ML20033F872
| ML20033F872 | |
| Person / Time | |
|---|---|
| Issue date: | 03/29/1990 |
| From: | Lohaus P NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Boyle R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-3 NUDOCS 9004040010 | |
| Download: ML20033F872 (6) | |
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W/V COMM TRANS u
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.J HAR 2 91990
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MEMORANDUM FOR: Rt.gis Boyle, Acting Chief Regulatory Branch Division of Low-Level Waste Management and Decommissioning, NMSS FROM:
Paul H. Lohaus, Chief Operations Branch Division of Low. Level Waste Management and Decommissioning, NMSS e
SUBJECT:
REVIEW 0F WEST VALLEY SITE CLOSURE DOCUMENTS In accordance with your recent request, Ted Johnson has conpleted a review of 1
surface water hydrology and erosion protection aspects of the subject documents. Our consnents are enclosed.
In general, the site closure design will be based on data gathered during
'various activities which characterize the site, as discussed in the Site i Characterization Plan (SCP). Our review of this dccument indicates that
historic erosion data will be collected and will then de relied upon to design erosion features at the site following closuto.
We firA this approach to have questionable nerit, since historic data is not utten indicative of ernsion problems which could occur in the future. Our ccmaann re:1ect this lack of confidence in historic data and provide suggee,tions for augmenting the site j'
characterization, in accordance with the long-term stability requirements of M'
10.CFR 61.
(SIGNE0} PAULH.LOHAUS Paul H. Lohaus, Chief Operations Branch 4
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Division of Low-Level Waste Management
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and Decommissioning, NMSS
Enclosure:
As stated Distribution: "Centrel;Eile;C20123%
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NO SUBJECT AB ACT: REVIEW E3T TALLEY SITE CLOSUF L'UMENTS
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1 MEMORANDUf', FOR: Regis Boyle, Ar. ting Chief Regulatory Branch Division of Low-Level Waste Management L
and Decommissioning, NMSS i
FROM:
Paul H. Lohaus, Chief i
Operations Branch Division of Low-Level Waste Management and Decommissioning, NHSS s
SUBJkCT:
REVIEW OF WEST VALLEY SITE CLOSURE DOCUMENTS N
r In accordance with your recent request, Ted Johnson has com)1eted a review of surface water hydrology and erosion protection aspects of tie subject documents.
Our conments are enclosed.
Ingeneral,theliteclosuredesignwillbebasedondatagatheredduring various activities'which characterize the site, as discussed in the Site CharacterizationPlag(SCP). Our review of this dccument inoicates that historic erosion data will be collected and will then be relied upon to design erosion features at the site following closure. We find this approach to have s
questionable merit, since, historic data is not often indicative of erosion problems which could occur in the future. Our consents reflect this lack of confidenceinhistoricdatkandprovidesuggestionsforaugmentingthesite characterization, in accordance with the long-term stability requirenents of i
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1 Paul H. Lohaus, Chief perations Branch D vision of Low-Level Waste Managenent d Decommissioning, NMSS
Enclosure:
M stated Distribution:
Central File f 201.3 NMSS r/f RBangart JGreeves RBoyle Surmeier PLohaus TJohnson MFliegel 4, gg /,g JJones PDR YES U.
N PDR NO
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SUBJECT AB ACT: REVIEW E M ALLEY SITE CLOSURE D ENTS l
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DATE:03//7/90
- 03/ /90
- 03/ /90
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OFFICIAL RECORD COPY
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y REVIEW Of WEST VALLEY SITE CLOSURE DOCUMENTS SURFACE WATER HYDROLOGY AND EROSION PROTECTION COMMENTS AND QUESTIONS Document
Title:
Site Characterization Plan, Phase 11 1.
Pacs 2-1. Section 2.1. Stud.y: Characterization of Erosion Poteatial on Streams which Drain the Site.
The Site Characterization Plan (SCP) indicates that the focus of the erosion study will be on defining the mechanism and rate of erosion on the various small streams which are located adjacent to the disposal areas. The study will attempt to use historical erosion data to detemine if mitirative designs are necessary to protect the site from long-term erosion problems.
< While historical erosion data are interesting and sometimes useful, such data have substantial limitations. The use of 30-50 years of erosion data, for example, to precict site p 300-500 years (in accordancewith10CFR61)erformanceforaperiodofmay result in the under-design of prot structures, unless very large floods have occurred and the rate of erotion for such large floods can be determined.
The questionable value of historic data is further evident when one examines t
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, the causative mechanisms of erosion. Typically, a slope, channel, or stream bank will be stable until a threshold value of velacity or tractive force is surpassed.
If this threshold value has not been exceeded during the period in which the data are gathered, one could be misled into thinking that stability l
1s a certainty, when, in fact, it is not.
Any long-term stability study should also consider that the designs selected must remain effective for hundreds of ;, ears following site closure, without reliance on routine maintenance. The study should thus consider various phenomena and failure modes which could occur over these long time periods, including gully erosion, sheet erosion, wind erosion, differential setlement creating preferential flow paths for drainage, headward gully advancement and l
lowering of base levels.
Indesigningagainstthesephenomena,theuseof models which are calibrated using historic data may not be adequate, due to the lack of a sufficient data base. Procedures that attempt to expand the data base such as age dating of flood terraces also have limitations, since it is I
generally not enough to know that the most recent large flood, for example, occurred 435 years ago. This type of information may be interesting but is of l
limited value when considering floco threats to the site. Only data with l
quantifiable hydrologic parameters, such as flow rate, rather than qualitative data, such as "large flood" can be used in estimating flood or erosion potential in a meaningful way.
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.W/V COMM i
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'Vsing historic erosion or flood data has severe limitations in selecting an appropriate design event. For example, if a small probability of occurrence of 5
0.10 is desired, the design flood for a period of 300 years would have a recurrence interval of 3000 years or more.
It is not likely that a 3000-year
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flood can be determined utilizing only 30-50 years of flood data. Similarly, using 30-50 years of erosion data may not be adeauate to design for a period of t
L 300-500. years with a very small probability of failure.
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f Our experience with long-term stability applications in the area of uranium.
mill tailings leads us'to conclude that the site design should be based on preventing the initiation of erosion by employing a stable design configuration. While we also believe that the data on historic erosion i
rates will provide some useful information, the data will need to be I
supplemented with a considerable amount of additional data. Specific comments addressing these data needs are given in Connents 2-5, below.
2.
Page 2-2. Section.2.1.1. Activit.y: Distribution.and Characteristics of Past and Present Eroston As discussed in Comment 1, above, the-usefulness of limited historic data for evaluating long term performance can be limited. Additional efforts should be devoted to determining the threshold values of velocity, tractive force, or flood discharge that a particular slope or channel has withstood in the past.
This value will vary across the site depending on stream and slope characteristics such as particle size, allowable tractive force, cohesion, vegetation density, etc.
(We note that that threshold value determinations are proposed to be developed as part of the activities discussed in Section 2.1.3 for use in evaluating current erosion.)
Based on staff experience with design of stable slopes and channels, the long-term stability design will be controlled by the capability of the site to withstand the effects of a single large flood event, such as the Probable Maximum Flood (PMF), for example.
It may be prudent to direct data-gathering efforts to determine the capability of the site to withstand such a large event. The determination of site stability can then be supplemented with the historic data gathered in this task.
3.
Page.2-5. Section 2.1.2..Activit.y: Flood Routing Calculation It is noted that the principal objective of this activity is to determine the discharges associated with floods of various magnitudes in the site vicinity.
As part of this activity, the staff suggests that the velocities and shear stresses produced by a range of floods also be determined. This will aid in determining the need for flood protection measures.
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The staff also emphasizes that the water levels and velocities associated with major floods will be significantly affected by the flood protection that is
- ultimately implemented at the site. For example, If.a diversion channel becomes part of the design for diverting floods away from waste disposal areas, it is likely that the diversion channel could cause significant changes to flood velocities and levels at various places in the site area. Therefore, the flood analyses proposed as part of this task may be of little value for final design purposes.
It may be prudent to cevelop a conceptual design at this point to determine the effects of flooding on a flood-protected site.
In short, it may be prudent to perform fewer flooding analyses for the site as it currently exists and more analyses for the site as it will exist in its post-closure state, based on at least a preliminary conceptual desigr..
It is also important to note that.the final design configuration could cause increases in flood velocities or flood levels which will need to be accounted for in assessing erosion rates imediately downstream of a diversion channel, for example..
3.
Page 2-7. Section 2.1.3. Activity: Monitoring of Current Erosion As discussed in Comment 1, the staff questions the value,of using current erosion rates to determine designs for site stability for long periods of tine.
However, the activities proposed in this task to determine threshold values of precipitation and discharge levels may be useful, particularly if a flood of 3
sufficient magnitude occurs and causes erosion. The staff suggests thet the L
proposed data.be supplemented with threshold values of stream velocity and tractive force, since the major design parameters are usually not flood levels.
4.
Pase 2-9, Section 2.1.4. Activity: Characterize Stability of Stream Bank anc Bed As discussed above, the characterization of stream materials should also include a determination of the allowable velocity or tractive force. These parameters are usually a function of the soil particle size, cohesion or clay content, and vegetation cover.
l We note that the suitability of using concrete as erosion protection will be l-evaluated (Section4.2.1). Based on staff experience with long-term stability l
designs, it is unlikely that concrete can be used as erosion. protection for l
long periods of time without maintenance (as required by 10 CFR Part 61).
Unless a type of concrete can be developed to withstand chemical and physical weathering for periods of 300-500 years, without the use of maintenance to repair and replace it, the staff suggests that rock sources in the area be L
1.nvestigated.
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Additional work under this activity should include a determination of the l
availability and quality of rock riprap to be used as erosion protection.
If no suitable rock exists onsite, a search should be conducted f or acceptable sources. NRC criteria currently exists for quantitativa1r cetermining the quality of rock riprap.
5.
Pace 2-12. Section 2.1.5. Activity: Develop. Erosion Model It is noted that models will be developed to determine erosion rates as a function of storm magnitude. However, it should be pointed out that other erosion models, such as the Universal Soil Lo s Equation, have been used to estimate soil losses unoer a wide range of conditions; it has been determinad that such models are of limited value when trying to predict soil losses in channels and gullies over a long period of time. The value of such models nay also be questionable where supercritical flow occurs. Models such as HEC-6, for example, may be of limited value when trying to predict the long term performance of steep stream channels for a long period of time and for a wide range of storm and flood events.
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