, in order to compare these dams It should be not necessary to cover the extreme values if there is a (or some subset of them) to the dam being modeled.
sound basis for limiting the range This does not appear to be a tractable approach in most cases.
Proposed Resolution:
Action:
It is useful to recognize that uncertainty in regression equations is associated with unexplained variance and that physical No change to text.
arguments/engineering justifications can be made as to where in the range of uncertainty a particular dam would be expected to fit given its physical characteristics that are not specifically included in the explained variance represented by the mathematical form of the regression equation. Therefore it may not be appropriate to perform sensitivity analyses over the entire range of uncertainty on predicted breach parameters (or predicted peak breach flow rates).
Understanding of Current Status: Not provided 69 Location: Sec 10.2, p. 84 Response:
[J. Riley]
Comment: Comment accepted 2nd complete sentence : Accurate estimates of flood elevation in areas Action:
of changing topography and near large objects in the flow field will Text modified to clarify that 2D analysis needed only Page 42 of 66
Comment No. Comment NRC Response typically require two-dimensional analysis. in regions where 2D effects are important.
Concern:
Suggest adding localized to sentence, as it is typically not necessary to perform two-dimensional analysis of the entire inundation area, which may be hundreds of miles long: .will typically require localized two-dimensional analysis.
Proposed Resolution: Not provided Understanding of Current Status: Not provided 70 Location: 1.3.2/page 5 Response:
[J.W. Shea, TVA]
Comment: With respect to debris, impact loads due to waterborne debris are the key issue. Impact loads on Step 5 -The reader is told to estimate the impacts of sediment and the dam and key appurtenances should be debris transport.
evaluated. In the event of dam failure, impact loads Concern: due to waterborne debris should be considered for The evaluation of debris and sediment transport and impacts to fluid exposed SSCs important to safety at the NPP site.
dynamics requires extensive, complex analysis. The main concerns regarding sediment transport Proposed Resolution: include: 1) impacts to predicted water surface elevations (e.g. sediment deposition will result in Clarify whether "impacts" refers to:(1) impacts on equipment for use higher water levels for a given discharge); 2) scour at during the integrated assessment; or (2) impacts on the fluid flow SSC structures; and 3) sediment accumulation in behavior itself (changes in fluid dynamics from additional sediment and UHS impoundment.
debris in the flood routing).
However, detailed guidance on sediment transport modeling is beyond the scope this ISG.
With respect to equipment for use in the integrated assessment, impact loads and sediment transport Page 43 of 66
Comment No. Comment NRC Response would be included in the associated effects of flooding that should be included in the hazard reevaluation. The impact that these associated effects have on the effectiveness of equipment and/or procedures relied upon for mitigation would be evaluated in the integrated assessment, if one is required.
Action:
Added section 9.3 which discusses general considerations for sediment transport modeling and provides references to the technical literature.
71 Location: 1.4.2/page 8 Response:
[J.W. Shea, TVA] Comment: This section was modified to remove requirement to evaluate seismic hazards at the 1e-6 annual Last bullet -Requires seismic analysis of 1 x 10-4 with sufficient margin exceedance level.
or seismic analysis to I x 10-6
- 1) For hydrologic failure analysis, the dam Concern:
should be able to pass the PMF via the The term 'sufficient margin" is not defined. As used, the term 'sufficient spillway and other discharge outlets. The margin" seems to imply more than meeting required factor of safety. structural loads/demands are hydrostatic and hydrodynamic loads of the reservoir Proposed Resolution: level associated with the PMF as well as Define "sufficient margin" to be the required factor of safety per Federal associated effects such as wind waves and Dam Safety Regulator's guidance. This term occurs in several places debris loads.
throughout the document.
Headwater/tailwater levels will be governed by inflow and the spillway conveyance relationships (and possibly backwater effects). Headwater/tailwater elevations will be calculated by the hydrologic or hydraulic Page 44 of 66
Comment No. Comment NRC Response routing model used.
The combination referred to in the comment is one alternative for deriving the PMF. PMF estimates typically include assumptions regarding antecedent base flow, soil moisture and rainfall conditions.
Factors of safety used in stability analysis should be consistent with accepted engineering practice and standards for the structure(s) in question.
- 2) For seismic failure, the load/demand are those effects (vibratory ground motion, displacement, liquefaction) associated with either the 1e-4 seismic hazard ground motion (or half of the 1e-4 hazard)
For the seismic stability calculation, default headwater elevation should be max normal pool level. Other levels can be used with justification. Tailwater should be average, nonflood levels. Flooding conditions should not be assumed to increase the stability of the dam.
Factors of safety used in stability analysis should be consistent with accepted engineering practice and standards for the structure(s) in question.
Page 45 of 66
Comment No. Comment NRC Response If the dam fails, the dam break flood wave should be combined with the 25 or 500 year flood (depending upon whether dam failed under the 1e-4 seismic hazard ground motion or 1/2 of the 1e-4 seismic hazard ground motion). When routing the flood wave, hydrologically consistent headwater/tailwater relationships, as calculated by the hydrologic or hydraulic routing model, should be used.
- 3) For sunny-day failure, the failure is simply assumed to occur. There is no specific load or demand. However, a breach scenario should be postulated.
For sunny-day failure flood routing, the default headwater elevation should be the max normal pool level. Other levels can be used with justification. Tailwater elevations will be calculated by hydrologic or hydraulic routing.
Action:
Section 1.4.2 modified to reflect the approaches for hazard evaluation described above.
72 Location: 3.1/page 22 Response:
[J.W. Shea, TVA] Comment: Removal of dams based only upon damage being Page 46 of 66
Comment No. Comment NRC Response Staff position says 'dams owned by licensees may not be removed" limited to the owners property does not apply to Concern: licensee owned dams (or onsite water control structures). In this situation additional analysis would There are licensee-owned dams that have minimal or no adverse failure be needed to justify that the dam or water control consequences beyond the owner's property. For example, there are structure meets the intent of the inconsequential holding ponds that are on the National Inventory of Dams that are category and may be removed from further owned by TVA. These are low hazard dams where failure under normal consideration.
(non-flood) conditions would result in environmental permit compliance issues with the state and is therefore deemed as a failure consequence Action:
to the owner's property. For flood analysis, these holding ponds would Section 3.1 modified to now read:
not increase the flood elevations at the sites and are inconsequential.
Removal of dams based only upon damage being Proposed Resolution: limited to the owners property does not apply to licensee owned dams (or onsite water control Given the situation described in the concern field and the fact that this structures). In this situation additional analysis would document is guidance, the proposed change to the statement is that the be needed to justify that the dam or water control "licensee owned dams should not be removed from consideration structure meets the intent of the inconsequential without justification."
category and may be removed from further consideration.
73 Location: 4.1.3/page 31 Response:
[J.W. Shea, TVA] Comment: The staff position is only meant to convey the general requirement for consideration of reduced The staff position requires engineering justification if failure of spillway conveyance capacity due to such failures. More gates and outlet works is not considered for hydrologic failure modes.
details are provided in subsequent sections of the Concern: ISG.
With a complex river system with multiple dams and many hydro units Action:
(IVA has 109 hydro units), the staff position is very difficult to implement without extensive analysis, such as an extensive uncertainty analysis The presumption of failure has been removed from with Monte Carlo simulations or other such analyses. The schedule for the staff position.
the flood hazard reevaluation doesn't support this type of analysis.
Page 47 of 66
Comment No. Comment NRC Response Proposed Resolution:
Recommend adding to the staff position, a third option, which would be a simplified system approach that allows probability of failure of gates and generating units to operate during flood events or application of an availability factor based on historical floods.
74 Location: 4.2/page 32, 5.2/page 45, & 6.1.1/page 66 Response:
[J.W. Shea, TVA] Comment:
The guidance is not clear on the establishment of loads/demands for 1) For hydrologic failure analysis, the dam detailed analysis. should be able to pass the PMF via the spillway and other discharge outlets. The Concern:
structural loads/demands are hydrostatic There is no separate section in the guidance for establishing the and hydrodynamic loads of the reservoir demands/loads for the detailed analysis as outlined in the flowchart in level associated with the PMF as well as Figure 2 of Section 1.3. The demands/loads are addressed in the associated effects such as wind waves and overtopping section of the document but only briefly. It is not clear to the debris loads.
reader if these loads are to include the combined effects from Appendix H of NUREG/CR-7046, e.g., Alternative 1 combination of mean monthly Headwater/tailwater levels will be governed base flow, median soil moisture, an antecedent or subsequent rain that by inflow and the spillway conveyance is the lesser of 40% PMP or 500 year rainfall, the PMP and waves relationships (and possibly backwater induced by 2-yearwind speed applied along the critical direction. effects). Headwater/tailwater elevations will Proposed Resolution: be calculated by hydrologic or hydraulic routing.
Provide clear guidance on the establishment of the demands/loads which are to be used for the detailed analysis of the dams. This same The combination referred to in the comment comment also applies to seismic load/demands for detailed analysis is one alternative for deriving the PMF. PMF and for sunny day loads/demands for detailed analysis. Guidance estimates typically include assumptions should include such specifics as the following: (1) headwater and regarding antecedent base flow, soil tailwater levels to be used in stability analysis; (2) whether antecedent moisture and rainfall conditions.
or subsequent rainfall must be combined with PMP; (3) whether or not Page 48 of 66
Comment No. Comment NRC Response to include 2-year wind speeds in the analysis; and (4) adequate factors Factors of safety used in stability analysis of safety. should be consistent with accepted engineering practice and standards for the structure(s) in question.
- 2) For seismic failure, the load/demand are those effects (vibratory ground motion, displacement, liquefaction) associated with either the 1e-4 seismic hazard ground motion (or half of the 1e-4 hazard)
For the seismic stability calculation, default headwater elevation should be max normal pool level. Other levels can be used with justification. Tailwater should be average, nonflood levels. Flooding conditions should not be assumed to increase the stability of the dam.
Factors of safety used in stability analysis should be consistent with accepted engineering practice and standards for the structure(s) in question.
If the dam fails, the dam break flood wave should be combined with the 25 or 500 year flood (depending upon whether dam failed under the 1e-4 seismic hazard ground motion or 1/2 of the 1e04 seismic hazard ground motion). When routing the flood wave, hydrologically consistent Page 49 of 66
Comment No. Comment NRC Response headwater/tailwater relationships, as calculated by the hydrologic or hydraulic routing model, should be used.
- 3) For sunny-day failure, the failure is simply assumed to occur. There is no specific load or demand. However, a breach scenario should be postulated.
For sunny-day failure flood routing, the default headwater elevation should be max normal pool level. Other levels can be used with justification. Tailwater elevations will be calculated by the hydrologic or hydraulic routing model used.
Action:
Figure 2 has been changed to clarify the relevant chapters that provide detailed information regarding the demands / loads.
75 Location: 4.2.21page 33 Response:
[J.W. Shea, TVA] Comment: NUREG-0800 states that dam failure should be In many cases the IDF is the probable maximum flood (PMF) developed evaluated using appropriate combination of antecedent flows as described by ANSI/ANS-by analyzing the impacts of the probable maximum precipitation (PMP) 2.8-1992. ANS-2.8-1992 states that each potentially Page 50 of 66
Comment No. Comment NRC Response event over the dams upstream watershed. critical dam should be subjected analytically to the Concern: PMF from their own contributing watershed. ANS-2.8-1992 further states that if an upstream dam The guidance seems to imply that the dams be evaluated for project would likely fail in the probable maximum flood from specific PMFs and if they are not able to pass the project specific PMFs its own watershed, it shall also be tested in the then they should be considered to fail without consideration for other probable maximum flood applicable to the total plant events. With a complex river system with multiple dams, the staff site watershed. If judged likely to fail in either case, position is unrealistic and overly conservative. The nuclear power plant the resulting flood wave shall be carried downstream PMP which produces the PMF is over a large watershed with smaller to the plant site for comparison and selection of the amounts of rainfall compared to the project specific PMFs which have critical case.
very high amounts of PMP over a smaller watershed. Action:
Proposed Resolution: Add the following text: to the section 4.2.9 Multiple Dam Failure due to Single Storm Scenario In addition to the evaluation of the dams for the IDF, allow the large watershed PMP and associated PMF to be used to evaluate the stability Operational rules may be considered but the starting water surface elevation at the most of the dams when there is a large watershed with many upstream dams. upstream dam under evaluation should be as This would be a further refinement in the hierarchical hazard analysis specified in Section 4.2.2.1. River flows downstream (HHA) process. of this dam should be based on the precipitation /
runoff from the basin encompassing the multiple dam scenario(s) under consideration.
76 Location: 4.2.2.2/page 34 Maximum normal pool elevation is defined as the
[J.W. Shea, TVA] elevation corresponding to the top of the active Comment:
storage.
Staff position talks about "maximum normal pool elevation" Action:
Concern:
Added a figure to Section 2.1.3 to clarify water levels Maximum normal pool elevation term is not defined. and storage volume definitions. Definitions are also Proposed Resolution: provided in the Terms and Definitions section (under storage)
Define what °maximum normal" is, and/or provide examples for different Page 51 of 66
Comment No. Comment NRC Response kinds of reservoirs. TVA defines maximum normal as the normal summer pool.
77 Location: 4.2.2.4/page 35 Response:
[J.W. Shea, TVA]
Comment: 5-10% capacity reduction is reasonable for dams with debris management. For dams that lack debris Consideration of debris blockage of spillway gates.
management, greater reductions may be Concern: appropriate. Capacity reductions as large as 35%
With a complex river system with multiple dams and many hydro units have been observed. This determination needs to be made on a case-by-case basis.
(TVA has 109 hydro units), the staff position is very difficult to implement without extensive analysis. The schedule for the flood hazard Action:
reevaluation doesn't support this level of analysis. Modified text to include description of Lake Lynn Proposed Resolution: Dam debris blockage. Modified staff position to include sensitively study using 5-10% capacity Recommend guidance provide percentage for spillway gate blockage. reduction for dams with debris management. Dams TVA's position that performance of sensitivity analyses on 5 percent and without debris management should consider greater 10 percent spillway gate blockage is appropriate. reductions on a case-by-case basis.
78 Location: 4.2.2.4/page 35 Response:
[J.W. Shea, TVA] Comment: Review of federal guidance on crediting discharge capacity through shows that different agencies take Last bullet -At least one turbine should always be assumed to be down different approaches. Therefore, turbine flows can in performing flood routings.
be credited, if engineering justification is provided.
Concern:
Action:
Dam operators typically perform their maintenance activities outside of Last bullet removed.
the flood season and the assumption that one unit is out of service for every hydro dam in a large system may be overly conservative. TVA Discussion of various federal guidelines on crediting has completed the hazard reevaluation input work for the darns (the turbine/powerhouse flows is added.
Page 52 of 66
Comment No. Comment NRC Response dam rating curves) assuming that all the hydro units are available until Discussion on use of site-specific and generic the turbine deck, switchyard or powerhouse is flooded. information on generating unit availability has been Proposed Resolution: added.
Recommend guidance allow a simplified system approach that considers probability of turbine outages or application of an availability factor based on maintenance data and/or historical floods.
79 Location: 4.2.6/page 39 Response:
[J.W. Shea, TVA] Comment: Text should have read:
Staff position -With regard to the fuse plugs, one should consider Fuse plugs are generally considered to be reliable, show that routing but there is some inherent uncertainty about the exact depth and duration of overtopping needed to Concern: initiate breach. There is also uncertainty about the exact rate of breach development. Understanding The sentence is incomplete and TVA is unable to understand the staff the magnitude of these uncertainties is important position regarding fuse plugs. because delayed operation of the fuse plug to lead to failure of the dam.
Proposed Resolution:
Staff position should have been:
Complete the sentence.
- With regard to fuse plugs, one should show that flood routings are not sensitive to the depth and duration of overtopping needed to initiate breach so that delayed operation does not lead to failure of a main dam.
Action:
Text corrected.
80 Location: 4.2.7.2/page 39 Response:
Page 53 of 66
Comment No. Comment NRC Response
[J.W. Shea, TVA] Comment: Typographic error.
No equation provided. Action:
Concern: Equation and definition of variables provided.
Editorial; no equation and variables provided.
Proposed Resolution:
Provide equation and define variables.
81 Location: 5/page 43 Response:
[J.W. Shea, TVA] Reference should be to Section 5.6 Comment:
Last paragraph, last sentence calls out a Section 0.
Concern: Action:
Editorial; user is not able to determine which section was meant to be Cross reference provided.
referenced.
Proposed Resolution:
Provide the correct section reference.
82 Location: 5.1/page 43 Response:
[J.W. Shea, TVA]
Comment: The modified approach to seismic states that the 500 year flood (or 1/2 PMF, whichever is less) should be When using the HHA process and assuming a seismic darn failure used when the dam fails under 1/2 of the 1e-4 seismic without a detailed analysis of the dam, there is no flood specified to use hazard ground motion. So, when dam failure is for hydrologic routing with the assumed failure.
assumed w/o any seismic analysis, the 500-year Concern: flood condition (or 1/2 PMF, whichever is less) should By not specifying to the user, the document seems to imply that a be used.
detailed seismic analysis is required to be performed which is contrary Action:
to NUREGICR-7046 HHA.
Page 54 of 66
Comment No. Comment NRC Response Proposed Resolution: Text in section modified to add staff position Provide the flood which is to be used for hydrologic routing when there Staff Position:
is an assumed failure of the dam under seismic loading.
- If seismic failure is simply assumed without analysis, the seismic failure should be assumed to occur under 500-year flood conditions (or 1/2 PMF, whichever is less).
83 Location: 5.2.3/page 48 Response:
[J.W. Shea, TVA] Comment: It is common practice to perform seismic analysis of spillway gates and other key appurtenances, Seismic analysis of appurtenant structures.
because their failure can lead directly to overtopping Concern: and failure of the dam.
This section implies the spillway gate system should be seismically However, the HHA approach includes the use of analyzed. It is not common practice to perform detailed seismic analysis conservative assumptions in lieu of more detailed of dam appurtenances within the dam safety industry. This is an analysis. It would be the responsibility of the extensive amount of work for the amount of dams within TVA's licensee to justify that some percentage of failure is watershed and very difficult for equipment installed 60-years ago. The conservative.
documentation of material and installation details will be a challenge.
Action:
The schedule for the flood hazard reevaluation doesn't support this type of analysis. No change to text.
Proposed Resolution:
Recommend staff guidance provide an option to consider some conservative percentage of failure of spillway gates, outlet works and other appurtenances instead of comprehensive detailed analysis.
84 Location:5.3.3/page 51 Response:
[J.W. Shea, TVA] Comment: Proposed revised text is the same as the existing Page 55 of 66
Comment No. Comment NRC Response Detailed investigations would include surveys and undisturbed sampling text in the document.
borings.
Action:
Concern:
Revised text to read:
This section implies the use of undisturbed sampling for direct "Detailed investigations would include surveys, in measurements of in situ densities and dynamic properties. However in situ field testing, and laboratory testing, as situ testing is often preferred for performing liquefaction analysis. appropriate, to (1) refine the preliminary Undisturbed sampling for laboratory testing of potentially liquefiable soil interpretation of the stratigraphy and the extent of often results in mixed results. potentially liquefiable soils, and (2) measure in situ Proposed Resolution: densities and dynamic properties for input to dynamic response analyses, (3) recover undisturbed Recommend paragraph be revised to read: "Detailed investigations samples for laboratory testing when site soils are not would include surveys and in situ field testing to (1) refine the adequately represented in the available data base.
preliminary interpretation of the stratigraphy and the extent of potentially liquefiable soils, and (2) measure in situ densities and dynamic properties for input to dynamic response analyses. Recover undisturbed samples for laboratory testing when site soils are not adequately represented in the available data base.
85 Location: 5.3.3/page 51 Response:
[J.W. Shea, TVA] Comment: The point of referencing RG-1.198 was to provide guidance with respect technical methodology. The Section 5.3.3 and Staff Position references NRC Regulatory Guide March 2012 Request for Information does not 1.198 Procedures and Criteria for Assessing Seismic Soil Liquefaction stipulate that Appendix B requirements apply to at Nuclear Power Plants" as providing guidance and detailed responses to the information request. However, if procedures for evaluating liquefaction.
the analyses submitted in response to the request for Concern: information are later used for certain licensing The staff position reference to RG 1.198 could imply that Appendix B purposes, Appendix B requirements may apply.
requirements are applicable to the ISG users in the future. These Action:
requirements are not applicable under this scope of work.
No change to text.
Page 56 of 66
Comment No. Comment NRC Response Proposed Resolution:
Recommend that the ISG clearly state that the Appendix B requirements do not apply or remove the reference to RG 1.198 and reference the engineering methods for liquefaction analysis directly.
86 Location: 5.4.1/page 52 Response:
[J.W. Shea, TVA]
Comment: Sufficient margin is usually defined in terms of a safety factor.
Staff position -Sufficient seismic margin in existing studies.
Action:
Concern:
Provided reference to FEMA guidelines on Sufficient seismic margin is not defined.
earthquake analysis of dams, which discusses Proposed Resolution: appropriate factors of safety.
Define 'sufficient margin" to be the required factor of safety per Federal Dam Safety Regulator's guidance. This term occurs in several places throughout the document.
88 Location: 5.5/page 54 Response:
[J.W. Shea, TVA] Comment: Third bullet in Figure 16 was meant to read:
Figure 16 - appears to have a repeat in the last bullet Ground motions causing failure at Dam 3 cannot be excluded from causing failure at Dam 2 Concern:
Action:
Editorial Text corrected in Figure 16.
Proposed Resolution:
Remove the repeat bullet or correct the bullet if it was meant to be a 3rd point in the figure.
89 Location:5.6/page 56 Response:
[J.W. Shea, TVA] In view of the uncertainties involved in estimating Page 57 of 66
Comment No. Comment NRC Response Comment: reservoir levels that might reasonably be expected to prevail at the time of failure, the default starting Staff position -Dam failure due to an earthquake should be considered water surface elevation used in flood routings for for both the maximum normal operating ('full-pool) and average evaluation of seismic failure consequences should reservoir levels. Normal, non-flood tailwater conditions should be used. be the maximum normal pool elevation (i.e. top of active storage pool). Other starting water surface Concern: elevations may be used, with appropriate justification. Justification should be based on It is not clear what is meant by "maximum normal pool'. The water operating rules and operating history of the elevation used in earthquake load case is generally the normal reservoir. The operating history used should be of operating level. The highest normal operating level is used when there sufficient length to support any conclusions drawn.
are seasonal fluctuations of the reservoir. But consideration should be given to possible instances where the operating history and/or rules Proposed Resolution: have been influenced by anomalous conditions such as drought.
It is recommended that the normal pool level with normal tailwater levels is used rather than maximum and average pool. The use of these levels Action:
aligns with the TVA (and other federal dam regulators) dam safety guidance for seismic stability analysis. Staff position reflecting the preceding discussion inserted in this section.
90 Location: 5.6/page 56 Response:
[J.W. Shea, TVA] Comment: Staff position has been modified to reflect the modified ANS-2.8 approach..
Staff position -The flood and seismic combinations to provide a 1x10-6 hazard frequency target. Action:
Concern: Staff position has been modified to reflect the modified ANS-2.8 approach.
This implies that a seismic fragility analysis is required for each dam and then flood inflows be developed to route with the failure of the dam.
This requires extensive analysis for a complex river system and is more difficult to implement than the two deterministic combinations that are defined in ANS 2.8. Modified ANS 2.8 combinations have been discussed with the staff for replacement of the deterministic earthquake Page 58 of 66
Comment No. Comment NRC Response with a probabilistic earthquake.
Proposed Resolution:
Recommend guidance include use of the modified ANS 2.8 combinations that have previously been discussed with the staff.
Those combinations are: 1) 1E-04 ground motion combined with 25 year flood and 2) 1/2 of IE-04 ground motion combined with lesser of 500 year flood or 1/2 PMF.
91 Location: 6.2.1/page 68 Response:
[J.W. Shea, TVA]
Comment: Staff position has changed to require analysis of sunny-day failure, so the comprehensive risk The ISG requires a comprehensive risk analysis to assess sunny day analysis to show nonfailure is no longer relevant.
failure modes.
Action:
Concern:
The discussion of comprehensive risk analysis for Significant resources will be required to complete these analyses. All of sunny-day failure has been removed.
TVA dams do not have existing Potential Failure Mode Analyses (PFMA) completed yet. The schedule for the flood hazard reevaluation doesn't support this type of analysis.
Proposed Resolution:
Allow use of simplified but conservative failure modes when there is a lack of an existing PFMA.
92 Location: 6.2.2/page 69 Response:
[J.W. Shea, TVA]
Comment: The staff position has been modified to be consistent with other statements about initial water levels. The The normal pool elevation (invert of the highest outlet or spillway) default water level is maximum normal pool (top of Page 59 of 66
Comment No. Comment NRC Response definition needs clarification. active pool) or other level, with justification.
Concern: Action:
This is confusing as it could be defined as a spillway sill elevation which Staff position:
would be significantly lower than normal pool. In view of the uncertainties involved in estimating Proposed Resolution: reservoir levels that might reasonably be expected to prevail at the time of failure, the default starting Suggest ISG document be revised to reflect a normal pool elevation water surface elevation used in flood routings for where reservoir is maintained for normal operations. evaluation of overtopping should be the maximum normal pool elevation (i.e. top of active storage pool). Other starting water surface elevations may be used, with appropriate justification. Justification should be based on operating rules and operating history of the reservoir. The operating history used should be of sufficient length to support any conclusions drawn (e.g., 20 years or more). But consideration should be given to possible instances where the operating history and/or rules have been influenced by anomalous conditions such as drought.
93 Location: 6.2.2/page 69 Response:
[J.W. Shea, TVA] Comment: Maximum normal pool elevation is defined as the top of active storage. Reference to maximum observed Last bullet - maximum observed pool elevation and maximum normal pool level is no longer used.
pool elevation Action:
Concern:
Staff position now reads:
These terms are not defined in the document.
In view of the uncertainties involved in estimating Proposed Resolution: reservoir levels that might reasonably be expected Suggest a definition be added to ISG document and/or provide to prevail at the time of failure, the default starting water surface elevation used in flood routings for examples. evaluation of overtopping should be the maximum Page 60 of 66
Comment No. Comment NRC Response normal pool elevation (i.e. top of active storage pool). Other starting water surface elevations may be used, with appropriate justification. Justification should be based on operating rules and operating history of the reservoir. The operating history used should be of sufficient length to support any conclusions drawn (e.g., 20 years or more). But consideration should be given to possible instances where the operating history and/or rules have been influenced by anomalous conditions such as drought.
94 Location:7.1/page 70 Response:
[J.W. Shea, TVA] Comment: The observation is correct.
4th bullet - loss of generation by flooding of switchyard. Action:
Concern: Modified statement to reflect loss of powerhouse, or switchyard. This discussion is now in Section 4.7.2.1.
In most cases the tail deck controls when generation is stopped.
Switchyard is usually at a higher elevation than the tail deck (the point at which the powerhouse is flooded due to high tailwater).
Proposed Resolution:
Suggest incorporating the consideration of loss of the switchyard or the powerhouse due to flooding, whichever is at a lower elevation.
95 Location: 10.1.2 and 10.2 /pages 82-84 Response:
[J.W. Shea, TVA] Comment: The ISG does not propose 2D analysis for the entire watershed or river system. 2D analysis is proposed NRC prefers use of 2-D analysis over a 1-D analysis.
for cases where it may have a significant effect on Concern: calculation of inundation water level and velocities at Efforts to address the issues discussed in this section can have a the NPP site. The NRC does not endorse specific significant impact on the time required to conduct the analyses. Effort modeling software. The use of a particular package Page 61 of 66
Comment No. Comment NRC Response to develop and calibrate a 2-D model is well beyond that for a 1-D should be justified by the licensee.
model and the current hazard reevaluation analysis schedule does not Action:
support a 2-D analysis for a large and complicated river system.
No change to text.
Proposed Resolution:
TVA intends to use a 1-D HECRAS analysis. Recommend guidance include a listing of 1-D and 2-D models for which appropriate analyses have been reviewed and approved by NRC staff.
96 Location: Section 1.4.2, from page 56 Response:
[K. Canvan, EPRI] Modeling Consequences of Seismic Dam Failure Section 1.4.2 has been revised to remove this Given the hazard frequency target of 1x10-6 discussed in Section approach of combining earthquake and flood. See 1.4.2, the dam failure flood wave at the site should be combined with response to comment 39 flows of a frequency that result in a combined annual probability of lx1 Action:
0-6. For example, if the dam fails under a 10-4 ground motion; combine the dam break flood wave with a 100-year flood. If the dam fails under a See response to comment 39.
10-3 ground motion, combine the dam break flood wave it with a 1000-year flood."
Comment:
The combining of an earthquake and a flood by simply multiplying their annual probabilities of occurrence does not allow for the very small duration within a year for the earthquake to coincide with a longer but still only a fairly small fraction of a year for the duration of most floods.
Proposed Resolution:
Recommend consideration of methodology in: Event Combination Analysis for Design and Rehabilitation of U.S. Army Corps of Engineers Navigation Structures by Bruce R. Ellingwood, Contract Report ITL Page 62 of 66
Comment No. Comment NRC Response 2, July 1995, US Army Corps of Engineers, Waterways Experiment Station.
97 Location: Page 76 Response:
[K. Canvan, EPRI] "However, their paper does not provide clear criteria for selecting the Same as comment 66 [J. Riley]. See response to erodibility index." comment 66 Comment: Action:
Xu and Zhang (2009) do not provide detailed criteria for selecting the See response to comment 66 erodibility index because they state that they used definitions in a paper by Briaud, which provides detailed definitions.
98 Location: page 76 - "In addition, anecdotal evidence suggests that their Response:
[K. Canvan, EPRI] relation for failure time may be biased Same as comment 67 [J. Riley]. See response to in favor of longer times (Wahl, 2013)." comment 67 Comment: Action:
Xu and Zhang define failure time differently than in other empirical See response to comment 67 breach parameter studies. This means that one must use their failure time estimates in a breach model (e.g. HEC-RAS) in a way that is consistent with their definition. It is not a fundamental deficiency or flaw in the method.
99 Location: pages 77 and 78 - Section 8.2.2.1, Uncertainty in Predicted Response:
[K. Canvan, EPRI] Breach Parameters and Hydrographs and Section 8.2.2.2, Performing Same as comment 68 [J. Riley]. See response to Sensitivity Analyses to Select Breach Parameters. comment 68 Comment: It is useful to recognize that "uncertainty" in regression equations is associated with "unexplained variance" and that physical Action:
arguments/engineering justifications can be made as to where in the No change to text.
range of "uncertainty" a particular dam would be expected to fit given its physical characteristics that are not specifically included in the "explained variance" represented by the mathematical form of the regression equation. Therefore it may not be appropriate to perform Page 63 of 66
Comment No. Comment NRC Response sensitivity analyses over the entire range of uncertainty on predicted breach parameters (or predicted peak breach flow rates).
100. Comment: Response:
[M.Conner] NRC is to be commended for considering dam safety implications in Flood risks at nuclear power plants due to potential light of the events that occurred during and following the Fukushima upstream dam failure has been considered by NRC earthquake. This effort demonstrates a commitment to better in reactor siting and licensing decisions since the understand dam safety risks as they relate to potential flood risks at early days of the commercial nuclear power industry.
downstream nuclear power plants. In light of the events at Fukushima, the NRC has asked commercial reactor licensee to reevaluate their design basis flood estimates. This exercise includes consideration of flooding hazards due to potential upstream dam failure.
Action: No change to text 101. Comment: Response:
[M.Conner] Reclamation and the Department of the Interior own, permit, operate The screening process described in this ISG is and/or maintain many dams within the watersheds upstream of nuclear intended to use readily available information from power plants. The, proposed guidance could lead to many requests for public sources such as the USACE National information about dams within our inventory. Given the difficult budget Inventory of Dams, and the USGS National challenges we are currently facing, we must be sure that the efforts of Hydrography Dataset. In addition, due to the very our staff are focused on the highest priority dam safety risk issues. It is small number of nuclear power plants downstream of likely that prior studies performed for our dams will not meet the exact large USBR dams, NRC does not anticipate that needs of NRC and would require considerable staff time to assure that considerable staff time by USBR staff will be any data and/or analysis results provided are being used and required.
represented appropriately. We believe that the screening process needs Action: No change to text to address an additional objective of limiting the impact of data Page 64 of 66
Comment No. Comment NRC Response collection efforts on dam owners to those dams that can be shown to have a reasonable likelihood of being significant contributors to the flood. risk at the nuclear power plants.
102. Comment: Response:
[M.Conner] We suggest that NRC consider an alternative strategy for assessing The alternative strategy outlined relies on a number flooding hazards. In most cases, the flooding hazard due to dam failure of plausible assumptions. However these will be dominated by one or a few dams immediately upstream of the assumptions are not a sufficient basis for nuclear power plant. A much more efficient use of resources for making demonstrating safety in the detailed manner required this assessment would be to start with an assessment of the dam by the NRC staff.
immediately upstream of the plant and progressively add assessments Action: No change to text of upstream dams until it is apparent that further assessments don't substantially alter the flooding hazard at the nuclear plant. This will minimize the investment of resources necessary for dams that will be found to make no substantial contribution to flooding hazards at the nuclear power plant.
103. Comment: Response:
[M.Conner] The guidance indicates a double standard for assessing risk. Hydrologic The guidance has been revised to remove the 1e-6 loads appear to be required to meet a standard 100 times more annual exceedance probability target for hydrologic conservative than acceptable seismic loads. While this may be failure. The ability to pass the PMF is now used as commonly accepted practice in the nuclear industry, it leaves an avenue the criteria for nonfailure under hydrologic loading for questioning the credibility of the assessments when there is a conditions. See the response to Comment #39.
systematic discounting of seismic Action: See response to Comment #39.
loads.
104. Comment: Response:
[M.Conner] Multiple dam failure scenarios are much more complex than portrayed Multiple dam failures, either cascading failures or in the guidelines: concurrent failure of dams on adjacent basins have
- The worst case scenarios described require a series of been observed. The likelihood of such events must simultaneous events that would likely place most of them well be considered on a case-by-case basis. General Page 65 of 66
Comment No. Comment NRC Response below the 1e-7 risk objective conclusions regarding the entire class of such events
- For hydrologic loads at dams large enough to be consequential, is not sufficient.
the likelihood of a storm of sufficient areal extent and intensity Action: No change to text to fail multiple dams on adjacent basins is much less likely than a storm that would fail a single damn.
105. Comment: Response:
[M.Conner] Communication of the flood hazard information is a significant concern. Many assumptions go into assessing potential While the many conservative assumptions incorporated into the nuclear power plant flooding after dam failure.
guidance provide a safety net for the assurance of nuclear safety, they NRCs new guidance generally uses very could have an unintended consequence of unnecessarily raising conservative assumptions, leading to conservative concern in the public regarding the safety of dams in a watershed. If the results. This approach is appropriate given the goal is to provide assurance that the nuclear plants can accommodate a potentially-severe and long-term consequences robust set of flood hazards, we recommend that communication of associated with nuclear power plant flooding.
design loads be limited to conveying a series of one or more stage- However, these assessments should in no way discharge. relationships for which the nuclear plant has been evaluated. reflect on any dam being evaluated as part of the The basis for the stage-discharge relationships could be simply nuclear power plant licensees evaluation. Questions described as a combination of operational flood releases and potential about a particular dam should be sent to the dam dam failure scenarios at upstream dams. Such an approach could avoid owner, operator, or regulator.
costly, difficult and unnecessary public affairs issues associated with the dams. Action: No change to text
- END of COMMENTS ***
Page 66 of 66
JAPAN LESSONS-LEARNED PROJECT DIRECTORATE
Guidance For Assessment of Flooding Hazards Due to Dam Failure Interim Staff Guidance Revision 0 July 29, 2013 ML13151A153
JAPAN LESSONS-LEARNED PROJECT DIRECTORATE
Guidance For Assessment of Flooding Hazards Due to Dam Failure Interim Staff Guidance Revision 0 ADAMS Accession No.: ML13151A153 *Via E-mail OFFICE NRR/JLD/PMB NRO/DSEA/RHMB* RES/DRA/ETB* NRR/JLD*
NAME GEMiller KSee JKanney SLent DATE 07/22/2013 07/24/2013 07/24/2013 07/23/2013 OFFICE NRR/JLD/PMB* NRO/DSEA/RHMB* QTE* OGC*
NAME MMitchell CCook JDougherty SClark (NLO)
DATE 07/25/2013 07/24/2013 07/19/2013 07/29/2013 OFFICE NRR/DE NRO/DSEA* NRR/JLD NAME PHiland NChokshi DSkeen DATE 07/25/2013 07/25/2013 07/29/2013
Official Record Copy July 29, 2013 ML13151A153
INTERIM STAFF GUIDANCE JAPAN LESSONS-LEARNED PROJECT DIRECTORATE GUIDANCE FOR ASSESSMENT OF FLOODING HAZARDS DUE TO DAM FAILURE JLD-ISG-2013-01 PURPOSE This interim staff guidance (ISG) is being issued to provide guidance acceptable to the staff of the U.S. Nuclear Regulatory Commission (NRC) for re-evaluating flooding hazards due to dam failure as described in NRCs March 12, 2012, request for information (Ref. 1) issued pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Section 50.54, Conditions of licenses, regarding Recommendation 2.1 of the enclosure to SECY-11-0093, Recommendations for Enhancing Reactor Safety in the 21st Century, the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident (Ref. 2). Among other actions, the letter dated March 12, 2012, requests that respondents reevaluate flood hazards at each site and compare the reevaluated hazard to the current design basis at the site for each flood mechanism. Addressees are requested to perform an integrated assessment if the current design-basis flood hazard does not bound the reevaluated flood hazard for all mechanisms. This ISG will assist operating power reactor respondents and holders of construction permits under 10 CFR Part 50 in performing flooding hazard assessments due to dam failure. The guidance provided in this ISG describes methods that can be used as part of performing the flooding hazard reanalysis requested in Enclosure 2 of the letter dated March 12, 2012.
BACKGROUND Following the events at the Fukushima Dai-ichi nuclear power plant, the NRC established a senior-level agency task force referred to as the Near-Term Task Force (NTTF). The NTTF conducted a systematic and methodical review of the NRC regulations and processes and determined if the agency should make additional improvements to these programs in light of the events at Fukushima Dai-ichi. As a result of this review, the NTTF developed a comprehensive set of recommendations, documented in the enclosure to SECY-11-0093 (Ref. 2). These recommendations were enhanced by the NRC staff following interactions with stakeholders. Documentation of the NRC staffs efforts is contained in SECY-11-0124, Recommended Actions to be Taken Without Delay From the Near-Term Task Force Report, dated September 9, 2011 (Ref. 3), and SECY-11-0137, Prioritization of Recommended Actions to be Taken in Response to Fukushima Lessons Learned, dated October 3, 2011 (Ref. 4).
As directed by the staff requirements memorandum in the enclosure to SECY-11-0093 (Ref. 5), the NRC staff reviewed the NTTF recommendations within the context of the NRCs existing regulatory framework and considered the various regulatory vehicles available to the NRC to implement the recommendations. SECY-11-0124 and SECY 0137 established the staffs prioritization of the recommendations based upon the potential safety enhancements. As part of the staff requirements memorandum for SECY-11-0124, dated October 18, 2011 (Ref. 6), the Commission approved the staff's proposed actions, 2
including the development of three information requests under 10 CFR 50.54(f). The information collected will be used to support the NRC staff's evaluation of whether further regulatory action should be pursued in the areas of seismic and flooding design and emergency preparedness. In addition to Commission direction, the Consolidated Appropriations Act, Public Law 112-074, which contains the Energy and Water Development Appropriations Act, 2012, was signed into law on December 23, 2011. Section 402 of the law requires a reevaluation of licensees' design basis for external hazards.
In response to the aforementioned Commission and Congressional direction, the NRC issued a request for information to all power reactor licensees and holders of construction permits under 10 CFR Part 50 on March 12, 2012 (50.54(f) letter)(Ref. 1). The March 12, 2012, 50.54(f) letter includes a request that respondents reevaluate flooding hazards at nuclear power plant sites using updated flooding hazard information and present-day regulatory guidance and methodologies. The letter also requests the comparison of the reevaluated hazard to the current design basis at the site for each potential flood mechanism. If the reevaluated flood hazard at a site is not bounded by the current design basis, respondents are requested to perform an integrated assessment. The integrated assessment will evaluate the total plant response to the flood hazard, considering multiple and diverse capabilities such as physical barriers, temporary protective measures, and operational procedures. The NRC staff will review the responses to this request for information and determine whether regulatory actions are necessary to provide additional protection against flooding. This ISG is specific to the assessment of flood hazards due to dam failure.
On April 25, 2013, the NRC staff issued a draft version of this ISG and published a notice of its availability for public comment in the Federal Register (78 FR 24439). The 30-day comment period ran April 25, 2013, through May 28, 2013, during that time the staff received 105 public comments in six submittals. Comments received were related to the following topical areas: (1) general comments; (2) comments specific to hydrologic dam failure; (3) comments specific to seismic dam failure; and (4) comments specific to sunny-day dam failure. In public meetings on May 2, 2013, and May 22, 2013, the NRC staff interacted with external stakeholders to discuss, understand, and resolve public comments.
Modifications were made to text of the ISG in response to the public comments and the outcomes of the public meetings. Full detail of the comments, staff responses, and the staffs bases for changes to the ISG are contained in NRC Response to Public Comments to JLD-ISG-2013-01 (Docket ID NRC-2013-0073) (Ref. 7).
RATIONALE On March 12, 2012, the NRC issued a request for information to all power reactor licensees and holders of construction permits under 10 CFR Part 50. The request was issued in accordance with the provisions of Sections 161c, 103b, and 182a of the Atomic Energy Act of 1954, as amended (the Act), and NRC regulation in Title 10 of the Code of Federal Regulations, Part 50, Paragraph 50.54(f). Pursuant to these provisions of the Act and this regulation, respondents were required to provide information to enable the staff to determine whether a nuclear plant license should be modified, suspended, or revoked.
The information request directed respondents to submit a reevaluated flooding hazard for their sites using updated information and present-day regulatory guidance and methodologies. This ISG describes approaches for assessment of flood hazards due to dam failure.
3
APPLICABILITY This ISG shall be implemented on the day following its approval. It shall remain in effect until it has been superseded or withdrawn.
PROPOSED GUIDANCE This ISG is applicable to holders of operating power reactor licenses and construction permits under 10 CFR Part 50. For combined license holders under 10 CFR Part 52, the issues in NTTF Recommendations 2.1 and 2.3 regarding seismic and flooding reevaluations and walkdowns are resolved and thus, this ISG is not applicable.
IMPLEMENTATION Except in those cases in which a licensee or construction permit holder under 10 CFR Part 50 proposes an acceptable alternative method for the assessment of flood hazards due to dam failure, the NRC staff will use the methods described in this ISG to evaluate the results of the assessment.
BACKFITTING DISCUSSION This ISG does not constitute backfitting as defined in 10 CFR 50.109 (the Backfit Rule) and is not otherwise inconsistent with the issue finality provisions in Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants, of 10 CFR. This ISG provides guidance on an acceptable method for responding to a portion of an information request issued pursuant to 10 CFR 50.54(f). Neither the information request, nor the ISG require the modification or addition to systems, structures, or components, or design of a facility.
Applicants and licensees may voluntarily use the guidance in JLD-ISG-2013-01 to comply with the request for information. The information received by this request may, at a later date, be used in the basis for imposing a backfit. The appropriate backfit review process would be followed at that time.
FINAL RESOLUTION The contents of this ISG, or a portion thereof, may subsequently be incorporated into other guidance documents, as appropriate.
ENCLOSURES Guidance for Assessment of Flooding Hazards Due to Dam Failure REFERENCES
- 1. U.S. Nuclear Regulatory Commission, Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, March 12, 2012, Agencywide Documents Access and Management System (ADAMS) Accession No. ML12053A340.
- 2. U.S. Nuclear Regulatory Commission, Recommendations for Enhancing Reactor Safety in the 21st Century, The Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, Enclosure to SECY-11-0093, July 12, 2011, ADAMS Accession No. ML111861807.
4
- 3. U.S. Nuclear Regulatory Commission, Recommended Actions to be Taken Without Delay From the Near Term Task Force Report, SECY-11-0124, September 9, 2011, ADAMS Accession No. ML11245A158.
- 4. U.S. Nuclear Regulatory Commission, Prioritization of Recommended Actions to be Taken in Response to Fukushima Lessons Learned, SECY-11-0137, October 3, 2011, ADAMS Accession No. ML11272A111.
- 5. U.S. Nuclear Regulatory Commission, Staff Requirements - SECY-11-0093 - Near-Term Report and Recommendations for Agency Actions Following the Events in Japan, August 19, 2011, ADAMS Accession No. ML112310021.
- 6. U.S. Nuclear Regulatory Commission, Staff Requirements - SECY-11-0124 -
Recommended Actions to be Taken Without Delay From the Near-Term Task Force Report, October 18, 2011, ADAMS Accession No. ML112911571.
- 7. U.S. Nuclear Regulatory Commission, NRC Responses to Public Comments, Japan Lessons-Learned Project Directorate Interim Staff Guidance (JLD-ISG-2013-01):
Guidance for Assessment of Flooding Hazards Due to Dam Failure in Response to the March 2012 Request for Information Letter, July 29, 2013, ADAMS Accession No. ML13151A153.
5
Guidance for Assessment of Flooding Hazards Due to Dam Failure Enclosure
Table of Contents Table of Contents ii Tables v Figures v
- 1. Introduction 1-1 1.1 Purpose 1-1 1.2 Scope 1-2 1.3 Framework for Dam Failure Flood Hazard Estimation 1-3 1.3.1 Screening 1-3 1.3.2 Detailed Analysis 1-5 1.4 Probabilistic and Deterministic Hazard Analysis 1-7 1.4.1 Historical Dam and Levee Failure Rates 1-7 1.4.2 Hydrologic Failure 1-8 1.4.3 Seismic Failure 1-8 1.4.4 Sunny-Day Failure 1-8 1.5 Interfacing with Owners and Regulators of Dams and Levees 1-9 1.5.1 Dam Safety Governance 1-9 1.5.2 Dam Safety Guidance by Other Federal Agencies 1-10 1.5.3 Obtaining Information on Dams and Levees 1-11 1.6 Organization of guidance 1-12
- 2. Background 2-1 2.1 Classification of Dams and Levees 2-1 2.1.1 Concrete Dams 2-1 2.1.2 Embankment Dams 2-3 2.1.3 Water Levels and Storage Volumes 2-6 2.1.4 Levees 2-6 2.2 Classification of Dam Failures 2-8 2.2.1 Influence of Dam Type on Failure Modes 2-8 2.2.1.1 Concrete Dams 2-8 2.2.1.2 Embankment Dams 2-9 2.2.2 Failure of Spillways, Gates, Outlet Works and Other Appurtenances 2-9 2.2.3 Operational Failures and Controlled Releases 2-10 2.3 Multiple Dam Failures 2-10
- 3. Screening and Simplified Modeling Approaches for Watersheds with Many Dams 3-1 3.1 Criteria for Inconsequential Dams 3-2 3.2 Simplified Modeling Approaches 3-3 3.2.1 Representing Clusters of Dams 3-9
- 4. Hydrologic Dam Failure 4-1 4.1 Hydrologic Failure by Structure Type 4-1 4.1.1 Concrete Dams 4-1 4.1.2 Embankment Dams 4-1 ii
4.1.3 Spillways, Gates, Outlet Works and Other Appurtenances 4-2 4.1.4 Levees 4-2 4.2 Analysis of Hydrologic Failure Modes 4-2 4.2.1 Internal Pressure 4-3 4.2.2 Overtopping 4-3 4.2.2.1 Reservoir Capacity 4-4 4.2.2.2 Starting Reservoir Elevation 4-4 4.2.2.3 Reservoir Surcharge Capacity 4-5 4.2.2.4 Spillway Discharge Capacity 4-5 4.2.2.5 Wave Action 4-7 4.2.3 Structural Overstressing of Dam Components 4-7 4.2.4 Surface Erosion from High Velocity and Wave Action 4-8 4.2.5 Failure of Spillways 4-8 4.2.6 Failure of Gates 4-9 4.2.7 Operational Failures and Controlled Releases 4-10 4.2.7.1 Operational Failures 4-10 4.2.7.2 Controlled releases 4-11 4.2.8 Waterborne Debris 4-11 4.2.9 Multiple Dam Failure due to Single Storm Scenario 4-12 4.2.10 Levee Failures 4-Error!
Bookmark not defined.Error! Bookmark not defined.
- 5. Seismic Dam Failure 5-1 5.1 Overview 5-1 5.1.1 Seismic Hazard Characterization 5-2 5.1.1.1 Use of USGS National Seismic Hazard Maps 5-2 5.1.2 Structural Considerations 5-2 5.1.3 Probabilistic Seismic Hazard Analysis 5-3 5.2 Seismic Failure by Structure Type 5-3 5.2.1 Concrete Dams 5-3 5.2.2 Embankment Dams 5-4 5.2.3 Spillways, Gates, Outlet Works and Other Appurtenances 5-5 5.2.4 Levees 5-5 5.3 Analysis of Seismic Hazards Using Readily Available Tools and Information 5-6 5.3.1 Ground Shaking 5-7 5.3.2 Fault Displacement 5-8 5.3.3 Liquefaction 5-8 5.4 Assessment of Seismic Performance of Dams Using Existing Studies 5-9 5.4.1 Ground Shaking 5-9 5.4.2 Fault Displacement 5-10 5.4.3 Liquefaction 5-10 5.5 Multiple Dam Failure Due to a Single Seismic Event 5-11 5.6 Modeling Consequences of Seismic Dam Failure 5-13 5.7 Detailed Site Specific Seismic Hazard Analysis 5-14 5.7.1 Ground Shaking 5-14 5.7.1.1 Seismic Source Characterization 5-14 5.7.1.2 Ground Motion Attenuation 5-15 5.7.1.3 Site Response 5-16 5.7.1.4 Development of Uniform Hazard Spectra 5-17 5.7.1.5 Development of Acceleration Time-Histories 5-17 5.8 Detailed Analysis of Seismic Capacity of the Dam 5-17 iii
5.8.1 Concrete Dams 5-18 5.8.1.1 Sliding and Overturning Stability 5-18 5.8.1.2 Dynamic Analysis 5-19 5.8.2 Embankment Dams 5-20 5.8.2.1 Deformation Analyses 5-20
- 6. Other (Sunny Day) Failures 6-1 6.1 Overview of Sunny Day Failures by Structure Type 6-2 6.1.1 Concrete Dams 6-2 6.1.2 Embankment Dams 6-2 6.2 Analysis of Sunny Day Failures 6-3 6.2.1 Sunny- Day Failure Modes 6-3 6.2.2 Initial Water Surface Elevation 6-4
- 7. Dam Breach Modeling 7-1 7.1 Breach Modeling for Concrete Dams 7-1 7.2 Breach Modeling of Embankment Dams 7-1 7.2.1 Regression Equations for Peak Outflow from the Breach 7-3 7.2.2 Regression Equations for Breach Parameters 7-4 7.2.2.1 Uncertainty in Predicted Breach Parameters and Hydrographs 7-6 7.2.2.2 Performing Sensitivity Analyses to Select Final Breach Parameters 7-7 7.2.3 Physically-Based Combined Process Breach Models 7-8
- 8. Levee Breach Modeling 8-1
- 9. Flood Wave Routing 9-1 9.1 Applicability and Limitations of Hydrologic Routing Models 9-1 9.1.1 Backwater Effects 9-1 9.1.2 Floodplain Storage 9-2 9.1.3 Interaction of Channel Slope and Hydrograph Characteristics 9-2 9.1.4 Configuration of Flow Networks 9-2 9.1.5 Occurrence of Subcritical and Supercritical Flow 9-3 9.1.6 Availability of Calibration Data Sets 9-3 9.2 Hydraulic Models 9-3 9.3 Sediment Transport Modeling 9-4 9.4 Inundation Mapping 9-4
- 10. Terms and Definitions 10-1
- 11. References 11-1 iv
Tables Table 1. Roles of Federal Agencies in Dam Safety 1-10 Table 2. Possible Dam Clustering Combinations 3-10 Figures Figure 1. Levels of Analysis 1-4 Figure 2. Overview of Detailed Dam Failure Flood Hazard Analysis 1-7 Figure 3. Section View of Concrete Gravity Dam (British Dam Society, 2013) 2-2 Figure 4. Typical Concrete Arch Dam Cross-Section and Plan View (Youssef, 2013) 2-2 Figure 5. Concrete Buttress Dam Cross-Section and Plan View.
(SimScience, 2013) 2-3 Figure 6. Concrete Multi-Arch Dam (Bartlett Dam. U.S. Bureau of Reclamation; Photo: National Park Service, 2013) 2-3 Figure 7. Typical Earthfill Embankment Dams (USACE, 2004) 2-4 Figure 8. Typical Rockfill Dams (USACE, 2004) 2-5 Figure 9. Reservoir water levels and corresponding storage volumes. 2-6 Figure 10. Typical Levee Cross-Section (Wikimedia Commons, 2013) 2-7 Figure 11. Screening approach for watersheds with many dams. 3-2 Figure 12. Screening Method Flowchart (a) - Method 1 (Volume) 3-6 Figure 13. Screening Method Flowchart (b) - Method 2 (Peak Flow without Attenuation) 3-7 Figure 14. Screening Method Flowchart (c) - Method 3 (Peak Flow with Attenuation) 3-8 Figure 15. Screening Method Flowchart (d) - Method 4 (Hydrologic Method) 3-9 Figure 16. Hypothetical Dam Representing Storage Upstream 3-10 Figure 17. Debris Upstream of Lake Lynn Dam after 1985 Flood Event (Schadinger et al., 2012) 4-6 Figure 18. Seismic Dam Failure Analysis Options 5-7 Figure 19. Using Knowledge about the Attenuation of Ground Motion with Distance 5-12 Figure 20. Refinement of Seismic Influence Using Deaggregation 5-13 Figure 21. Generalized Trapezoidal Breach Progression (Gee, 2008) 7-3 v
Guidance for Assessment of Flooding Hazards Due to Dam Failure
- 1. INTRODUCTION When evaluating flooding hazards for nuclear power plants, floods resulting from dam failures need to be considered. In engineering terms, dams and levees fail when they do not deliver the services for which they are designed, such as flood protection, water supply, and hydropower. However, this interim staff guidance (ISG) defines failure from a point of view at the nuclear power plant (NPP). Therefore, in this ISG dam failure refers to flooding caused by any uncontrolled release of water that threatens to impact structures, systems and components (SSCs) important to safety at a NPP site.
It should also be noted that there may be instances where a controlled release of water from a dam can also lead to the inundation of a NPP site. Examples include, but are not limited to: (a) releases performed in order to prevent dam failure during flood conditions; (b) releases performed to rapidly drawdown a reservoir to prevent incipient failure after a seismic event; and (c) releases performed to rapidly drawdown a reservoir to prevent incipient sunny day failure.
In some cases, the elevation of the site provides the principle protection from flooding hazards. In some cases, SSCs important to safety are protected by passive (e.g.,
structures), or active (e.g., equipment), flood protection features. In other cases, flood protection is provided by procedures. NPPs may also use some combination of the protection methods outlined above. Therefore, the site elevation and the lowest flood protection elevation of SSCs important to safety are the primary criteria for flood hazard assessment.
In general, failure of any dam upstream from the plant site is a potential flooding mechanism (consideration of upstream dams should include all water-impounding structures, whether or not they are defined as dams in the traditional sense). Dams that are not upstream from the plant, but whose failure would impact the plant because of backwater effects, may also present potential flooding hazards. Failures of dikes or levees in the watershed surrounding the site may contribute to or ameliorate flooding hazards, depending on the location of the levee and the circumstances under which it fails.
Failure of a dam or levee that impounds the ultimate heat sink constitutes a hazard to the plant. In addition, failures of onsite water-storage or water-control structures (such as onsite cooling or auxiliary water reservoirs and onsite levees) that are located at or above the grade of SSCs important to safety are potential flooding mechanisms.
The dam failure itself may be due to flooding or some other cause such as a seismic event, a structural defect, or human performance related issues. The potential for these mechanisms to initiate dam failure, as well as the potential failure modes must be evaluated to fully characterize the dam failure flooding hazard.
1.1 Purpose The purpose of this ISG is to provide guidance on methods acceptable to U.S. Nuclear Regulatory Commission (NRC) staff for re-evaluating flooding hazards due to dam failure for the purpose of responding to the March 2012 request for information (Agencywide Documents Access and Management System (ADAMS Accession No. ML12053A340).
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However, licensees are not required to follow this guidance. Approaches and methods that differ from those presented in this ISG will be evaluated on a case-by-case basis. It should be noted that dam failures discussed as a result of applying this guidance are postulated solely to ensure the safety of a NPP. This guidance should in no way supersede or be used in place of guidance developed by any agency that owns, operates or regulates the dam(s) of interest.
This ISG supplements and clarifies other NRC guidance that discusses dam failure, such as:
- Regulatory Guide 1.59, Design Basis Floods for Nuclear Power Plants, Revision 2 (USNRC, 1977)1
- NUREG-0800, Review of Safety Analysis Reports for Nuclear Power Plants (USNRC, 2007)
- NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, (USNRC, 2011)
This ISG is intended to provide guidance that is broadly consistent with published federal guidance from agencies that have direct responsibility for ownership, operation or regulation of dams, or direct responsibility for emergency planning and response for dam failure incidents. Therefore, this guide draws from guidelines developed by the Federal Emergency Management Agency (FEMA), Federal Energy Regulatory Commission (FERC),
the Bureau of Reclamation (USBR), and the U.S. Army Corps of Engineers (USACE).
Some portions of this guidance draw from dam safety guidelines developed by states, including California, Colorado, and Washington. A draft white paper on dam failure prepared by the Nuclear Energy Institute was also used.
Although this ISG is broadly consistent with best practices identified in the Federal and State guidance discussed above, there may be differences. In some cases, guidance is not uniform across agencies. In some cases, variance between this ISG and guidance of other agencies is due to differences in risk tolerance levels between the nuclear power sector and sectors such as water resources and flood control.
Certain widely-used modeling software packages are mentioned in this ISG for illustrative purposes, but the NRC does not recommend or endorse specific software packages. In general, hydrologic, hydraulic, geotechnical and structural simulation models accepted in standard engineering practice by Federal agencies and other authorities responsible for similar design considerations may be used. Justification for selection and use of a particular modeling method, approach or software package is the responsibility of the licensee.
1.2 Scope A prioritization memo dated May 11, 2012 (ADAMS Accession No. ML12097A509) specified three categories for submittal of reevaluated flooding hazard reports. Those plants in Category 1 should have already submitted their flood reevaluation report by March 11, 2013 (unless an extension has been granted), which predates issuance of this ISG. Therefore, this ISG is not strictly applicable to Category 1 sites that have submitted their completed flood reevaluation report, and their dam failure flood hazard evaluations will be reviewed using present-day methodologies and regulatory guidance, as described in the request for information. This ISG is applicable to Category 2 and Category 3 sites (i.e., sites with 1
Regulatory Guide 1.59, Rev. 2 included ANSI Standard N170-1976, Standards for Determining Design Basis Flooding at Power Reactor Sites as an appendix. ANSI-N170-1976 was superseded by ANSI/ANS-2.8, Determining Design Basis Flooding at Power Reactor Sites in 1981. ANSI/ANS-2.8 was last updated in 1992. It has lapsed as an ANSI standard, but is still used by NRC staff.
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submittal dates of March 12, 2014 and 2015, respectively), as well as most Category 1 sites that have been granted an extension. Instances where Category 1 sites have been granted a very short extension (e.g. a few weeks), will be considered on a case-by-case basis.
This ISG is applicable to estimating flood hazards due to failure of all offsite and some onsite water control structures and impoundments. For offsite structures, hydrologic, seismic and sunny-day failure mechanisms are within the scope of the R2.1 Flooding Hazard Reevaluation and this ISG. This ISG provides guidance that is applicable to the evaluation of onsite dams and levees, including dam- or levee-like structures associated with onsite reservoirs (e.g., earthen cooling reservoir impoundments). Thus, while Section 2.4.4 of NUREG-0800 includes failure of all onsite water control or storage structures (e.g.,
levees, dikes, and any engineered water storage facilities that are located above site grade and may induce flooding at the site such as tanks and basins), this ISG provides guidance applicable to only a subset of those onsite structures. For example, even though the evaluation of site flooding from structures such as concrete cooling tower basins is within the scope of the NTTF Recommendation 2.1 flood hazard reevaluations, provision of guidance to support evaluation of such structures is not within the scope of this ISG.
Moreover, evaluation of flooding from tanks is not within the scope of the NTTF Recommendation 2.1 flood hazard reevaluations and associated guidance is not provided in this ISG. Seismic failure of onsite structures may require input from the R2.1 Seismic Reevaluations.
1.3 Framework for Dam Failure Flood Hazard Estimation 1.3.1 Screening Any sufficiently large watershed in the U.S. typically contains many dams (hundreds to thousands for major watersheds). It is generally not practicable to perform detailed failure analysis on each dam in the watershed. Even if it were a tractable problem, a large number of the dams in the watershed will have no impact on flooding at a NPP due to some combination of small size or large distance from the NPP. Therefore, it is useful to perform a screening level analysis to identify these dams. Section 3 describes several procedures for identifying the small/distant dams whose failure would likely have negligible impacts on flooding at a NPP site. The approach identifies several classes of dams for the purposes of this ISG. Dams that can be removed from consideration without analysis because they meet criteria described in ISG Section 3.1 (e.g., dams not owned by a NPP licensee and identified by Federal or State agencies as having minimal or no adverse failure consequences beyond the dam owners property), are called inconsequential dams.
Dams that can be shown to have little impact on flooding at a NPP site using simplified analyses (as described in Section 3) are termed noncritical dams. All other dams are considered potentially critical. Detailed analyses will be required to further assess these dams. Detailed analysis will show which of the potentially critical dams are truly critical to flood hazard estimates at a NPP site. Critical dams are those whose failure, either alone, or as part of a cascading or multiple dam failure scenario, would cause inundation of a NPP site. Figure 1 illustrates the screening concept and the various dam classes. Details of the screening methods are provided in Section 3.
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Figure 1. Levels of Analysis 1-4
1.3.2 Detailed Analysis For potentially critical dams (e.g., those not screened out as discussed in the preceding section), the first step in detailed dam failure flood hazard estimation is determining the demand or loading cases that will be applied to the dam. For the purposes of responding to the March 2012 Request for Information (NRC, 2012), failure under hydrologic loadings associated with extreme floods, as well as ground motions associated with earthquakes must be considered. In addition, failure due to non-hydrologic, non-seismic causes (i.e.,
sunny day failures) must be considered. Sunny day failures encompass a wide variety of mechanisms (e.g., geologic or structural defects, misoperation, etc.).
Detailed dam failure flood hazard estimation will require collecting data on the dam(s) to be analyzed (e.g., design documents, construction records, maintenance, and inspection program, planned modifications) as well as hydrometeorological and hydrologic data (e.g.,
design storms, topography, rainfall-runoff characteristics) on the river basin(s) in question.
Typically, information about the dam is obtained from the dam owner and/or regulator. In the U.S., there is no single entity responsible for regulation of dams. Instead, dam regulation is distributed among various Federal agencies and State authorities. Dams may be privately owned, or owned by Federal, State or local agencies. Many large dams on major rivers are owned by self-regulating Federal agencies (e.g., the USACE, USBR, or the Tennessee Valley Authority (TVA)). Information on the physical characteristics and flooding history of many watersheds can be obtained from Federal agencies (e.g., the U.S. Geological Survey (USGS), or National Oceanographic and Atmospheric Administration/National Weather Service (NOAA/NWS)), states, and organizations such as river basin commissions and flood plain managers.
Existing estimates for design storms and floods (e.g., probable maximum precipitation (PMP) and probable maximum flood (PMF)) in the region of interest developed by Federal, State or other agencies may be used. However, some of these reports may be quite old (e.g., the NOAA/NWS Hydrometeorological Report 51 for the Eastern United States was published in 1978). The licensee should exercise due diligence and examine the record of extreme storms and floods in the region of interest to provide assurance that the existing estimates are still valid.
Once the demand or loads have been estimated, the capacity of the dam to withstand the estimated loads is considered. The level of detail and effort expended will depend on several factors including, but not limited to, consequence of failure (e.g. a very large dam or a dam very near an NPP site versus a very small dam or one that is very far away),
availability of design and construction information, and availability of recent studies to support capacity estimates (e.g., spillway capacity ratings, seismic capacity ratings, inspection and maintenance records, etc.). In place of a detailed analysis, one can simply assume that the dam fails under appropriate loading and move on to estimation of the consequences.
Comparison of the estimated capacities to the applied loads is used to assess the credibility of failure modes associated with those cases. The assessment may consider factors of safety incorporated into the dam design or dam capacity assessments, with appropriate justification. Likewise, uncertainties in capacity and loading estimates should be considered to arrive at an appropriately conservative decision. If it cannot be demonstrated that the dam-failure likelihood over the expected remaining life of the nuclear power plant is extremely low (or that the consequences of failure are negligible), failure should be postulated and the flooding consequences estimated. It is recognized that such 1-5
assessments will often require a combination of deterministic, qualitative probabilistic, and/or quantitative probabilistic analysis. For example, current NRC guidance accepts deterministic analysis of hydraulic hazards (e.g., PMP, PMF). Deterministic analyses of capacity to withstand loads that were arrived at by probabilistic or deterministic analysis are also accepted. Detailed guidance on identifying potential failure modes is beyond the scope of this ISG. The USBR and the USACE have jointly developed guidance on this topic (USBR 2011).
Dam failure consequence analysis will generally include estimating the reservoir outflow hydrograph (discharge hydrograph) resulting from dam failure (dam-breach analysis) and routing of the dam breach discharge to the plant site. The flood routing analysis should consider any potential for domino-type or cascading dam failures. Transport of sediment and debris by floodwaters should be considered.
In summary, the dam-failure flood hazard analysis for potentially critical dams will comprise the following steps (see also Figure 2):
- 1. Data collection
- a. Compile information on the dam(s) (design, construction, inspection, maintenance, etc.)
- b. Compile information on the river basin upstream and downstream from each dam (topography, bathymetry, reservoir volumes, reservoir flood inflows, etc.)
- 2. Estimation of demand and loads
- a. Flooding case
- b. Seismic case
- c. Sunnyday case (assumed to occur)
- 3. Assess credible failure modes/scenarios under the various loading cases (flooding, seismic, sunny day), including potential for multiple or cascading failures
- a. Compare loadings to estimated capacities, taking into account uncertainties as well as factors of safety
- b. For each credible failure, perform steps 4 through 6
- c. If the failure is not considered credible, the analysis is complete
- 4. Breach analysis
- a. Estimate breach parameters (geometry and failure time)
- b. Compute reservoir routing and breach hydrograph
- 5. Flood routing
- a. Establish initial and boundary conditions
- b. Select hydrodynamic modeling approach and develop basin model (one-dimensional, two-dimensional, or hybrid)
- c. Perform flood-routing simulations
- d. Estimate impacts of sediment and debris transport
- e. Estimate Water-Surface Elevation (WSE) at plant site.
- 6. Inundation mappingdevelop maps delineating the areas and structures at the plant site that would be inundated in the event of dam failure 1-6
Figure 2. Overview of Detailed Dam Failure Flood Hazard Analysis 1.4 Probabilistic and Deterministic Hazard Analysis The current state of practice in dam safety analysis uses a combination of deterministic and probabilistic approaches. Probabilistic seismic hazard analysis is accepted current practice in both the nuclear and dam safety communities, as reflected in Federal guidance and industry consensus standards. Probabilistic approaches for estimating the extreme rainfall and flood events of interest in this ISG (e.g. 1x 10-4 per year or lower annual exceedance probability) exist, but there are no industry consensus standards or Federal guidance that defines current accepted practice. NRC has established probabilistic screening criteria for man-related hazards (e.g., between 1x10-7 and 1x10-6 annual exceedance probability) that are, in theory, applicable to sunny-day dam failures. However, no widely accepted methodology exists for estimating sunny-day dam failure probabilities on the order of 1x10 1x10-6 annual exceedance probability.
1.4.1 Historical Dam and Levee Failure Rates Nearly 1,500 dam failures have been recorded in the United States since the middle of the 19th century (records are unreliable for prior periods). Over this period, the long-term average rate of dam failures is about 10 per year, although this figure represents dams of all sizes and types, including small dams, whose failures have little or no consequences (National Academy of Sciences (NAS), 2012). If instead one looks at a running 10-year average of the dam failure rates since 1850, the failure rate has been over 20 per year for most of the period since the late 1970s (NAS, 2012). Expressed in terms of dam years, 1-7
numerous studies of dam failures in the U.S. and worldwide have indicated an average failure rate on the order of 10-4 per dam year (e.g., Baecher et al., 1980). These historical rates for dam failure provide useful information about generic failure probabilities, but generic dam failure rates are not sufficient to screen out dam failure for the purpose of the Recommendation 2.1 flooding hazard reevaluations (see ADAMS Accession No. ML090510269).
1.4.2 Hydrologic Failure Probabilistic approaches for estimating the extreme rainfall and flood events of interest in this ISG (e.g. 1x 10-4 per year or lower annual exceedance probability) exist, but there are no industry consensus standards or Federal guidance that defines current accepted practice. Therefore, for the purpose of the Recommendation 2.1 Flooding Hazard reevaluation, a deterministic approach based on the PMP and PMF should be used. The PMP is an estimate of the maximum possible precipitation depth over a given size catchment for a given length of time (Stedinger et al., 1996). The PMF is the flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in the drainage basin under study. Consult Section 4 for additional detail on hydrologic failure.
Staff Position:
A dam should be assumed to fail due to hydrologic hazard if it cannot withstand its basin specific PMF, with associated effects.
1.4.3 Seismic Failure Probabilistic seismic hazard analysis (PSHA) is considered to be present-day methodology in both the dam safety and the nuclear safety communities. Estimation of seismic hazards (e.g., vibratory ground motion, fault displacement, loss of strength) at annual exceedances of 10-4 per year is routine. Widely accepted earthquake source characterization data sets, ground motion prediction equations, and site amplification factors are publicly available.
Section 5 provides additional detail on analysis of seismic dam failures.
Staff Position:
A dam should be assumed to fail if it cannot withstand the relevant seismic hazards (e.g., vibratory ground motion at spectral frequencies of importance, fault displacement, loss of strength) with an annual exceedance probability of 1x10-4 per year. Although the probability of extreme flooding occurring at the same time as an earthquake is extremely low, the probability of lesser floods should not be neglected. In addition, if the seismic capacity of the dam is considerably less than what is required to withstand the 10-4 seismic hazard, the possibility of large (though not extreme) floods should be considered. Therefore, the dam should be assumed to fail due to seismic hazard if it cannot withstand the more severe of the following combinations:
- 10-4 annual exceedance seismic hazard combined with a 25-year flood
- half of the 10-4 ground motion, combined with a 500-year flood.
1.4.4 Sunny-Day Failure The hydrologic and seismic failures discussed in the previous subsections require a natural hazard initiator, and can therefore be considered, at least in part, a natural hazard. On the other hand, sunny-day failures are clearly a purely man-related hazard.
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The NRCs traditional approach to assessing impacts from man-related hazards is provided in the Standard Review Plan for the Safety Review of Nuclear Power Plants (NUREG-0800),
Sections 2.2.1 through 2.2.3. The NRC considers that design-basis events resulting from the presence of hazardous materials or activities in the vicinity of the plant is acceptable based on estimated annual frequency. If a postulated accident type meets the NRC staff objective (with an order of magnitude of 10-7 per year) then the potential exposures are considered to meet the requirements of Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a)(1) as it relates to the requirements of 10 CFR Part 100, Reactor Site Criteria. If data are not available to make an accurate estimate of the event probability, an expected rate of occurrence of potential exposures resulting in radiological dose in excess of 10 CFR 50.34(a)(1) as relates to the requirements of 10 CFR Part 100, by an order of magnitude of 10-6 per year is acceptable if, when combined with reasonable qualitative arguments, the realistic probability can be shown to be lower. This exception is made because data are often not available to enable the accurate calculation of probabilities given the low probabilities associated with the events under consideration.
The approach outlined in the preceding paragraph has been applied to man-related hazards such as those associated with industrial and transportation activities. However, as discussed above in the introductory paragraph to Section 1.4, even when failure probability estimates are developed based on site and dam specific data and information, no widely accepted current engineering practice exists for estimating failure rates on the order of at the 1x 10-6 per year.
Staff Position:
Because no widely accepted current engineering practice exists for estimating failure rates on the order of at the 1x 10-6 per year, sunny-day failure should be assumed to occur and the consequences estimated.
1.5 Interfacing with Owners and Regulators of Dams and Levees There are roughly 84,000 dams (USACE, 2011b) and over 100,000 miles of levees (National Committee on Levee Safety (NCLS), 2009) in the U.S., constructed by a variety of public sector agencies (local, State and Federal) as well as numerous private sector entities (e.g., individuals, groups, and corporations). Dam and levee safety program governance in the United States is shaped by laws, policies, and practice, and is similar to the governance that has evolved for emergency response in the U.S. (NAS, 2012).
1.5.1 Dam Safety Governance In general, State and local governments have responsibility for dam safety governance of non-Federally owned or operated dams. Almost all states have formal dam safety programs tied to Federal guidelines. FEMA has recently published a summary of existing dam safety guidance that provides information on individual states dam safety programs (FEMA, 2012).
Federal regulatory authority for non-Federal dams is limited to the roughly 2,100 dams used for hydropower projects regulated by the Federal Energy Regulatory Commission (FERC),
and mine-tailings dams regulated by the Mine Safety and Health Administration. In a few cases, states have jurisdiction over dams that are also regulated by a Federal agency (e.g.,
California regulates hydropower dams).
Federally owned dams are regulated not by an independent agency but according to the policies and guidance of the individual Federal agencies that own the dams. Table 1 summarizes Federal dam ownership and dam safety roles.
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1.5.2 Dam Safety Guidance by Other Federal Agencies At the Federal level, FEMA has been charged with encouraging the establishment and maintenance of effective Federal and State programs, policies and guidelines. It implements this charge through leadership of the National Dam Safety Program (NDSP),
the National Dam Safety Review Board (NDSRB), and the Interagency Committee on Dam Safety (ICODS). ICODS has generated and released a series of guidance documents in an attempt to provide a uniform and consistent dam safety framework for Federal, State, and private dam owners and regulators. However, adherence to this guidance is not mandatory.
For example, FEMA has oversight responsibility for developing guidance, but no direct regulatory authority for dam safety. Other Federal agencies such as USACE, the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service, the U.S. Department of the Interior (USDOI) Bureau of Reclamation, and FERC have published dam safety guidelines. FEMA has recently published a summary of existing dam safety guidance that provides information on Federal dam safety programs (FEMA, 2012).
Table 1. Roles of Federal Agencies in Dam Safety1 Agency Primary Roles Dams under Jurisdiction U.S. Department of Lead agency for National Does not own any dams Homeland Security, Federal Dam Safety Program; Emergency Management Chairs National Dam Safety Agency (FEMA)
Review Board and Interagency Committee on Dam Safety U.S. Department of Owns or regulates dams; More than one-third of dams Agriculture (USDA) Supports private owners with in the National Inventory of planning, design, finance, Dams (NID) are associated and construction with the USDA U.S. Department of Defense Plans, designs, finances, DOD has total of 267 dams (DOD) constructs, owns, operates, under its jurisdiction on and permits dams; limited to military lands military lands with exception of USACE civil-works programs U.S. Army Corps of Plans, designs, constructs, Jurisdiction over USACE Engineers (USACE) operates, and regulates dams, dams constructed by dams; permits and inspects USACE but operated by dams others, and other flood-control dams subject to Federal regulation; 631 dams in the NID are associated with USACE U.S. Department of the Plans, designs, constructs, About 2,000 dams in the NID Interior, (USDOI) operates and maintains under five bureaus, mainly dams Bureau of Reclamation 1-10
U.S. Department of Labor Regulates safety and health- About 1,400 dams under (USDOL) related aspects of miners Mine Safety and Health Administration Federal Energy Regulatory Issues licenses, performs 2,530 dams in NID affecting Commission (FERC) inspections, and regulates navigable waters non-Federal dams with hydroelectric capability Tennessee Valley Authority Plans, designs, constructs, Approximately 49 major operates, and maintains dams in Tennessee River dams Valley 1
NRC regulates dams providing ultimate heat sink at NPPs as well as tailings dams at uranium mill tailings sites.
Source: NRC (2012) and FEMA (2009) 1.5.3 Obtaining Information on Dams and Levees Obtaining detailed information to support the dam failure flood hazard evaluation may be challenging because of the dispersed nature of dam ownership and regulation in the United States. In most cases, licensees do not operate or own the dams or levees that potentially may contribute to flooding hazards at a NPP site.
National and State dam inventories can be used to identify dams within the watershed of a stream or river and to obtain characteristics for each dam (location, height, and volume).
The USACE maintains the National Inventory of Dams (NID), which provides information on thousands of dams (USACE, 2011b). Following Hurricane Katrina, Congress authorized the USACE to develop a National Levee Database (NLD). Initially, the NLD contained information only for USACE levees. However, integration of levee data collected by the FEMA National Flood Insurance Program (NFIP) into the NLD, which is under way, will increase the total number of miles of levee systems in the NLD. These databases and inventories are useful sources of basic geographic and physical information on dams and levees in the U.S.
Staff Positions:
- In the case of dams and levees owned or operated by U.S. Federal agencies, the Federal agency responsible for (owner or operator of) the dam should be involved in any discussions, including possibly reviewing any analysis performed. Evaluation of dams is complex, requiring extensive expertise and site-specific knowledge. It is critical for the owner or operator of the dam to assist the NRC or its licensees when modifying the assumptions or methods used to develop the inundation maps for a specific area. If a Federally owned dam is identified as critical to the flooding reanalysis, the licensee should contact the NRC promptly. The NRC will act as the interface between these agencies and licensees. Memoranda of Agreement or other mechanisms are being developed to facilitate sharing of data (including necessary safeguards to protect sensitive information) between the NRC and the appropriate Federal agencies. It is important to note that in many cases Federal agencies that own or operate dams have conducted detailed failure analysis. To the extent that these analyses are applicable, they should be used in the Recommendation 2.1 flooding reanalysis.
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- In some cases, the dam or levee will be owned or operated by a private entity, but regulated by a Federal agency. In this case, the NRC will interface with the Federal regulatory agency to obtain available information. Interactions between the licensee and the owner should be coordinated with the NRC and the Federal regulator.
- In most cases, dams and levees will be owned and operated by private entities and regulated by a State agency. In this case, the licensee should interact directly with the owner and regulator. The licensee should notify the NRC if it encounters difficulties in obtaining information. On a case-by-case basis, the NRC might be able to provide some assistance in interfacing with State agencies.
1.6 Organization of guidance ISG Section 1 provides an overview of the flood hazard reevaluation process; discusses dam safety governance and existing guidance and discusses procedures for obtaining information on dams and levees, particularly in regard to those that are Federally owned or regulated. ISG Section 2 presents an overview of dams and levee types and causative mechanisms for dam failure. ISG Section 3 discusses screening methods and simplified analysis approaches for drainage basins with many small dams. ISG Section 4 discusses dam failure due to hydrologic mechanisms. ISG Section 5 discusses dam failure due to seismic mechanisms. ISG Section 6 discusses dam failure resulting from causes other than hydrologic or seismic mechanisms (e.g., sunny-day failures). ISG Section 7 provides details of dam breach modeling. ISG Section 8 discusses levee breach modeling. ISG Section 9 provides details of flood routing. ISG Section 10 contains a list of terms and definitions.
ISG Section 11 lists the references used to develop this guidance.
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- 2. BACKGROUND This section of the ISG provides a brief overview of the various types of dams in common use, and the principal classes of mechanisms that initiate damage and failure. Failure mechanisms vary with the type of dam, as well as materials used in the construction.
2.1 Classification of Dams and Levees Dams can be classified using several criteria (e.g., purpose, size, construction material, hazard potential, etc.). This ISG will use a classification system based mainly on construction material since modes of failure, as well as susceptibility to a given initiating mechanism, are generally correlated with dam construction material. The two major categories discussed in this ISG are concrete and embankment dams. There are also a large number of so-called composite dams comprised of both concrete and embankment sections currently in service.
A levee or dike is a manmade barrier (embankment, floodwall, or structure) along a watercourse constructed for the primary purpose to provide hurricane, storm, and flood protection relating to seasonal high water, storm surges, precipitation, and other weather events; and that normally is subject to water loading for only a few days or weeks during a year. Almost all levees are earthen embankments, although some may have parapets or floodwalls comprised of other materials built on their crest.
2.1.1 Concrete Dams As the name implies, concrete dams are typically constructed of concrete or other masonry components. The major types of concrete dams include gravity, arch, multi-arch, and buttress dams (as shown in Figure 3 through Figure 6). Some dams are hybrids being combinations of the major types. For example, gravity-arch dams combine the features of arch and gravity designs. Multi-arch dams typically employ buttresses to support the arches (Figure 6).
Concrete gravity dams typically consist of a solid concrete structure that maintains the stability against design loads by relying on the geometric shape, and the mass and strength of the concrete (Figure 3). Conventionally placed mass concrete and roller-compacted concrete (RCC) are the two general concrete construction methods for concrete gravity dams (USACE, 1995a,b). Gravity dams depend primarily on their own weight for stability.
Generally, gravity dams are sized and shaped to resist overturning, sliding, and crushing at the dam toe. If the moment around the turning point caused by the hydraulic load of the reservoir is smaller than the moment caused by the weight of the dam, the dam will not overturn. This is the case if the resultant force of hydraulic load and weight of the dam falls within the base of the dam. Typically, gravity dams are constructed on a straight axis, though they may be slightly angled or curved, in an arch shape. In earlier periods of dam design, gravity dams were built of masonry materials such as stone, brick, or concrete blocks jointed with mortar. Additionally, gravity dams can have a hollow interior with concrete or masonry used on the outside. Engineering manuals published by the U.S. Army Corps of Engineers (USACE, 1995a), the Bureau of Reclamation (USBR, 1976, 1977b), and the Federal Energy Regulatory Commission (FERC, 2002) provide more detailed discussion of concrete gravity dam engineering and design.
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Figure 3. Section View of Concrete Gravity Dam (British Dam Society, 2013)
An arch dam is a structure that is designed to curve upstream so that the force of the water in the upstream reservoir presses against the arch, compressing and strengthening the structure as it pushes into its foundation or abutments (Figure 4). Because they are thinner than any other dam type, they require less material to construct, making them both economical and practical in remote areas. There are two basic designs for an arch dam:
constant-radius dams, which have constant radius of curvature, and variable-radius dams, which have both upstream and downstream curves that systematically decrease in radius below the crest. Arch dams can be double-curved in both horizontal and vertical planes.
Arch dams with more than one contiguous arch or plane are described as multiple-arch dams (see Figure 6). The foundation or abutments for an arch dam must be very stable with strength proportionate to that of the concrete. Engineering manuals published by several Federal agencies provide more detailed discussion of concrete arch dam engineering and design (USACE, 1994; USBR, 1977a,b; FERC, 1999).
Figure 4. Typical Concrete Arch Dam Cross-Section and Plan View (Youssef, 2013)
Buttress dams are concrete structures consisting of two basic features: an upstream water barrier and buttresses (Figure 5). Buttress dams are typically designed as reinforced concrete structures. The upstream water barrier can be a flat slab or massive heads. The upstream water barrier transfers the reservoir load into the buttresses that then transfer the load into the foundation through frictional resistance like a gravity dam. Buttress dams can be thought of as hollowed-out gravity dams with a sloping upstream face. The sloping 2-2
upstream face allows the buttresses to efficiently carry static loads because the weight of the water on the dam adds to the vertical force transmitted to the foundation and therefore the stability of the dam. Depending on the thickness of the concrete members, buttress dams might or might not have reinforcing steel. Engineering manuals published by several Federal agencies provide more detailed discussion of concrete buttress dam engineering and design (USBR, 1976, FERC, 1997).
Figure 5. Concrete Buttress Dam Cross-Section and Plan View. (SimScience, 2013)
Figure 6. Concrete Multi-Arch Dam (Bartlett Dam. U.S. Bureau of Reclamation; Photo: National Park Service, 2013) 2.1.2 Embankment Dams Embankment dams are made from compacted natural (earthen) materials. Earthfill dams are typically trapezoidal in shape and rely on their weight to resist the hydrostatic loads created by the water, similar to concrete gravity dams. The two most common types of embankment dams are rockfill and earthfill dams (see Figure 7 and Figure 8).
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Figure 7. Typical Earthfill Embankment Dams (USACE, 2004)
Earthfill dams (Figure 7) are composed of suitable soils that are spread and compacted in layers by mechanical means. Earthfill dams can be constructed with homogenous layers (homogeneous dam) or zones of different materials of varying characteristics (zoned-earth dam). Typical zones include a clay core and filter and drain zones.
Rockfill dams (Figure 8) are constructed from compacted earth fill that contains a high percentage of rocks and other larger aggregate materials. To prevent seepage, rockfill dams have an impervious zone on the upstream side of the dam or within the embankment. The impervious zone can be made from a variety of materials including masonry, concrete, 2-4
plastic, steel pile sheets, timber, or clay. If clay is used, it is often separated from the fill by a filter to prevent erosion of the clay into the fill material.
Figure 8. Typical Rockfill Dams (USACE, 2004)
Earthfill or rockfill dams can include a watertight core made from asphalt concrete. Dams with this type of core are called concrete-asphalt core embankment dams. Most concrete-asphalt dams use rock or gravel as the main fill material. These types of dams are considered especially appropriate for areas susceptible to earthquakes because of the flexible (elastic) nature of the asphalt core.
Engineering manuals published by several Federal agencies provide more detailed discussion of embankment dam engineering and design (USACE, 2004; USBR,1987; FERC, 1991).
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2.1.3 Water Levels and Storage Volumes The vocabulary and terminology used to describe storage volumes and corresponding water levels (pool levels) in the water resources and dam safety literature vary between agencies and practitioners. Figure 9 illustrates the terminology that has been adopted for use in this ISG (see also Section 10, Terms and Definitions).
Reservoir storage consists of dead storage, inactive storage, active storage, and flood surcharge storage. Dead storage is the volume in a reservoir below the lowest controllable level. Inactive storage is the capacity below which the reservoir is not normally drawn.
Active (or usable) storage is the total amount of reservoir capacity normally available for release from a reservoir. Active storage is usually composed of conservation storage (used for water supply, irrigation, recreation, hydropower, navigation, etc.), and flood-control storage. The top of the active storage is referred to as the full pool or maximum normal pool elevation. Flood surcharge storage is volume between the maximum water surface elevation for which the dam is designed and the crest of an uncontrolled spillway (or the normal full-pool elevation of the reservoir with the crest gates in the normal closed position).
Figure 9. Reservoir water levels and corresponding storage volumes.
2.1.4 Levees A levee or dike (Figure 10) is a manmade barrier (embankment, floodwall, or structure) along a water course that is 1) constructed primarily to provide hurricane, storm, and flood protection relating to seasonal high water, storm surges, precipitation, and other weather events; and 2) normally subject to water loading for only a few days or weeks during a year.
Embankments that are subject to water load for prolonged periods (longer than normal flood-protection requirements) or permanently are sometimes referred to as frequently loaded levees; such levees should be designed in accordance with earth dam criteria rather than levee criteria. The potential failure of levees intended to provide flood protection to a NPP site should be considered. For the purposes of this ISG, distant levees are generally not of great concern. In general, levees should be assumed to fail when they are overtopped as a result of any mechanism (e.g., floods or dam break flow waves).
Levees might also include embankments, floodwalls, and similar types of structures intended to provide flood protection to lands below sea level and other lowlands and that might be subject to water loading for much, if not all, portions of the year, but do not constitute barriers across watercourses or constrain water along canals.
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As with dams, levees incorporate features and appurtenances that are critical to proper functioning. Examples include floodwall sections, closure structures, pumping stations, interior drainage works, and flood damage reduction channels.
Most levees and dikes are constructed using clay, silt, or sand with a clay core or cover, often on a foundation of substrata that is also subject to erosion. The levee definition used here does not include shoreline protection or riverbank protection systems such as revetments, barrier islands, etc. Such shoreline or riverbank protection systems are hardened structures that inhibit erosion but do not necessarily hold back water. Natural coastal barriers often consist mostly of sandy material.
Even though levees are functionally similar to small earth dams they differ from earth dams in the following important respects: a) a levee embankment may become saturated for only a short period of time beyond the limit of capillary saturation; b) levee alignment is dictated primarily by flood protection requirements, which often leads to construction on poor foundations; and c) borrow soil is generally obtained from shallow pits or from channels excavated adjacent to the levee, which produce fill material that is often heterogeneous and far from ideal.
A levee system comprises one or more levee segments that collectively reduce flood damage to a defined area. A levee segment is a discrete portion of a levee system that is owned, operated and maintained by a single entity, or discrete set of entities. A levee segment might have one or more levee features. Highway and railroad embankments can be considered levees only if they are designed to function as part of a flood control system.
More detail on the design, maintenance, and inspection of levees and floodwalls can be found in several engineering manuals and technical letters published by the U.S. Army Corps of Engineers (USACE, 1989, 2000, 2011b).
Figure 10. Typical Levee Cross-Section (Wikimedia Commons, 2013)
(1) design high water level (HWL); (2) low water channel; (3) flood channel; (4) riverside slope; (5) riverside banquette; (6) levee crown; (7) landside slope; (8) landside banquette; (9) berm; (10) low water revetment; (11) riverside land; (12) levee; (13) protected lowland; (14) river zone 2-7
2.2 Classification of Dam Failures Dam failure is a complex phenomenon and the root causes of actual dam failures are sometimes difficult to determine precisely. Identification of potential failure modes can only be performed after thoroughly reviewing all relevant background information on a dam, including geology, design, analysis, construction, flood and seismic loadings, operations, dam safety evaluations, and performance and monitoring documentation. Many failure modes are progressive and can be subdivided into phases, typically including initiation, progression and, finally, breach and uncontrolled reservoir release.
Hydrologic dam failure refers to those failures that are initiated by a hydrologic event (e.g.
inflow flood). The most common scenario is a large flood that overwhelms the dam spillway discharge capacity, with floodwaters overtopping the dam crest, leading to erosion of the downstream dam face, foundation materials, or abutments and eventual failure (breach).
Seismic dam failure occurs as the result of an earthquake (e.g., ground shaking, surface faulting, landsliding, or liquefaction). Strong ground shaking is the most common earthquake effect. Ground-shaking may directly damage the dam structure and appurtenances or induce subsequent failure modes (e.g., failure of gates leading to overtopping, reservoir landslide or seiche leading to overtopping, cracking or deformation of the embankment that leads to overtopping or an internal erosion failure).
In spite of their progressive nature, failures are often broadly categorized according to the predominant initiating mechanism for failure: (a) hydrologic dam failure, (b) seismic dam failure, and (c) dam failure from other causes. However, these categories are not mutually exclusive.
Dam failures not caused by a concurrent extreme flood or seismic event can arise from a wide variety of causes. Examples include, but are not limited to the following:
- latent design or construction errors
- age-related weakness or deterioration of embankment material, foundations, abutments or spillways
- malfunction or misoperation of appurtenances such as floodgates, valves, conduits, and other components may also lead to dam failure Dam failures caused by hydrologic events are discussed in Section 4. Seismic dam failure is discussed in Section 5. Dam failures due to nonhydrologic, nonseismic events are discussed in Section 6.
2.2.1 Influence of Dam Type on Failure Modes As mentioned in Section 2.1, predominant causes for failure, failure modes, and failure progression for dams depend on the dam type. If the dam fails, the breach shape and timing will also depend on the dam type.
2.2.1.1 Concrete Dams In general, concrete dams are much stronger in compression than in tension. With the exception of buttress dams, concrete dams are typically made of plain concrete that possesses limited tensile strength. Therefore, dam structural response to tensile loads is best characterized using a classic stress-strain relationship composed of elastic and inelastic strain ranges followed by a complete loss of strength (failure). The inelastic-strain 2-8
range provides only limited inelastic behavior. The dam response beyond this range is governed by complete loss of strength, sliding, and nonlinear response behavior of discrete blocks bounded by opened joints and cracked sections.
Because of the nature of their design and manner of construction, concrete dams can usually withstand some degree of overtopping. Some concrete dams actually are designed to be overtopped. However, overtopping can lead to erosion of the dam foundation and its abutments.
Observation and analysis indicate that degree and speed of failure for concrete dams depends on dam type. Because of their strength and mass, concrete gravity, dams are typically subject to partial rather than complete failure; failure typically is not instantaneous.
By contrast, concrete arch dams typically fail completely, and almost instantaneously. For buttress and multi-arch dams, failure of one or more sections is much more common than complete failure. Failure of the sections is typically treated as essentially instantaneous.
2.2.1.2 Embankment Dams Failure modes for embankment dams are heavily influenced by the design (e.g.,
homogenous vs. zoned), the materials (e.g., cohesive vs. noncohesive soils), and construction methods (e.g., degree of compaction) used.
Causes of failure for embankment dams under hydrologic load associated with flooding mainly fall into three categories: a) increased internal seepage rates; b) overtopping which initiates embankment erosion; and c) structural overstressing. The design, materials and construction methods employed will heavily influence failure initiation and progression in each of these categories. For example, cohesionless soils are less able to withstand erosion caused by overtopping, internal seepage pressures or structural overstressing than cohesive soils. The degree to which a given soil is compacted is an important factor determining its load-bearing capacity and resistance to erosion. Zoned dams with internal drainage layers and filters are better able to accommodate significant internal seepage rates.
The ability of an embankment dam to withstand earthquake shaking without loss of strength or liquefaction of foundation or embankment soils (leading to deformation, sliding, cracking or other failures) is very dependent on the materials and degree of compaction used.
Design features such as conduits passing through the dam can be an important consideration in its behavior under seismic loading.
2.2.2 Failure of Spillways, Gates, Outlet Works, and Other Appurtenances There are a number of dam features, not unique to any one dam type, important to dam functioning for which loss of function could directly cause uncontrolled release of the reservoir or lead to uncontrolled release of the reservoir through overtopping, erosion, or some combination thereof. Chief among these are spillways, gates and other outlet works.
Spillway discharge capacity is usually the critical component in passing large floods. More details on spillway failure are provided in Section 4, which discusses hydrologic failure mechanisms A variety of gates is used to control spillways. Gates range in complexity from simple slide gates (e.g., fixed-wheel gates or roller gates), to float-type gates (e.g., drum gates and ring gates), to gates which use hydrodynamic forces of flowing water to assist in actuation (e.g.,
radial or tainter gates). Uncontrolled releases could result from gates failing open, while overtopping and eventual loss of the entire dam could result from gates failing closed. Gate failures may be associated with hydrologic, seismic or sunny-day failure mechanisms. ISG 2-9
Sections 4, 5, and 6 discuss gate failures associated with hydrologic, seismic and sunny-day failure mechanisms, respectively.
Outlet works are typically less important because they involve smaller flows. They will not be discussed further in this guide, but the potential impact of this type of failure should be considered.
2.2.3 Operational Failures and Controlled Releases Certain operational failures and even certain controlled releases can lead to flooding at the NPP site. These failures can occur in a variety of situations and cannot be neatly categorized as having or being part of hydrologic, seismic or sunny-day failure. They might be a compounding factor in any operational failure. Operational failures occur when equipment, instrumentation, control systems (including both hardware and software), or processes fail to perform as intended. This, in turn, can lead to uncontrolled reservoir release. Instances where controlled releases can lead to inundation at a NPP site include, but are not limited to: a) releases performed in order to prevent dam failure during flood conditions; b) releases performed to rapidly draw down a reservoir to prevent incipient failure after a seismic event; and c) releases performed to rapidly draw down a reservoir to prevent incipient sunny-day failure. ISG Section 4.2.7 further discusses operational failures and controlled releases.
2.3 Multiple Dam Failures At some NPP sites, the potential might exist for flooding caused by essentially simultaneous failure of multiple dams or the domino failure of a series of dams. For example, the site might be located in a region in which dams are located close enough to one another that a single storm or seismic event can cause multiple failures. Failure of a critically located dam storing a large volume of water may produce a flood wave that triggers domino-type failures of downstream dams. ISG Section 4.2.5 provides additional detail on multiple dam failure due to hydrometeorological phenomena. Section 5 provides additional detail on multiple dam failure due to a seismic event.
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- 3. SCREENING AND SIMPLIFIED MODELING APPROACHES FOR WATERSHEDS WITH MANY DAMS Section 1.3.1 and Figure 1 provided an overview of screening approaches intended to reduce the analysis burden for watersheds with many dams. This section discusses this issue in detail, including the criteria used to identify those dams that may be removed and not given further consideration in the analysis (i.e., inconsequential dams) and conservative screening approaches based on simplified empirical and mechanistic methods.
These screening approaches are intended to reduce the amount of effort required to show that failure of certain upstream dams does not result in water levels above the flood protection level of structures, systems, and components (SSCs) important to safety, or plant grade, if appropriate (i.e., the approaches screen out noncritical dams). The guidance in this section may be applied to both single dams and groups of dams. All other dams should be considered potentially critical dams and subjected to further evaluation. Refer to Figure 11.
These screening methods are intended to be used with publicly available information.
Online information on thousands of dams is available from the U.S. Army Corps of Engineers (USACE) National Inventory of Dams (NID; http://nid.usace.army.mil). Other Federal agencies, as well as State and local government agencies may also maintain information on dams that they own or regulate. Data used to delineate and describe watersheds (e.g., topographic maps, digital elevation datasets, watershed boundaries) are available from the U.S. Geological Survey (USGS) (at http://ned.usgs.gov, and http://nhd.usgs.gov/wbd.html).
A justification for using simplified methods should be developed on a site-specific basis and included in the flood hazard reevaluation report. Note that other methods can be used and will be reviewed on a case-by-case basis.
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Figure 11. Screening approach for watersheds with many dams.
3.1 Criteria for Inconsequential Dams Those dams identified by the USACE as meeting the requirements described in Appendix H, Dams Exempt from Portfolio Management Process, to ER 1110-2-1156, Safety of Dams - Policy and Procedures, (USACE, 2011c) may be removed from consideration for site impacts. The USACE states that there is essentially no concern with their possible failure, and thus, expenditure of scarce dam safety resources thereon is to be minimized. Non-routine management will generally take place Additionally those dams that upon failure would only cause damage to the property of the dam owner may be removed. In some cases, dams in this category have been identified by State dam safety programs. For example, the State of Colorado identifies such dams as No Public Hazard (NPH), while the Commonwealth of Virginia uses the term low hazard with special criteria.
These dams are referred to as inconsequential dams in this ISG. Removal of dams from the Recommendation 2.1 reevaluation based only on damage being limited to the owners property does not apply to licensee-owned dams (or onsite water-control structures). In this situation, additional analysis would be needed to justify that the dam or water-control structure meets the intent of the inconsequential category and may be removed from further consideration.
Staff Positions:
- Dams identified by Federal or State agencies as having minimal or no adverse failure consequences beyond the owners property may be removed from further consideration in the Recommendation 2.1 reevaluation. Dams owned by licensees may not be removed. Other inconsequential dams may be removed with appropriate justification (e.g., if they can be easily shown to have minimal or no adverse downstream failure consequences).
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- Continued consideration should be given to the failure consequences for clusters of dams that individually meet the above criteria if engineering judgment indicates that their collective failure would exceed the removal criteria.
3.2 Simplified Modeling Approaches Several optional methods discussed below provide a quantitative basis for simplified modeling of upstream dams. The methods are presented in a gradation of conservatism that is considered hierarchical-hazard-assessment (HHA) (see NUREG/CR-7046), and are applicable to all initiating events (hydrologic, seismic, and sunny-day).
SSCs important to safety located below site grade must also be confirmed to have flood protection to the elevation of the site in order to apply the screening methods. If SSCs important to safety do not have this level of flood protection, then replace site grade in the screening discussion with the lowest flood-protection elevation of SSCs important to safety.
The following methods may be applied sequentially in a HHA-type gradation of conservatism. Alternatively, a single method or a subset of the methods may be applied, as appropriate. The methods are described below and illustrated in Figure 12 through Figure 15.
- 1. Volume Method: This calculation is representative of having the total upstream storage volume simultaneously transferred to the site without attenuation. The following steps illustrate the method (see also Figure 12):
- a. Estimate and sum the storage volume for all upstream dams (inconsequential dams may be excluded) in the watershed, assuming pool levels are at levels corresponding to the maximum storage volume (i.e.
corresponding to the top of the dam).
- b. The 500-year flood is used to capture antecedent flood conditions at a NPP site. Current information on 500-year water surface elevations may be used, if available. Existing stage-discharge functions or USGS streamflow rating curves may also be used to estimate the flood stage at the site corresponding to the 500-year return period. If neither estimates of 500-year water surface elevations nor stage-discharge functions exist, then they may be developed using appropriate methods (e.g., using hydrologic and hydraulic models).
- c. Using available topographic data (e.g., LiDAR datasets or USGS digital elevation models), develop the stage-storage function at the site. The lowest stage should correspond to the 500-year flood elevation estimated in step (b).
The stage-storage function should exclude remote floodplain storage areas that could not be accessed by overbank floodwaters. Compute the flood elevation at the site by applying the total storage volume for all upstream dams (step a) to the stage-storage function.
- d. If the resulting water surface elevation is above the flood protection level of SSCs important to safety (or plant grade, if appropriate), iteratively repeat the process, removing volumes from largest dams, to segregate potentially critical dams from dams with small cumulative effect of failure at the site (small in the sense that detailed modeling is not required to conservatively account for their effect). The dams that are removed are potentially critical and should be evaluated separately, using refined methods. The cumulative effect of the noncritical dams will be carried forward and eventually added to refined estimates for the critical dams.
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- 2. Peak Outflow without Attenuation Method: This method is based on summing estimated discharges from simultaneous failures of upstream dams arriving at the site without attenuation. The following steps illustrate the method (see also Figure 13):
- a. Use applicable regression equations for estimating the peak breach outflow.
For those equations that use water level behind the dam at time of failure, assume pool levels corresponding to the maximum storage volume (i.e.
corresponding to the top of the dam). Because of the potentially large number of dams at this stage of the analysis, justification of applicability for individual dams will not be practical. Therefore, use of demonstrated conservative regression relations such as those developed by the Bureau of Reclamation (USBR, 1982) is recommended.
- b. Sum the peak failure outflows for all upstream dams (i.e., assume flows from all of the upstream dams reach the site simultaneously, ignoring attenuation).
As in step 1.a, inconsequential dams may be excluded.
- c. Using an existing stage-discharge function (e.g., from available hydraulic models of the watershed or USGS streamflow rating curves), estimate the flood stage at a NPP site corresponding to the 500 year return period. If stage-discharge functions do not exist, they may be developed using appropriate methods.
- d. Using the stage-discharge function developed in step (c), estimate the flood stage corresponding to the peak failure outflow sum (step b), using the 500-year flood elevation estimated in step (c) as the initial stage. Compare the estimated flood stage to the flood protection level of SSCs important to safety (or plant grade, if appropriate).
- e. If the resulting water surface elevation is above the flood protection level of SSCs important to safety (or plant grade, if appropriate), iteratively repeat the process, removing peak flow rates from largest dams, to segregate potentially critical dams from dams with small cumulative effect of failure at the site (see step 1d). The dams that are removed are potentially critical and should be evaluated separately, using refined methods. The cumulative effect of the noncritical dams will be carried forward and eventually added to refined estimates for the critical dams.
- 3. Peak Outflow with Attenuation Method: This method is based on summing estimated discharges from simultaneous failures of upstream dams arriving at the site with attenuation (i.e., using Method 2 with attenuation). The following steps illustrate the method (see also Figure 14):
- a. Same as Method 2, Step (a).
- b. Sum the peak failure outflows for all upstream dams (i.e., assume flows from all of the upstream dams reach the site simultaneously, taking into account attenuation based on distance). As in step 1.a, inconsequential dams may be excluded. The distance from the dam(s) to the site can be determined using GIS tools. Either the distance from the dam(s) through the river network to the site or the straight-line distance from the dam(s) to the site (more conservative) may be used. Regression equations for attenuation provided in USBR (1982) may be used, but should be tested against available models and/or studies to justify their applicability to the river/floodplain system.
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- c. Same as Method 2, Step (c).
- d. Same as Method 2, Step (d).
- e. Same as Method 2, Step (e).
- 4. Hydrologic Model Method (see Figure 15): Use an available rainfall-runoff-routing software package (e.g., HEC-HMS) to assess dam failure scenarios. The advantage to this approach is a more realistic representation of the effects of multiple upstream dam failures and attenuation to the site. The use of simplified hydrologic routing must be justified and shown to be appropriate for use (Section 9). Additionally, this method requires additional basin-specific inputs (e.g., watershed topography, roughness, unit hydrographs, and antecedent conditions), as well as dam breach parameters. Appropriate justification for these inputs should be provided.
For watersheds with many dams, setting up a single hypothetical dam to conservatively represent multiple dams in a rainfall-runoff-routing model involves much less effort than modeling actual dams. The hypothetical dam(s) should include representative situations of dams in series and cascading failures (see example illustration in Figure 16). The hypothetical dams should conserve the impounded volume of the dams they represent. The stage-storage relationship of the hypothetical dam should be based on the topography of its chosen location. As in Method 2, use available topographic data (e.g., LiDAR datasets or USGS digital elevation models). See Section 3.2.1 for additional detail on dam clustering and hypothetical dams.
Compare the estimated flood stage to the flood protection level of SSCs important to safety (or plant grade, if appropriate). As with Methods 1 through 3, it might be necessary to iteratively remove dams (hypothetical or real), larger to smaller, to the point at which the resultant water surface elevation is below the flood protection level of SSCs important to safety (or plant grade, if appropriate). The dams that are removed are potentially critical and should be evaluated separately, using refined methods. The cumulative effect of the noncritical dams will be carried forward and eventually added to refined estimates for the critical dams.
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Figure 12. Screening Method Flowchart (a) - Method 1 (Volume) 3-6
Figure 13. Screening Method Flowchart (b) - Method 2 (Peak Flow without Attenuation) 3-7
Figure 14. Screening Method Flowchart (c) - Method 3 (Peak Flow with Attenuation) 3-8
Figure 15. Screening Method Flowchart (d) - Method 4 (Hydrologic Method) 3.2.1 Representing Clusters of Dams To reduce the level of effort necessary to evaluate the flood levels occurring due to a dam breach, dams may be grouped together or clustered and represented as a larger hypothetical dam. The volume of this larger hypothetical dam would be the cumulative volume of the real dams it is intended to represent. The location of this hypothetical dam must be at either the location of the most downstream (DS) dam in the cluster or even further downstream toward the site (see Figure 16). Note that clustering of dams into fictitious configurations is only to be used in screening. It is not appropriate to apply this technique in detailed analyses.
Topographic information from LiDAR or a digital elevation map (DEM) at the location of the hypothetical dam is used to develop a stage-storage function for the hypothetical dam. This stage-storage function is used to determine the water surface elevation of the hypothetical dam.
As an alternative, if topographic information is not used to develop a stage-storage curve for the hypothetical dam, the stage-storage curve may be derived by summing the storage curves of the individual dams. The height of a hypothetical dam developed in this manner would be equal to the height of the tallest actual individual dam with a maximum storage equal to the summed storage of the individual dams. The invert elevation of the hypothetical dam would be derived from the topographic information.
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While choosing which dams to cluster and where to place the hypothetical dam representing the actual dams, one must keep in mind that the clustering must make hydrologic sense.
For example in Figure 16, the following dams may be represented or clustered according to Table 2.
Table 2. Possible Dam Clustering Combinations Location of Dam Comment hypothetical dam Dams 1, 2, and 3 DS of 1, 2, and 3 Illustrated in Figure 15 Dams 1 and 2 At or DS of 2 Dam 2 is closer to the site Dams 1 and 3 At or DS of 3 Dam 3 is closer to the site Dams 2 and 3 At or DS of 2 Dam 2 is closer to the site Figure 16. Hypothetical Dam Representing Storage Upstream 3-10
- 4. HYDROLOGIC DAM FAILURE Hydrologic dam failures can be induced by extreme rainfall or snowmelt events that can lead to natural floods of variable magnitude. The main causes of hydrologic dam failure include overtopping, structural overstressing, and surface erosion due to high velocity flow and wave action. Section 4.1 provides an overview of hydrologic failure by dam type. Section 4.2 provides more detail on analysis of various hydrologic failure modes.
4.1 Hydrologic Failure by Structure Type 4.1.1 Concrete Dams Concrete dams are generally perceived to be relatively resistant to overtopping failure.
Nonoverflow sections of concrete dams (i.e. sections not designed to be overtopped) are typically able to withstand some overtopping due to the inherent structural properties of their concrete. However, the foundation or abutments may be susceptible to significant erosion during overtopping flows (e.g. due to weak/fractured rock, or erodible soils), and if foundation or abutment support is lost due to overtopping erosion, the dam could fail. Other portions of the concrete dam or appurtenances may be vulnerable to flood-induced hydrologic loading. Examples include, but are not limited to: (a) erosion of an unlined tunnel or spillway chute; (b) erosion of a channel downstream from a stilling basin due to flow in excess of capacity; and (c) erosion of the spillway foundation where floor slabs have been damaged or lost; and (d) cavitation damage to lined tunnels or spillway chutes.
Overstressing of a dam may occur under flood conditions. As the reservoir rises during flood loading, there may be a level at which the heel of the dam goes into tension (based on effective stress), in which case the potential for cracking along a lift joint at that elevation may increase. At some point, the estimated tensile strength of the concrete may be exceeded, leading to failure of the dam.
Overstressing of an abutment may be a concern for concrete arch dams. An abutment foundation block on which the dam rests could become unstable under increased loading due to flood conditions. The increase in reservoir level not only affects the dam loads on the block, but also the hydraulic forces on the block bounding planes (joints, faults, shears, bedding plane partings, foliation planes, etc.).
Staff Position:
Concrete dams should be evaluated for potential hydrologic failure modes including, but not limited to:
- overtopping of the main dam, and overtopping erosion of a dam abutment or foundation
- erosion of an unlined tunnel or spillway chute
- erosion of a channel downstream from a stilling basin due to flow in excess of capacity
- erosion of the spillway foundation where floor slabs have been damaged or lost
- overstressing of the dam, foundation, or abutments
- cavitation damage to spillway and outlet flow surfaces 4.1.2 Embankment Dams Hydrologic loadings on embankments associated with flooding mainly fall into two categories: (a) increased internal seepage pressures; and (b) overtopping which initiates 4-1
embankment erosion. Overtopping may be due to stillwater elevation alone, or in combination with wave action. Overtopping may also be due to failure of gates, outlet works, or other appurtenances. Deterioration or plugging of drains may lead to increased internal seepage pressures.
Staff Position:
Embankment dams should be analyzed for conditions leading to, and the effects of:
- overtopping
- increases in internal seepage pressures 4.1.3 Spillways, Gates, Outlet Works, and Other Appurtenances There are a number of dam features, not unique to any one particular dam type, for which loss of function during flooding events could directly cause uncontrolled release of the reservoir or lead to uncontrolled release of the reservoir because of overtopping, erosion, or some combination of these. Chief among these are spillways, gates and other outlet works.
Sections 4.2.5 and 4.2.6 further discuss treatment of spillways and gates, respectively (as well as other outlet works), in the analysis of hydrologic failures.
Staff Position:
- Analysis of hydrologic failure modes should consider the potential for loss or degraded function of spillways, gates, outlet works, and other appurtenances. If failure is not assumed, provide an engineering justification.
4.1.4 Levees Levees that provide flood protection to a NPP site should be evaluated. Should they be overtopped in the event of a flood, it can be assumed that the levee has failed. Stability of the levee when not overtopped by a flood should be handled on a case-by-case basis.
Distant levees are generally not of great concern.
Overtopping can lead to significant landside erosion of the levee or even be the mechanism for a complete breach. Often earthen embankment levees are armored or reinforced with rocks or concrete to minimize erosion and prevent failure. In the riverine context, levee overtopping will initiate when floodwaters exceed the lowest crest of the levee system.
Wind waves and setup may contribute to the overtopping. In the coastal context, overtopping from the seaward side is most often caused by sustained high water levels and waves, due to a combination of storm surges and tide (and potentially tsunamis).
Overtopping can also occur from the landward side if the water level is raised under extreme precipitation in the basin.
Except for so-called frequently loaded levees, levees are generally not designed to withstand high water levels for long periods. A frequently loaded levee is one that experiences a water surface elevation of one foot or higher above the elevation of the landside levee toe at least once a day for more than 36 days per year on average (CADWR, 2012). Frequently loaded levees are generally designed to earthen dam standards.
4.2 Analysis of Hydrologic Failure Modes Overtopping is the most widely recognized hydrologic failure mode. Other common modes include overstressing of the dam or its abutments caused by hydrologic loads, erosion of embankments due to wave action, and erosion or cavitation in spillways. The next several subsections discuss in more detail the analysis of these and potential other failure modes 4-2
associated with flooding, as well as the potential failure of multiple dams as a result of a single storm event.
4.2.1 Internal Pressure Estimating internal seepage pressures associated with various reservoir levels is an essential element of embankment dam design. However, deterioration or plugging of drains, as well as internal erosion mechanisms, can lead to increased internal pressures and seepage. These conditions can compromise the structural integrity of the dam.
Staff Position:
Embankment dams should be evaluated for potential failures due to internal pressures from a large hydrologic inflow event (flood). Potential failure modes that should be evaluated include deterioration or plugging of drains and internal erosion mechanisms.
Evaluation should generally include reviewing the dam design to assure that appropriate filters, drains, and monitoring points are included. Monitoring records from piezometers, observation wells or other observation methods can be used to infer an absence of deficiencies.
4.2.2 Overtopping Overtopping occurs when the water surface elevation in the reservoir exceeds the height of the dam, allowing it to flow over the crest of the dam, an abutment, or a low point in the reservoir rim.
During a severe overtopping event, the foundation and abutments of concrete dams may also be eroded, leading to a loss of support and failure from sliding or overturning (FEMA, 2004a). Overtopping of a dam because of flooding, leading to erosion and breach of the embankment, is the most common failure mode for embankment dams. The details of breach modeling are discussed in Section 7.
Dams are typically designed to accommodate the so-called inflow design flood (IDF). In many cases, the IDF is the Probable Maximum Flood (PMF) developed by analyzing the impacts of the Probable Maximum Precipitation (PMP) event over the dams upstream watershed. In some cases, a lesser flood is considered. Inadequacy of the dam/spillway system and reservoir storage capacity to handle the inflow design flood is the most common cause of overtopping (inflow design flood estimates often change over time as more data is acquired or changes occur in the watershed). An overtopping failure may also occur when a reservoirs outlet system is not functioning properly, thereby raising the water surface elevation of the reservoir.
Staff Position:
- Dams unable to pass their individual PMF should be considered for failure.
o Embankment dams should generally be assumed to fail when overtopped. If failure is not assumed when a dam is overtopped, justification should include a detailed engineering analysis supported by site-specific information, including material properties of the embankment and foundation soils, material properties of embankment protection (if any), dam condition, etc.
o Concrete dams are not assumed to fail due to minor overtopping, but should be evaluated for failure due to loss of foundation or abutment support.
Impact of the flood flows on structures such as tunnels, spillways, chutes, and stilling basins should be examined.
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- The potential for overtopping due to nonfunctioning gates, outlets and other appurtenances should be evaluated to determine what the appropriate failure assumptions are with appropriate engineering justification.
- Onsite or offsite temporary structures can continue to be credited in the Recommendation 2.1 flood hazard reevaluation if such credit has been evaluated and accepted by the NRC staff prior to the request for information letter (NRC, 2012). No other temporary structures or measures (including mitigation or compensatory measures) should be credited in the flood hazard reevaluation.
Temporary structures or measures not credited in the hazard reevaluation may be proposed as interim actions and discussed in the appropriate section(s) of the hazard reevaluation response as described in the request for information letter (NRC, 2012).
4.2.2.1 Reservoir Capacity The reservoir capacity will influence the maximum water surface elevation as well as the rate of change in elevation during floods. Consideration should be given to the potential for reductions in reservoir capacity over the life of nuclear power plant. The most common reason for reservoir capacity reduction is sedimentation. Other potential mechanisms, although much less likely, include mud or debris flows (e.g., from fire-impacted watersheds),
failure of upstream coal-ash and mine-tailings impoundments.
Staff Position:
Consideration should be given to the potential for reductions in reservoir capacity due to sedimentation over the life of Nuclear Power Plant (NPP). Records from periodic bathymetric surveys of the reservoir, records of sediment production in upstream reaches, or estimates for sediment production rates for the upstream watershed can be used to support modeling assumptions.
4.2.2.2 Starting Reservoir Elevation The starting reservoir water surface elevation at the beginning of a flood can impact the maximum reservoir water surface elevation, and thus the potential for overtopping. A lower starting reservoir water surface elevation can lower the maximum water surface achieved in flood routings due to the additional surcharge space within the reservoir. Some reservoirs are operated to provide more surcharge storage during flood season.
Staff Position:
In view of the uncertainties involved in estimating reservoir levels that might reasonably be expected to prevail at the beginning of a flooding event, the default starting water surface elevation used in flood routings for evaluation of overtopping should be the maximum normal pool elevation (i.e., the top of the active storage pool). Other starting water surface elevations may be used, with appropriate justification. Justification should be based on the operating rules and operating history of the reservoir. For example, if the flood being considered is associated with a distinct season and the operation of the dam has seasonal variations that are codified and have historically been followed, it may be reasonable to select a starting reservoir elevation consistent with the operating rules and history. The operating history used should be of sufficient length to support any conclusions (e.g., 20 years or more). However, consideration should be given to possible instances in which the operating history or rules have been influenced by anomalous conditions such as drought.
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4.2.2.3 Reservoir Surcharge Capacity Reservoir surcharge capacity can affect the ability to pass large floods at a dam. Reservoir surcharge space can be used to store a portion of the incoming flood and in combination with the spillway capacity, can attenuate the peak of the flood (the peak outflow released through the spillway may be significantly less than the peak flood inflow). The amount of the peak inflow attenuation is a function of the reservoir surcharge volume in comparison to the flood volume, in addition to the spillway type and capacity. If the reservoir surcharge volume is large in comparison to the flood volume, significant attenuation will occur.
Staff Position:
Reservoir surcharge capacity can be credited in flood routings for evaluation of overtopping, with appropriate justification and documentation.
4.2.2.4 Spillway Discharge Capacity Spillway discharge capacity is one of the most significant factors in the ability of a dam to pass floods. Spillway discharge capacity is usually the critical component in passing large floods, but in some cases, release capacity through other waterways (outlets, turbines, etc.)
may be significant and will contribute to the total available release capacity. The term spillway discharge capacity as used in this document is intended to include spillway discharge capacity and any additional release capacity that would be available through other release structures at the dam.
In general, if the spillway discharge capacity is roughly equal to the peak inflow from large floods (approaching the PMF), dam overtopping is usually not an issue. If the spillway discharge capacity is significantly less than the peak inflow of a large flood, and if the volume of the flood is large in comparison to the surcharge capacity of the reservoir, dam overtopping could occur. The likelihood of these floods and erodibility of the dam or foundation materials controls the risk.
With regard to crediting release capacity through appurtenances other than the spillway (e.g., outlets or turbines), existing Federal guidance is not consistent. For example, USACE engineering manual EM 1110-2-1603, Hydraulic Design of Spillways, states that a powerhouse should not be considered as a reliable discharge facility when considering the safe conveyance of the spillway. Conversely, FERC Engineering Guidelines for the Evaluation of Hydropower Projects states that those release facilities that can be expected to operate reliably under the assumed flood condition can be credited for flood routing.
USBR best practice guidelines (USBR, 2011) suggest that at least one turbine should always be assumed to be down (e.g., for maintenance or other reasons) in performing flood routing.
Operational history on generating unit outages (e.g., maintenance, planned, and forced outages) can be used to inform assumptions about release capacity through turbines. The North American Electric Reliability Corporation (NERC) provides reports on generating unit availability for North America (e.g., NERC, 2012), which can be used if site-specific information is not available. Site-specific records from past flooding events, if available, should be reviewed.
Staff Positions:
- Release capacity through appurtenances other than the spillway (e.g., outlets and turbines) may be credited as part of the total available release capacity, with appropriate engineering justification that these appurtenances will be available and remain 4-5
operational during a flood event. Access to the site during a flood event should be considered.
- The generators and transmission facilities to support the credited turbine(s) must be shown to be operational under concurrent flood and expected prevailing weather conditions if the turbines are credited as part of the total available release capacity.
Potential for Reservoir Debris to Block Spillway Watershed runoff following a major storm event typically includes a large amount of debris and this debris has the potential to block spillway bays. Figure 17 shows debris build up at Lake Lynn Dam on the Cheat River (West Virginia) during a large flood in 1985. The spillway capacity was reduced by approximately 35% from the theoretical flow and thirteen out of twenty-six Tainter gates were almost fully blocked (Schadinger et al., 2012).
Figure 17. Debris Upstream of Lake Lynn Dam after 1985 Flood Event (Schadinger et al., 2012)
Many dams have debris management facilities (e.g., trash booms or trash gates) and programs. Sturdy trash booms may be able to capture debris before it reaches the spillway, but if not, the debris may clog the spillway opening. Trash gates may be used to route debris way from spillway structures and pass it downstream.
Historical information for debris production on the watershed (or similar watersheds) can be used to gauge the potential for debris blockage. Periodic debris studies are often performed by dam owners, dam regulators, or river basin commissions.
Staff Position:
- The potential for flood-borne debris to reduce spillway capacity should be considered. Historical information on debris production in the watershed or similar watersheds should be used to assess the potential debris volumes.
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- For dams that have debris management, a sensitivity study assuming a 5 to 10%
reduction in capacity should be performed. Describe structures, equipment, and procedures used to prevent spillway blockage by waterborne debris.
- For dams that lack debris management, greater capacity reductions should be considered. The appropriate capacity reduction will vary on a case-by-case basis.
Justification for the reduction used should be provided (e.g., debris studies for the watershed or similar watersheds).
4.2.2.5 Wave Action In addition to stillwater levels associated with flood flows, wind-generated wave action may lead to overtopping of a dam. In extreme circumstances, overtopping of the dam solely due to wave action could initiate erosion of the embankment and ultimately breach the dam.
Part of the evaluation should be to determine the potential for waves to exceed the dam freeboard (based on the prevailing wind direction, the wind speeds and the fetch of the reservoir).
Parapet walls are sometimes employed to contain waves that might overtop the dam and may need to be evaluated for a sustained water load. If a parapet wall is constructed on the dam crest across the entire length of the dam, dam overtopping will initiate when the reservoir water surface exceeds the elevation of the top of the parapet wall. If a parapet wall overtops, the impinging jet from overtopping flows may erode the dam crest and undermine the parapet wall. If the parapet wall or a section of the wall fails, the depth of flows overtopping the dam crest could be significant and the embankment will likely erode rapidly.
Staff Position:
Overtopping due to wave action should be evaluated, in addition to stillwater levels.
Coincident wind waves should be estimated at the dam site based on the longest fetch length and a sustained 2-year wind speed and added to the stillwater elevation.
4.2.3 Structural Overstressing of Dam Components Higher loading conditions are typically found in dams where the reservoir elevation is increased due to a hydrologic event. While the dam itself may not be overtopped, the surcharge may be increased, overstressing the dams structural components. This overstressing may then result in an overturning failure, sliding failure, or failure of specific components of the dam.
Embankment dams may be at risk when increased water surface elevations produce increased pore pressures and seepage rates that exceed the design seepage control measures for the dam. Concrete dams may be at risk due to potential failure of specific components of the dam, such as overturning or slipping of a slab section (FEMA, 2004a).
Staff Position:
Static stability of the dam and key appurtenances under hydrologic loads associated with the dams PMF should be demonstrated using current methods and standards. If the dam cannot withstand the applied loads, the dam should be assumed to fail. If the appurtenance cannot withstand the load, assume failure of the appurtenance and estimate the impact of its failure on stability of the dam. If the dam stability is not impacted, one still should consider the downstream impact of uncontrolled release (if any) associated with appurtenance failure.
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4.2.4 Surface Erosion from High Flow Velocity and Wave Action Surface erosion can occur along earthen spillways, the upstream or downstream embankment slopes, or along other inlet and outlet channels of appurtenant structures.
Surface erosion is primarily caused by high velocity runoff, reservoir wave action, and ice action. High flow velocities may cause headcutting along spillway sides that can progress towards the spillway crest, eventually leading to a full dam breach (FEMA, 2004a).
Staff Position:
Surface erosion of earthen embankments, spillways, channels, etc. due to wave action, high velocity flows, and ice effects should be considered.
4.2.5 Failure of Spillways Concrete-lined spillways, as well as unlined or grass-lined earthen spillways and unlined spillways excavated through rock, are subject to processes that may lead to failure during high flow events such as flooding.
Concrete-lined spillways are subject to stagnation pressure related failures that occur because of water flowing into cracks and joints during spillway releases. If water entering a joint or a crack reaches the foundation, failure can result from excessive pressure and/or flow into the foundation. If the foundation has no drainage system, or if the drainage system is inadequate, and the slab is insufficiently tied down, the buildup of hydrodynamic pressure under a concrete slab can cause hydraulic jacking. If drainage paths are available, but are not adequately filtered, erosion of foundation material is possible and structural collapse may occur.
Concrete-lined spillways are also subject to cavitation related failures. Cavitation occurs in high-velocity flow where the water pressure is reduced locally because of an irregularity in the flow surface. If the pressure drops below the vapor pressure, the water boils at ambient temperatures and water vapor bubbles form in the flow. As the vapor cavities move into a zone of higher pressure, they rapidly collapse as they return to the liquid state, sending out high-pressure shock waves. If the cavities collapse near a flow surface, there may be damage to the surface material. Cracks, offsets, surface irregularities and/or open joints in chute slabs and the lower portions of chute walls exposed to flow, may allow this failure mode to initiate. The geometry of the flow surface irregularities will affect the initiation of cavitation. The more abrupt the irregularity, the more prone the spillway will be to the initiation of cavitation. Once a flow surface is damaged by cavitation, the intensity of cavitation produced by the roughened surface increases, so damage can become severe in a short time.
Concrete deterioration in the form of delamination, alkali-silica reaction, freeze-thaw damage and sulfate attack can exacerbate failures related to stagnation pressure or cavitation by initiating cracks, opening cracks and joints in the chute concrete, creating offsets into the flow at joints, and causing separation of the chute from the supporting foundation.
Unlined (soil or grass-covered) spillways are subject to erosion phenomena similar to those associated with overtopping of embankments. The most common scenarios involve: (a) failure of the grass or vegetation cover in the spillway; (b) concentrated erosion that initiates a headcut; and (c) deepening and upstream advance of the headcut. The U.S. Department of Agricultures Agricultural Research Service (USDA/ARS) and Natural Resources Conservation Service (USDA/NRCS) have developed tools to assess erosion in earthen 4-8
and vegetated auxiliary spillways of dams. The Water Resource Site Analysis Computer Program (SITES) model and the Windows Dam Analysis Modules (WinDAM) are publicly available (NRCS, 2009, 2011). Both computer programs implement similar technology for evaluating spillway integrity. They are able to indicate whether breach of a spillway due to headcutting is likely, but do not model the consequences of the breach (i.e., the simulations stop when spillway breach initiation is predicted; enlargement of the spillway breach and release of the reservoir storage are not modeled). A detailed discussion of causative mechanisms and predictive models for erosion of unlined soil or grass-covered spillways is provided in USSD (2006).
For spillways excavated in rock, the models discussed in the previous paragraph have some ability to accommodate rock-like materials through their use of the headcut erodibility index, which is defined for both soil-like and rock-like materials. Appropriate conservatism should be exercised when applying this model to a rock channel, because it was not originally developed in that environment. Another alternative for dealing with scour of rock materials is the use of a curve relating the headcut erodibility index and the required stream power to produce scour. Variations of this type of curve have been proposed by Annandale (2006),
and Wibowo et al. (2005). USSD (2006) discusses in detail the causative mechanisms and predictive models for erosion of unlined spillways excavated in rock.
Staff Positions:
- Dams should be evaluated for potential failure due to spillway failure.
- Concrete spillways should be evaluated for relevant failure modes including stagnation pressure failures, cavitation, concrete deterioration (e.g., delamination, alkali-silica reaction, freeze-thaw damage, and sulfate attack) and other relevant modes.
- Other (non-concrete) spillways should be evaluated for potential failures including failure of the grass or vegetation cover in the spillway; concentrated erosion that initiates a headcut, deepening and upstream advance of the headcut, and other relevant modes.
4.2.6 Failure of Gates A variety of gates is used to control spillways. Gates range in complexity from simple slide gates (e.g., fixed-wheel gates or roller gates) to float-type gates (e.g. drum gates and ring gates) to gates which are shaped to balance hydrostatic forces (e.g., radial or tainter gates).
Another class of spillway gate is the fuse plug, which is a collapsible dam installed on spillways to increase the dam's capacity. The principle behind the fuse plug is that the majority of water that overflows a dam's spillway can be safely dammed except during high flood conditions. The fuse plug may be a sand-filled container, a steel structure or a concrete block. Under normal flow conditions, water will spill over the fuse plug and down the spillway. In high flood conditions, where the water velocity may be so high that the dam itself may be put in danger, the fuse plug breaches, and the floodwaters safely spill over the dam.
Gates may fail to operate due to mechanical or power failures. Gates may also fail to operate when needed in flooding situations due to excessive friction or corrosion. This is more common with gates that are not properly maintained or seldom used. There is also the potential for actual gate operations to differ from planned operations (e.g., inability of an operator to access gate controls or an operator decision to delay opening the gates due to downstream flooding concerns).
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Fuse plugs are generally considered reliable, but there is some inherent uncertainty about the exact depth and duration of overtopping needed to initiate their breach. There is also uncertainty about the exact rate of breach development. Understanding the magnitude of these uncertainties is important because delayed operation of the fuse plug could lead to failure of the dam.
Staff Position:
- The evaluation should consider the potential for gate failure under flooding conditions to lead to an uncontrolled release of the reservoir.
- With regard to fuse plugs, one should show that flood routings are not sensitive to the depth and duration of overtopping needed to initiate a breach so that delayed operation does not lead to the failure of the main dam.
4.2.7 Operational Failures and Controlled Releases Certain operational failures and even certain controlled releases can lead to flooding at a NPP site. They may occur in a variety of situations, but the primary concern is that operational failures or controlled releases may be a compounding factor in flooding situations.
4.2.7.1 Operational Failures Operational failures can occur at dams when equipment, instrumentation, control systems (including both hardware and software), or processes fail to perform as intended. This, in turn, can lead to uncontrolled reservoir release. Some illustrative examples of these types of failures include:
- Failure of a log boom allows reservoir debris to drift into and plug the spillway, leading to premature overtopping of the dam.
- Gates fail to operate as intended causing premature overtopping of the dam. This could result from mechanical or electrical failure, control-system failure, or failure of the decision process for opening the gates. Gates may also fail to operate when needed due to excessive friction or corrosion. This is more common with gates that are not maintained or used very seldom.
- Loss of access to operate key equipment during a flood leads to overtopping of the dam or other uncontrolled releases.
- Loss of release capacity leads to overtopping of the dam. For example, if releases through the power plant are a major component of the release capacity and the switchyard is taken out during a flood or earthquake, that release capacity will be lost. If the powerhouse is lower than the switchyard, loss of the powerhouse without loss of the switchyard would also result in loss of release capacity.
- Mechanical equipment failure due to changes in operation without a corresponding change in maintenance leads to premature dam overtopping. For example, if river operation requires frequent gate opening to enhance fisheries without a corresponding increase in the frequency of gate lubrication, component failure could occur when the gate is needed to pass a flood.
- Overfilling pumped-storage reservoirs can lead to overtopping and failure of the dam. This could happen due to faulty instrumentation, control system issues, or operator error.
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- Failure to detect hazardous flows or a breakdown in the communication process to get people out of harms way leads to failure of the dams safety measures. For example, power and phone lines may be cut by a large earthquake or flood. This may result in inability to warn people in advance of life-threatening downstream flows.
An exhaustive analysis of all potential operational failures is generally not required. Instead, the intent is to understand the site-specific relationship between potential operational failures and existing safety margins (e.g., available freeboard).
Staff Position:
- Operational failures that may lead to uncontrolled releases and threaten to inundate a NPP site should be considered. Applicable operational failures should be identified, and consequences for the most likely failures should be evaluated.
Operational history of similar dams, equipment, and procedures should be used to identify and rank operational failures.
4.2.7.2 Controlled Releases There may be instances where controlled releases can lead to inundation at a NPP site.
Examples include, but are not limited to: 1) releases performed in order to prevent dam failure during flood conditions; 2) releases performed to rapidly drawdown a reservoir to prevent incipient failure after a seismic event; and 3) releases performed to rapidly drawdown reservoir to prevent incipient sunny day failure.
Consideration of the potential for controlled releases to cause flooding at a NPP site may include examination of spillway and gate discharge capacities and examination of reservoir/dam operating rules and procedures. Communication plans and systems for warning downstream entities of impending release should also be considered.
Staff Position:
- The potential for controlled releases that may threaten SSCs important to safety at a NPP site should be considered.
4.2.8 Waterborne Debris Waterborne debris (e.g., trees, logs, or other objects) produces drag and impact loads that may damage or destroy buildings, structures, or parts thereof. The magnitude of these loads is very difficult to predict, yet some reasonable allowance must be made for them in evaluating dam performance. The loads are influenced by where the structure is in the potential debris stream:
- immediately adjacent to or downstream from another building
- downstream from large floatable objects (e.g., exposed or minimally covered storage tanks)
- among closely spaced buildings Building standard ASCE 7-10 developed by the American Society of Civil Engineers (ACSE 2010) describes a methodology for determining impact loads based on the momentum-impulse method. The methodology differs from the classic impulse-momentum approach (USACE 1995d, FEMA, 2011) in that it assumes a half-sine form for the applied load and includes several coefficients to allow design professionals to adapt the resulting force to local flood, debris, and building characteristics. The ASCE 7-10 methodology incorporates an importance coefficient to represent the risk category of the impacted structure. For critical 4-11
or potentially critical dams (and for SSCs important to safety at NPP sites), an importance coefficient of 1.3, corresponding to Risk Category IV is appropriate. The ASCE 7-10 methodology also uses a depth coefficient meant to take into account the structures location within the flood hazard zone and flood depth. For dam and NPP sites, this coefficient should be based on stillwater depth (i.e., disregard the selection criteria based on flood insurance rate map zones). ASCE 7-10 also provides a method for estimating drag loads on structures.
ACSE 7-10 provides guidance regarding the debris object weight selection for impact loads.
The standard states that large woody debris with weights typically ranging from 1,000 to 2,000 lbs are appropriate for riverine floodplains in most areas of the U.S. In the Pacific Northwest, larger tree and log sizes suggest a typical 4,000 lb debris weight. Debris weights in riverine areas subject to floating ice typically range from 1,000 to 4,000 lb. ASCE considers the 1,000 lb object to represent a reasonable weight for other types of debris ranging from small ice floes, to boulders, to manmade objects. However, licensees should consider regional and/or local conditions before the final debris weight is selected.
Staff Positions:
- Drag and impact loads due to waterborne debris carried by flood waters should be considered with regard to impacts on the dam (i.e., to gates and associated mechanical equipment, appurtenances, parapets, etc.).
- In the case of dam break flood waves, debris loads on SSCs important to safety should be considered.
- The methodologies for debris load estimation described in ASCE 7-10, with the caveats described above, are acceptable to NRC staff.
- Licensees should consider regional and/or local conditions before the final debris weight is selected. On navigable waterways, for example, the potential for impact from watercraft and barges should be considered in addition to that from trees, logs and common manmade objects.
4.2.9 Multiple Dam Failure due to Single Storm Scenario At some NPP sites, there may be potential for flooding due to multiple dam failures (e.g.
dams on different reaches or tributaries above the NPP site) or the domino failure of a series of dams on the same reach. For example, the site may be located in a watershed where dams are located close enough to one another that a single storm event can cause multiple failures that have a compound effect on flood waves reaching the site. Failure of a critically located dam storing a large volume of water may produce a flood wave compounded by domino-type failures of downstream dams (e.g., failure of an upstream dam may generate a flood that would become an inflow to the reservoir impounded by a downstream dam and may cause failure by overtopping the downstream dam; if several such dams exist in a river basin, each sequence of dams within the river basin could fail in a cascade).
Staff Positions:
- Those dams unable to be removed as inconsequential or screened out as noncritical (see Section 3) remain potentially critical dams. These dams should be evaluated for hydrologic failure that leads to cascading downstream failures and/or simultaneous failures of tributary dams that ultimately produce flood conditions at the site. Operational rules may be considered but the starting water surface elevation at the most upstream dam under evaluation should be as specified in Section 4.2.2.1.
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River flows should be based on the precipitation / runoff from the basin encompassing the multiple dam scenario(s) under consideration. Flood waves from multiple dam failures should be assumed to reach a NPP site simultaneously unless appropriate justification for differing flood arrival times is provided.
- Three cases of multiple dam failure should be considered: (a) failure of individual dams on separate tributaries upstream from the site, (b) cascading or domino-like failures of dams upstream from the site, and (c) a combination of cases (a) and (b).
o In the first case, one or more dams may be located upstream from the site but on different tributaries, so the flood generated from the failure of an individual dam would not flow into the reservoir impounded by another dam.
These individual dam failures should be analyzed together because of the potential for a severe storm to cause large floods on multiple tributaries.
o In the second case, failure of an upstream dam may generate a flood that would become an inflow to the reservoir impounded by a downstream dam and may cause failure by overtopping the downstream dam. If several such dams exist in a river basin, each sequence of dams within the river basin could fail in a cascade. Each of these cascading failure sequences should be investigated to determine one or more sequences of dam failures that may generate the most severe flood at the site. Simplified estimates of the total volume of storage in each of the potential cascades should provide a good indication of the most severe combination. If multiple cascading dam failures that cannot be separated by simple hydrologic reasoning, all of the candidate cascades that are comparable in terms of their potential to generate the most severe flood at the site should be simulated using the methods described in Section 9. The most severe flood at the site resulting from these cascades should be considered in determining the design-basis flood.
- Depending on the storage capacities of the reservoirs impounded by dams in a given cascading scenario, it may be reasoned that the scenario that would release the largest volume of stored water would likely lead to the most severe flooding scenario. However, the distance a flood has to travel to reach a plant site can also affect the severity of the flood at the site. If a definite conclusion cannot be reached, all possible cascading scenarios should be simulated to determine the most severe scenario.
4.2.10 Levee Failures Failure of levees that provide flood protection to a NPP site should be considered. Such levees should be considered to fail when overtopped. Their stability when they are not overtopped should be handled on a case-by-case basis. Distant levees are generally not of great concern.
Earthen levees (the most common type) are designed to withstand flood conditions, but typically for limited durations, discharges and water surface elevations. Under flooding conditions, pore pressures within the embankment soils may increase to the point where the embankment slopes become unstable. Slope failure and subsequent breaching may be quite sudden. Similar instability may arise in the foundation soils. Such conditions are often accompanied by levee boils, or sand boils, in which underseepage resurfaces on the landside, in the form of a volcano-like cone of sand. Boils signal a condition of incipient instability which may lead to erosion of the levee toe or foundation or sinking of the levee into the liquefied foundation below (i.e., the boils may be the result of internal erosion or piping or they may also be a symptom of generalized instability of the foundation).
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Lack of inspection, maintenance, and control is often a major contributing factor in levee failure. Uncontrolled vegetation growth (especially trees) or animal burrows may be sources of local weaknesses.
Natural geomorphic processes associated with channel migration may endanger a riverine levee system. For example, the downstream end of bends and areas across from tributary inflows are areas of high-energy river flow and significant erosion may occur resulting in bank retreat and eventual levee failure. Embankments constructed across ancient riverbeds or stream channel meanders can provide weak points for seepage and pipe formation.
In some cases, levees are breached intentionally, in order to protect other areas. In most cases, an intentional breach is not initiated without significant planning and notification.
Not all levees are earthen embankment type. Concrete and sheet piles are sometimes used. Some earthen levees have sheet pile or concrete parapets.
Staff Positions:
- In general, earthen embankment levees should be assumed to fail when overtopped.
The case for nonfailure should be developed using detailed engineering analysis supported by site-specific information, including material properties of the embankment and foundation soils, material properties of embankment protection (if any), levee condition, etc. Other forms of levees (e.g., pile walls and concrete flood walls) should be evaluated for potential failures applicable to the particular type of levee.
- Levees are generally not designed to withstand high water levels for long periods.
However, no generally accepted method currently exists for predicting how long a levee will continue to function under high loading conditions. Therefore, historical information is the best available basis for predicting levee performance. The historical information should be from levees that have similar design and construction characteristics as the levee being analyzed.
- If the performance of levees is potentially important to estimation of inundation at a NPP site, failures should be treated in a conservative, but realistic, manner.
- If credit is taken for a specific levee behavior (either failure or nonfailure), an engineering justification should be provided.
- Crediting intentional levee breaching will generally not be accepted due to large uncertainties in implementation of such plans (e.g., decisions about such actions are often political in nature).
- Assumptions regarding conveyance and off-stream storage should be supported with engineering justifications.
- The potential for loss or degraded function of levee control works should be considered.
- Because levees are typically designed to function as a system, the potential for failure of an individual segment should be evaluated for its impact on the functioning of the levee system as a whole.
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- 5. SEISMIC DAM FAILURE Seismic hazard is generally defined as the physical effects that occur as the result of an earthquake (e.g., ground shaking, surface faulting, landsliding, or liquefaction). The severity of these effects depends on factors such as the intensity and spectral characteristics of ground motions, dam type, dam construction materials and methods, and local site conditions. For some dam sites, the potential for surface fault displacement through the site is a major concern, but strong ground shaking is the most common earthquake effect.
Ground shaking may directly damage the dam structure and appurtenances or induce subsequent failure modes. For example, seismically induced soil liquefaction can lead to embankment failure for earthen embankment dams or foundation failure for other types of dams.
Another possibility concerns an active fault passing through the reservoir of a dam. Fault offset within the reservoir could create a seiche wave capable of overtopping and eroding the dam. Seiche waves can be generated by large fault offsets beneath the reservoir or by regional ground tilting that encompasses the entire reservoir. Sloshing can lead to multiple overtopping waves from these phenomena.
Note that the seismic dam failure scenario is one in which load combinations come into play (e.g., a more frequent earthquake combined with a flood event). This is discussed in Section 5.6. In some instances (e.g., when downstream consequences are likely to be small), the licensee may elect to simply assume that the dam fails seismically in lieu of conducting a seismic analysis. In this case, the question arises of what flood event to assume, since no frequency is assigned to the seismic event. In Section 5.6, the 500-year flood is used in conjunction with the lower of the two seismic hazard levels. Therefore, a lesser flood would not be appropriate in this case.
Staff Position:
If seismic failure is simply assumed without analysis, the seismic failure should be assumed to occur under 500-year flood conditions (or 1/2 PMF, whichever is less).
5.1 Overview A complete seismic evaluation of an existing dam typically includes: 1) an assessment of site-specific geological and seismological conditions to determine seismic potential and associated ground motions (a field and laboratory testing program may be needed to characterize the distribution and properties of the soils if they are not known); and 2) an analysis of the effects of earthquake shaking on the dams structure and its appurtenances.
The basic steps in analyzing the seismic failure problem are as follows:
- 1. Estimate earthquake ground motions:
- a. Characterize earthquake sources
- b. Apply attenuation relations to estimate bedrock motion at the site
- c. Determine site amplification function to estimate ground surface response at the site
- 2. Estimate the loadings imposed on the dam by the earthquake ground motions
- 3. Analyze the ability of the dam to withstand the earthquake-induced loadings The behavior of dams and their foundations under earthquake loading is an extremely complex problem. It is therefore essential that seismic investigations be conducted by knowledgeable seismic engineers following the state of the practice in the profession.
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5.1.1 Seismic Hazard Characterization Seismic parameters represent one of several ground-motion-related variables or characteristics, such as peak ground acceleration (PGA), peak ground velocity (PGV) displacement, response spectra, acceleration time histories, or duration. They can be obtained using deterministic or probabilistic seismic hazard analysis (PSHA) procedures.
Preferably, seismic evaluation parameters should be specified using site-dependent considerations, making use of existing knowledge and actual observations that pertain to earthquake records obtained from sites with similar characteristics. In particular, attenuation characteristics should not be applied blindly due to differences in earthquake focal depths, transmission paths, and tectonic settings. It is important to indicate if seismic parameters predicted by attenuation relationships take into account the effect of surface soil layers since soft deposits can alter the bedrock motions dramatically. The duration of shaking is a significant seismic evaluation parameter, as it has been shown to be directly related to the extent of damage, especially in the case of embankment dams. This is even more critical when the foundation or the embankment contain soils that are prone to accumulate excess pore pressures during an earthquake. Local conditions may affect the expected duration of earthquake shaking and should be considered on a case-by-case basis.
Vertical peak and spectral accelerations are usually considered less critical than horizontal motions for embankment dams that are distant from the earthquake source(s). They are more important for concrete dams and concrete appurtenances. Vertical motions are sometimes estimated by scaling horizontal accelerations, along with corresponding frequencies in the case of spectral values, using factors in the range of 1/2 to 2/3. When vertical accelerations are critical, it is preferable to rely on attenuation relationships developed specifically for vertical accelerations.
While definition of seismic parameters by peak values and spectral shapes is sufficient for some dam applications, in other cases a time history analysis may be required. This will be the case when induced stresses approach the strength of the dam or foundation materials, or when it is necessary to consider the inelastic behavior of the dam.
5.1.1.1 Use of USGS National Seismic Hazard Maps The USGS National Seismic Hazard Maps (USGS, 2008) are developed from seismic sources and ground-motion equations specific to the Central and Eastern United States earthquakes, to the Western United States crustal fault earthquakes, and to subduction-zone interface and in-slab earthquakes. In the Central and Eastern United States, the USGS generally calculates ground motions from sources that are up to 1,000 km from the site. In the Western United States, the USGS calculates ground motion from crustal sources less than 200 km and subduction sources less than 1,000 km from the site. The USGS also maintains a Web site where the maps (and the data and software used to create them) are available (http://earthquake.usgs.gov/hazards/?source=sitenav).
5.1.2 Structural Considerations It is important for the engineers who will be doing the fragility evaluation to coordinate with the seismologist on generating the hazard curves, uniform hazard spectra, and time-history accelerograms. These products should reflect the parameters that control the structural response of the dam and/or appurtenant structures. Typically, this is the spectral acceleration at the predominant period of a structure, or perhaps the area under a response spectrum curve covering more than one structural vibration period if several modes contribute to the structural response. It may be necessary to ask for different hazard curves for different structures forming the reservoir retention system. In some cases, a certain 5-2
combination of acceleration and velocity may be critical to the structural response, and hazard curves would need to be developed that relate to simultaneous exceedance of given acceleration and velocity levels.
5.1.3 Probabilistic Seismic Hazard Analysis A probabilistic seismic hazard analysis (PSHA) involves relating a ground-motion parameter to its probability of exceedance at the site. The value of the ground-motion parameter to be used for the seismic evaluation is then selected after defining a probability level, applicable to the dam and site considered. PSHA considers the contributions from all potential sources of earthquake shaking collectively. Uncertainty is treated explicitly, and the annual probability of exceeding specified ground motions (commonly expressed as response spectra acceleration(s) at the period of interest), are computed. Alternatively, the analysis may be performed for a specified duration of time (such as the operating life of the dam).
PSHA involves a thorough mathematical and statistical process that takes into account local and regional geologic and tectonic settings, as well as applicable historic and geologic rates of seismic activity. The results are typically expressed in terms of peak ground acceleration (PGA), peak ground velocity (PGV), or spectral amplitudes at specified periods.
Whereas in the past deterministic approaches have been favored in dam engineering, there has been a gradual shift to probabilistic methods for determining ground-motion parameters.
Therefore, the rest of this document will concentrate on the PSHA approach. Any PSHA study has three basic components: 1) seismic source characterization, 2) development of ground-motion estimates, and 3) development of the site response. Additional steps include the development of uniform hazard spectra and development of acceleration time histories.
Staff Position:
- PSHA is considered the state of practice for evaluating seismic hazards for dam failure.
5.2 Seismic Failure by Structure Type The impact of seismic ground motions and key failure modes of interest will depend on the design and construction of the dam. The following sections discuss the key concerns and seismic failure modes for concrete and embankment dams, as well as spillways, gates, and other appurtenances. This is followed by a short discussion of levees.
5.2.1 Concrete Dams Under earthquake loading, concrete dams will respond to the level and frequency of the ground shaking, combined with the forces due to water in the reservoir. The tensile strength of the concrete under such dynamic loading is typically an important consideration.
However, both structural and foundation failure modes may be important. If the shaking is severe enough, cracking and subsequent partial or complete separation of the contact surface between blocks may propagate through the structure. Ground surface displacement along a fault or liquefaction of foundation soils could lead to cracking and failure. It should be noted that the post-earthquake stability of the dam and foundation may be reduced depending on the level and duration of shaking experienced. An earthquake may also damage the dams drainage system. The stability of the dam could be threatened if the drain functions are impaired to the point that uplift pressure increases significantly.
Although no concrete dam foundations are known to have failed as a result of earthquake shaking, unprecedented seismic loads would in effect be a first-loading condition that could trigger movement and failure of arch dam foundation blocks. Therefore, it is important to 5-3
analyze and evaluate the risks associated with potential earthquake-induced foundation instability.
Historically, concrete arch dam failures have resulted primarily from sliding of large blocks within the foundation or abutments. However, since there have been no known arch dam failures as a result of earthquake shaking (USBR 2011), there is no direct empirical evidence to indicate how an arch dam would structurally fail under seismic loading. Shake table model studies and numerical simulations (e.g., three-dimensional dynamic finite element analysis) provide the basis for postulated failure modes. These studies indicate that structural failure is initiated by cantilever cracking across the lower central portion of the dam, followed by diagonal cracking parallel to the abutments. This type of cracking eventually leads to isolated blocks within the dam. The isolated blocks may subsequently rotate (swing downstream or upstream), catastrophically failing the dam and releasing the reservoir.
The design of older buttress dams generally considered only the gravity and water pressure loads, and the buttress configuration is remarkably efficient in providing the resistance required for such loading. However, in the interest of efficiency, the buttresses were made very slender and thus they have very little strength for resisting cross-stream accelerations.
Under strong shaking, it is conceivable that an older slab and buttress or multiple arch dam designed in this manner may suffer significant cracking/buckling and fail in domino fashion through the successive collapse of its buttresses. Typically both structural and foundation failure modes should be considered. The foundation stiffness can have a large effect on the rotations at the base of the buttresses and the dynamic response of the dam.
Under earthquake loading, concrete buttress and multi-arch dams will respond to the level and frequency of the ground shaking, combined with the forces due to water in the reservoir.
Forces due to the water will depend upon the details of the design. The entire mass of water directly over the dam face will move with the vertical seismic motion. For flat slabs, there will be little or no cross-canyon hydrodynamic forces generated. Designs with cylindrical arches, domes or massive head buttresses will be subject to cross-canyon hydrodynamic forces. Depending upon the element of the structure under consideration, either the tensile, shear, or compressive strength will be an important consideration. For struts (which provide lateral support to the buttresses, when present) compressive strength is important. Slab-type water barriers supported by a corbel carry load by compression, shear and moment. Shear and tensile strength is important for the buttresses and the supporting corbels.
Staff Positions:
- Seismic analysis of concrete dams should include assessment of ground shaking, surface displacement, and forces due to water in the reservoir.
- Both structural and foundation failure modes should be considered.
- Foundation liquefaction/deformation potential should be considered.
- Structural failure modes considered should take into account the unique concerns for the type of dam in question.
5.2.2 Embankment Dams Although many embankment dams have been exposed to earthquake shaking, there have been few instances where an earthquake has damaged an embankment dam enough to result in the uncontrolled release of reservoir water. Either the damage caused by the earthquake was not extensive enough, or in the rare cases where damage was extensive, 5-4
the reservoir was far below the damage and uncontrolled releases did not occur. However, in spite of the relatively few failures experienced, it remains true that earthquakes can initiate a wide variety of potential failure modes in embankment dams. Shaking can cause loss of strength or even liquefaction of foundation or embankment soils, leading to deformation, sliding, or cracking failures.
Extensive shear strength reduction beneath an embankment slope can trigger a flow slide that, in turn, can produce a very rapid dam failure. Many cycles of low-amplitude loading can also induce a fatigue-like shear strength loss in dense, saturated, materials. A translational failure can occur if the entire foundation beneath an embankment liquefies and the reservoir pushes the embankment downstream far enough to create a gap in the vicinity of an abutment.
There are many ways in which cracking can occur due to seismic shaking, such as differential settlement upon shaking, general disruption of the embankment crest, offset of a foundation fault, or separation at spillway walls. Surface displacements can lead to cracking of the dam foundation, embankment or conduits passing through the dam. Shearing of a conduit passing through an embankment dam due to fault displacement can allow transmission of high-pressure water into the dam, leading to increased gradients and potential for internal erosion.
Staff Positions:
- Seismic analysis of embankment dams should include assessment of ground shaking and surface displacement.
- Both structural and foundation failure modes should be considered.
- Deformation and liquefaction potential of both the dam and the foundation should be considered.
5.2.3 Spillways, Gates, Outlet Works and Other Appurtenances There are a number of facilities, not unique to any one dam type, for which loss of function during or following a seismic event could directly cause uncontrolled release of the reservoir through the failed gate or lead to uncontrolled release of the reservoir via overtopping, erosion, or some combination of these. Chief among these are spillways, gates and other outlet works.
Gates may fail to operate for a variety of reasons during seismic events. Dynamic loading may cause buckling of the gate itself. The seismic event could damage a gate hoist mechanism mounted above the gates, or cause shear or moment failure of supporting structures such as the piers in which the gates are mounted. Inoperability of a gate can cause a reservoir to fill beyond its design maximum water level (causing failure due to increased hydrostatic forces or overtopping) if not corrected in a timely manner.
Staff Position:
Seismic evaluation of dams should include consideration of whether a seismic event could lead to dam failure and subsequent uncontrolled release of the reservoir due to loss or degraded function of spillways, gates, outlet works and other appurtenances.
5.2.4 Levees Earthquakes can damage or cause complete failures of levees. The most common mode of earthquake-induced damage is expected to be lateral spreading and cracking associated with earthquake shaking. As for earthen dams, shaking may cause liquefaction of soils 5-5
within the levee or in the foundation soils. Design of levee systems for seismic performance has generally had low priority in the past, except for so-called loaded levees with a high likelihood of having coincident high water and earthquake loading. (e.g., levees in the Sacramento-San Joaquin Delta of California).
Staff Position:
- As with hydrologic levee failures, seismic failure of distant levees is not of concern.
Failure of an offsite levee that provides flood protection to the NPP site, where applicable, is of interest (seismic analysis of onsite water control structures including levees, if applicable, are part of the Recommendation 2.1 Seismic Reevaluation.
- Levees without seismic design criteria should be assumed to fail during a seismic event. Survival of a loaded levee during an earthquake event should be justified through appropriate engineering analysis.
- In general, levees are not designed to withstand significant seismic loads.
Therefore, to examine consequences of seismic failure, assume a starting water level elevation corresponding to a 500-year flood or top of the levee, whichever is less.
- Levees should not be assumed to fail in a beneficial manner, without appropriate engineering justification.
5.3 Analysis of Seismic Hazards Using Readily Available Tools and Information Because there will generally be insufficient time and resources to perform detailed seismic analyses for all dams upstream from a NPP site, the following approach may be applied. It is assumed that the screening approach described in Section 3 has already been applied (i.e., inconsequential dams have been removed and non-critical dams have been screened out).
The analysis approach outlined in this section and in Section 5.4 is meant to take advantage of existing information for the dam (e.g. seismic design information or seismic qualification studies), along with a consistent level analysis of the seismic hazard (e.g. existing seismic hazard curves or seismic hazard assessments developed using readily available tools and data). In order to apply this approach, the seismic capacity of the dam (i.e., based on seismic design or post-construction seismic capacity studies) must be known. The seismic capacity should be characterized for frequencies of importance to the dam (e.g. design response spectrum). Note that there may be different capacities depending on the failure mode. For example, the capacity for concrete cracking for a concrete dam may be different from the capacity for sliding.
The licensee has the option of using this approach or conducting a more detailed site-specific characterization of seismic hazards at the dam site (as discussed in Section 5.7) as well as more detailed analysis of the seismic capacity of the dam (discussed in Section5.8 ).
The options for performing seismic hazard analysis are outlined in Figure 18.
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Figure 18. Seismic Dam Failure Analysis Options 5.3.1 Ground Shaking Ground shaking is one of the most common seismic loads that should be considered for dams. As discussed in Section 1.4.2, it acceptable to use the 1x10-4 annual frequency ground motions, at spectral frequencies important to the dam, for seismic evaluation.
Uniform hazard spectra (UHS) are computed or developed from the seismic hazard curves.
This is done by developing hazard curves (i.e., spectral acceleration versus exceedance probability) for several vibration periods to define the response spectra. Then, for a given exceedance probability or return period, the ordinates are taken from the hazard curves for each spectral acceleration, and an equal hazard response spectrum is generated. Thus, the response spectra curves are generated for specified annual exceedance frequencies of interest.
Staff positions:
- The seismic hazard at the dam site should be characterized using probabilistic seismic hazard assessment (PSHA) for the spectral frequencies of interest to the dam:
o The data and software tools available from USGS, which were used to develop the most recent version of the National Seismic Hazard Maps (this is the 2008 version as of the publishing of this guidance) are suitable for developing bedrock hazard curves and uniform hazard spectra at 1x10-4 annual frequency of exceedance. (USGS, 2008). However, due diligence should be applied to demonstrate the continued validity of the data used in USGS (2008) for sites in the western US (see Section 5.7.1.1).
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o The site amplification functions developed by the Electric Power Research Institute (EPRI, 1989) should be used with the bedrock hazard curves (developed using methods described above) to obtain site-specific soil hazard curves as described in NUREG/CR-6728 (USNRC, 2001).
- As an alternative to the use of the USGS seismic hazard curves, it is acceptable to perform a detailed site-specific PSHA consistent with the methodologies suitable for use in characterizing seismic hazard at U.S. nuclear power plant sites, as described in Regulatory Guide 1.208 (USNRC, 2007). General aspects of a site-specific seismic hazard analysis are discussed in Section 5.7. Regulatory Guide 1.208 provides more definitive and detailed guidance.
5.3.2 Fault Displacement For some dam sites, the potential for surface fault displacement through the dam site or foundation is a concern. Another possibility concerns an active fault passing through the reservoir of an embankment dam. Fault offset within the reservoir could create a seiche wave capable of overtopping and eroding the dam. Seiche waves can be generated by large fault offsets beneath the reservoir or by regional ground tilting that encompasses the entire reservoir. Sloshing can lead to multiple overtopping waves from these phenomena.
Two types of surface faulting are generally recognized: principal (or primary) and distributed (or secondary) surface faulting. Principal faulting occurs along the main fault plane(s) that is the locus of release of seismic energy. Distributed, or secondary faulting, is displacement that occurs on a fault or fracture away from the primary rupture and can be quite spatially discontinuous.
Probabilistic fault displacement hazard analyses (PFDHA) can be performed in a manner analogous to that used for probabilistic ground motion. The results are represented by a hazard curve, which shows annual occurrence of fault displacement values (i.e., the annual frequency of exceeding a specified amount of displacement). A recent example is the analysis conducted for Lauro Dam near Santa Barbara, California (Anderson and Ake, 2003). This analysis followed the methodology that was used for the proposed Yucca Mountain, Nevada nuclear waste repository (Stepp et al. 2001; Youngs et al., 2003).
Staff Position:
- Dam sites should be evaluated for the potential for surface fault displacement to cause damage to the dam.
- The potential for primary and secondary surface faulting should be considered.
- It is acceptable to utilize existing analyses that demonstrate that a dam is not susceptible to fault displacement.
5.3.3 Liquefaction During an earthquake, soils may undergo either transient or permanent reduction in undrained shear resistance because of excess pore water pressures or disruption of the soil structure accompanying cyclic loading. Such strength degradation may range from slight diminution of shear resistance to the catastrophic and extreme case of seismically induced liquefaction, which is a transient phenomenon. In this guide, the term seismically induced liquefaction includes any drastic loss of undrained shear resistance (stiffness and/or strength) resulting from repeated rapid straining, regardless of the state of stress prior to loading. The term is interchangeably applied to the development of either excessive cyclic 5-8
strains or complete loss of effective stress within an undrained laboratory specimen under cyclic loading (sometimes referred to as initial liquefaction).
An initial assessment of the potential for earthquake-induced ground failure typically includes:
- 1. Geomorphology of the site.
- 2. A soil profile, including classification of soil properties and the origin of soils at the site
- 3. Water level records, representative of both current and historical fluctuations.
- 4. Evidence obtained from historical records, aerial photographs, or previous investigations of past ground failure at the site or at similar (geologically and seismologically) nearby areas (including historical records of liquefaction, topographical evidence of landslides, sand boils, effects of ground instability on trees and other vegetation, subsidence, and sand intrusions in the subsurface).
- 5. Seismic history of the site.
Detailed investigations would include surveys, in situ field testing, and laboratory testing, as appropriate, to (a) refine the preliminary interpretation of the stratigraphy and the extent of potentially liquefiable soils, (b) measure in situ densities and dynamic properties for input to dynamic response analyses, and (c) recover undisturbed samples for laboratory testing when site soils are not adequately represented in the available database.
Regulatory Guide 1.198, Procedures and Criteria for Assessing Seismic Soil Liquefaction at Nuclear Power Plants (USNRC, 2003) provides guidance on acceptable methods for evaluating the potential for earthquake-induced instability of soils resulting from liquefaction and strength degradation. It provides descriptions of screening techniques as well as procedures for detailed analysis.
Staff positions:
- The dam site should be evaluated for liquefaction potential.
- Regulatory Guide 1.198 provides guidance on acceptable methods for evaluating the potential for earthquake-induced instability of soils resulting from liquefaction and strength degradation.
5.4 Assessment of Seismic Performance of Dams Using Existing Studies In lieu of performing a new seismic hazard evaluation of dam performance, it is acceptable to utilize existing studies or design documentation to demonstrate the seismic capability of a dam.
Staff Positions Existing studies will be accepted on a case-by-case basis. However, studies utilized should ideally consider seismic capacity for both the maximum normal operating pool level (i.e. top of active storage) and average pool level (i.e. 50% exceedance duration pool level calculated using average daily water levels for the period of record). The average non-flood tailwater level should be used with both headwater conditions above.
5.4.1 Ground Shaking In order to utilize existing studies to demonstrate the seismic capability of a dam, the seismic capacity of the dam (e.g., based on seismic design or post-construction seismic 5-9
capacity studies) must be known for spectral frequencies of importance to the dam (e.g.,
using design response spectrum).
Staff positions:
- The seismic demands on the structure should be defined using the site-specific hazard spectrum (based on the UHS and accounting for site amplification) as described in Section 5.3.1. The design spectrum (or spectrum determined by other seismic analyses) is compared against the site-specific hazard spectrum to assess the failure potential of the dam. If the capacity of the structure exceeds the site-specific seismic demands at the spectral frequencies of relevance to the dam, with appropriate margin to account for uncertainties in the analysis, the dam can be assumed not to fail due to seismic ground shaking. Appropriate margin is usually expressed as a factor of safety, which will depend upon the type of dam and failure mode under consideration. FEMA guidelines on earthquake analysis of dams (FEMA 2005) provide additional detail. Note that the factor of safety should be applied relative to the ground motion criteria defined in this ISG. If the relevant Federal agency guidance proposes a factor of safety of 1.4 under the ground motions they consider, then the licensee would show that this factor of safety (1.4) is maintained when the dam is subjected to the 1e-4 ground motion.
- In cases where information does not exist to characterize the capacity of the dam by response spectrum or define capacities at the frequencies of relevance to the dam (e.g., in the case when the dam design was based on pseudo-static analysis using a single demand such as peak ground acceleration and the dam has not been reevaluated to define capacity in terms of other intensity measures), the licensee may leverage such analysis with appropriate justification. Examples of appropriate justification include demonstration of the conservatism and applicability of the analysis, in light of the UHS developed in Section 5.3.1 including effects of site amplification of a range of spectral frequencies.
- Dams that cannot be shown to have sufficient capacity should be assumed to fail and breach parameters computed as described in Section 7 Moreover, dams that are susceptible to seismic failure should be evaluated for the potential for multiple dams to fail during a single seismic event as described in Section 5.5. Alternatively, it is acceptable to perform a more detailed assessment of the performance of the dam (i.e., performing new assessments) as described in Section 5.8.
5.4.2 Fault Displacement Staff position:
- Existing studies or data on dam or foundation materials can be used to assess performance of the dam with respect to surface displacement, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties.
5.4.3 Liquefaction Staff position:
- Existing studies or data on dam or foundation soils can be used to assess performance of the dam with respect to liquefaction or loss of strength, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties.
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5.5 Multiple Dam Failure Due to a Single Seismic Event Comparison of the seismic capacity of a dam to the dam-specific seismic hazard, as described above, may produce a set of dams that are vulnerable to failure at or below the ground motion level associated with a 1E-4 annual frequency of exceedance. For these dams, it is necessary to consider the potential for a single seismic event to cause multiple dam failures. In general, the potential for multiple dam failures can be addressed through consideration of the distance between the dams, as described below.
In some cases, using knowledge about the attenuation of ground motion with distance relative to the distance between dams may provide useful information to assess the potential for common failure. For example, by considering one or more ground motion prediction equations (GMPEs) and amplification relations applicable for the location and characteristics of the site, it is possible to evaluate how, for a large magnitude event (e.g.,
M=6.5), the ground motion attenuates with distance for relevant ground motion measures (e.g., spectral accelerations at predominant frequencies of the dam). By considering a conservative estimate of ground motion attenuation (e.g., use of 84th percentile versus median values), it may be shown that two dams are sufficiently far apart that an earthquake affecting one dam will be unlikely to affect the other dam because the ground motion would likely attenuate to negligible level. Figure 19 provides a graphical illustration of the above concept. A GMPE is used to conservatively select a distance beyond which the ground motion (conservatively) attenuates to a negligible level (i.e., relative to the design capacity of the dam). This distance is used to define a ring around the dam with radius defined according to the selected distance. If the circles do not overlap for two dams, then failure during the same event could be considered unlikely.
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Figure 19. Using Knowledge about the Attenuation of Ground Motion with Distance If the attenuation distance approach outlined above does not rule out combined failure, the potential for multiple failures may be further refined through deaggregation of the seismic hazard at the location of the dams. Deaggregation provides information and insight into the seismic sources that impact the hazard at a particular site. As a result, insights into scenarios leading to multiple dam failures may be gained through deaggregation of seismic hazard for relevant ground motion measures and at ground motion levels corresponding to the multiple annual frequencies (e.g., 10-2, 10-3, 10-4). When considering relevant ground motion measures, multiple annual frequencies, and site characteristics, if the deaggregation of the hazard indicates that a large portion of the hazard (e.g., greater than 85-90%) comes from scenarios associated with earthquakes within a specified distance, this distance can be used, in combination with similar information for other dams, to justify that dams are sufficiently far apart such that it is unlikely they will fail during a single seismic event.
Graphically, this corresponds to modifying the radius of the ring around a dam site in accordance with the results of the hazard deaggregation (see Figure 20).
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Figure 20. Refinement of Seismic Influence Using Deaggregation Staff positions:
- The approaches outlined above can be used to identify the collection of dams that should be considered in a multiple-failure scenario, but not the precise sequence of the failures.
- Once the critical dams have been identified, various failure sequences should be considered to arrive at a suitably conservative estimate of the multiple-failure consequences.
5.6 Modeling Consequences of Seismic Dam Failure Once a dam has been assumed to fail under seismic load, the consequences of dam failure will be developed through breach modeling and flood wave routing as discussed in Sections 7 and 9, respectively. However, assumptions regarding headwater and tailwater levels, as well as coincident flood flows are discussed here 5-13
Staff Positions:
- If the dam failed under the 10-4 annual exceedance probability seismic hazard (ground motion), assume that failure coincides with the peak water level from a 25-year flood in the watershed above the dam.
- If the dam failed at half of the 10-4 annual exceedance probability seismic hazard (ground motion), assume that failure coincides with the peak water level from a 500-year flood (or 1/2 PMF, whichever is less) in the watershed above the dam.
- Water level estimates at the site should include effects of a 2-year wind speed from the critical direction.
- In view of the uncertainties involved in estimating reservoir levels that might reasonably be expected to prevail at the time of failure, the default starting water surface elevation used in flood routings for evaluation of seismic failure consequences should be the maximum normal pool elevation (i.e. top of active storage pool). Other starting water surface elevations may be used, with appropriate justification. Justification should be based on operating rules and operating history of the reservoir. The operating history used should be of sufficient length to support any conclusions drawn. However, consideration should be given to possible instances where the operating history and/or rules have been influenced by anomalous conditions such as drought.
- Reservoir and downstream tributary inflows should be consistent with the selected reservoir level. Hydrologically consistent headwater or tailwater relationships should be used in routing the flood wave.
5.7 Detailed Site Specific Seismic Hazard Analysis When the analysis using readily available information (as described above) indicates that a dam cannot be screened out based upon its design-basis information, a more detailed, site-specific seismic hazard evaluation is required if failure is not assumed. Approaches for such a detailed analysis are discussed below.
Staff Position:
Because each dam and its immediate environment form a unique system, it is not feasible to provide detailed guidance that will be applicable in all cases. Therefore, detailed, site-specific seismic hazard analyses will be reviewed on a case-by-case basis. The following discussion is meant to provide a general overview of the pieces that would normally be part of a detailed seismic hazard evaluation.
5.7.1 Ground Shaking Detailed estimates for ground shaking at the dam site can be developed using the same PSHA procedures discussed earlier, only using more detailed, site-specific information. This may take the form of more detailed seismic source characterization, attenuation relations or site response functions.
5.7.1.1 Seismic Source Characterization Seismic source characterization is concerned with the identification of all the relevant potential earthquake sources. Earthquake sources typically consist of: 1) faults, and 2) areal or background seismic source zones. Fault sources are usually modeled as planar surfaces, where the parameters such as activity (expressed by either slip rate or recurrence interval), geometry (location, length, dip, and down-dip extent), sense of slip, segmentation 5-14
(some segments may be more active than others), maximum magnitude (Mmax), and recurrence model (characteristic earthquake or maximum magnitude) are specified. For areal source zones, the parameters of interest are maximum magnitude, associated rate of activity, and recurrence model (e.g., exponentially truncated Gutenberg-Richter relationship, or just truncated exponential). The fault sources are primarily characterized using geological data, while the areal or background source zones are characterized using historical seismicity occurrence and magnitude data.
Many definitions exist for what constitutes an active or potentially active fault and hence what faults should be considered potential seismic sources. Simply put, most faults considered in a PSHA to be potential seismic sources are faults with evidence of Quaternary activity; i.e., faults that have documented or suspected evidence of displacement during roughly the last 1.6 million years. For most studies, all faults within approximately 50 km of the site are characterized, but for some sites, faults as far as 1000 km or more distant are included if they can have a significant impact on the hazard (such as large earthquakes originating on the Cascadia Subduction zone of western Washington, Oregon, and northern California, or the San Andreas fault in California). The U.S.
Geological Survey (USGS), many state geological surveys, and various consulting companies have conducted studies of potentially active faults in the United States. The USGS has produced catalogues describing the faults and other seismic sources used to develop the National Seismic Hazard Maps (e.g., the Quaternary Fault and Fold Database of the United States).
Seismic Sources for the CEUS In addition to the USGS seismic source catalogues (USGS, 2008), for the region designated as the central and eastern U.S. (CEUS), a regional study was jointly conducted by the U.S. Nuclear Regulatory Commission (NRC), EPRI, and DOE during the period 2009-2011, to develop a comprehensive representation of seismic sources for nuclear plant seismic evaluation purposes. The results were published by the NRC in 2012, and provide an acceptable source characterization model for use in seismic hazard studies.
Seismic Sources for the WUS In the Western United States (WUS), numerous studies have been conducted over the last 30 years or so that identify and characterize in one form or another most of the known or suspected Quaternary faults. For example, the U.S.
Bureau of Reclamation has a large inventory of studies in the Western United States that include detailed seismic source characterizations. Much of this data has been compiled by the USGS and can be accessed at http://earthquake.usgs.gov/hazards/qfaults/imsintro.php.
It should be emphasized that the USGS Quaternary fault compilation was done primarily to facilitate the development of the national seismic hazard maps, and many faults are not in the database, such as those that have low slip rates, were characterized recently, or were characterized for private companies. In addition, the quality of the data and level of study varies greatly for many faults. Finally, it also must be emphasized that for a critical facility, such as a dam, even faults with fairly low slip rates (~0.01 mm/year) can be important if the fault is located close to the site and if the downstream consequences are significant.
Therefore, in the WUS, development of seismic sources and the adequacy of the USGS National Seismic Hazard Map (USGS, 2008) should be determined on a site-specific basis.
5.7.1.2 Ground Motion Attenuation Ground-motion prediction equations (GMPEs) or attenuation relations relate the source characteristics of the earthquake and propagation path of the seismic waves to the ground motion at a site. The predicted ground motion is typically quantified in terms of a median value (a function of magnitude, distance, style of faulting, and other factors) and a 5-15
probability density function of peak horizontal ground acceleration or spectral accelerations.
GMPEs are statistical models developed by combining geophysical attenuation models with regression analysis of recorded strong motion databases. Ground motion parameters are typically expressed as functions of earthquake magnitude, distance from the rupture zone, the type of faulting, and site conditions. Because they are based on recorded strong motion data, GMPEs change with time as more strong motion data becomes available.
The state of the art in GMPEs for shallow crustal earthquakes is well represented by relationships developed as part of the Next Generation Attenuation (NGA) Project sponsored by the Pacific Earthquake Engineering Research (PEER) Center Lifelines Program. The NGA models are based on an extensive database of strong ground motion recordings and were developed through the efforts of five selected attenuation relationship developer teams working in a highly interactive process with other researchers. These relationships have a substantially better scientific basis than earlier ground motion attenuation relationships. In order to model site conditions, most of the NGA ground motion attenuation relationships incorporate the input parameter VS30, which is the average shear-wave velocity in the upper 30 m at the site. Development of the database of strong motion recordings is discussed in Chiou et al. (2008) and the attenuation relationships are available on the PEER website: http://peer.berkeley.edu/products/rep_nga_models.html.
In 2004, EPRI published a set of GMPEs for the CEUS, which was subsequently updated in 2006. A second update is scheduled to be released in late 2015.
Staff Position:
Ground motion prediction equations approved by the NRC for Recommendation 2.1 Seismic are acceptable for use in dam failure analysis for Recommendation 2.1 Flooding.
5.7.1.3 Site Response Local site conditions can profoundly influence important characteristics of strong ground motion (e.g., amplitude, frequency content, direction). The extent of the influence depends upon the geometry and the material properties of subsurface materials, site topography, and the characteristics of the input motion. In particular, the characteristics of local soil deposits can have a significant impact on the ground motions experienced at the surface. Critical parameters that determine which frequencies of ground motion might experience significant amplification (or de-amplification) are the layering of soil and/or soft rock, the thicknesses of these layers, the initial shear modulus and damping of these layers, their densities, and the degree to which the shear modulus and damping change with increasing ground motion.
The site response is typically addressed by developing amplification functions or amplification factors that relate the bedrock ground motions to motions at the ground surface. Methods to calculate possible site amplification are well established, but at some sites, the characterization of the subsurface may be limited.
It is well known that topographic irregularities and alluvial basin geometry can have significant effects on ground motions (Kramer, 1996). Due to where they are typically sited, this effect should be considered in developing the site response for dams. In fact, the best-known example of apparent topographic effects was observed at a dam site during the 1971 San Fernando earthquake (Trifunac and Hudson, 1971). Evaluation of these effects requires two- and in some cases three-dimensional analysis. At this time, only linear 2-D and 3D analyses are standard practice.
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5.7.1.4 Development of Uniform Hazard Spectra Uniform hazard spectra (UHS) are computed or developed from the seismic hazard curves.
This is done by developing hazard curves (i.e. spectral acceleration vs. exceedance probability) for several vibration periods to define the response spectra. Then, for a given exceedance probability or return period, the ordinates are taken from the hazard curves for each spectral acceleration, and an equal hazard response spectrum is generated. Thus, the response spectra curves are generated for specified annual exceedance frequencies of interest.
5.7.1.5 Development of Acceleration Time-histories For higher-level studies, acceleration time histories are developed for the sites that represent the seismic hazard at the return periods of interest. The selected ground motions are then used for dynamic analyses. A wavelet-based method is currently employed by Reclamation to produce acceleration time-histories through spectral matching to the 5%
damping mean UHS at the return period of interest. Because the UHS calculated from the PSHA curves is only available for the horizontal component of the ground motion, the vertical-component response spectra used for spectral matching is found by scaling the UHS using estimated V/H ratios.
In addition to the response spectra, additional characteristics of the time history, such as phase spectra and strong shaking duration, are also needed to produce a time history using spectral matching. A suitable recording from an historic earthquake is used as the initial time history to provide the required characteristics for the spectral matching. This earthquake and the recording station should be of similar magnitude and distance as the earthquake event dominating the UHS. In some cases, the records from several different events should be used because of the differences in the ground motions produced by earthquakes with even similar magnitudes and distances. If there are no near-field strong ground motion recordings from historical earthquakes, synthetic accelerations generated by stochastic methods are used.
5.8 Detailed Analysis of Seismic Capacity of the Dam Once the earthquake ground motions or displacements have been determined, the impact they have on the dam and its appurtenances must be determined. The extent and type of analysis required for the seismic evaluation of a dam depends on the hazard potential classification, level of seismic loading, the site conditions, type and height of dam, construction methods, as-built as well as current material properties, and engineering judgment. Consistency should be maintained between the level of analysis and level of effort given to the development of seismotectonic data, the ground motion parameters, and the site investigation. For example, a highly refined structural analysis based on an assumed earthquake loading is not reasonable in most cases. Likewise, a highly refined structural analysis should use site-specific ground motions, not assumed values.
In general, it is the most cost-effective for seismic analyses to begin with the simplest conservative method appropriate to the problem. If the structure is judged able to resist the earthquake loading within certain safety margins from the initial analysis, then further analysis should not be necessary. If further studies are needed, they would be progressively more detailed and the structure evaluated accordingly. Regardless of the method of analysis, the final evaluation of the seismic safety of the dam will include engineering judgment and experience, not just numerical results of the analyses.
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In some cases, the analyses may indicate that the dam is either clearly safe or clearly unsafe. Frequently, however, judgments concerning the safety of the dam must take into account not only the results of the analyses but also the level of confidence that can be put in those analyses and underlying assumptions and, to some extent, the consequences of misjudging the level of uncertainty.
Staff Position:
Seismic capacity studies should consider seismic capacity for both the maximum normal operating pool level (i.e. top of active storage) and average pool level (i.e. 50%
exceedance duration pool level calculated using average daily water levels for the period of record). The average non-flood tailwater level should be used with both headwater conditions above.
5.8.1 Concrete Dams Concrete dams should be analyzed for the effects of ground shaking and surface displacements. Cracking in the dam as well as displacement of foundation materials, leading to sliding or overturning are common concerns.
5.8.1.1 Sliding and Overturning Stability Concrete dams (gravity, arch, or buttress) and sites are very unique and should be evaluated for stability under earthquake loading.
Excessive cracking is a safety concern for concrete gravity dams subjected to earthquakes, which can lead to potential instability of the dam from sliding or overturning. Sliding can be on an existing plane of weakness in the dam or foundation or along planes of weakness formed by excessive cracking of the concrete above or at the foundation-dam interface. For concrete dams, sliding instability is possible due to an earthquake-induced vibratory motion on a plane of weakness at, above, or below the foundation-dam interface.
For an arch dam, sliding instability is more likely to occur by failure of the abutment support because the arching effect provides additional resistance to sliding within the dam. In general, instability of gravity and arch dams caused by excessive cracking of the concrete is most likely to occur in the upper half of the dam.
Buttress dams are also particularly vulnerable to cross-valley shear motions that can result in tipping of the buttresses and loss of support for the reinforced concrete slab.
Of the two possible types of instability discussed above, historical experience shows that foundation (abutment) induced failure is often the chief source of concern for concrete dams. In contrast to the dam itself, the supporting medium consists of natural materials of varying composition, irregular joints, and planes of weakness. The strength of this medium is generally estimated from exploratory borings and tests on only a small fraction of the material present. Key zones of weakness are critical and often difficult to detect.
In the past, pseudostatic methods were commonly used to analyze dam stability. However, given the widespread availability of structural dynamics software, pseudostatic methods are generally discouraged today and should only be used for screening from further consideration those dams where a seismic stability failure is highly improbable. An example of such a screening analysis is given in FEMA (2005):
Structures that fail to meet the prescribed pseudostatic stability requirements (i.e., sliding safety factors and resultant location) should be subjected to in-depth study using dynamic analyses to assess the demands placed on the dam and foundation during an earthquake.
Dynamic time-history analyses are used to determine the displacements and stresses 5-18
experienced by the dam and foundation. Evaluation of the results is used to determine if there is a risk of a stability failure.
Staff Positions:
- Pseudostatic methods are generally discouraged from use in stability analysis of structures.
- Structures that fail to meet prescribed pseudostatic stability requirements (i.e.,
sliding safety factors and resultant location) should be subjected to in-depth study using dynamic analyses to assess the performance of the dam and foundation during an earthquake.
- Detailed evaluation of the seismic performance of a concrete dam should be performed using (as appropriate) linear-elastic response spectrum analysis, linear-elastic time-history analysis, or nonlinear time-history methods. Guidance provided on these methods in FEMA Dam Safety Guidelines (FEMA, 2005) should be used to perform the evaluation.
5.8.1.2 Dynamic Analysis Results of dynamic analyses are generally evaluated in terms of compressive and tensile strengths of the concrete. The compressive stresses resulting from the combination of static and earthquake loads usually remain below the dynamic strength of the concrete.
However, since the mere occurrence of tensile stresses does not necessarily lead to failure, the significance of predicted tensile stresses is not evaluated as easily. The number and amplitudes of stress cycles that exceed the dynamic tensile strength are taken into account for this purpose in linear analysis.
To evaluate the effects of stresses that exceed the tensile strength, sound engineering judgment is required and should be based on the expected effects of nonlinear behavior and the past performance of dams under similar earthquake loadings. To estimate the extent of cracking, one must consider nonlinear behavior leading to stiffness degradation and energy absorption. Nonlinear behavior from cracking reduces the stiffness of the dam and shifts the dams response into other frequency ranges of the ground motion. The energy level of the earthquake corresponding to the frequency of the cracked structure may or may not be more severe than when uncracked. As a result, the peak values of tensile stress and the extent of tensile zones may increase or decrease, and large tensile stresses given by linear elastic analysis may or may not necessarily indicate an unsafe condition. They may, in fact, be artifacts of the analysis rather than real behavior.
A nonlinear analysis should be performed if the response of the dam would be influenced significantly by nonlinearity from material behavior or changes in geometry. For an arch dam, this might include 1) cantilever tensile stresses larger than the tensile strength of the concrete over significant areas of the dam; 2) a long duration earthquake; 3) opening and closing of contraction joints indicated by simultaneous arch tensions on the upstream and downstream faces; and 4) large displacements or distortions of the arch.
Staff Positions:
- A nonlinear analysis should be performed if the response of the dam would be influenced significantly by nonlinearity from material behavior or changes in geometry.
- Detailed evaluation of the seismic performance of a concrete dam should be performed using (as appropriate) linear-elastic response spectrum analysis, linear-elastic time-history analysis, or non-linear time-history methods. Guidance provided 5-19
on these methods in FEMA Dam Safety Guidelines (FEMA, 2005) should be used to perform the evaluation.
5.8.2 Embankment Dams The most common concern for embankment dams is deformation and/or liquefaction of the embankment or foundation. These may fail the dam directly due to overtopping or indirectly due to cracking and subsequent internal erosion. The deformation may be due to surface displacement, but ground shaking is a more common concern.
If a dam is deformed by earthquake excitation or fault displacement, the deformations can cause cracks in the dam and/or disrupt internal filters, either of which could lead to failure of the dam by erosion. Cracks are most likely to occur at interfaces with concrete structures (e.g., spillway walls) or at abrupt changes in the embankments cross section. There is also evidence that shaking could precipitate piping even without formation of a crack if the dam is already on the verge of piping. The amount of deformation a dam can withstand without risk of failure by erosion through cracks depends on the materials in the dam and foundation, the details of internal zoning (filters, drains, and cutoff), the reservoir elevation at the time of the earthquake, and the nature and location of appurtenant structures. Should there be conduits through the embankment, deformation of the dam can rupture them or cause joints to separate, leading to erosive failure by either creating an unfiltered exit for seepage or exposing the embankment or foundation to full reservoir head where not intended. Erosion along intact conduits has also caused dam failures.
5.8.2.1 Deformation Analyses Direct methods of assessing deformation model the design earthquake, the dam, and foundation to calculate the expected deformation. There are also indirect methods to predict the response of the embankment and foundation based on empirical observations.
Post-earthquake stability analysis can, in a sense, also be considered an indirect prediction of deformation - if the post-earthquake factor of safety is high, the deformations should be limited to a few feet except under very severe loading. The magnitude of deformations is very dependent on the strengths of the materials involved. During strong shaking, permanent deformations (usually small) may occur simply because the dynamic stresses temporarily exceed the available strength. In saturated soils, there is frequently some loss of shearing resistance due to an increase in pore water pressure when shaken. This increases the dynamic deformations over what they would be with no strength loss. In very loose, contractive soils, the strength may become a small fraction of its static, drained value due to excess pore-water pressure (liquefaction). Very large deformations can result, driven by gravity even after the shaking ends. There is an intermediate condition known as "cyclic mobility," in which the shearing resistance is initially very low due to excess pore pressure, but increases with larger shear strains, helping to prevent gross instability but still permitting significant deformation.
Deformation analyses can be made for three conditions: 1) liquefaction would not occur; 2) liquefaction may occur but instability would not; and 3) liquefaction may occur, resulting in instability. In the first two conditions, judgment is required to determine whether the predicted deformations along the critical failure surfaces are small enough that cracking of the embankment/foundation materials that could eventually cause a piping failure of the dam does not occur. A determination also must be made whether the post-earthquake sliding factors of safety and available freeboard are adequate to ensure the dam would not be overtopped and would be able to safely retain the reservoir. If there are no potentially liquefiable materials present, this can usually be done by the simple Newmark sliding-block 5-20
approach. In situations where excess pore pressure could develop, it may be necessary to conduct more rigorous finite-element or finite-difference analyses.
If the results of post-earthquake sliding stability analyses for critical failure surfaces indicate a safety factor well above 1.0 (e.g., 1.25 or greater) using the strengths expected after the earthquake, experience from past earthquakes suggests that deformations will be small and the dam will perform satisfactorily (FEMA, 2005). Confidence in the safety of the dam decreases when the factor of safety against triggering of liquefaction is 1.0 or less and a post-earthquake sliding factor of safety less than or approaching 1.0 is calculated using residual shear strengths for materials assumed to be liquefied. In general, many analyses have shown that when a wedge or circular sliding surface has a low post-earthquake sliding factor of safety, the deformations on these sliding planes will be excessive. If these failure planes are critical to the overall integrity of the dam, deformations may lead to failure of the dam by overtopping or internal erosion.
FEMA (2005) provides a screening-level analysis for embankment dams similar to that discussed for concrete dams above. FEMA (2005) suggests that for a dam and foundation not subject to liquefaction, minor deformation may take place but should not lead to failure if all of the following conditions are satisfied:
- Dam and foundation materials are not subject to liquefaction and do not include loose soils or sensitive clays.
- The dam is well built and compacted to at least 95% of the laboratory maximum dry density (modified Proctor test), or to a relative density greater than 80%.
- The slopes of the dam are 3:1 (H:V) or flatter, and/or the phreatic line is well below the downstream slope of the embankment.
- The peak horizontal acceleration at the base of the embankment is no more than 0.2 g.
- The static factors of safety for all potential failure surfaces (other than shallow surficial slides) are greater than 1.5 under loading and pore-pressure conditions expected immediately prior to the earthquake.
- The freeboard at the time of the earthquake is at least 3% to 5% of the embankment height and not less than 3 feet (0.9 m). (Freeboard requirements to accommodate reservoir seiche waves or coseismic movement of faults at the dam site or in the reservoir must be considered as a separate issue.)
- There are no critical appurtenant features that would be harmed by small movements of the embankment, or that have the potential to cause cracks that would allow internal erosion.
If these conditions are not satisfied, more detailed study is required. This may include assessment of liquefaction potential, post-earthquake stability analysis, and/or deformation analysis.
A comprehensive review of the factors to be considered in the earthquake resistance design of dams, as well as a review and commentary on the field performance of dams during earthquakes, can be found in Seed (1979). Regulatory Guide 1.198, Procedures and Criteria for Assessing Seismic Soil Liquefaction at Nuclear Power Plants (USNRC, 2003) provides guidance on acceptable methods for evaluating the potential for earthquake-induced instability of soils resulting from liquefaction and strength degradation.
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Staff Positions:
- Detailed seismic evaluation of embankment dams should include the following (as appropriate): post-earthquake stability analysis, deformation analysis, and assessment of liquefaction potential.
- If there are no potentially liquefiable materials present, evaluation can usually be done using the Newmark sliding-block approach. In situations where excess pore pressure could develop, more rigorous finite-element or finite-difference analyses should be conducted.
- Embankment dams should be evaluated to ensure sufficient factors of safety against sliding of critical failure surfaces.
- Embankment dams should be evaluated to ensure sufficient factors of safety against triggering of liquefaction.
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- 6. OTHER (SUNNY DAY) FAILURES Dam failures not associated with a concurrent extreme flood or seismic event may arise from a variety of causes. These failures are often referred to as sunny day or fair weather failures. These dam failures may occur because of failures of embankment material, foundations, or appurtenances such as floodgates, valves, spillways, conduits, and other components. The potential for these failures to occur should be carefully evaluated.
American National Standards Institute/American Nuclear Society Standard 2.8-1992, Determining Design Basis Flooding at Nuclear Power Plant Sites (ANSI/ANS, 1992), lists the potential nonhydrologic and nonseismic causes for partial or complete dam failure. That list, with minor modifications, is:
- Deterioration of concrete (e.g., weathering, cracking, chemical growth)
- Deterioration of embankment protection (e.g., grass cover, riprap, or soil cement)
- Excessive saturation of downstream face or toe of embankment.
- Excessive embankment settlement.
- Cracking of embankment due to uneven settlement.
- Excessive pore pressure in structure, foundation, or abutment.
- Excessive loading due to buildup of silt load against dam.
- Excessive leakage through foundation.
- Embankment slope failure.
- Leakage along conduit in embankment.
- Channels from tree roots or burrowing.
- Landslide in reservoir.
More detailed discussion of failure modes by dam type is provided below.
Staff Positions:
- Sunny-day failures cannot be screened-out. If no other failure mechanisms exist, sunny-day failures should be the default failure scenario for the purposes of this ISG (see discussion in Section 1.4).
- An exception to the preceding staff position is that dams failed due to hydrologic and seismic events shown to have negligible impacts at the site do not require evaluation for the sunny-day scenario since the sunny-day scenario is bounded by the other two events. The level of effort required for evaluating sunny-day failure is typically lower since it only involves identifying the worst-case individual or cascading failure scenario.
- Sunny-day failures such as those listed above should be carefully evaluated to ensure that all plausible mechanisms for flooding from dam breaches and failures at and near a site are considered.
- A sunny-day breach can be used to model piping failures for hydrologic, geologic, structural, seismic, and human-influenced failure modes.
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6.1 Overview of Sunny Day Failures by Structure Type 6.1.1 Concrete Dams Several potential failure initiators are common to all types of concrete dams. These include plugging of drains (leading to increased uplift pressures), gradual creep that reduces the shear strength on potential sliding surfaces, and degradation of the concrete from alkali-aggregate reaction, freeze-thaw, or sulfate attack.
For concrete gravity dams founded on bedrock, the leading cause of dam failures has been related to sliding on planes of weakness within the foundation, most typically weak clay or shale layers within sedimentary rock formations. For concrete gravity dams founded on alluvial soils, the leading cause of failure is piping or blowout of the soil material from beneath the dam. Failures have also occurred along weak lift joints within dams.
Historically, arch dam failures have resulted primarily from foundation deficiencies. The predominant mode of failure is sliding of large blocks (bounded by geologic discontinuities) within the foundation or abutments. Typically, these failures have been sudden, brittle, and have occurred on first filling of the reservoir.
Plugging of the drains may lead to an increase in the foundation uplift pressure. This may lead to creep along sliding surfaces. Additionally, degradation of the concrete may occur from alkali-aggregate reactions, freeze-thaw cycles, or sulfate attack. All of these may lead to dam failure. Some of these mechanisms may be difficult to detect. A review of instrumentation results can be helpful. For example, if piezometers or uplift pressure gauges indicate a rise in pressures, and weirs indicate a reduction in drain flows, the drains may be plugging leading to higher uplift and potentially unstable conditions. If conditions appear to be changing, risk estimates are typically made for projected conditions as well as current conditions.
Because of high unit loads underneath the buttresses, concrete buttress dams are subject to failures initiated by weakness within the foundation. Such weakness may cause the foundation to undergo unacceptable settlement or shearing. Sliding on planes of weakness within the foundation may also occur. Concrete buttress dams founded on alluvial soils are subject to failure initiated by piping or blowout of the soil material from beneath the dam.
Deformation of the abutment can also lead to failure since unforeseen movement in the abutment will induce stresses in a buttress that may not have been considered in its design.
Particular attention must be paid to the quality and performance of the concrete in the face slab. Because of its relative thinness it cannot withstand excessive deterioration, pitting, or spalling that will decrease the strength of the slab. Exposure and corrosion of reinforcing steel can reduce the capacity of reinforced concrete elements.
6.1.2 Embankment Dams Historically, the most common failure modes for embankment dams are initiated by or heavily influenced by various seepage-related internal erosion phenomena. Internal erosion phenomena are the predominant mechanism for sunny-day (nonhydrologic, nonseismic) failures of embankment dams. The term internal erosion is used here as a generic term to describe erosion of particles by water passing through a body of soil. Piping is often used generically in the literature, but actually refers to a specific internal erosion mechanism.
Several types of internal erosion have been observed in embankments.
Classical piping occurs when soil erosion begins at a seepage exit point, and erodes backwards, supporting a pipe or roof along the way. Progressive erosion can occur when the soil is not capable of sustaining a roof or a pipe. Soil particles are eroded and a 6-2
temporary void grows until a roof can no longer be supported, at which time the void collapses. This mechanism is repeated progressively until the core is breached or the downstream slope is over-steepened to the point of instability. Suffosion or internal instability occurs when the finer particles of a soil are eroded through the coarser fraction of that soil, leaving behind a coarsened and more permeable soil skeleton. The loss of material can lead to voids and sinkholes.
Scour occurs when tractive seepage forces along a surface (i.e. a crack within the soil, adjacent to a wall or conduit, or along the dam foundation contact) are sufficient to move soil particles into an unprotected area. Once this begins, a process similar to piping or seepage erosion could result.
Heave can occur where an impervious layer overlies more pervious material near the downstream toe of a dam. A buildup of hydrostatic pressure beneath the impervious layer can lead to high uplift forces capable of moving material from and breaching of the impervious layer. This in turn can lead to rapid development of piping or seepage erosion (unless the pressure is relieved to the point where the seepage velocities are insufficient to move soil particles). This is sometimes referred to as blowout, especially if it occurs in a local area.
The various internal erosion phenomena discussed above may affect the embankment (including spillway walls), the foundation or both. The zone of contact between earth materials and conduits through the embankment or its foundations and around drains is an area prone to internal erosion phenomena. More detailed discussion of internal erosion and piping for earthen dams is provided in USBR (2011).
6.2 Analysis of Sunny Day Failures Analysis of sunny day failure can be organized into three basic steps:
Step 1. Assessment of potential failure modes Step 2. Breach modeling Step 3. Flood wave routing Failure modes and assumptions regarding initial water surface elevation (Step 1) used in breach modeling (Step 2) and flood routing (Step 3) are discussed below. The details of breach modeling are discussed in Section 7 and details of flood routing are discussed in Section 9.
Base flow conditions for a sunny day failure are typically ignored because of the small discharge and volume compared to that of a dam breach. As a general guidance, base flow can be ignored if the dam breach flow is two times greater than the base flow. Where base flow is considered, the discharge is typically estimated based on reported base flows through the dams outlet works or from stream gage records. Additional inflow (e.g. from a storm event) is not required when analyzing a sunny-day breach.
6.2.1 Sunny- Day Failure Modes An essential element in evaluating the potential for sunny-day failure is assessment of credible failure modes. Common sunny-day failure modes for various dam types are discussed in Section 6.1. That discussion is not meant to be exhaustive. The purpose of the discussion is to inform the process of identifying potential failure modes. In general, identifying potential failure modes will require a thorough review of all relevant background information on a dam, including geology, design, analysis, construction, operations, dam safety evaluations, and performance monitoring documentation.
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6.2.2 Initial Water Surface Elevation Breaching should be assumed and breaching scenario(s) should be assessed, if sunny day failure modes cannot be ruled out. Section 7 discusses breach modeling in detail, but does not discuss assumptions regarding initial water surface elevations used in the breach modeling.
Staff Position:
In view of the uncertainties involved in estimating reservoir levels that might reasonably be expected to prevail at the time of failure, the default starting water surface elevation used in flood routings for evaluation of overtopping should be the maximum normal pool elevation (i.e. top of active storage pool). Other starting water surface elevations may be used, with appropriate justification. Justification should be based on operating rules and operating history of the reservoir. The operating history used should be of sufficient length to support any conclusions drawn (e.g., 20 years or more). However, consideration should be given to possible instances where the operating history and/or rules have been influenced by anomalous conditions such as drought.
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- 7. DAM BREACH MODELING The breach is the opening formed in a dam when it fails, the aim of a breach analysis is to estimate the resulting reservoir outflow hydrograph. Modeling of the breach formation (or development) process has typically been one of the greatest sources of uncertainty in dam failure analysis, and is especially important when the dams distance from the location(s) or population(s) of interest is small, and routing effects are minimized (Gee, 2008; Wahl 2004, 2010).
The simplest approach to breach modeling is to assume that the dam fails completely and instantaneously. While this assumption is convenient when applying simplified analytical techniques for analyzing dam-break flood waves, and is somewhat appropriate for concrete arch dams, it is not considered realistic for either earthen or concrete gravity dams, which tend to fail partially and/or progressively. Concrete gravity dams tend to have a partial breach (as one or more monolith concrete sections are forced apart by the escaping water),
although the time for breach formation is in the range of a few minutes. Earthen dams do not tend to fail completely, nor do they tend to fail instantaneously. Dam breach analysis of composite dams (dams that include both concrete and earthen sections) should consider the failure of the portion or portions of the dam that would produce the largest peak outflow.
7.1 Breach Modeling for Concrete Dams In most cases where breach of a concrete dam is considered, one or more sections of the dam deemed most susceptible to failure (based on engineering analysis) are assumed to fail instantaneously. The breach size and shape are determined by considering the size and shape of the failed section(s), and then a weir formula or hydraulic simulation software package is used to compute the outflow hydrograph and/or peak outflow.
Concrete gravity dams tend to have a partial breach as one or more monolith sections formed during construction of the dam are forced apart and overturned by the escaping water. The time for breach formation depends on the number of monoliths that fail in succession, but is typically on the order of minutes. The challenge of modeling breach of concrete dams is in predicting the number of monoliths that may be displaced or fail.
However, by using a dam-breach flood prediction model and running several trials of the model, wherein the breach width parameter representing the combined lengths of assumed failed monoliths is varied in each trial, the resulting reservoir water surface elevations can be used to indicate the extent of reduction in the loading pressures on the dam. Because the hydraulic loadings diminish as the breach width increases, a limiting-safe loading condition, which would not cause further failure, may be estimated (Fread, 2006).
Unlike concrete gravity dams, concrete arch dams tend to fail completely and are assumed to require only a few minutes for the breach formation. The geometry of the breach is usually approximated as a rectangle or a trapezoid. Buttress and multi-arch dams can be modeled in a similar fashion, where sections are assumed to fail completely.
7.2 Breach Modeling of Embankment Dams For earthen dams, the failure process often begins when appreciable amounts of water flow over or around the dam face and begin to scour the face of the dam. In general, the most erosive flow occurs on the downstream slope, where the velocity is highest and where the slope makes it easier to remove material.
On dams that have been overtopped by floods, severe erosion has often been observed to begin where sheet flow on the slope meets an obstacle, such as a structure, a large tree, or 7-1
groin, creating local turbulent flow. Erosion generally continues in the form of "headcutting,"
upstream progression of deep eroded channel(s) that can eventually reach the reservoir.
Pavement on the crest of the dam may be of some value in slowing head cutting once the gullies reach the crest, but should not be expected to affect initiation.
For cohesive soils, the failure mechanism is typically headcut initiation and advance. A small headcut is typically formed near the toe of the dam and then advances upstream until the crest of the dam is breached. For cohesionless soils, the failure process typically initiates because of tractive stresses from the flow removing material from the downstream face, but then progresses as headcut advance once a surface irregularity is formed.
Predicting whether breach initiation and formation will occur can be a complicated procedure. Several factors have been shown to be important, including:
- Depth and duration of overtopping
- Potential concentration of overtopping flows at dam crest due to camber or low spots
- Potential concentration of overtopping flows on the dam face, along the groins or at the toe of the dam
- Erosional resistance of materials on the downstream face and in the downstream zones of the embankment
- Whether the dam crest is paved
- Whether a parapet wall is provided and the potential for the wall to fail before or after the dam is overtopped For embankment dams, failure typically begins at a point on the top of the dam and expands in a generally trapezoidal shape. The water flow through the expanding breach acts as a weir; however, depending on conditions such as headwater and tailwater, various flow characteristics can be observed during a breach development including weir flow, converging flow, and channel flow.
Breach analysis for earthen and rockfill embankment dams is typically more complex than for other dam types. As mentioned above, failure of embankment dams is typically progressive. Failure progression differs for overtopping and piping failures (the two most common failure modes for earthen dams), as described below.
- In the case of overtopping, once a breach has been initiated, the discharging water will progressively erode the breach until either the reservoir water is depleted or the breach resists further erosion. Erosion processes typically result in the progressive widening and deepening of the breach. In some cases, the breach will deepen until bedrock or some other erosion resistant strata is encountered. At this point, the breach depth stays approximately constant while the breach continues to widen. The final breach shape is often modeled as trapezoidal (see Figure 21).
- Piping is a term used to describe an array of internal erosion failure modes that can be very different in their initial stages, but all cause the breach opening in the dam to initially form at some point below the top of the dam. As erosion proceeds, a pipe" through the dam enlarges until the top of the dam collapses, or the breach becomes large enough that open channel flow occurs. Beyond this point, breach enlargement is similar to the overtopping case.
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Figure 21. Generalized Trapezoidal Breach Progression (Gee, 2008)
Breach widths for earthen dams are usually much less than the total length of the dam. The breach also requires a finite interval of time for its formation through erosion of the dam materials by the escaping water. The total time of failure may range from a few minutes to a few hours, depending on the height of the dam, the type of materials used in construction, and the magnitude and duration of the flow of escaping water.
There are two widely used approaches to breach modeling of embankment dams, both based on regression analysis of data from dam failures. The first approach is direct estimation of the breach outflow hydrograph by simple equations that relate the peak outflow discharge and time for breach development to basic reservoir and embankment parameters. Once the peak outflow is estimated, it can be used to complete the analysis of flooding impacts. The second approach uses regression equations to predict parameters of the breach opening (e.g., size, shape, and rate of development) when given input data such as reservoir volume, initial water height, dam height, dam type, configuration, failure mode, and material erodibility. These breach parameters are then used in a computational model that determines the breach outflow through the parameterized opening using a weir or orifice flow equation (e.g., Gee and Brunner, 2005; Xiong, 2011).
7.2.1 Regression Equations for Peak Outflow from the Breach For screening-level analyses, direct estimation of the breach outflow hydrograph by simple equations that relate the peak outflow discharge and time for breach development to basic reservoir and embankment parameters is often adequate. The equations are developed by regression of case-study data. A number of such regression equations have appeared in the literature in the past 35 years. Some attempt to provide conservative estimates by developing equations that envelop the case study data, while others provide a best fit to the data. The number and types of dams included in the studies vary, but typically the studies have been small (~10-20 dams) and skewed towards small dams. The following list provides a description of some, but not all, existing equations for peak outflow:
- Kirkpatrick (1977) presented data from 13 embankment dam failures and six additional hypothetical failures and provided a best-fit relation for peak discharge as a function of depth of water behind the dam at failure. This study included data from the failure of one concrete gravity dam (St. Francis Dam, in California) because, at the time of the study, this dam was thought to have failed due to piping in the abutment. More recent studies have questioned that explanation.
- The Soil Conservation Service (SCS, 1981) used the 13 case studies cited by Kirkpatrick to develop a power law equation relating the peak dam failure outflow to 7-3
the depth of water behind the dam at time of failure. This was meant to be an enveloping relation although three data points are slightly above the curve.
- USBR (1982) extended this work and proposed a similar relation for peak outflow using case study data from 21 dams. This study also proposed a relation for attenuation of the peak flow with distance downstream. USBR (1983) later analyzed six case studies with large storage-to-height ratios and proposed a modification to the USBR (1982) peak discharge equation that included reservoir storage as a parameter.
- Singh and Snorrason (1982, 1984) used 10 real dam failures and eight simulated failures to develop peak dam failure outflow as functions of dam height and reservoir storage.
- MacDonald and Langridge-Monopolis (1984) developed best fit and envelope curves for peak outflow from studies of 42 breached earthfill dams.
- Costa (1985) developed envelope curves and best-fit regression equations for peak flow from 31 breached dams as functions of dam height, storage volume at time of failure and the product of these two parameters. Costas study included both constructed and natural dams, as well as the St. Francis Dam failure.
- Froehlich (1995a) developed a best-fit regression equation for peak discharge based on reservoir volume and head, based on 22 case studies. He also presented a procedure for determining confidence intervals for the estimates.
- Pierce et al. (2010) conducted a study that compares several of the regression equations described above to each other and to a database developed by the U.S.
Bureau of Reclamation (USBR, 1998). This study provides insights into the degree of conservatism in the equations studied.
Staff Position:
- For screening-level analysis, the use of simple equations that relate the peak outflow discharge to basic reservoir and embankment parameters is acceptable with adequate justification. The list above describes several available regression equations for peak outflow. Selection of methods should consider the assumptions inherent in the models and their applicability to the dam failure scenario being considered. Sensitivity studies should be performed on a reasonable variation of input parameters, when applicable. If there are multiple applicable models available for use, a study should be performed to evaluate the effect of model selection (and input parameter sensitivity, when applicable) on the results of the analysis.
Justification for the preferred model and input parameters should be documented, including results of sensitivity studies.
7.2.2 Regression Equations for Breach Parameters Regression equations have been developed to predict parameters of the breach opening (e.g., size, shape, and rate of development) when given certain input data are available such as reservoir volume, initial water height, dam height, dam type, configuration, failure mode, and material erodibility. These parameters are then used in a computational model that determines the breach outflow through the parameterized opening using a weir or orifice flow equation. Wahl (2010) suggests that one of the main advantages of using empirical parametric regression equations is that the analyst can exert some control over the breach parameters used in the model, and thus account for site-specific factors. A large number of relationships have been published in the last 35 years. The U.S. Bureau of 7-4
Reclamation, the U.S. Army Corps of Engineers, and others have compiled extensive reviews of the most widely used regression-based approaches for breach parameter estimation, including discussions of uncertainties in the methods [Gee, 2008; Wahl, 2004,2010; Washington State Department of Ecology (WSDE), 2007; Colorado Department of Natural Resources (CODNR), 2010].
The list below provides descriptions of some, but not all, of the available regression models for breach parameters:
- Johnson and Illes (1976) published a classification of failure configurations for earthfill, gravity, and arch dams. The breach shape for earthen dams was described as varying from triangular to trapezoidal as the breach progressed.
- Singh and Snorrason (1982) conducted a study of 20 dam failures and noted that breach width was generally between two and five times the dam height. They also found that the breach formation time was generally 15 minutes to 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and the maximum overtopping depth prior to failure (for overtopping failures) ranged from approximately 0.5 foot to 2 feet.
- Based on 42 dam failure case studies, MacDonald and Langridge-Monopolis (1984) proposed a breach formation factor, defined as the product of the volume of breach outflow and the depth of water above the dam. They related this factor to the volume of material eroded from the dams embankment. The amount of water that passes through the breach is not known before breach analysis occurs; however, the entire volume of the reservoir can be used as a starting estimate (Gee, 2008).
Based on their study, MacDonald and Langridge-Monopolis also concluded that the breach could be assumed to be trapezoidal with a side slope of 0.5H:1V. The study further presented an envelope equation for the breach formation time for earthfill dams.
- Based on a study of 52 earthen embankment dam breaches, Singh and Scarlatos (1988) determined that the ratio of top and bottom breach widths ranged from 1.06 to 1.74, with an average value of 1.29. They also noted that the majority of breach formation times were less than 1.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and most were less than 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />.
- In 1995, Froehlich developed equations for breach width and breach formation time based on 63 case studies (Froehlich, 1995a). Froehlich suggested using a breach side slope factors of 1.4 for overtopping failures and 0.9 for other failure modes.
Froehlich (2008) provided updated breach parameter equations based on data collected from 74 embankment dam failures. He also used the new equations and their uncertainties in a Monte Carlo simulation to estimate the degree of uncertainty in predictions (Froehlich, 2008).
- Xu and Zhang (2009) used a database of 182 earth and rockfill dam failure cases. A multiparameter nonlinear regression was used to develop empirical relationships between five breaching parameters (breach depth, breach top width, average breach width, peak outflow rate, and failure time) and five selected dam and reservoir control variables (dam height, reservoir shape coefficient, dam type, failure mode, and dam erodibility). A significant feature of this study is that nearly one-half of the 182 case studies focused on dams higher than 15 m (a commonly used metric for large dams). However, the majority of these case studies are of dam failures in China, which have not been subject to independent review by the U.S. dam safety community. Another feature is that Xu and Zhangs relations explicitly include erodibility of embankment soils. Their study showed that breach parameters are 7-5
very sensitive to the selected erodibility index (dam erodibility was found to be the most important factor, influencing all five breaching parameters).
- Review of the Xu and Zhang paper by NRC staff has revealed several concerns.
The Xu and Zhang paper does not provide clear criteria for selecting the erodibility index. Their paper also has an internally inconsistent treatment of breach formation time and time to failure, which leads to uncertainty in how these definitions were applied in the case studies supporting their regression analysis. In addition, discussions with staff from USBR and USACE indicate that the Xu and Zhang treatment of failure time is not consistent with how this parameter is applied in widely used hydraulic models (e.g., HEC-RAS). Use of the Xu and Zhang failure time could thus lead to invalid results. There is also concern that the Xu and Zhang relation for failure time may be biased in favor of longer times (Wahl, 2013; Brunner, 2013).
Staff Positions:
- The state of practice in dam breach modeling shows a clear preference for regression-based approaches. The preferred approach uses regression equations to predict final parameters of the breach opening (.e.g. size, shape, time to fully develop) when given input data such as reservoir volume, initial water height, dam height, dam type, failure mode, and material erodibility.
- Based on discussions with technical experts at USACE and USBR, as discussed above, and the lack of independent review of the case studies for Chinese dam failures, there is concern about the use of breach parameters computed using Xu and Zhang (2009). Given the time necessary to conduct a sufficient review of the model in conjunction with other Federal agencies, and the uncertainty regarding its ultimate acceptability, the NRC staff does not recommend use of the breach parameter model developed in this paper (Xu and Zhang 2009) for the purposes of conducting the Recommendation 2.1 hazard review. This position applies to Category 2 and 3 plants, while Category 1 sites that used the Xu and Zhang relationship (and submitted their Recommendation 2.1 flood hazard reevaluation report in March 2013) will be reviewed on a case-by-case basis (see Section 1.2).
7.2.2.1 Uncertainty in Predicted Breach Parameters and Hydrographs Predicting the reservoir outflow hydrograph remains a great source of uncertainty, especially for embankment dams in which dam failure is usually a complex progressive process that is difficult to model (Wahl, 2010). Since the scale of estimated consequences associated with a dam failure can be sensitive to the choice of breach parameters, careful consideration should be given to the selection of the proper method(s) of determining breach parameters and the associated uncertainty, not only with the parameters themselves, but also of the overall result of the breach modeling efforts.
Numerous regression equations, summarized in the preceding sections of this ISG, have been developed for peak discharge and breach parameters. The available equations vary widely depending on the analyst and the types of dam failures studied. Regression equations suffer from a lack of well-documented case study data as well as a high level of uncertainty in the data used to develop the equations. Approximately 75 percent of the dam failures used to develop the equations are less than 50 feet in height; therefore, these equations may not be very representative of dams greater than 50 feet in height. According to Wahl (2010), the best methods of breach width prediction are empirically derived parametric equations (e.g., USBR, 1988; Von Thun & Gillette, 1990; and Froehlich, 1995a).
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These methods were found to have uncertainties of about +/- one-third of an order of magnitude.
In general, predictions of the side slope of dam breach openings have a high uncertainty, although this is of secondary importance since breach outflows are relatively insensitive to the selection of side slopes. In the case studies used to develop most regression equations, observed final breach openings are generally sloped. However, based on laboratory and scale model studies, researchers believe that side slopes are nearly vertical while the breach is actively eroding and enlarging.
Predictions of breach formation time may also be subject to great uncertainty due to a lack of reliable case study data; many dams fail without eyewitnesses, and the problem of distinguishing between breach initiation and breach formation phases has likely tainted much of the data (USBR, 1998). An analysis by Wahl (2004) found that most of the empirically developed equations for predicting time of failure had uncertainties of about +/-1 order of magnitude; the best predictions were obtained with the equation by Froehlich (1995b). Newer equations by Froehlich (2008) and Xu and Zhang (2009) which are based on more case studies and additional parameters (e.g., erodibility) may be marginally improved, but breach failure time predictions should still be considered highly uncertain.
7.2.2.2 Performing Sensitivity Analyses to Select Final Breach Parameters With a wide range of methods available that can produce a wide range of results for breach width and breach formation time, a sensitivity analysis should be performed prior to selecting the final breach parameters. The sensitivity analysis should not be restricted to identifying the impact of varying the breach parameters on the peak discharge and breach hydrograph at the dam. It should also identify the effect of breach parameters on the calculated water surface elevations at locations of interest downstream from the dam. While a model may indicate that the stage and outflow at the dam vary greatly depending on the selected breach parameters, the effect on the stage, flow, and travel time to an area of interest downstream of the dam may be smaller due to routing effects (e.g., flood attenuation and floodplain hydraulics).
Significant engineering judgment must be exercised in interpreting breach parameter and/or breach peak flow results. The sensitivity analysis could involve using several widely used predictor equations to establish breach parameters. However, it should be noted that the importance attached to details of the breach model are highest for those sites that are closest to the dam. As the distance between the dam and the site of interest increases, the attenuation of the flood wave reduces the influence of the breach details. Sensitivity analysis can be used to study this effect.
Staff Positions:
- Because of the large uncertainties, inconsistencies and potential biases associated with breach modeling, licensees should not rely on a single modeling method.
Instead, licensees should compare the results of several models judged appropriate. Justification should be provided for the selection of the candidate models used as well as the value(s) for the specific model. Model and parameter uncertainty as well as parameter sensitivity in final results should be explicitly addressed.
- Studies have shown that failure time uncertainties can be quite large. Contributions to uncertainty include: (a) observations of failure time in case studies generally originate from non-professional eyewitness; and (b) lack of clear and consistent definition of failure time across (and sometimes within) studies. Therefore, licensees 7-7
should describe how failure time is defined and discuss how failure time was appropriately applied in the numerical model selected to simulate the breach formation and outflow hydrograph.
7.2.3 Physically-Based Combined Process Breach Models Another approach to dam breach analysis for earthen dams is to use a combined process model that simulates specific erosion processes and the associated hydraulics of flow through the developing breach to yield a breach outflow hydrograph. These combined process models attempt to simulate the progression of a dam breach using sediment detachment and/or sediment transport equations that in turn rely on estimated erosion rates and soil mechanics relations to predict mass slope failures. Several models have been developed (e.g., Fread, 1991; Mohamed, 2002; Temple et al., 2006; Hanson et al. 2011; Visser et al. 2012; Wu, et al. 2010), but they are not nearly as widely used as the previously described regression approaches. The work required to develop site-specific parameters needed for application of these models is likely to be significant.
Staff Position:
The state of practice in dam breach modeling tends to emphasize regression-based approaches. However, use of physically based breach modeling will be considered on a case-by-case basis. If used, the parameters describing erosion and hydraulic properties should be developed from site-specific studies. Generic values or values obtained from the literature are, in general, not sufficient. Uncertainty and sensitivity studies should be performed to evaluate the effect of model and input parameter selection on the results of the analysis. Justification for the preferred model and values for input parameters should be provided, including documentation of uncertainty and sensitivity studies.
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- 8. LEVEE BREACH MODELING The breaching process for levees can be quite different from that of earthen dams. The principal differences include: (a) breach sensitivity to upstream and downstream conditions; (b) dimensionality of the outflow; and (c) flow direction relative to the structure.
One of the most significant differences is the effect of the upstream and downstream water conditions. In a dam breaching event, the upstream reservoir water level drops and the breach outflow discharge increases as the breach enlarges. At some point, the discharge will decrease as water level decreases and storage volume in the reservoir is depleted. The dam breach size and outflow are thus usually limited by reservoir characteristics, and downstream tailwater conditions are generally of secondary importance. In a scenario involving a levee failure bordering a large lake, only a minimal drop in water level can be expected. The breach size and outflow continue to increase until the tailwater downstream from the breach rises to reduce hydraulic stresses on the breach opening below the threshold for continued erosion. Thus, downstream tailwater rise is much more important than for dams. Tailwater rise has a similar effect on a riverine levee breach, but upstream river inflow (and hence catchment size) also affects the breach size and outflow by sustaining the water level in the river.
Outflow from a dam breach often flows into a narrow valley and is often as a one-dimensional flow. However, outflow from a levee breach usually spreads into a relatively flat plain. The diverging flow as water inundates land behind the levee is essentially two-dimensional in nature. This two-dimensional flow can be important when the levee is close to the site. When the levee breach is distant from the site, simplified modeling approaches are more appropriate.
In a coastal context, the presence of waves in the incipient breach increases sediment mobilization and transport. The breach flow may be affected by the tidal cycle, and water conditions on both sides of the embankment. In addition, a barrier breach may be closed naturally by the sediments transported from adjacent beaches and shores attributable to littoral drift, or it may increase in size and become a new inlet or estuary In contrast to earthen embankment dams, very few studies have been carried out to derive regression equations for levee breach parameters. The parametric dam breach models discussed above may not be strictly applicable to levee breaches because water conditions upstream and downstream from levee breaches may be very different from those at dam breaches.
Staff Positions:
- In general, earthen embankment levees should be assumed to fail when overtopped.
The case for nonfailure must be developed using detailed engineering analysis supported by site-specific information, including material properties of the embankment and foundation soils, material properties of embankment protection (if any), levee condition, etc. Other forms of levees (e.g., pile walls, concrete flood walls) should be evaluated for potential failures applicable to the particular type of levee.
- Levees are generally not designed to withstand high water levels for long periods.
However, no generally accepted method currently exists for predicting how long a levee will continue to function under high loading conditions. Therefore, historical information is the best available basis for predicting levee performance. The 8-1
historical information should be from levees that have similar design and construction characteristics as the levee being analyzed.
- Because there is no widely accepted method for modeling breach development in the case of levees, conservative assumptions regarding the extent of the breach and the failure time should be used.
- In general, inundation mapping of a NPP site from an onsite or nearby levee will require two-dimensional modeling.
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- 9. FLOOD WAVE ROUTING Regardless of the type of dam failure, the dam-break flood hydrographs represent dynamic, unsteady flow events. Therefore, a dynamic hydraulic model should generally be used to route the dam failure flood wave to the plant. Sensitivity of flood stage and water velocity estimates to reservoir levels, reservoir inflow conditions, and tailwater conditions before and after dam failure should be examined. Transport of sediment and debris by the floodwaters should be considered. However, as discussed in Section 3, there may be situations where using a simplified approach is appropriate.
This section describes h y d r o l o g i c routing methods that are appropriate for use in modeling dam breach in hydrologic modeling software packages. Several commonly used methods are:
- Muskingum
- Modified Puls (also known as storage routing)
- Muskingum Cunge Each of these models computes a downstream hydrograph, given an upstream hydrograph as a boundary condition. Each does so by solving the continuity equation and simplified version of the momentum equations.
An important consideration in selecting a routing method is the nature of the flood wave exiting the reservoir. The flood wave will rise from a fairly low value, very quickly to a value much greater than the initial flow. The extremely large flows will generally overflow the channel and enter the floodplain. Therefore, the routing method selected must be capable of nonlinear routing.
While some of the hydrologic routing methods include attenuation, none of them include acceleration. Acceleration can be a significant source of attenuation during a large floodwave in a flat channel. The attenuation by hydrologic methods is approximate. A hydraulic model may be necessary to accurately predict the attenuation.
9.1 Applicability and Limitations of Hydrologic Routing Models Each routing model discussed above involves solving both the momentum and continuity equations. However, each omits or simplifies certain terms of those equations to arrive at a solution. To select a routing model, one must consider the routing method's assumptions and reject those models that fail to account for critical characteristics of the flow hydrographs and the channels through which they are routed. These include (but are not limited to) the effects discussed in the following subsections.
9.1.1 Backwater Effects Tidal fluctuations, significant tributary inflows, dams, bridges, culverts, and channel constrictions can cause backwater effects. A flood wave that is subjected to the influences of backwater will be attenuated and delayed in time.
Practically, none of the hydrologic routing models will simulate channel flow well if the downstream conditions have a significant impact on upstream flows. The structure of the methods is such that the computations progress from upstream watersheds and channels to those downstream. Thus, downstream conditions are not yet known when routing computations begin. Only a full unsteady-flow hydraulic system model can accomplish this.
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9.1.2 Floodplain Storage If flood flows exceed the channel carrying capacity, water flows into overbank areas.
Depending on the characteristics of the overbanks, the overbank flow can be slowed greatly, and often ponding will occur. This can significantly a f f e c t the translation and attenuation of a flood wave.
To analyze the transition from main channel to overbank flows, the model should account for varying conveyance between the main channel and the overbank areas. For one-dimensional flow models, this is normally accomplished by calculating the hydraulic properties of the main channel and the overbank areas separately, then combining them to formulate a composite set of hydraulic relationships. The Muskingum model parameters are assumed constant. However, as flow spills from the channel, the velocity may change significantly, so the Muskingum K should change. While the Muskingum model can be calibrated to match the peak flow and timing of a specific flood magnitude, the parameters cannot easily be used to model a range of floods that may remain in bank or go out of bank. Similarly, the kinematic wave model assumes constant celerity, an incorrect assumption if flows spill into overbank areas.
In fact, flood flows through extremely flat and wide flood plains may not be modeled adequately as one-dimensional flow. Velocity of the flow across the floodplain may be just as large as that of flow down the channel. If this occurs, a two-dimensional flow model will better simulate the physical processes.
9.1.3 Interaction of Channel Slope and Hydrograph Characteristics As channel slope approaches zero assumptions in some of the hydrologic models will be violated (e.g., momentum-equation terms that were omitted become significant).
For example, the simplification for the kinematic-wave model is appropriate only if the channel slope exceeds 0.002. The Muskingum-Cunge model can be used to route slow-rising flood waves through reaches with flat slopes. However, it should not be used for rapidly rising hydrographs in the same channels, because it omits acceleration terms of the momentum equation that are significant in that case. Ponce et al. (1978) established a numerical criterion to judge the likely applicability of various routing models. He suggested that the error due to the use of the kinematic wave model is less than 5 percent if:
where T = hydrograph duration, u0 is the reference mean velocity, and d0 = reference flow depth. (These reference values are average flow conditions of the inflow hydrograph.) He suggested that the error with the Muskingum- Cunge model is less than 5 percent:
where g = acceleration of gravity.
9.1.4 Configuration of Flow Networks In a dendritic stream system, if the tributary flows or the main channel flows do not cause significant backwater at the confluence of the two streams, any of the hydraulic or hydrologic routing methods can be applied. However, if significant backwater does occur at confluences, then models that can account for backwater must be applied. For networks, 9-2
where the flow divides and possibly changes direction during the event, none of the simplified hydrologic models are recommended.
9.1.5 Occurrence of Subcritical and Supercritical Flow During a flood, flow may shift between subcritical and supercritical regimes. If the supercritical flow reaches are short, this shift will not have a noticeable impact on the discharge hydrograph. However, if the supercritical-flow reaches are long, these should be identified and treated as separate routing reaches. If the shifts are frequent and unpredictable, then a hydraulic model is recommended.
9.1.6 Availability of Calibration Data Sets In general, if observed data are not available, the physically based routing models will be easier to set up and apply with some confidence. Parameters such as the Muskingum X can be estimated, but the estimates should be verified with observed flows. Thus, these empirical models should be avoided if the watershed and channel are ungauged.
Staff Position
- The use of simplified hydrologic routing must justified and shown to be appropriate for use on a case-by-case basis.
- When available, records from the largest observed floods should be used to calibrate hydrologic and hydraulic models. Flood records from nearby hydrologically similar watersheds may also be useful.
- When flood records are not available, USGS regression equations for ungauged watersheds may be used to inform modeling.
9.2 Hydraulic Models As stated above, hydraulic routing methods are preferred when routing flood waves from dam breach. Hydraulic routing models provide more accuracy when modeling flood waves from a dam breach. These models include terms that hydrologic models neglect. Typically, a dynamic hydraulic model should be used to route the dam failure flood wave to the plant.
There are many readily available dynamic (unsteady flow) hydraulic models that have been used for dam breach outflow hydrograph computation and downstream routing. Recent case studies of dam-break flood routing using hydraulic models developed by Federal agencies are available in the hydraulic engineering literature. Models to route the flood can be one- or two-dimensional or can be a combination of both. In general, as the flood plain widens, one-dimensional analysis becomes less reliable. Accurate estimates of flood elevation in areas of changing topography and near large objects (i.e. buildings and other structures) in the flow field will typically require localized two-dimensional analysis, in areas of particular interest or sensitivity.
Staff Positions:
- For estimating inundation at or near a NPP site, two-dimensional models are generally preferred by the NRC staff. However, use of one-dimensional models may be appropriate in some cases. Therefore, use of one-dimensional models will be accepted on a case-by-case basis, with appropriate justification.
- Large uncertainty exists in relationships between water elevation and discharge (rating curves), especially at high river discharges. Typically, observed data are extrapolated well beyond field-observed data when discussing dam breach 9-3
scenarios. Some estimation of the likely variation in maximum water surface elevation at a NPP site should be reported to account for this uncertainty in the rating curve.
9.3 Sediment Transport Modeling Sediment transport effects such as erosion and sedimentation should be considered.
Ignoring sediment deposition on or near the site may result in underestimates of water level elevations. Conversely, ignoring sediment erosion may mean that potentially dangerous scouring around structures is not analyzed. However, detailed guidance on sediment transport modeling is beyond the scope of this ISG. The reader should consult one or more standard references in this discipline (Yalin 1977, Julien 2010, Lick 2008). Some hydrologic and hydraulic modeling packages include sediment transport modules (e.g., HEC-RAS, SRH-1D, SRH-2D, FLO-2D). In many cases, simplified conservative estimates for sediment transport, erosion and sedimentation may be used in place of detailed analysis.
Staff Position:
Transport of sediment and debris by the flood waters should be considered.
9.4 Inundation Mapping Inundation maps have a variety of uses including EAPs, mitigation planning, emergency response, and consequence assessment. Each map application has unique information requirements tailored to a specific end need. For the purpose of this ISG, inundation maps provide assistance in identifying SSCs important to safety that may require protective and/or mitigation measures from flooding due to dam breach.
The use of Geographic Information Systems (GIS) has emerged as the most common method to develop mapping products associated with hydrologic/ hydraulic engineering, including inundation maps from dam breach modeling. Several software packages are commercially available and some are free of charge. It should be noted that the NRC does not endorse a particular software package.
Several Federal agencies have developed guidance documents on producing inundation maps (USDOI, 2010; USACE, 2011d). These documents provide guidance based on their specific mission and are mentioned for information only.
Staff Positions:
- Inundation map(s) should be developed for NPP sites that are flooded because of dam failure The inundation map(s) should reflect the bounding flood scenario (greatest depth of inundation at the NPP site) from the dam breach analysis. The inundation map(s) should contain the following features:
o Topographic elevation / contour information related to a stated datum.
o Streets and highways bordering and within the NPP site o SSCs important to safety and other key structures and landmarks.
o Orthophotographic imagery of the NPP site and nearby areas.
o Cross sections / Grid (if applicable) from hydraulic model(s) o Water Surface elevations for the NPP site and nearby areas related to a stated datum.
o Local water depths across the site and nearby areas.
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o Velocity data across the NPP site and nearby areas.
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- 10. TERMS AND DEFINITIONS Abutment. That part of the valley side against which the dam is constructed. An artificial abutment is sometimes constructed, as a concrete gravity section, to take the thrust of an arch dam where there is no suitable natural abutment. The left and right abutments of dams are defined with the observer viewing the dam looking in the downstream direction, unless otherwise indicated.
Appurtenant structure. Ancillary features of a dam such as outlets, spillways, power plants, tunnels, etc.
Attenuation. A decrease in amplitude of the seismic waves with distance caused by geometric spreading, energy absorption, and scattering, or decrease in the amplitude of a flood wave caused by channel geometry and energy loss.
Average reservoir level. The 50% exceedance duration pool level calculated using average daily water levels for the period of record.
Axis of dam. The vertical plane or curved surface, chosen by a designer, appearing as a line, in plan or in cross-section, to which the horizontal dimensions of the dam are referenced.
Base thickness. Also referred to as base width. The maximum thickness or width of the dam measured horizontally between upstream and downstream faces and normal to the axis of the dam, but excluding projections for outlets or other appurtenant structures.
Bedrock. Any sedimentary, igneous, or metamorphic material represented as a unit in geology; being a sound and solid mass, layer, or ledge of mineral matter; and with shear wave threshold velocities greater than 2500 feet/second.
Borrow area. The area from which natural materials, such as rock, gravel or soil, used for construction purposes is excavated.
Breach. An opening through a dam that allows the uncontrolled draining of a reservoir. A controlled breach is a constructed opening. An uncontrolled breach is an unintentional opening allowing discharge from the reservoir. A breach is generally associated with the partial or total failure of the dam. A breach opening could be formed by many processes.
Channel. A general term for any natural or artificial facility for conveying water.
Compaction. Mechanical action that increases the density by reducing the voids in a material.
Conduit. A closed channel to convey water through, around, or under a dam.
Construction joint. The interface between two successive placements or pours of concrete where bond, and not permanent separation, is intended.
Core. A zone of low permeability material in an embankment dam. The core is sometimes referred to as central core, inclined core, puddle clay core, rolled clay core, or impervious zone.
Core wall. A wall built of relatively impervious material, usually of concrete or asphaltic concrete in the body of an embankment dam to prevent seepage.
Crest length. The measured length of the dam along the crest or top of the dam.
Crest of dam. See top of dam.
Critical damping. The minimum amount of damping that prevents free oscillatory vibration.
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Cross section. An elevation view of a dam formed by passing a plane through the dam perpendicular to the axis.
Cutoff trench. A foundation excavation to be filled later with impervious material in order to limit seepage beneath a dam.
Cutoff wall. A wall of impervious material usually of concrete, asphaltic concrete, or steel sheet piling constructed in the foundation and abutments to reduce seepage beneath and adjacent to the dam.
Cyclic mobility. A phenomenon in which a cohesionless soil loses shear strength during earthquake ground vibrations and acquires a degree of mobility sufficient to permit intermittent movement up to several feet, as contrasted to liquefaction where continuous movements of several hundred feet are possible.
Dam. An artificial barrier that has the ability to impound water, wastewater, or any liquid-borne material, for the purpose of storing or controlling the material.
Critical dam. A dam (or set of dams) that is shown to have flooding impacts at a NPP site (i.e., flood elevations at or above systems, structures, and components important to safety).
Inconsequential dam. A dam identified by federal or state agencies as having minimal or no adverse failure consequences beyond the owners property. Also, a dam that can be shown to have minimal or no adverse downstream failure consequences.
Noncritical dam. A dam (or set of dams) that can be shown to have no flooding impacts at a NPP site (i.e., flood elevations below systems, structures, and components.
Potentially critical dam. A dam (or set of dams) that is under evaluation to determine if it is either a non-critical or critical dam for a NPP site (i.e., flood evaluations below systems, structures, and components important to safety).
Dam failure. Catastrophic type of failure characterized by the sudden, rapid, and uncontrolled release of impounded water or the likelihood of such an uncontrolled release. It is recognized that there are lesser degrees of failure and that any malfunction or abnormality outside the design assumptions and parameters that adversely affect a dam's primary function of impounding water is properly considered a failure. These lesser degrees of failure can progressively lead to or heighten the risk of a catastrophic failure. They are, however, normally amenable to corrective action. Dams may be classified according to the broad level of importance in estimating the flooding hazard at a NPP site:
Damping. Resistance that reduces vibrations by energy absorption. There are different types of damping such as viscous, Coulomb, and geometric damping.
Damping ratio. The ratio of the actual damping to the critical damping.
Design water level. The maximum water elevation, including the flood surcharge that a dam is designed to withstand.
Design wind. The most severe wind that is reasonably possible at a particular reservoir for generating wind setup and run-up. The determination will generally include the results of meteorologic studies that combine wind velocity, duration, direction and seasonal distribution characteristics in a realistic manner.
Dike. See saddle dam.
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Drain, blanket. A layer of pervious material placed to facilitate drainage of the foundation and/or embankment.
Drain, chimney. A vertical or inclined layer of pervious material in an embankment to facilitate and control drainage of the embankment fill.
Drain, toe. A system of pipe and/or pervious material along the downstream toe of a dam used to collect seepage from the foundation and embankment and convey it to a free outlet.
Drainage area. The area that drains to a particular point on a river or stream.
Drainage curtain. A line of vertical wells or boreholes to facilitate drainage of the foundation and abutments and to reduce water pressure.
Drainage wells or relief wells. Vertical wells downstream of or in the downstream shell of an embankment dam to collect and control seepage through and under the dam. A line of such wells forms a drainage curtain.
Duration of strong ground motion. The "bracketed duration" or the time interval between the first and last acceleration peaks that are equal to or greater than 0.05g.
Dynamic routing. Hydraulic flow routing based on the solution of the St.-Venant Equation(s) to compute the changes of discharge and stage with respect to time at various locations along a stream.
Earthquake. A sudden motion or trembling in the earth caused by the abrupt release of accumulated stress along a fault.
Emergency gate. A standby or reserve gate used only when the normal means of water control is not available for use.
Energy dissipator. A device constructed in a waterway to reduce the kinetic energy of fast flowing water.
Epicenter. The point on the earth's surface located directly above the point where the first rupture and the first earthquake motion occur.
Erosion. The wearing away of a surface (bank, streambed, embankment, or other surface) by floods, waves, wind, or any other natural process.
Failure. See Dam, Failure.
Failure mode. A potential failure mode is a physically plausible process for dam failure resulting from an existing inadequacy or defect related to a natural foundation condition, the dam or appurtenant structures design, the construction, the materials incorporated, the operations and maintenance, or aging process, which can lead to an uncontrolled release of the reservoir.
Fetch. The-straight-line distance across a body of water subject to wind forces. The fetch is one of the factors used in calculating wave heights in a reservoir.
Filter (filter zone). One or more layers of granular material graded (either naturally or by selection) so as to allow seepage through or within the layers while preventing the migration of material from adjacent zones.
Flashboards. Structural members of timber, concrete, or steel placed in channels or on the crest of a spillway to raise the reservoir water level but intended to be quickly removed, tripped, or fail in the event of a flood.
Flood. A temporary rise in water surface elevation resulting in inundation of areas not normally covered by water. Hypothetical floods may be expressed in terms of average 10-3
probability of exceedance per year such as one-percent-chance-flood, or expressed as a fraction of the probable maximum flood or other reference flood.
Flood, Inflow Design (IDF). The flood flow above which the incremental increase in downstream water surface elevation due to failure of a dam or other water impounding structure is no longer considered to present an unacceptable threat to downstream life or property. The flood hydrograph used in the design of a dam and its appurtenant works particularly for sizing the spillway and outlet works and for determining maximum storage, height of dam, and freeboard requirements.
Flood, Probable Maximum (PMF). The flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the drainage basin under study.
Flood plain. An area adjoining a body of water or natural stream that may be covered by floodwater. Also, the downstream area that would be inundated or otherwise affected by the failure of a dam or by large flood flows. The area of the flood plain is generally delineated by a frequency (or size) of flood.
Flood routing. A process of determining progressively over time the amplitude of a flood wave as it moves past a dam or downstream to successive points along a river or stream.
Flood storage. The retention of water or delay of runoff either by planned operation, as in a reservoir, or by temporary filling of overflow areas, as in the progression of a flood wave through a natural stream channel.
Flume. An open channel constructed with masonry, concrete or steel of rectangular or U shaped cross section and designed for medium or high velocity flow. Also, a channel in which water is conveyed through a section of known size and shape for purposes of measurement.
Foundation. The portion of the valley floor that underlies and supports the dam structure.
Freeboard. Vertical distance between the surface elevation of specified stillwater (or other) reservoir and the top of the dam, without camber.
Gallery. A passageway in the body of a dam used for inspection, foundation grouting, and/or drainage.
Gate. A movable water barrier for the control of water.
Bascule gate. See flap gate.
Bulkhead gate. A gate used either for temporary closure of a channel or conduit before dewatering it for inspection or maintenance or for closure against flowing water when the head difference is small, e.g., for diversion tunnel closure.
Crest gate (spillway gate). A gate on the crest of a spillway to control the discharge or reservoir water level.
Drum gate (roller drum gate). . A type of spillway gate consisting of a long hollow drum. The drum may be held in its raised position by the water pressure in a flotation chamber beneath the drum.
Emergency gate. A standby or auxiliary gate used when the normal means of water control is not available. Sometimes referred to as guard gate.
Fixed wheel gate (fixed roller gate) (fixed axle gate). A gate having wheels or rollers mounted on the end posts of the gate. The wheels bear against rails fixed in side grooves or gate guides.
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Flap gate. A gate hinged along one edge, usually either the top or bottom edge.
Examples of bottom-hinged flap gates are tilting gates and fish belly gates so called from their shape in cross section.
Flood gate. A gate to control flood release from a reservoir.
Outlet gate. A gate controlling the flow of water through a reservoir outlet.
Radial gate (Tainter gate). A gate with a curved upstream plate and radial arms hinged to piers or other supporting structures.
Regulating gate (regulating valve). A gate or valve that operates under full pressure flow conditions to regulate the rate of discharge.
Roller drum gate. See drum gate.
Roller gate (stoney gate). A gate for large openings that bears on a train of rollers in each gate guide.
Skimmer gate. A gate at the spillway crest whose prime purpose is to control the release of debris and logs with a limited amount of water. It is usually a bottom hinged flap or Bascule gate.
Slide gate (sluice gate). A gate that can be opened or closed by sliding in supporting guides.
Gate chamber (valve chamber). A room from which a gate or valve can be operated, or sometimes in which the gate is located.
Hazard. A situation that creates the potential for adverse consequences such as loss of life, property damage, or other adverse impacts.
Hazard potential. The possible adverse incremental consequences that result from the release of water or stored contents due to failure of the dam or misoperation of the dam or appurtenances. Impacts may be for a defined area downstream of a dam from floodwaters released through spillways and outlet works of the dam or waters released by partial or complete failure of the dam. There may also be impacts for an area upstream of the dam from effects of backwater flooding or landslides around the reservoir perimeter.
Head, static. The vertical distance between two points in a fluid.
Head, velocity. The vertical distance that would statically result from the velocity of a moving fluid.
Headwater: The water immediately upstream from a dam. The water surface elevation varies due to fluctuations in inflow and the amount of water passed through the dam.
Heel. The junction of the upstream face of a gravity or arch dam with the ground surface.
For an embankment dam, the junction is referred to as the upstream toe of the dam.
Height, above ground. The maximum height from natural ground surface to the top of a dam.
Height, hydraulic. The vertical difference between the maximum design water level and the lowest point in the original streambed.
Height, structural. The vertical distance between the lowest point of the excavated foundation to the top of the dam.
Hydrograph, breach or dam failure. A flood hydrograph resulting from a dam breach.
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Hydrograph, flood. A graph showing, for a given point on a stream, the discharge, height, or other characteristic of a flood with respect to time.
Hydrograph, unit. A hydrograph with a volume of one inch of runoff resulting from a storm of a specified duration and areal distribution. Hydrographs from other storms of the same duration and distribution are assumed to have the same time base but with ordinates of flow in proportion to the runoff volumes.
Hydrology. One of the earth sciences that encompasses the natural occurrence, distribution, movement, and properties of the waters of the earth and their environmental relationships.
Hydrometeorology. The study of the atmospheric and land-surface phases of the hydrologic cycle with emphasis on the interrelationships involved.
Hypocenter. The location where the slip responsible for an earthquake originates; the focus of an earthquake.
Inflow Design Flood (IDF). See Flood.
Intake. A component placed at the beginning of an outlet-works waterway (power conduit, water supply conduit), the intake establishes the ultimate drawdown level of the reservoir by the position and size of its opening(s) to the outlet works. The intake may be vertical or inclined towers, drop inlets, or submerged, box-shaped structures. Intake elevations are determined by the head needed for discharge capacity, storage reservation to allow for siltation, the required amount and rate of withdrawal, and the desired extreme drawdown level.
Landslide. The unplanned descent (movement) of a mass of earth or rock down a slope.
Leakage. Uncontrolled loss of water by flow through a hole or crack.
Length of dam. The length along the top of the dam. This also includes the spillway, power plant, navigation lock, fish pass, etc., where these form part of the length of the dam. If detached from the dam, these structures should not be included.
Lining. With reference to a canal, tunnel, shaft, or reservoir, a coating of asphaltic concrete, reinforced or unreinforced concrete, shotcrete, rubber or plastic to provide water tightness, prevent erosion, reduce friction, or support the periphery of the outlet pipe conduit.
Liquefaction. A condition whereby soil undergoes continued deformation at a constant low residual stress or with low residual resistance, due to the buildup and maintenance of high pore water pressures, which reduces the effective confining pressure to a very low value.
Pore pressure buildup leading to liquefaction may be due either to static or cyclic stress applications and the possibility of its occurrence will depend on the void ratio or relative density of a cohesionless soil and the confining pressure.
Logboom. A chain of logs, drums, or pontoons secured end-to-end and floating on the surface of a reservoir to divert floating debris, trash, and logs.
Low-level outlet (bottom outlet). An opening at a low level from a reservoir generally used for emptying or for scouring sediment and sometimes for irrigation releases.
Maximum flood control level. The highest elevation of the flood control storage.
Maximum wind. The most severe wind for generating waves that is reasonably possible at a particular reservoir. The determination will generally include results of meteorologic studies that combine wind velocity, duration, direction, fetch, and seasonal distribution characteristics in a realistic manner.
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Meteorological homogeneity. Climates and orographic influences that are alike or similar.
Meteorology. The science that deals with the atmosphere and atmospheric phenomena, the study of weather, particularly storms and the rainfall they produce.
Minimum operating level. The lowest level to which the reservoir is drawn down under normal operating conditions. The lower limit of active storage.
Non-overflow dam (section). A dam or section of dam that is not designed to be overtopped.
Orographic. Physical geography that pertains to mountains and to features directly connected with mountains and their general effect on storm path and generation of rainfall.
Outlet. An opening through which water can be freely discharged from a reservoir to the river for a particular purpose.
Outlet works. A dam appurtenance that provides release of water (generally controlled) from a reservoir.
Overflow dam (section). A section or portion of a dam designed to be overtopped.
Parapet wall. A solid wall built along the top of a dam (upstream or downstream edge) used for ornamentation, for safety of vehicles and pedestrians, or to prevent overtopping caused by wave run-up.
Peak flow. The maximum instantaneous discharge that occurs during a flood. It is coincident with the peak of a flood hydrograph.
Pervious zone. A part of the cross section of an embankment dam comprising material of high permeability.
Phreatic surface. The free surface of water seeping at atmospheric pressure through soil or rock.
Piezometer. An instrument used for measure water levels or pore water pressures in embankments, foundations, abutments, soil, rock, or concrete.
Piping. The progressive development of internal erosion by seepage.
Probability. The likelihood of an event occurring.
Probable. Likely to occur; reasonably expected; realistic.
Probable Maximum Flood (PMF). See Flood.
Probable Maximum Precipitation (PMP). Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location during a certain time of the year.
Reservoir. A body of water impounded by a dam for storage.
Reservoir regulation procedure (Rule Curve): The compilation of operating criteria, guidelines, and specifications that govern the storage and release function of a reservoir. It may also be referred to as operating rules, flood control diagram, or water control schedule.
These are usually expressed in the form of graphs and tabulations, supplemented by concise specifications and are often incorporated in computer programs. In general, they indicate limiting rates of reservoir releases required or allowed during various seasons of the year to meet all functional objectives of the project.
Reservoir rim. The boundary of the reservoir including all areas along the valley sides above and below the water surface elevation associated with the routing of the IDF.
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Reservoir surface area. The area covered by a reservoir when filled to a specified level.
Response spectrum. A plot of the maximum values of acceleration, velocity, and/or displacement response of an infinite series of single-degree-of-freedom systems subjected to a time-history of earthquake ground motion. The maximum response values are expressed as a function of natural period for a given damping.
Riprap. A covering layer of large chunks of rock or concrete used to protect abutments, shorelines, and other vulnerable surfaces from erosion or scour.
Scaling. An adjustment to an earthquake time-history or response spectrum where the amplitude of acceleration, velocity, and/or displacement is increased or decreased, usually without change to the frequency content of the ground motion.
Seepage. The internal movement of water that may take place through the dam, the foundation or the abutments.
Seiche. An oscillating wave in a reservoir caused by a landslide into the reservoir or earthquake-induced ground accelerations or fault offset or meteorological event.
Sensitivity analysis. An analysis in which the relative importance of one or more of the variables thought to have an influence on the phenomenon under consideration is determined.
Settlement. The vertical downward movement of a structure or its foundation.
Slope. Inclination from the horizontal. Sometimes referred to as batter when measured from vertical.
Slope protection. The protection of a slope against wave action or erosion.
Sluice. An opening for releasing water from below the static head elevation.
Smooth response spectrum. A response spectrum devoid of sharp peaks and valleys that specifies the amplitude of the spectral acceleration, velocity, and/or displacement to be used in the analyses of the structure.
Spillway. A structure over or through which flow is discharged from a reservoir. If the rate of flow is controlled by mechanical means, such as gates, it is considered a controlled spillway.
If the geometry of the spillway is the only control, it is considered an uncontrolled spillway.
Spillway, auxiliary (spillway, emergency). Any secondary spillway that is designed to be operated infrequently, possibly in anticipation of some degree of structural damage or erosion to the spillway that would occur during operation.
Spillway, emergency. See Spillway, auxiliary.
Spillway, service (spillway, principal). A spillway that is designed to provide continuous or frequent regulated or unregulated releases from a reservoir, without significant damage to either the dam or its appurtenant structures. This is also referred to as principal spillway.
Spillway capacity: The maximum spillway outflow that a dam can safely pass with the reservoir at its maximum level.
Spillway channel. An open channel or closed conduit conveying water from the spillway inlet downstream.
Spillway chute. A steeply sloping spillway channel that conveys discharges at supercritical velocities.
Spillway crest. The lowest level at which water can flow over or through the spillway.
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Spillway Design Flood. See Flood, Inflow Design.
Spillway, fuse plug. A form of auxiliary spillway with an inlet controlled by a low embankment designed to be overtopped and washed away during an exceptionally large flood.
Spillway, shaft. A vertical or inclined shaft into which water spills and then is conveyed through, under, or around a dam by means of a conduit or tunnel. If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a bellmouth or morning glory spillway.
Stability. The condition of a structure or a mass of material when it is able to support the applied stress for a long time without suffering any significant deformation or movement that is not reversed by the release of the stress.
Stilling basin. A basin constructed to dissipate the energy of rapidly flowing water, e.g.,
from a spillway or outlet, and to protect the riverbed from erosion.
Stillwater level. The elevation that a water surface would assume if all wave actions were absent.
Stoplogs. Large logs, timbers, or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit to provide a cheaper or more easily handled means of temporary closure than a bulkhead gate.
Storage. The retention of water or delay of runoff by planned operation, as in a reservoir, or by temporary filling of overflow areas, as in the progression of a flood wave through a natural stream channel. Definitions of specific types of storage in reservoirs are:
Active storage. The volume of the reservoir that is available for some use such as power generation, irrigation, flood control, water supply, etc. The bottom elevation is the minimum operating level. The top elevation is the maximum normal operating level.
Dead storage. The storage that lies below the invert of the lowest outlet and that, therefore, cannot readily be withdrawn from the reservoir.
Flood surcharge. The storage volume between the top of the active storage and the design water level.
Inactive storage. The storage volume of a reservoir between the crest of the invert of the lowest outlet and the minimum operating level.
Live storage. The sum of the active-and the inactive storage.
Reservoir capacity. The sum of the dead and live storage of the reservoir.
Surcharge. The volume or space in a reservoir between the controlled retention water level and the maximum water level. Flood surcharge cannot be retained in the reservoir but will flow out of the reservoir until the controlled retention water level is reached.
Surface waves. Waves that travel along or near the surface and include Rayleigh (Sv) and Love (SH) Waves of an earthquake.
Tailwater. The water immediately downstream from a dam. The water surface elevation varies due to fluctuations in the outflow from the structures of a dam and due to downstream influences of other dams or structures. Tailwater monitoring is an important consideration because a failure of a dam will cause a rapid rise in the level of the tailwater.
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Toe of the dam. The junction of the downstream slope or face of a dam with the ground surface; also referred to as the downstream toe. The junction of the upstream slope with ground surface is called the heel or the upstream toe.
Topographic map. A detailed graphic delineation (representation) of natural and manmade features of a region with particular emphasis on relative position and elevation.
Top thickness (top width). The thickness or width of a dam at the level of the top of dam (excluding corbels or parapets). In general, the term thickness is used for gravity and arch dams, and width is used for other dams.
Trashrack. A device located at an intake to prevent floating or submerged debris from entering the intake.
Tributary. A stream that flows into a larger stream or body of water Unit Hydrograph. See Hydrograph, unit.
Valve. A device fitted to a pipeline or orifice in which the closure member is either rotated or moved transversely or longitudinally in the waterway to control or stop the flow.
Watershed. The area drained by a river or river system or portion thereof. The watershed for a dam is the drainage area upstream of the dam.
Watershed divide. The divide or boundary between catchment areas (or drainage areas).
Wave protection. Riprap, concrete, or other armoring on the upstream face of an embankment dam to protect against scouring or erosion due to wave action.
Wave run-up. Vertical height above the stillwater level to which water from a specific wave will run up the face of a structure or embankment.
Weir. A notch of regular form through which water flows.
Weir, broad-crested. An overflow structure on which the nappe is supported for an appreciable length in the direction of flow.
Weir, sharp-crested. A device for measuring the rate of flow of water. It generally consists of a rectangular, trapezoidal, triangular, or other shaped notch, located in a vertical, thin plate over which water flows. The height of water above the weir crest is used to determine the rate of flow.
Wind setup. The vertical rise in the stillwater level at the face of a structure or embankment caused by wind stresses on the surface of the water.
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- 11. REFERENCES Anderson, L.W., and Ake, J.P. (2003), Probabilistic Fault Displacement Hazard Analysis for Lauro Dam, Cachuma Project, California: Technical Memorandum No. D8330-2003-12, Bureau of Reclamation, Denver, Colorado, 19 p.
Annandale, G. W. (2006) Scour Technology: Mechanics and Engineering Practice, McGraw-Hill.
ANSI/ANS (1992), Determining Design Basis Flooding at Nuclear Power Plant Sites, ANSI/ANS Standard 2.8, American National Standards Institute/American Nuclear Society, La Grange Park, IL.
ASCE (2005a), Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities, ASCE/SEI 43-05, American Society of Civil Engineers, Reston, VA.
ASCE (2010), Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-10, American Society of Civil Engineers, Reston, VA.
Baecher, G. B., M. E. Paté, and R. De Neufville (1980), Risk of Dam Failure in Benefit-Cost Analysis, Water Resources Research, 16(3), 449-456.
British Dam Society (2103), British Dam Society Web Site, www.britishdams.org, Accessed April 7, 2013.
Brunner, G. W. (2013), Personal communication, ADAMS Accession No. ML13193A280.
CADWR (2012), Urban Levee Design Criteria, State of California, Natural Resources Agency, Department of Water Resources, Sacramento, CA Chiou, B., R. Darragh, N. Gregor, and W. Silva (2008), NGA Project Strong-Motion Database, Earthquake Spectra, 24(1), 23-44.
CODNR (2010), Guidelines for Dam Breach Analysis, State of Colorado, Department of Natural Resources, Division of Water Resources, Office of the State Engineer, Dam Safety Branch, Denver, CO.
Costa, John E. (1985), Floods from Dam Failures, Open-File Report 85-560, U.S.
Department of Interior, Geological Survey, Denver, CO, 54 p.
EPRI (1989), Probabilistic Seismic Hazard Evaluation at Nuclear Plant Sites in the Central and Eastern United States: Resolution of the Charleston Issue, NP-6395-D, Electric Power Research Institute, April 1989.
FEMA (2004), Federal Guidelines for Dam Safety: Glossary of Terms, U.S. Department of Homeland Security, Federal Emergency Management Agency, Washington, D.C.
FEMA (2005), Federal Guidelines for Dam Safety: Earthquake Analyses and Design of Dams, U.S. Department of Homeland Security, Federal Emergency Management Agency, Washington, D.C.
FEMA (2009), Dam Safety in the United States: A Progress Report on the National Dam Safety Program Fiscal Years 2006 and 2007, FEMA Report P-759, U.S. Department of Homeland Security, Federal Emergency Management Agency, Washington, D.C.
FEMA (2011), Coastal Construction Manual, FEMA Report P-55, U.S. Department of Homeland Security, Federal Emergency Management Agency, Washington, D.C.
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FEMA (2012), Summary of Existing Guidelines for Hydrologic Safety of Dams, FEMA Report P-919, U.S. Department of Homeland Security, Federal Emergency Management Agency, Washington, D.C.
FERC (1991), Engineering Guidelines for the Evaluation of Hydropower Projects, Chapter 4:
Embankment Dams, Federal Energy Regulatory Commission, Washington, DC.
FERC (1997), Engineering Guidelines for the Evaluation of Hydropower Projects, Chapter 10: Other Dams, Federal Energy Regulatory Commission, Washington, DC.
FERC (1999), Engineering Guidelines for the Evaluation of Hydropower Projects, Chapter 11: Arch Dams, Federal Energy Regulatory Commission, Washington, DC.
FERC (2002), Engineering Guidelines for the Evaluation of Hydropower Projects, Chapter 3:
Gravity Dams, Federal Energy Regulatory Commission, Washington, DC.
Fread, D.L. (1991), BREACH: An Erosion Model for Earthen Dam Failures, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Office of Hydrology, Hydrologic Research Laboratory: Washington, D.C.
Fread, D.L. (2006), Dam-Breach Floods, in ADSO Advanced Technical Seminar: Dam Failure Analysis, jointly sponsored by the U.S. Department of Homeland Security, Federal Emergency Management Agency, and the Association of State Dam Safety Officials, July 25-28, 2006, Burlington VT.
Froehlich, David C. (1995a), Peak Outflow from Breached Embankment Dam, Journal of Water Resources Planning and Management, 121(1), pp. 90-97.
Froehlich, David C., (1995b), Embankment Dam Breach Parameters Revisited, Water Resources Engineering, Proceedings of the 1995 ASCE Conference on Water Resources Engineering, San Antonio, Texas, August 14-18, 1995, pp. 887-891.
Froehlich, David C. (2008), Embankment Breach Parameters and Their Uncertainties, Journal of Hydraulic Engineering, 134(12), pp. 1708-1721.
Gee, D.M. (2008), Comparison of Dam Breach Parameter Estimators, U.S. Army Corps of Engineers Hydrologic Engineering Center, Davis, CA.
Gee, D.M. and G.W. Brunner (2005), Dam Break Flood Routing Using HEC-RAS and NWS-FLDWAV, ASCE World Water and Environmental Resources Congress: Impacts of Global Climate Change, May 15-19. 2005, Anchorage, Alaska Hanson, G.J., Temple, D.M., Hunt, S.L., Tejral, R.D. (2011). Development and Characterization of Soil Material Parameters for Embankment Breach. Applied Engineering in Agriculture. 27(4):587-595.
Hanson, G.J., Temple, D. M., Morris, M., Hassan, M., Cook, K. (2005), Simplified breach analysis model for homogeneous embankments: Part II, Parameter inputs and variable scale model comparisons, in Proceedings of 2005 the U.S. Society on Dams Annual Meeting and Conference. 2005: Salt Lake City, Utah Johnson, F.A., and Illes, P. (1976), A Classification of Dam Failures, International Water Power and Dam Construction, December Issue, pp. 43-46.
Julien, P. Y. (2010), Erosion and Sedimentation, 2nd Edition, Cambridge University Press.
Kirkpatrick, G. W. (1977), Evaluation Guidelines for Spillway Adequacy, The Evaluation of Dam Safety, Engineering Foundation Conference, Pacific Grove, California, ASCE,
- p. 395-414.
Kramer, S. L. (1996), Geotechnical Earthquake Engineering, Prentice-Hall.
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Lick, W. (2008), Sediment and Contaminant Transport in Surface Waters, CRC Press.
MacDonald, Thomas C., and Jennifer Langridge-Monopolis (1984), Breaching Characteristics of Dam Failures, Journal of Hydraulic Engineering, vol. 110, no. 5, p.
567-586.
Mohamed, M.A.A. (2002), Embankment Breach Formation and Modelling Methods, Doctoral Dissertation, Open University: UK.
NAS (2012), Dam and Levee Safety and Community Resilience: A Vision for Future Practice, National Academy of Sciences, National Research Council, Washington, DC.
NERC (2012), Generation Availability Report, 2007-2011, North American Electric Reliability Corporation, http://www.nerc.com/pa/RAPA/gads/Pages/Reports.aspx, Accessed June 17, 2013.
National Park Service (2013), Discover Our Shared Heritage Travel Itinerary Bureau of Reclamation Historic Dams and Water Projects, http://www.nps.gov/nr/travel/ReclamationDamsAndWaterProjects/Bureau_of_Recla mation_Engineering_Achievements.html, Accessed April 7, 2013.
NCLS (2009), Recommendations for a National Levee Safety Program: A Report to Congress, National Committee on Levee Safety NRCS (2009), WinDAM B: Quick Start Guide, U.S. Department of Agriculture, Natural Resources Conservation Service, Washington, DC.
NRCS (2011), WinDAM B Dam Overtopping Model, U.S. Department of Agriculture, Natural Resources Conservation Service, Washington, DC.
Pierce, M.W., Thorton, C.I. and Abt, S.R. (2010), Predicting Peak Outflow from Breached Embankment Dams, Prepared for: National Dam Safety Review Board Steering Committee on Dam Breach Equations, Colorado State University, Fort Collins, CO.
Ponce, V. M., R. M. Li, and D. B. Simons (1978), Applicability of Kinematic and Diffusion Models, ASCE Journal of the Hydraulics Division, 104(HY3), 355-360.
SCS (1981), Simplified Dam-Breach Routing Procedure, Technical Release No. 66 (Rev. 1),
U.S. Department of Agriculture, Soil Conservation Service.
SimScience (2013), SimScience Cracking Dams Web Site, http://www.simscience.org/cracks/advanced/butt_anat1.html, Acessed April 7, 2013.
Seed, H. B. (1979), Considerations in the Earthquake-Resistant Design of Earth and Rockfill Dams, 19th Rankine Lecture of the British Geotechnical Society, Geotechnique, XXIX(3), 215-263.
Singh Krishan P and Scarlatos (1988), Analysis of Gradual Earth-Dam failure, Journal of Hydraulic Engineering, Vol 114, No. 7 pp 21-42 Singh, Krishan P., and Snorrason, Arni (1982), Sensitivity of Outflow Peaks and Flood Stages to the Selection of Dam Breach Parameters and Simulation Models, SWS Contract Report 288, Illinois Department of Energy and Natural Resources, State Water Survey Division, Surface Water Section at the University of Illinois, 179 p.
Singh, Krishan P., and Snorrason, Arni (1984), Sensitivity of Outflow Peaks and Flood Stages to the Selection of Dam Breach Parameters and Simulation Models, Journal of Hydrology, vol. 68, p. 295-310.
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Stepp, J. C., I. Wong, J. Whitney, R. Quittmeyer, N. Abrahamson, G. Toro, R. Youngs, K.
Coppersmith, J. Savy, and T. Sullivan (2001), Probabilistic Seismic Hazard Analyses for Ground Motions and Fault Displacement at Yucca Mountain, Nevada, Earthquake Spectra, 17(1), 113-151.
Temple, D.M., Hanson, G. J., Neilsen, M. L., Cook, K. R. (2005), Simplified breach analysis model for homogeneous embankments: Part I, Background and model components, in Proceedings of the 2005 U.S. Society on Dams Annual Meeting and Conference, Salt Lake City, Utah.
Temple, D.M., Hanson, G.J., Neilsen, M.L. (2006), Windam-analysis of Overtopped Earth Embankment Dams. In: ASABE Annual International Meeting, July 9-12, 2006, Portland, Oregon. Paper No. 06-2105. 2006 CDROM.
USACE (1989), Retaining and Flood Walls, Engineer Manual 1110-2-2502, U.S. Army Corps of Engineers, Washington, D.C.
USACE (1994), Arch Dam Design, Engineer Manual 1110-2-2201, U.S. Army Corps of Engineers, Washington, D.C.
USACE (1995a), Gravity Dam Design, Engineer Manual 1110-2-2200, U.S. Army Corps of Engineers, Washington, D.C.
USACE (1995b), Seismic Design Provisions for Roller Compacted Concrete Dams, Engineer Pamphlet 1110-2-12, U.S. Army Corps of Engineers, Washington, D.C.
USACE (1995c), Earthquake Design and Evaluation for Civil Works Projects, Engineer Regulation 1110-2-1806, U.S. Army Corps of Engineers, Washington, D.C.
USACE (1995d), Flood Proofing Regulations, Engineer Pamphlet 1165-2-314, U.S. Army Corps of Engineers, Washington, D.C.
USACE (2000), Design and Construction of Levees, Engineer Manual 1110-2-1913, U.S.
Army Corps of Engineers, Washington, D.C.
USACE (2004), General Design and Construction Considerations for Earth and Rock-Fill Dams, Engineering Manual 1110-2-2300, U.S. Army Corps of Engineers, Washington, D.C.
USACE (2011a), Engineering and Design: Evaluation of I-Walls, Engineering Technical Letter 1110-2-575, U.S. Army Corps of Engineers, Washington, D.C.
USACE (2011b ), National Inventory of Dams Fact Sheet, U.S. Army Corps of Engineers, Army Geospatial Center, Alexandria, VA.
USACE (2011c), Safety of Dams - Policy and Procedures, Engineering Regulation 1110 1156, U.S. Army Corps of Engineers, Washington, D.C.
USACE (2011d), Modeling, Mapping and Consequence Standard Operating Procedures (SOP [April 2011])USBR (1976), Design of Gravity Dams, U.S. Department of Interior, Bureau of Reclamation, Denver, CO.
USBR (1977a), Design of Arch Dams, U.S. Department of Interior, Bureau of Reclamation, Denver, CO.
USBR (1977b), Design Criteria for Concrete Arch and Gravity Dams, Engineering Monograph No. 19, U.S. Department of Interior, Bureau of Reclamation, Denver, CO.
11-4
USBR (1982), Guidelines for Defining Inundated Areas Downstream from Bureau of Reclamation Dams, Reclamation Planning Instruction No. 82-11, June 15, 1982, U.S. Department of Interior, U.S. Bureau of Reclamation, Denver, CO USBR (1983), Guidelines for Defining Inundated Areas Downstream from Bureau of Reclamation Dams, Reclamation Planning Instruction No. 83-05, April 6, 1983, U.S.
Department of Interior, U.S. Bureau of Reclamation, Denver, CO.
USBR (1987), Design of Small Dams, U.S. Department of Interior, Bureau of Reclamation, Denver, CO.
USBR (1988), ACER Technical Memorandum No 11: Downstream Hazard Classification Guidelines, U.S. Department of Interior, Bureau of Reclamation, Denver, CO.
USBR (1998), Prediction of Embankment Dam Breach Parameters - A Literature Review and Needs Assessment, Report DSO-98-004, U.S. Department of Interior, Bureau of Reclamation, Denver, CO.
USBR (2011), Dam Safety Risk Analysis Best Practices Training Manual, Version 2.2, U.S.
Department of Interior, Bureau of Reclamation, Denver, CO.
USCOLD (1999), Updated Guidelines for Selecting Seismic Parameters for Dam Projects, United States Committee on Large Dams, Denver, CO.
USDOI (2010;Draft), Inundation Mapping/Modeling Subproject Report, Implementation Phase 1: Launch Risk Reduction.
USGS (2008), Documentation for the 2008 Update of the United States National Seismic Hazard Maps, Open-File Report 2008-1128, U.S. Department of Interior, U.S.
Geological Survey, Reston, VA.
USNRC (1977), Design Basis Floods for Nuclear Power Plants, Regulatory Guide 1.59, Rev. 2, U.S. Nuclear Regulatory Commission, Washington, DC.
USNRC (2001), Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard- and Risk-consistent Ground Motion Spectra Guidelines, NUREG/CR-6728, U.S. Nuclear Regulatory Commission, Washington, DC.
USNRC (2003), Procedures and Criteria for Assessing Seismic Soil Liquifaction at Nuclear Power Plant Sites, Regulatory Guide 1.198, U.S. Nuclear Regulatory Commission, Washington, DC.
USNRC (2007a), Review of Safety Analysis Reports for Nuclear Power Plants, NUREG-0800, U.S. Nuclear Regulatory Commission, Washington, DC.
USNRC (2007b), A Performance-Based Approach to Define the Site Specific Earthquake Ground Motion, Regulatory Guide 1.208, U.S. Nuclear Regulatory Commission, Washington, DC.
USNRC (2011), Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, NUREG/CR-7046, Prepared by Pacific Northwest National Laboratory, U.S. Nuclear Regulatory Commission, Washington, DC.
USNRC (2012), Central and Eastern United States Seismic Source Characterization for Nuclear Facilities, NUREG-2115, U.S. Nuclear Regulatory Commission, Washington, DC.
USNRC (2012), Information Notice 2012-2: Potentially Nonconservative Screening Value for Dam Failure Frequency in Probabilistic Risk Assessments, (ML090510269) 11-5
Visser, K., Hanson, G.J., Temple, D., Neilsen, M. 2012. Earthern Embankment Overtopping Analysis using the WinDAM B Software. In: Dam Safety 2012. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 16-20, 2012, Denver, CO. CDROM.
USSD (2006), Erosion of Unlined Spillways, United States Society on Dams, Denver, CO.
Von Thun, J. L., and Gillette, D. R. (1990), Guidance on Breach Parameters unpublished internal memorandum, U.S. Department of Interior, Bureau of Reclamation. Denver, Colorado.
Wahl, T.L. (2004), Uncertainty of Predictions of Embankment Dam Breach Parameters, Journal of Hydraulic Engineering, 130(5): p. 389-397.
Wahl, T.L. (2010), Dam Breach Modeling - An Overview of Analysis Methods, Parameters, Joint Federal Interagency Conference on Sedimentation and Hydrologic Modeling, June 27 - July 1, Las Vegas, NV Wahl, T.L. (2013), Personal communication.
Wibowo, J.L., D.E. Yule, E. Villanueva, D.M. Temple, Earth and Rock Surface Spillway Erosion Risk Assessment, Alaska Rocks 2005, The 40th U.S. Symposium on Rock Mechanics (USRMS), June 25 - 29, 2005 , Anchorage, AK, American Rock Mechanics Association WSDE (2007), Dam Safety Guidelines Technical Note 1: Dam Break Inundation Analysis and Downstream Hazard Classification, Washington State Department of Ecology, Olympia, WA.
Wu, W. and S.S.Y. Wang (2010), An Introduction to the Latest Developments in Soil Erosion and Sediment Transport Modeling, in 11th International Symposium on River Sedimentation, September 6-9, 2010: Stellenbosch, South Africa.
Xiong, Y. (2011), A Dam Break Analysis Using HEC-RAS, Journal of Water Resource and Protection, 3: p. 370-379.
Xu, Y. and Zhang, L.M., 2009. Breaching parameters for earth and rockfill dams, Journal of Geotechnical and Geoenvironmental Engineering, vol. 135, no. 12, p. 1957-1970.
Yalin, S. M. (1977), Mechanics of Sediment Transport, Pergamon Press,.
Youngs, R. R., et al. (2003), A Methodology for Probabilistic Fault Displacement Hazard Analysis (PFDHA), Earthquake Spectra, 19(1), 191-219.
Youssef, Tahani Fahim (2013), Heading-Up Structures Web Site, http://osp.mans.edu.eg/tahany/dams1.htm, Accessed April 7, 2013.
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