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NRC-2017-000688 (Formerly FOIA/PA-2017-0690) - Resp 5 - Final, Agency Records Subject to the Request Are Enclosed, Part 13 of 15
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B. L. "Pete" Ivey Southern Nuclear Vice President Operating Company, Inc.

Regulatory Affairs 40 Inverness Center Par1<way Post Office Box 1295 Birmingham. AL 35242 Tel 205.992.7619 Fax 205 992 5217 SOUTHERN.\.

COMPANY March 5, 2013 Docket Nos.: 50-42'4 NL-13-0117 50-42'5 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D. C. 20555-0001 Vogtle Electric Generating Plant - Units 1 and 2 Flooding Recommendation 2.1 Hazard Reevaluation Report Requested by NRC Letter, 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 fr9m the Fukushima Daiichi Accident, dated March 12, 2012

Reference:

1. NRC Letter, 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 Daiichi Accident. dated March 12, 2012

2. NRC Letter, Prioritization of Response Due Dates for Request for Information to Title 10 of the Code of Federal Regulations 50.54(f)

Regarding Flooding Hazard Reevaluations for Recommendation 2.1 of the Near-Term Task Force Review of Insights from the Fukushima Daiichi Accident, dated May 11, 2012 Ladies and Gentlemen:

On March 12, 2012, the U. S. Nuclear Regulatory Commission (NRC) issued Reference 1 to all power reactor licensees and holders of construction permits in active or deferred status. Enclosure 2 of Reference 1 requests licensees to perform a reevaluation of all appropriate external flooding sources, including the effects from local intense precipitation on the site, probable maximum flood on stream and rivers, storm surges, seiches, tsunamii, and dam failures. The NRC requested information for the following purposes:

To gather information with respect to Near-Term Task Force Recommendation 2.1, as amended by staff requirements memoranda associated with SECY-11-0124 and SECY-11-0137, and the Consolidated

U.S. Nuclear Regulatory Commission NL-13-0117 Page 2 Appropriations Act, for 2012 (Pub Law 112-74), Section 402, to reevaluate seismic and flooding hazards at operating reactor sites

To collect information to facilitate NRC's determination if there is a need to update the design basis and systems, structures, and components important to safety to protect against the updated hazards at operating reactor sites

To collect information to address Generic Issue 204 regarding flooding of nuclear power plant sites following upstream dam failures Enclosure 2 of Reference 1 requires each addressee to submit the Hazard Reevaluation Report within one to three years from the date of the information request. Enclosure 1 of Reference 2 assigned Vogtle Electric Generating Plant (VEGP) Units 1 and 2 as a Category 1 site, meaning VEGP is required to submit its flooding hazard reevaluation report within one year. The Flooding Hazard Reevaluation Report for VEGP Units 1 and 2 is provided in the enclosure to th1is letter, as required by Reference 1.

This completes the flooding hazard reevaluation for VEGP Units 1 and 2 as required by Reference 1. If an integrated assessment is requested by the NRC as specified in Reference 1, it will be complete in 2015 or per the priority determined by the NRG after all ireevaluations requested by Reference 1 are complete and

. submitted by Reference 1 recipients.

This letter contains no new regulatory commitments. If you have any questions, please contact Ken McElroy at {205) 992-7369.

Mr. 8 . L. Ivey states he is a Vice President of Southern Nuclear Operating Company, is authorized to execute this oath on behalf of Southern Nuclear Operating Company and, to the best of his knowledge and belief, the facts set forth in this letter are true.

Respectfully submitted, 8 . L. Ivey Vice President - Regulatory Affairs BLI/CLN S-1-lt 11, I Sworn to and subscribed before me this _ _ £ttl=f'i_c._'~11~~, 2013.

day of _ ---'/-=

~ l,IC/- ~~ ~v NBtary Public My commission expires: !vrardt 1,,3t UJ I 1/

U. S. Nuclear Regulatory Commission NL-13-0117 Page 3

Enclosure:

Vogtle Electric Generating Plant Units 1 and 2 Flooding Hazard Reevaluation Report cc: Southern Nuclear Operating Company Mr. S. E. Kuczynski, Chairman , President & CEO Mr. D. G. Bost, Executive Vice President & Chief Nuclear Officer Mr. T. E. Tynan, Vice President - Vogtle Mr. B. J. Adams, Vice President - Fleet Operations Mr. C. R. Pierce, Director - Regulatory Affairs RType: CVC7000 U. S. Nuclear Regulatory Commission Mr. V. M. Mccree, Regional Administrator Mr. R. E. Martin, NRA Senior Project Manager - Vogtle Mr. L. M. Cain, Senior Resident Inspector - Vogtle Mr. G. E. Miller, NRA Project Manager-JLD/JPMB State of Georgia Mr. J. H. Turner, Environmental Director Protection Division

Vogtle Electric Generating Plant - Units 1 and 2 Flooding Recommendation 2.1 Hazard Reevaluation Report Requested by NRC Letter, Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(1) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Daiichi Accident, dated March 121 2012 Enclosure Vogtle Electric Generating Plant Units 1 and 2 Flooding Hazard

Reevaluation Report

SOUTHERN COMPANY A.

Vogtle Electric Generating Plant Units 1 & 2 Flooding Hazard Reevaluation Report Response to Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendation 2.1:

Flooding of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident Report Number: - - SNC453475 Revision: 0

Vogtle Electric Generating Plant, Units 1 & 2 50.54{f) Enclosure 2 Flooding Hazard Reevaluation Report Report Number: SNC453475 Rev. 0 Flooding Reevaluation Report Team Signature Date Mahmood Nagash o4 Narr.Ii 1.013 Bechtel Power Preparer Craig Talbot Bechtel Power Verifier SNC Technical Manager Uav; .J. LAJkf f3/4~

SNC Project Manager This report provides information in response to NRC's March 12, 2012, 10 CFR 50.54(f) letter requesting that nuclear power plant licensees assess plant vulnerability beyond design basis floods pursuant to the recommendations in NRC's Near-Term Task Force review of the accident at the Fukushima Dai-ichi nuclear facility.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 TABLE OF CONTENTS 0.0 Introduction ...................................................................................................................................... 6 1.0 Site Information Related to the Flood Hazard .................................................................................. 6 1.1 Detailed Site Information ............................................................................................................... 6 1.1 .1 Settlement of VEGP 1 & 2 Power Block Structures .............................................................. 7 1.1 .2 Recalculated Flood Heights .................................................................................................. 8 1.2 Current Design Basis Flood Elevations ...................... .................................................................. 8 1.2.1 Local Intense Precipitation .................................................................................................... 9 1.2.2 Flooding in Streams and Rivers .......................................................................................... 19 1.2.3 Dam Failures ....................................................................................................................... 20 1.2.4 Storm Surge and Seiche ..................................................................................................... 20 1.2.5 Tsunami ............ ................................................................................................................... 20 1.2.6 Ice Induced Flooding ........................................................................................................... 20 1.2.7 Channel Diversion ............................................................................................................... 20 1.2.8 Combined Effects ................................................................................................................ 20 1.3 Licensing Basis Flood-Related and Flood Protection Changes .................................................. 20 1.4 Watershed and Local Area Changes .......................................................................................... 20 1.5 Current Licensing Basis Flood Protection and Mitigation Features ............................................ 21 1.6 Additional Site Details ................................................................................................................. 21 2.0 Flooding Hazard Reevaluation ....................................................................................................... 21 2.1 Local Intense Precipitation .......................................... ................................................................ 21 2.2 Flooding in Streams and Rivers .................................................................................................. 22 2.2.1 Probable Maximum Flood (VEGP 3 & 4 UFSAR Section 2.4.3) ......................................... 22 2.2.2 Review of Studies for Units 1 and 2 (VEGP 3 & 4 UFSAR Section 2.4.3.1) ...................... 23 2.2.3 Savannah River Watershed Hydrologic Model (VEGP 3 & 4 UFSAR Section 2.4 .3.1.1 ) ... 24 2.2.4 Dynamic Hydraulic Model Check on Hydrologic Model Results (VEGP 3 & 4 UFSAR Section 2.4.3.1.2) ................................................................................................................ 24 2.2.5 Estimation of PMF by Approximate Methods (VEGP 3 & 4 UFSAR Section 2.4.3.2) ........ 24 2.2.6 Estimation of Flood Stage at VEGP Site for PMF (VEGP 3 & 4 UFSAR Section 2.4.3.3) ......... 25 2.2.7 Conclusions (VEGP 3 & 4 UFSAR Section 2.4.3.4) ........................................................... 26 2.3 Dam Failures ............................................................................................................................... 26 2.3.1 Potential Dam Failure (VEGP 3 & 4 UFSAR Section 2.4.4) ................ ,.............................. 26 2.3.2 Dam Failure Permutations (VEGP 3 & 4 UFSAR Section 2.4.4.1) ..................................... 27 2.3.3 Unsteady Flow Analysis of Potential Dam Failures (VEGP 3 & 4 UFSAR Section 2.4.4.2) ...... 28 2.3.4 Water Level at the Plant Site (VEGP 3 & 4 UFSAR Section 2.4.4.3) ................................. 33 2.4 Storm Surge and Seiche ............................................................................................................. 34 2.4.1 Probable Maximum Storm Surge and Seiche Flooding (VEGP 3 & 4 UFSAR Section 2.4.5) ... 34 3 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 2.5 Tsunami. ......................................................................................................................................35 2.5.1 Probable Maximum Tsunami Flooding (VEGP 3 & 4 UFSAR Section 2.4.6) ..................... 35 2.6 Ice Induced Flooding ................................................................................................................... 35 2.6.1 Ice Effects (VEGP 3 & 4 UFSAR Section 2.4.7) ................................................................. 36 2.6.2 Ice Jam Events (VEGP 3 & 4 UFSAR Section 2.4.7.2) ...................................................... 36 2.7 Channel Diversions ..................................................................................................................... 36 2.7.1 Channel Diversions (VEGP 3 & 4 UFSAR Section 2.4.9) ................................................... 37 2.8 Combined Effects ........................................................................................................................ 38 3.0 Comparison of Current Design Basis and Reevaluated Flood Causing Mechanisms .................. 38 3.1 Local Intense Precipitation .......................................................................................................... 38 3.2 Probable Maximum Flooding ...................................................................................................... 38 3.3 Dam Failures ............................................................................................................................... 39 3.4 Storm Surge and Seiche ............................................................................................................. 39 3.5 Tsunami. ................... ................................................................................................................... 39 3.6 Ice Induced Flooding ................................................................................................................... 39

3. 7 Channel Diversion .... ................................................................................................................... 39 3.8 Combined Effects ..... ................................................................................................................... 39 4.0 Interim Evaluation and Actions Taken or Planned ......................................................................... 39 5.0 Additional Actions ........................................................................................................................... 39 6.0 References ..................................................................................................................................... 40 4 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 LIST OF TABLES Table 1- 1. Current Design Basis Flood Elevations ............................................. ......................... ................. 9 T able 1-2. Stage-Volume Relationship for VEGP 1 & 2 Area inside the VBS ............................................ 11 Table 1-3. Elevation-Discharge Rating Curves for the VEGP 1 & 2 Perimeter VBS .... .. ............................ 12 Table 1-4. Probable Maximum Precipitation for VEGP 1 & 2 ..................................................................... 13 Table 1-5. PMP Rainfall Depth Used as Input for Frequency Storm HEC-HMS Model ........... .. ................ 14 Table 1-6. SCS Hydrologic Parameters for VEGP 1 & 2 HEC-HMS Model ............................................... 16 Table 1-7. VEGP 3 & 4 Sub-basin HEC-HMS Model Parameters .............................................................. 17 Table 1-8. PMP Sub-basin Flow Hydrograph Input to HEC-RAS ............................................................... 19 Table 3-1. Current Design Basis and Reevaluation Flood Elevations ........................................................ 38 LIST OF FIGURES Figure 1- 1. Stage-Discharge Curves for VEGP 1 & 2 Perimeter VBS ....................................... .. ............... 13 Figure 1-2. PMP Depth and Duration for Point Rainfall .............................................................................. 14 Figure 1-3. PMP Hyetograph Determined in Design Basis PMP ...................... .. ........................................ 15 Figure 1-4. HEC-HMS Schematic for Units 1 & 2 PMP Simulation ............................................................ 15 Figure 1-5. VEGP 3 & 4 Drainage Basin Model Used in HEC- HMS and HEC-RAS .................................. 17 Figure 1-6. Flood Routing Model Used in HEC-RAS .................................................................................. 18 Figure 2- 1. PMP Hyetograph for Reevaluation Rainfall Event and per NUREG/CR-7046 ......................... 22 LIST OF ATTACHMENTS , VEGP 3 & 4 UFSAR Tables, 12 pages , VEGP 3 & 4 UFSAR Figures, 21 pages 5 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 0.0 Introduction Following the accident at the Fukushima Dai-ichi nuclear power plant resulting from the 2011 Great Tohoku Earthquake and tsunami, the Nuclear Regulatory Commission (NRC) established the Near-Term Task Force (NTTF) aind tasked it with conducting a systematic and methodical review of NRC processes and regulations to determine whether improvements are necessary.

The resulting NTTF report concludes that continued U.S. nuclear plant operation does not pose an imminent risk to public health and safety and provides a set of recommendations to the NRC. The NRC directed the staff to determine which recommendations should be implemented without unnecessary delay (Staff Requirements Memorandum [SRM] on SECY-1 1-0093).

The NRC issued its request for information pursuant to 10 CFR 50.54(f) on March 12, 2012, based on the following NTTF flood-related recommendations:

Recommendation 2.1 : Flooding

Recommendation 2.3: Flooding Enclosure 2 of the NRC 50.54(f) letter addresses Recommendation 2.1 for the following purposes:

1. To gather information with respect to NTTF Recommendation 2.1, as amended by SRM associated with SECY-11-0124 and SECY-11-0137, and the Consolidated Appropriations Act, for 2012 (Pub Law 112-74), Section 402, to reevaluate seismic and flooding hazards at operating reactor sites.

2. To collect information to facilitate NRC's determination if there is a need to update the design basis and systems, structures, and components (SSCs) important to safety to protect against the updated hazards at operating reactor sites.

3. To collect information to address Generic Issue 204 regarding flooding of nuclear power plant sites following upstream dam failures.

This report was prepared for the Vogtle Electric Generating Plant Units 1 & 2 (VEGP 1 & 2) in response to NTTF Recommendation 2.1 only.

1.0 Site Information Related to the Flood Hazard 1.1 Detailed Site Information The VEGP site consists of 3,169 acres located on a coastal plain bluff on the southwest side of the Savannah River in eastern Burke County, Georgia. The plant is directly across the river from the Department of Energy's Savannah River site (Barnwell County, South Carolina). The VEGP site is approximately 26 mi south-southeast of Augusta, Georgia, and approximately 15 mi east-northeast of Waynesboro, Georgia. It is approximately 100 mi north-northwest of Savannah, Georgia, approximately 150 river mi. from the mouth of the Savannah River. The contributing drainage area of the Savannah River at the site is 8,304 sq mi as estimated from digital mapping (Section 2.4.1.1 of Reference 2).

The Savannah River basin and its sub-basins, as delineated by the National Weather Service (NWS) and further subdivided into U.S. Geological Survey (USGS) Hydrologic Unit Code (HUC-

12) Watershed Boundary Dataset, are shown in VEGP Units 3 & 4 (VEGP 3 & 4) Updated Final Safety Analysis Report (UFSAR) Figure 2.4-204 (Reference 2) and included in this report in Attachment 2. The VEGP 3 & 4 data are used because they are the most recent data and valid 6 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 information. The drainage areas of the NWS sub-basins are given in VEGP 3 & 4 UFSAR Table 2.4-208 (Reference 2) and included in this report in Attachment 1.

VEGP 1 & 2 are Westinghouse pressurized water reactors that began commercial operation in May 1987 and May 1989, respectively. VEGP 3 & 4 are Westinghouse AP1000 reactors and will be located west of and adjacent to the existing VEGP 1 & 2 as shown in VEGP 3 & 4 UFSAR Figure 1.1-202 (Reference 2) and included in Attachment 2 of this report. The combined license (COL) for VEGP 3 & 4 was issued by the NRC on February 10, 2012.

The VEGP site is a dry site, which is not subject to flooding from the nearby streams and the Savannah River (including postulated dam break scenarios). The normal water elevation of the Savannah River is approximately 80 ft from mean sea level (msl).

During the construction of VEGP 1 & 2, the VEGP 3 & 4 site area was included in the grading plan . The VEGP 3 & 4 area was rough-graded to the plan, which included a drainage ditch south-southwest of the VEGP 3 & 4 area, designed to accommodate the runoff from the 100-year storm. This ditch also functions as the local probable maximum precipitation (PMP) drainage path for the VEGP 3 & 4 area during storms and a postulated PMP event.

The site grade elevation for all safety-related structures of all four units is located at nominal elevation 220 ft mean sea level (msl) (Section 2.4.1.1 of Reference 1 and Section 2.4.1.1 of Reference 2).

1.1.1 Settlement of VEGP 1 & 2 Power Block Structures Section 2.5.4.10 of the VEGP 1 & 2 FSAR (Reference 1) addresses the predicted heave and settlement of the VEGP 1 & 2 power block area during the initial excavation through the construction of the facility. A heave-and-settlement monitoring program existed during the entire construction and has been maintained during the operational period as described in Section 2.5.4.13 of the VEGP 1 & 2 FSAR (Reference 1). Currently, the settlement monitoring program data are collected and reported to the NRC at a reduced frequency (annually) because settlement of the structures has essentially ceased. During the excavation period, the heave at selected depths below the excavation resulting from the removal of the overburden was recorded. From the measured heave data, corrected for depth and loading effects, an average heave of approximately 1.8 in. (Appendix B of Reference 11) was determined for the excavation floor in the power block area.

Measured settlements attributed to backfilling of the excavations are not reported in the FSAR. VEGP 1 & 2 FSAR Figure 2.5.4-1 (Reference 1) presents estimated settlements for power block structures with a maximum settlement of 4.2 in (0.35 ft). The maximum measured total settlement at the power block structures reported in Bechtel's VEGP Report on Settlement, August 1986 (Reference 9), was 3.6 in. (0.3 ft). These values, along with the calculated differential settlements, are within the allowable limits.

The soil column between the bedrock and the blue bluff marl (BBM), the competent foundation layer, is approximately 1,000 ft deep. The power block excavation extends down, about 90 ft, to the BBM. The BBM is typically from 60 ft to 70 ft thick. Some of the power block structures are founded on and in the BBM , and some are foundedl in the backfill. Depending on the foundation loading and other factors, the measured settlements vary within the expected range.

As a result of the settlement, the nominal design e levations (NOE) are not the actual elevations of the structures. As part of the preparation of this flooding hazard reevaluation report (FHRR), a survey was performed to conf irm the actual elevations of floors at the NOE of elevation 220 ft msl. The minimum elevation reported in the recent survey was 7 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 elevation 219.6 ft msl (Reference 12). For the purposes of this FHRR, the NOE of 220 ft msl, the elevation of the grade level structures, is established as elevation 219 .6 ft msl, which is the lowest of any of the settlements in the FSAR and the results of the recent survey.

1.1 .2 Recalculated Flood Heights During the preparation of this FHRR and the VEGP 1 & 2 Flooding Walkdown Report (FWR) (Reference 10) in accordance with NTTF Recommendation 2.3, the PMP calculations were revisited , and the need for several updates was identified after the completion of the walkdowns at the VEGP Site but before the submittal of the FWR report to the NRC on November 27, 2012. The following activities were undertaken:

The then-current PMP baseline calculation was determined to have a flood height (elevation 2 19.6 ft msl) that was calculated in an excessively conservative manner when settlement was included in the evaluation of available physical margin (APM).

APM is defined as the difference between t he elevation of the openings (elevation 219.6 ft msl) into the structures and the calculated flood height. Because t here was no margin available with the then-current analysis, the calculation was revisited and revised to include the dynamic model of the Units 3 and 4 areas to provide a more realistic flood flow behavior across the western side of the VEGP 1 & 2 vehicle barrier system (VBS).

The preliminary estimate was that the flood height would be reduced by at least 0.4 ft to elevation 219.2 ft msl (a reduction from elevation 219.6 ft msl, which is t he same value as the worst case settlement). The FWR and the flooding walkdown packages (FWP) were revised to indicate that the APMs remained as documented in the FWP because both elevations had been reduced by the same amount, 0.4 ft.

Before the FWR was submitted to the NRC, the design basis PMP calculation was completed , and the calculated flood height was finalized at elevation 219. 1 ft msl (Reference 13). Because the flood depth was less than the estimated flood height elevation o f 219.2 ft msl, the FWR and the FWP were not revised to reflect the flood height reduction of 0.1 ft. Section Ill of the FWR does report the finalized increase in flood height of 0.2 ft, calculated for higher 1-hour rainfall intensity, above the finalized design basis flood height of elevation 219. 1 ft msl. This FHRR reports the finalized design basis flood height of e levation 219.1 ft msl and revaluated flood elevation 219.3 ft msl for beyond design basis rainfall intensity (Reference 14). The elevation 219.1 ft msl flood height provides 0.5 ft of margin between the design basis flood height and the lowest elevation of the openings into the structures.

The FWR provides the results of the updated rainfall event, which was used in the VEGP 3 & 4 COL/UFSAR (19.2 in. during the worst case 1-hour rainfall period). The FWR indicates that the revised rainfall event increases the flood height by 0.2 ft.

Section 2.1 of this FHRR provides the calculated flood height for the updated rainfall event as elevation 219.3 ft msl. This flood height provides 0.3 ft of margin between the reevaluation flood height and the lowest elevation of the openings into the structures.

1.2 Current Design Basis F lood Elevations The current design basis flood elevations from various flood causing mechanisms (except the local intense precipitation flooding) are listed in Table 1-1 below and are taken from Section 2.4 of the VEGP 1 & 2 Final Safety Analysis Report (FSAR) (Reference 1).

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 Table 1-1. Current Design Basis Flood Elevations Flood Flood Elevation Flood Causing Elevation coincident with Wind Source Mechanism (msl) Wave (msl)

Local Intense (1) N/A (1)

Precipitation Flooding in Reference 1 Sections Streams and 138 ft 165 ft 2.4.3.4 and 2.4.3.6 Rivers Upstream Dam Reference 1 Sections 141 ft 168 ft Failures 2.4.4.2 and 2.4.4.3 Storm Surge and N/A N/A Reference 1 Section 2.4.5 Seiche Tsunami N/A N/A Reference 1 Section 2.4.6 Ice Induced N/A N/A Reference 1 Section 2.4.7 Floodino Channel N/A N/A Reference 1 Section 2.4.9 Diversion (1) Section 2.4.2.3 of Reference 1 provides the s1mpllf1ed methodology for determining the PMP flood height but it does not report the vallue. Section 1.2.1 provides the current PMP analysis methodology and results.

1.2.1 Local Intense Precipitation The PMP for VEGP 1 & 2 was analyzed for use in this report for a double row VBS around the perimeter, considering the VEGP 3 & 4 construction phase period. The PMP values (shown in Table 1-4) were obtained from VEGP 1 & 2 FSAR (Reference 1) and are the current design basis. These rainfall data , which have a maximum intensity of 15 in. of rainfall for a 1-hour duration, were used to develop the hyetograph for simulating the flood event and determining the maximum runoff flows. Runoff is directed away from the plant and over the surrounding areas by a sloped ground surface that leads to natural drainage channels and subsequently to the Savannah River. The VEGP 1 & 2 Site Rough Grading and Drainage Plan is shown in Drawing CX2D46V003 of Reference 1, and the VEGP 1 & 2 Site Drainage Plan for PMP Storm is shown in Drawing CX2D46V004 of Reference 1. It was assumed that all catch basins and culverts are plugged for the PMP flood analysis and that all flow is overland. The overland drainage system for the site precludes flooding of safety-related SSCs.

After the construction phase for VEGP 3 & 4 was started, an additional perimeter VBS row was added and security features were implemented for VEGP 1 & 2, the effects of PMP on site drainage were again evaluated. The evaluation considered the double row VBS and a worst case configuration for the VEGP 3 & 4 construction phase. From a PMP analysis standpoint, the construction phase of VEGP 3 & 4 is more onerous than the operational phase of VEGP 3 & 4 because of the features addressed in Section 5.0 of this FHRR. The VBS rows were added at different times so that currently there are two rows of VBSs surrounding the VEGP 1 & 2 power block. The newer VBS row is mostly located outside the older VBS row but not at all locations.

Water levels in the VEGP 1 & 2 power block area during the PMP storm event were modeled using the U.S. Army Corps of Engineers (USAGE) computer program HEC-HMS (Hydrologic Engineering Center-Hydrologic Modeling System) (Reference 8) using lumped-parameter reservoir routing. The area within the perimeter of the double row VBS is modeled as a lumped-parameter reservoir, i.e., it is represented by one stage-volume curve and one stage-discharge curve.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 The stage-volume relation for the Units 1 & 2 power block area was developed based on the contours of topographic mapping of the site, augmented by spot elevations obtained by field survey. Table 1-2 below shows the volume of water stored inside the VBS from elevation 214 ft msl to elevation 220 ft msl.

The drainage features of VEGP 1 & 2 power block area are as follows:

The power block structures generally have a first floor NOE of 220 ft msl. The power block is surrounded by a perimeter road system at elevation 218.5 ft msl (inside the protected area (PA) fence). Access to all of the power block structures is provided from this perimeter road. The railroad system, which is also at elevation 218.5 ft msl, is generally outside the PA fence except for where it enters the turbine buillding and auxiliary building.

The VBS is outside the railroad tracks on the north and south sides and inside the railroad tracks on the west. On the east side, the VBS is partially inside and partially outside the railroad tracks. The west and s-outh sides of the power block include drainage ditches, which are crossed by the railroad and VBS. The culverts under these crossings are generally blocked for the PMP evaluation to increase the flood depth , except for the culvert at the southwest corner, which allows water to enter into the power bock area.

Beyond the crossings on the west, the drainage ditch is the 100-year flood conveyance. For this PMP analysis, a temporary diversion ditch was used to replace a section of this ditch affected by the VEGP 3 & 4 construction activities. In addition, a culvert crossing on the 100-year drainage ditch is conservatively assumed to be blocked in the PMP model, which increases the flood depth (see Figure 1-6).

To the north and east of the VEGP 1 & 2 power block, the overland drainage system grade elevations are well below the railroad tracks and VBS crest elevations and consequently are not included in the PMP model. West of the power block, the VEGP 3 & 4 area is shown to pond during the PMIP event, due to construction activities, and VEGP 3 & 4 flood waters flow over the VBS and into the VEGP 1 & 2 power block area. On the south side of the VEGP 1 & 2 power block, the terrain grade elevations increases, and only a small drainage area contributes runoff to the ponded area.

The area inside the perimeter roads is below elevation 218.5 ft msl, and the entire area is served by catch basins and storm drain piping that discharge into the 100-year drainage ditches. The finished grade elevation at the VBS is generally well below elevation 218.5 ft msl , and the yard e levation varies along its 6,600-ft-long length based on the pre-existing grading and drainage provisions on either side of the VBS. For PMP modeling, it is assumed conservatively that all storm drains are blocked, and the only means of discharge to the outside drainage is flow over the VBS.

The accumulation of water inside the VBS normally does not occur except for the hypothetical PMP evaluation where all culverts and roof down-spouts, which enter below grade drainage pipes, are assumed to be 100 percent blocked. Using the PMP analysis assumptions, the area inside the power block perimeter road ponds until the water level reaches elevation 218.5 ft msl. Similarly, the area between the perimeter road and the VBS ponds, as determined by the PMP model. Once elevation 218.5 ft msl is reached on the inside and outside of the perimeter road, the entire area inside the VBS ponds as a single free-surface area to the top of the VBS.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 Table 1-2. Stage-Volume Relationship for VEGIP 1 & 2 Area inside the VBS Elevation Volume (ft, msl) (ac-ft) 214.00 1.287 214.50 1.896 215.00 3.401 215.50 6.197 216.00 10.27'9 216.50 17.018 217.00 24.770 217.50 33.72'9 218.00 44.681 218.50 61.348 219.00 82.827 219.50 107.596 220.00 135.610 A stage-discharge curve was developed for VEGP 1 & 2 based on two rows of VBSs placed along the perimeter of the power block area with consideration of a detailed plan and profiles of the double row VBS based on top of the barrier wall elevations. Periodic pedestrian access openings exist in both VBS rows but are not accounted for in this report because the openings for the inner VBS do not generally coincide with the openings for the outer VBS. This means that there is a barrier upstream or downstream of each opening that would act as a weir. The additional outflows along the serpentine flow path between the inner and outer openings are ignored. Because these openings are all outflows, except one at the southwest corner of the power block, this is a conservative assumption with respect to calculated water levels for VEGP 1 & 2 power block area.

The hydraulic performance of the double row VBS for the western perimeter is represented by an inflow rating curve, with flows from the VEGP 3 & 4 boundary into the VEGP 1 & 2 site, assuming conservatively that there would be no submergence of this portion of the VBS by water levels in the VEGP 1 & 2 power block. A separate rating curve for flows out of the VEGP 1 & 2 power block along the north and east perimeter double VBS rows was also developed. Outflows over the southern VBS boundary are prevented by topography.

Discharge over the VBS was treated as a case of broad-crested weir discharge. A weir discharge coefficient of 2.62 was estimated based on guidance from the !AHR Hydraulic Structures Design Manual (Reference 3).

The addition of a second line of weirs means that the head on the outer VBS w ill in some cases create a tailwater elevation causing submergence of the inner VBS weir and reducing the discharge for a given head. Submergence is checked along each segment of the VBS. Using methods described in Reference 4, where submergence occurs along the VBS, a discharge reduction coefficient was applied to determine the actual discharge over the double row weir.

Using the weir relationships described above, stage discharge rating curves foir the inflow from VEGP 3 & 4 into the VEGP 1 & 2 power block area and for the outflow from VEGP 1

& 2 to the outside were developed. The rating curves are shown in Table 1-3 and plotted in Figure 1-1 .

11 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 Table 1-3. Elevation-Discharge Rating Curves for the VEGP 1 & 2 Perimeter VBS Flow from VEGP 3 1 Flow Out of VEGP Stage (ft) & 4 to VEGP 1 & 2 1 & 2 Area (cfs)

Area (cfs) 217.0 0.0 0.0 217.1 0.0 0.0 217.2 0.0 0.0 217.3 0.0 0.0 217.4 0.0 0.0 217.5 0.0 0.0 217.6 0.0 0.0 217.7 0.0 0.0 217.8 0.0 1.1 217.9 0.0 4.4 218.0 0.0 22.3 218.1 0.0 64.4 218.2 0.0 125 218.3 0.0 205 218.4 0.3 330 218.5 5.3 519 218.6 21.8 765 218.7 46.5 1,078 218.8 88.1 1,451 218.9 181 1,913 219.0 305 2,466 219.1 472 3,101 219.2 675 3,816 219.3 914 4,609 219.4 1,195 5,485 219.5 1,519 6,473 219.6 1,882 7,603 219.7 2,284 8,853 219.8 2,729 10,218 219.9 3,222 11 ,700 220.0 3,760 13,297 1

The stages indicated in Column 1 represent ( 1) the VEGP 3 & 4 water levels in the ditch adjacent to the western perimeter VBS of VEGP 1 & 2 (Column 2) and (2) the VEGP 1 & 2 water levels inside the power block (Column 3).

12 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluration Report Report Number: SNC453475 Rev. 0 Figure 1-1. Stage-Discharge Curves for VEGP 1 & 2 Perimeter VBS Runoff hydrographs for the area to the southwest that drains to the VEGP 1 & 2 power block and for Units 3 & 4 (under construction) were developed in HEC-HMS, version 3.5.0 (Reference 8). The inputs required for modeling included a design rainstorm and unit hydrographs to translate rainfall into runoff.

The design rainfall hyetograph was developed in HEC-HMS utilizing the frequency storm option in the Meteorologic module (Reference 8). This option requires the input of PMP point depth for durations 5, 15, 60, 120, 180, and 360 minutes.

The PMP rainfall depths used to estimate water levels in the VEGP 1 & 2 site were developed from the permit values reported in Section 2.4.2 of Reference 1.

Depth-duration pairs and associated intensities from the VEGP 1 & 2 FSAR are shown in Table 1-4.

Table 1-4. Probable Maximum Precipitation for VEGP 1 & 2 Maximum Corresponding Time Rainfall Intensity (min)

(in.) (in./hr) 1 2.0 120 2 2.8 84 4 4.0 60 6 4 .9 49 8 5 .5 41 10 6.2 37 20 9 .0 27 40 12.5 19 60 15.0 15 120 22.0 11 240 31 .0 8 13 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevalu1ation Report Report Number: SNC453475 Rev. 0 A curve is fit to the data in Table 1-4 to obtain a continuous relation that is used to obtain the additional HEC-HMS- required depth-duration points. The curve is shown in Figure 1-2.

35.0 30.0 E

~ ~ ...... Y =1.984x 0,4999

~ _ 25.0

ii::§. .....

.,,. Ir R2 =0.9998 >-

E ~ 20.0 .....

-~

.Cl

"' ,,,.V ~

0 ~

~ .e- 15.0 GI

, f

,,)

r"

- Q.

~

-l

.5 10.0 5.o JI ii *

- - Pow Plv1P Ra infall er (PMP Rainfall) 0.0 0' 50 100 150 Duration (min) 200 250 300 I

Figure 1-2. PMP Depth and Duration for Point Rainfall Based on the power function fit to the data, the PMP total depth was estimated for the missing durations as demonstrated in Table 1-5. An intensity position of 50 percent was selected for the HEC-HMS calculation , consistent with the alternating block method described by Chow, Maidment, and Mays (Reference 5).

Table 1-5. PMP Rainfall Depth Used as Input for Frequency Storm HEC-HMS Model Time Time Intensity Rainfall Depth (min) (hr) (in./hr) (in.)

5 0.08 53.2 4.4*

15 0.25 30.7 7.7*

60 1.00 15.0 15.0 120 2.00 11 .0 22.0 180 3.00 8.9 26.6*

360 6.00 6.3 37.6*

Estimated from the Power Function The 5-minute interval rainfall hyetograph for Table 5 rainfall event is shown in Figure 1-3.

14 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 PMP HEC-HMS Generated Hyetograph 5 ~------------------------~

4.5

'2 4

-; 3.5

t - -- - - - - - - - - - - --11 -- - - - - - - - - - -- - i Q. 3

~

iii 2.5

~ 2 E

..,f 1.5

.: 1 0.5 0fW , LUO,U,lLU.Ll.U.U.U-'UUJ

~~~~~~~~~~##~#$#~~

Time Increment (5 min)

Figure 1-3. PMP Hyetograph Determined in Design Basis PMP Runoff hydrographs for the VEGP 1 & 2 site were developed in HEC-HMS using the Soil Conservation Service (SCS) unit hydrograph methodology and the PMP depths in Table 1-5. Flow elements within the VEGP 1 & 2 HEC-HMS basin model include: (1) the VEGP 1 & 2 power block area watershed, (2) the VEGP 3 & 4 inflow hydrograph, and (3) a small watershed discharging into the VEGP 1 & 2 power block area. The HEC-HMS model combined these hydrographs and routed them through the power block area, which acts as a reservoir since it is surrounded by VBS walls on all sides. The modeled reservoir storage attenuated the flood and determined outflow from the perimeter VBS using stage-storage and stage-discharge relations. The HEC-HMS model is shown schematically in Figure 1-4.

C 0(1S1n Modl!I 1U1 0 "I lwo VDS Siruul(')lmn] ~~~~

A SB-Un~1&2 V38* jnnow 1,.; UI &2 Power Block

~

V1S2Cul V

Figure 1-4. HEC-HMS Schematic for Units 1 & 2 PMP Simulation 15 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 Runoff hydrographs were developed for the sub-basins using the SCS Hydrograph Methodology (Reference 8), which requires the following parameters as input:

Drainage Area (sq mi)

Base Flow (cfs)

Runoff Curve Number

Lag Time (minutes)

Drainage areas were measured from topographic maps. SCS runoff curve numbers were conservatively estimated based on a paved land cover condition (Reference 8). The lag time was estimated to be 0.6 times the sub-basin time of concentration value as determined using SCS methodologies (Reference 8). Base flow was estimated at 100 cfs per square mile of drainage area for sub-basins with natural vegetation cover, based on a 40 percent PMP storm occurring 3 days prior to the PMP storm event. Values for these parameters are summarized in Table 1-6.

Table 1-6. SC$ Hydrologic Parameters for VEGP 1 & 2 HEC-HMS Model Sub-basin Area Runoff Base Lag Tc Sub-basin Curve Flow Time (min) sq.ft acres sq. mi. No. (cfs) (min)

SB-Units 1 & 2 3,379,468 77.58 0.1364 98 0.0 6.4 3.9 Units 1 & 2 Cul 218,465 5.02 0.0078 98 0.8 5.0 3.0 The inflow hydrograph from VEGP 3 & 4 was determined based on water levels established in a runoff analysis for the VEGP 3 & 4 site considering the construction phase when the VEGP 3 & 4 power block had been backfilled and before the main east-west VEGP 3 & 4 drainage ditch had been improved. In this phase, flow in the main ditch was routed through a diversion ditch located adjacent to the main ditch . The runoff analysis considered a combined hydrologic runoff model using HEC-HMS and an unsteady flow hydraulic model using the USACE computer program HEC-RAS (Hydrologic Engineering Center- River Analysis System) (Reference 6). The analysis determined VEGP 3 & 4 runoff overflows to the VEGP 1 & 2 site area while considering the reduction in subsequent runoff volume due to overflows into the VEGP 1 & 2 area.

A schematic of the HEC-HMS runoff model for the VEGP 3 & 4 construction phase is shown in Figure 1-5.

SCS methodologies were also used for the VEGP 3 & 4 runoff analysis. The input parameters for drainage area, base flow, runoff curve number, and lag time were determined using the same procedures described previously for the VEGP 1 & 2 analysis. These input parameters are shown in Table 1-7.

16 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevalu1ation Report Report Number: SNC453475 Rev. 0 SB-A1a A1 -82 8B-A3 88-A2 SB-A1 b

<)

Q

...,0 8 8-C2 SB-C1 b SB-C1a 88-A7 A7-C7 88-C7 Figure 1-5. VEGP 3 & 4 Drainage Basin Model Used in HEC-HMS and HEC-RAS Table 1-7. VEGP 3 & 4 Sub-basin HEC-HMS Model Parameters Sub-basin Area Runoff Base Tc Lag Time Sub-basin Curve Flow (min) (min) sq. ft acres sq. mi. Number (cfs)

SB-A1 137,843 3.16 0.0049 98 0.5 6.0 3.6 SB-A1a 172,915 3.97 0.0062 98 0.6 5.6 3.4 SB-A1b 617,740 14.18 0.0222 98 2.2 10.5 6.3 SB-A2 1,267,745 29.10 0.0455 98 4.5 14.2 8.5 SB-A3 2,560,586 58.78 0.0918 98 9.2 43.9 26.3 SB-A5 5,733,214 131 .62 0.2057 98 20.6 57.7 34.6 SB-A6 622,122 14.28 0.0223 98 2.2 10.1 6. 1 SB-A? 264,535 6.07 0.0095 98 0.9 10.1 6. 1 SB-C1a 259,321 5.95 0.0093 98 0.9 6.8 4.1 SB-C1b 338,879 7.78 0.0122 98 1.2 5.0 3.0 SB-C2 492,040 11 .30 0.0176 98 1.8 6.0 3.6 SB-C3 273,160 6.27 0.0098 98 1.0 5.6 3.4 SB-C4 273,160 6.27 0.0098 98 1.0 5.6 3.4 SB-C7 1,697,983 38.98 0.0609 98 6.1 5.6 3.4 SB-N3 1,269,515 29.14 0.0455 98 4.6 11.4 6.9 17 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluration Report Report Number: SNC453475 Rev. 0 The 15 sub-basins within the VEGP 3 & 4 drainage area are connected in a drainage network defined by 8 junctions and 7 reaches as shown in Figure 1-5. The sub-basin hydrographs were computed in the HEC-HMS model. Then, the sub-basin hydrographs were added at respective junctions as lateral inflow hydrographs and routed thrnugh the reaches using the USACE computer program HEC-RAS, version 4.0.0 (Reference 6).

For HEC-RAS modeling, the unsteady option was applied for simulating the actual hydrographs as time series. For modeling purposes, the VEGP 3 & 4 drainage ditch conveyance system was modeled applying the surveyed ditch cross sections, and the flood routing was used for this dynamic simulation . From computer modeling, the PMP water surface elevation time series in the VEGP 3 & 4 drainage area and along the VEGP 1 & 2 boundary were determined for the PMP rainfall event. A schematic of the channel layout is shown in Figure 1-6.

Construction Crossing ~

Bypassed channel ~ 8

- - - - - ~- - - - - - - - - - - - - -*-

- *. ,..t,...... .*

~* * .

Ditch 5348 5298

~ t ,ot

l 4671

~ 2800 4600 I>'

_., -9 2600 2700 4500 4400

2417 3000 3400 3650 3600 4200 Blocked Culvert 2317 36951773 2217

~

Stream Line 2017 River stalion Left & R~t Bank St*oons 18 17 0 Levet 17 17 16 17 15 17 Figure 1-6. Flood Routing Model Used in HEC-RAS fneff'ectf\le FIO'i,v Are&.

During the VEGP 3 & 4 construction period, runoff is diverted from the bypassed channel to a temporary channel, shown in Figure 1-6. Cross-section locations along the existing and temporary channel are also shown in Figure 1-6. Cross-section geometry was obtained from site surveys and construction drawings. Manning's n values for the ditch were estimated to be 0.015 for the ditch channel and 0.03 for the ditch overbank area (Reference 7). Contraction and expansion coefficients are 0.0 1 and 0.03, respectively, for straight uniform flow areas, and 0.03 and 0.05, respectively, for cross sections near bends or just upstream or downstream of constrictions or expansions in the flow.

PMP flow hydrographs from the VEGP 3 & 4 HEC-HMS model were directly read into the HEC-RAS unsteady model at the cross sections indicted in Table 1-8.

18 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 Table 1-8. PMP Sub-basin Flow Hydrograph Input to HEC-RAS HEC-RAS Cross HEC-HMS Sub-basin Section (River Flow Hydrograph Station - RS) 5698 SB-A1 5498 SB-A1a 5348 SB-C1a 5298 SB-A1b 5071 SB-C1b 4600 SB-A2 4500 SB-C2 4400 SB-C3 4200 SB-N3 3400 SB-C4 3000 SB-A3 2800 SB-A5 2600 SB-A6 1817 SB-A?

1617 SB-C7 With the inflow PMP hydrographs and the cross-section geometry, the unsteady HEC-RAS model determined stage hydrographs at each cross section. The stage elevations between cross sections 5698 and 5498 were conservatively used as the water level along the entire boundary between VEGP 1 & 2 and VEGP 3 & 4. The lowest VBS elevations are located downstream of these cross sections, at approximately crnss section 5450. The lateral structure in HEC-RAS model was located at this cross section.

Using the stage hydrograph and the weir rating data in Table 1-3 and Figure 1-1 , the inflow hydrograph from VEGP 3 & 4 was determined and used as the input into the VEGP 1 & 2 HEC-HMS model.

The development of the VEGP 3 & 4 inflow hydrograph assumes that fl ow is from VEGP 3 & 4 to VEGP 1 & 2. This assumption was verified by examining the flood stage hydrographs on each side of the divide and noting that the instantaneous VEGP 3 & 4 water level is a lways higher than the VEGP 1 & 2 water level. Additionally, the VEGP 3 &

4 inflow hydrograph was developed conservatively, assuming free overflow into VEGP 1

& 2 without reducing inflows as a result of downstream submergence effects from VEGP 1 & 2 water levels.

The results of the VEGP 1 & 2 HEC-HMS model indicate a maximum water level in the VEGP 1 & 2 power block at elevation 219.1 ft msl during the local PMP event. This elevation is below the elevation of the safety-re lated structures of 219.6 ft msl and therefore safety-related SSCs are not flooded during the construction phase for VEGP 3

& 4, considering the VBSs that have been placed around VEGP 1 & 2.

1.2.2 Flooding in Streams and Rivers T he probable maximum flood (PMF) elevation on the Savannah River at VEGP 1 & 2 is 138 ft msl (Section 2.4.3.4 of Reference 1). When combined with coincident wind wave run-up, the flood elevation would reach 165 ft msl (Section 2.4.3.6 of Reference 1). This is below the elevation of the safety-related structures of 219.6 ft msl.

19 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 1.2.3 Dam Failures The peak flood stage elevation for dam failures at VEGP 1 & 2 is 141 ft msl (Section 2.4.4.2 of Reference 1). When combined with coincident wind wave run-up, the dam failure surge wave would reach an elevation of 168 ft msl along the plant side of the river (Section 2.4.4.3 of Reference 1). This is less than the lowest elevation of the safety-related structures of 219.6 ft msl.

1.2.4 Storm Surge and Seiche Storm surge and seiche events are not expected to affect the site, and no flood elevation is given (Section 2.4.5 of Reference 1).

1.2.5 Tsunami Tsunamis are not expected to affect the site, and no flood elevation is given (Section 2.4.6 of Reference 1).

1.2.6 Ice Induced Flooding Ice induced flooding is not expected to affect the site, and no flood elevation is given (Section 2.4. 7 of Reference 1).

1.2. 7 Channel Diversion Channel diversions are not expected to produce a flood event that could affect the site.

No flooding elevation is given in the VEGP 1 & 2 FSAR (Section 2.4.9 of Reference 1).

1.2.8 Combined Effects Combined flooding effects (PMP, PMF, dam failure, and/or wind-driven waves) are discussed in Sections 1.2.2 and 1.2.3 of this report.

1.3 Licensing Basis Flood-Related and Flood Protection Changes Deficiencies and non-conforming or unanalyzed conditions were not observed during the walkdown of flood protection features that are the subject of the FWR.

The flood walkdown process verified the flood protection for the current licensing basis and determined that no additional or enhanced flood protection features are necessary. (Reference 10) 1.4 Watershed and Local Area Changes Changes to the local area will result from the construction of VEGP 3 & 4, located to the west and adjacent to the existing VEGP 1 & 2 as shown in VEGP 3 & 4 UFSAR Figure 1.1-202 (Reference

2) and included in this report in Attachment 2. This figure shows the finished site layout for all four units. However, the changes that would affect the local PMP flooding water levels in the VEGP 1

& 2 area the most would be from construction phase changes for VEGP 3 & 4. These construction phase changes, which reflect the worst case scenario, are discussed in Section 2.1 of this report.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 1.5 Current Licensing Basis Flood Protection and Mitigation Features The safety-related structures at the VEGP site are not susceptible to flooding by the most severe flood at the site, and therefore no flood protection measures are necessary. One flood source is the river, and the highest postulated flood level due to an upstream dam failure is elevation 141 ft msl (Section 2.4.4.2 of Reference 1). Coincident wind-wave run-up is postulated to reach a maximum elevation of 168 ft msl, which is below the finished grade elevation of 219.6 ft msl for safety-related structures. Therefore, flooding is not probable from this source. (Section 2.4.10 of Reference 1)

Another source of flooding is severe rainfall. The power block area for VEGP 1 & 2 is on a high plateau and is not in the path of any adjacent watershed. The topography is such that the runoff is directed away from the power block by a combined system of culverts and open ditches to natural drainage channels. The system has been evaluated to ensure that flooding of safety-related equipment would not occur as a result of the local intense PMP. The highest flood level due to the local PMP is at elevation 219.1 ft msl, which is below the finished grade elevation of 219.6 ft msl, for safety-related structures.

1.6 Additional Site Details There are no additional site details.

2.0 Flooding Hazard Reevaluat ion The reevaluations in Section 2.0 are not part of the VEGP 1 & 2 licensing basis as documented in Reference 1. Except for the local intense precipitation flooding hazard described in Section 2.1 of this report, the following flooding hazard reevaluations are adopted directly from the VEGP 3 & 4 UFSAR (Section 2.4 of Reference 2), Hydrologic Engineering.

The VEGP 3 & 4 UFSAR incorporates the VEGP 3 & 4 early site permit (ESP) and the VEGP 3 & 4 COL, which were issued by the NRC on August 26, 2009, and February 10, 2012, respectively. The flooding causal mechanisms and design basis flood elevation described in the VEGP 3 & 4 UFSAR are applicable to the flood reevaluation for VEGP 1 & 2 because VEGP 1 & 2 and VEGP 3 & 4 are located physically adjacent to each other and share a common site. The floor elevation of safety-related SSCs for all four units is located at NOE elevation 220 ft msl. The site-specific hydrologic evaluations for flooding in streams and rivers, dam failures, storm surge and seiche, tsunami, and ice-induced flooding, as reported in the VEGP 3 & 4 UFSAR, utilize the current applicable ESP and COL methodologies. These evaluation sections from the VEGP 3 & 4 UFSAR (Reference 2) are included in Sections 2.2 through 2.7 of this report for completeness.

The applicable VEGP 3 & 4 Tables (Attachment 1) and Figures (Attachment 2) are attached at the end of this report.

2.1 Local Intense Precipitation The flood water elevation due to a maximum local intense precipitation (also known as PMP) was reevaluated. The analysis evaluated the maximum water surface elevation within the VEGP 1 & 2 power block area resulting from the occurrence of the PMP during the worst case scenario for the construction configuration for storm runoff for the VEGP 3 & 4 site, with the double row VBS in place around the VEGP 1 & 2 power block perimeter.

This reevaluation differs from the design basis analysis of Section 1.1 of this report by utilizing the maximum peak rainfall intensity of 19.2 in . during the worst 1-hour rainfall period . This value was obtained from the VEGP 3 & 4 PMP analysis and is also in accordance with NUREG/CR-7046.

The values shown in Table 2.4-201 in Attachment 1 were used in the HEC-HMS model for the 21 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard ReevalU1ation Report Report Number: SNC453475 Rev. 0 present reevaluation. The 5-minute interval rainfall hyetograph for this rainfall event based on the rainfall data shown in Attachment 1 is shown in Figure 2 -1 .

-* ***-** ***- -*.. --... -~*-** -**- **- _______.. ______*. ... ------*- *--** . *- .. ******- .. .. ***-----... ____-*- .. -**- ***** **-** ...

..................'.............Ir.tr..................................................................................................................................................... .

..:.-.-.-.-.-.-.-.:.:.:.:.:.:_. ..:_._..::.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:o.:.:.:.:.:_:_.? .Q.:.:.:.:.15.9.:..:.:.:Q.O.::.:.J ::2.Q*:.1.!5.9:.:.t ?.9*.~.:1_0...:2.*1.9.:.'J.7.:9..~QQ::1:3.9.::~:IS.0...-::::::::::::.-.-:..**

.:::.. ::::::::::**:::*.-:::*::':::::..:::::.:.::::::::.:::..:...:*::?tlme-:trom*.:si3r£ofSto*rm;*::m.,nut.e,i:~:_;:::_;::~** ::::::.:.::.::::~:*~::..:::.~.-*

Figure 2-1. PMP Hyetograph for Reevaluation Rainfall Event The impact of the higher rainfall design basis PMP event, as specified in NUREG/CR-7046, was analyzed considering VEGP 3 & 4 construction phase conditions and a double row of VBSs around the perimeter of the VEGP 1 & 2 power block. The same HEC-HMS and HEC-RAS models were used as for the design basis rainfall presented in Section 1.1 of this report for VEGP 1 & 2.

The maximum PMP flood water surface elevation assoc iated with the increased PMP event is conservatively estimated to be 219.3 ft msl. This elevation is 0.3 ft below the elevation of the safety-related structures of 219.6 ft msl, and therefore safety-related SSCs are not subject to flood during the construction phase of VEGP 3 & 4, considering the VBSs that have been placed around the VEGP 1 & 2 site.

The worst case scenario for the configuration of VEGP 3 & 4 was assumed in the local intense PMP analysis. As discussed in Section 5.0, the PMP flood water surface elevation of VEGP 1 & 2 during the construction phase of VEGP 3 & 4 is higher than for any subsequent condition during the operation of VEGP 1 & 2.

2.2 Flooding in Streams and Rivers To represent the reevaluation of flooding in streams and rivers for VEGP 1 & 2, Section 2.4.3 of the VEGP 3 & 4 UFSAR (Reference 2), Probable Maximum Flood, is provided below. The subsections are renumbered, and references are removed for clarity. The VEGP 3 & 4 UFSAR integrates the VEGP 3 & 4 ESP, which was issued by the NRG on August 26, 2009, and the VEGP 3 & 4 COL, which was issued by the NRG on February 10, 2012.

2.2.1 Probable Maximum Flood (VEGP 3 & 4 UFSAR Section 2.4.3)

In this section, the hydrometeorological design basis of any necessary flood protection measures is presented for those structures, systems, and components necessary to ensure the capability to shut down the proposed VEGP Units 3 and 4 and maintain them in a safe shutdown condition. One of the scenarios investigated to determine the design basis flood for ensuring the safety of nuclear power plants is the Probable Maximum 22 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 Flood (PMF). PMF flows and stages at a site can be the result of local flooding, as discussed in Subsection 2.1 of this report, or riverine flooding, as described below.

The location of VEGP Units 3 and 4 is adjacent to and generally to the west of the existing VEGP units, as illustrated in VEGP 3 & 4 UFSAR Figure 1.1-202 (Attachment 2).

The site is located on a high bluff on the west bank of the Savannah River. The site grade for the new units is established as Elevation 220 ft msl, similar to the existing VEGP units, which is well above the probable maximum flood stage of the Savannah River.

Based on calculations, site visits, an assessment of site conditions, and a review of previous studies, it was determined that the maximum water surface elevation resulting from the PMF on the Savannah River at the VEGP site and the additional combined action of wind setup and wave run-up would be substantially below El. 220 ft msl.

Considering this assessment, the VEGP site can be characterized as a "flood-dry site,"

as described in Section 5.1.3 of the American National Standard Report, Determining Design Basis Flooding at Power Reactor Sites, because the safety-related structures of both the existing VEGP and proposed AP1000 units are or will be so high above the Savannah River that safety from flooding is "obvious or can be documented with minimum analysis".

A review of studies and analysis performed for the existing units was carried out to confirm that the conclusions continue to be valid for Units 3 and 4. This characterization of the VEGP site is reported in Subsection 2.2.2.

A calculation of the PMF discharge using approximate methods was developed for the ESP application from Regulatory Guide 1.59, Design Basis Floods for Nuclear Power Plants, Revision 2, August 1977, reported in Subsection 2.2.5, and the calculation of the associated flood stage using a steady-state hydraulic model and wave run-up, reported in Subsection 2.2.6. These calculations indicate t hat the maximum flood stage associated with Savannah River flooding is approximately 70 ft below the base slab elevat ion of the proposed units , confirming the assessment of the VEGP site as "flood dry."

2.2.2 Review of Studies for Units 1 and 2 (VEGP 3 & 4 UFSAR Section 2.4.3.1)

As part of the hydrologic study carried out for Units 1 & 2, the PMF values for the Savannah River at the site were first estimated! using a hydrologic model of the entire upstream watershed and then were checked with a dynamic hydraulic model of the reach of the Savannah River between the last storage reservoir and the VEGP site, as summarized below:

1. The HEC-1 Flood Hydrograph Computer Program, developed by the USACE, was used to develop the PMF hydrograph of the Savannah River near the VEGP site, using the unit hydrographs of the 10 sub-basins developed by the National Weather Service (NWS) together with Probable Maximum Precipitation (PMP) estimates derived from methodology outlined in National Weather Service Hydrometeorological Reports (NWS HMR 51 and HMR 52). Valley storage was accounted for by separately modeling the Strom Thurmond Dam HEC-1 outflow hydrograph with the NWS DAMBRK program .

2. The HEC-1 model was independently verified by routing the USAGE-derived PMF outflow hydrograph from the Strom Thurmond Dam down to the VEGP site and combining it with the PMF hydrographs from the intervening drainage areas developed from HEC-1.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 The results of these previous modeling efforts are summarized in VEGP 3 & 4 UFSAR Table 2.4-221 (Attachment 1) and are described in more detail below.

2.2.3 Savannah River Watershed Hydrologic Model (VEGP 3 & 4 UFSAR Section 2.4.3.1 .1)

In the HEC-1 hydrologic model, the watershed for the Savannah River at the VEGP site was subdivided into 10 sub-basins with a total drainage area estimated at that time as 8,015 sq mi (the sub-watershed areas used by the NWS for the current flood forecasting model of the Savannah River basin are different from the values used in previous modeling; the updated watershed areas are presented in VEGP 3 & 4 UFSAR Table 2.4-208 (Attachment 1) and are used for the PMF approximation described in Subsection 2.2.5). The PMF hydrograph for each sub-basin was developed using the unit hydrograph obtained from NWS for the respective sub-basins and the corresponding PMP estimates pertaining to the sub-basin in question.

Starting from the most upstream sub-basin, the PMF hydrograph was then routed and combined in succession in the downstream direction to the VEGP site, including reservoir routing through the upstream Burton, Hartwell, Strom Thurmond, and Stevens Creek dams.

Below Augusta, Georgia, significant floodplain storage exists that could significantly reduce the flood peak. Two PMF values at the VEGP site are presented in the study for licensing Units 1 and 2: a value of 540,000 cfs, with valley storage effects considered ,

and a value of 895,000 cfs without storage. Without the wind wave activities included, the maximum Savannah River PMF water levels at the VEGP site were estimated to be at El.

126 ft msl and 136 ft msl, respectively, for these two cases.

2.2.4 Dynamic Hydraulic Model Check on Hydrologic Model Results (VEGP 3 & 4 UF SAR Section 2.4.3.1 .2)

An independent check of the reliability of the HEC-1-based estimate of the PMIF at the VEGP site was carried out by routing the USACE-derived PMF outflow hydrograph from the Strom Thurmond Dam down to the VEGP site using the NWS dynamic hydraulic model DAMBRK and combining it with the HEC- 1-derived PMF hydrographs from the intervening drainage areas between the Strom Thurmond Dam and the site.

The PMF outflow hydrograph at the Strom Thurmond Dam was obtained from the 1962 USAGE Reservoir Regulation Manual (revised in 1968) developed by the Savannah District before the HMR 51 and 52 PMP guidelines were published and before the closure of the upstream dams.

The PMF peak discharge at the VEGP site was found to be 710,000 cfs, with a corresponding maximum water level at EL 138 ft msl.

It appears that a PMF value of 710,000 cfs was adopted in the study for Units 1 and 2 because it gave a higher water level than the 540,000 cfs value derived from the HEC-1/NWS modeling effort, when valley storage effects were considered.

2.2.5 Estimation of PMF by Approximate Methods (VEGP 3 & 4 UFSAR Section 2.4.3.2)

An alternative method for estimating the PMF is described in the NRC Regulatory Guide 1.59 for flood dry sites. The method consists of obtaining a relationship for the PMF discharge as a function of drainage area , based on PMF iso-line maps developed for regions of the United States east of the 105th Meridian , and utilizing the drainage area at a given site, obtain the PMF from the relation determined for that region. No PMP is 24 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 required for this method. Calculations for the estimated PMF at the VEGP site are presented in VEGP 3 & 4 UFSAR Table 2.4-222 (Attachment 1 ).

The PMF values determined from the 100-, 500-, 1,000-, 5,000-, 10,000-, and 20,000 sq mi contributing area maps at the location of the Savannah River watershed upstream of the VEGP site are tabulated in VEGP 3 & 4 UFSAR Table 2.4-222 (Attachment 1).

A logarithmic plot of the power curve fit to these values is presented in VEGP 3 & 4 UFSAR Figure 2.4-211 (Attachment 2). Based on the curve fit to the data and the currently estimated drainage area of 8,304 sq mi (as discussed in Subsection 1.1), the estimated PMF for the VEGP site is about 920,000 cfs. This point is located on the curve in VEGP 3 & 4 UFSAR Figure 2.4-21 1 (Attachment 2), along with a data point for VEGP (reported as Alvin W. Vogtle), presented on page 4 of 17 in Table B.1 of Regulatory Guide 1.59 as 1,001,000 cfs for a drainage area of 6,144 sq mi. Considering current and previously reported measurements, the drainage area reported for the VEGP site in Table B.1 appears to be incorrect and inconsistent with the Regulatory Guide 1.59 method, which was used to derive the value. However, it is presented as a published reference value.

2.2.6 Estimation of Flood Stage at VEGP Site for PMF (VEGP 3 & 4 UFSAR Section 2.4.3.3)

A stage-discharge relationship or "rating curve" is required to estimate the water surface elevation of the Savannah River near the VEGP site associated with the PMF discharge.

This relationship was obtained from a steady-state hydraulic backwater analysis of the Savannah River run in HEC-RAS, a computer model developed by the USAGE.

The steady-state model was adapted from the dynamic model used for the analysis of the dam-break scenario described in Subsection 2.3.1, using the same channel roughness (Manning's n) values as in that model. All bridges were removed from the dynamic model; they were not put back into the steady-state model, which is equivalent to assuming that any downstream bridges are eitlner swept away or have a negligible impact on water surface elevations at the VEGP site during the PMF event.

Changes in the HEC-RAS model used to estimate stages at the VEGP site included:

The reaches of the model upstream of the Augusta City Dam (River Mile 199.667) were removed.

The model was converted from dynamic to steady-state mode with the downstream boundary condition at River Mile 99.406 determined by normal depth using an estimated energy slope of 0.0005 (the downstream water surface elevation will have a negligible impact on water surface elevations some 90 mi upstream near the VEGP site).

The PMF and reference discharges were input for the entire model reach.

The cross-section nearest the VEGP site (River Mile 150.906) was extended to the proposed top-of-slab elevation using 1:24,000-scale topography from 7.5-minute USGS quadrangles.

The results for the cross-section nearest to the VEGP site (River Mile 150.906 in the model) are shown in VEGP 3 & 4 UFSAR Table 2.4-223 (Attachment 1).

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 The longitudinal profile output for the Savannah River for this model is reproduced as VEGP 3 & 4 UFSAR Figure 2.4-212 (Attachment 2). The cross section developed for the VEGP site is shown in VEGP 3 & 4 UFSAR Figure 2.4-213 (Attachment 2).

The estimated maximum stages at the VEGP site for the PMF estimated per the approximate method outlined in Regulatory Guide 1.59 are shown in VEGP 3 & 4 UFSAR Table 2.4-224 (Attachment 1).

Based on the fact that the estimated maximum stage reached by the Savannah River at the site for the approximate PMF flood is over 69 feet below the minimum top-of-slab elevation of any safety-related systems, structures, or components at the VEGP site, the characterization of a flood-dry site should be established.

2.2.7 Conclusions (VEGP 3 & 4 UFSAR Section 2.4.3.4) 2.3 Dam Failures (b)(3) 16 USC § 824o-1(d), (b)(4), (b)(?)(F) 2.3.1 Potential Dam Failure (VEGP 3 & 4 UFSAR Section 2.4.4)

(b)(3) 16 USC § 8240 1(d) (b)(4) (b)(?)(F) 1 Editorial change in table number only.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluration Report (b)(3) 16 U S C § 8240-1 (d), (b)(4), (b)(7)(F) 2.3.2 Dam Failure Permutations (VEGP 3 & 4 UFSAR Section 2.4.4.1)

(b)(3) 16 U SC § 824o-1(d}, (b)(4), (b)(?)(F) 27 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 (b)(3)*16 USC § 824o-1(d) (b)(4), (b)(?)(F) 2.3.3 Unsteady Flow Analysis of Potential Dam Failures (VEGP 3 & 4 UFSAR Section 2.4.4.2)

(b)(3):16 USC § 824o-1(d). (b)(4). (b)(?)(F) 28 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluration Report Report Number: SNC453475 Rev. 0 (b)(3) 16 U SC § B24o-1 (d) (b)(4), (b)(7)ti- J 29 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0

(b)(3) 16 USC § 824o-1(d), (b)(4) (b)(7)(F) constan elevation. This means that bottom elevation of the entire (b)(3) 16 USC § 824o-1(d) (b)(4) (b)(?)(F)

As shown on VEGP 3 & 4 UFSAR Figure 2.4-220 (Attachment 2), the total dam width at the too of the dam is about 5,700 ft.I (b)(3) 16 USC § 8240-1 (d), (b)(4) (b)(7)(F)

I (b)(3) 16 U SC § 8240-1 (d), (b)(4), (b)(7)(F) ms to e conservative. he cross section s own 1n

,,......."T"ff..,,...,..,....,,....

ig-u-re~ 2.4-220 (Attachment 2) has been artificially widened a (b)(3) 16 U SC 8240-1 ( ) (b)(4) (b)(?)(F) 30 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0

The HEC-RAS dam breach model and the equations used to determine (b)(3) 16 USC§ 824o-1(d), (b)(4), (b)(7)(F)

(b)(3) 16 USC § 8240-1(0) (b)(4) (b)(f)(F)

Cross-section data for the Savannah River used in the HEC-RAS computer model were (O)(:J) 16 U S c.; § 8240-1(d) (0)(4) (b)(/)(1-J 31 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluration Report Report Number: SNC453475 Rev. 0 less than the actual cross-section. However downstream of the breached dam a (b)(3) 16 USC§ 824o-1(C!). (b)(4) (b)(7)(~)

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 (b)(3) 16 USC § 824o-1(d), (b)(4), (b)(7)(F) 2.3.4 Water Level at the Plant Site (VEGP 3 & 4 UFSAR Section 2.4.4.3)

(b)(3) 16 USC § 824o-1(d), (b)(4), (b)(7)(F) 33 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluration Report Report Number: SNC453475 Rev. 0 Various wind speed durations were analvzed to determine the maximum wave heiqht and b)(3) 16 Us C § ts24o 1(d) (b)(4) (0)(/)(f-)

2.4 Storm Surge and Seiche To represent reevaluation of flooding caused by storm surge and seiche for VEGP 1 & 2, VEGP 3

& 4 UFSAR Section 2.4.5, Probable Maximum Surge and Seiche Flooding, (Reference 2) is provided below. The subsections are renumbered and references are removed for clarity. The VEGP 3 & 4 UFSAR integrates the VEGP 3 & 4 ESP, which was issued by the NRC on August 26, 2009, and the VEGP 3 & 4 COL, which was issued by the NRC on February 10, 2012.

2.4.1 Probable Maximum Storm Surge and Seiche Flooding (VEGP 3 & 4 UFSAR Section 2.4.5)

The VEGP site is located on a coastal plain bluff on the west bank of the Savannah River approximately 151 River Miles inland from the Atlantic Ocean at grade El. 220 ft msl.

Since the site is not located on an open or large body of water, surge or seiche flooding will not produce the maximum water levels at the site.

The Savannah River estuary region is occasionally exposed to extreme mid-Atlantic hurricanes. Between 1841 and 2004, only three major hurricanes, Category 3 or over (measured using the Saffir/Simpson Hurricane Scale), hit the coast of Georgia. The most devastating hurricane on record with a landfall within approximately 100 miles of the Savannah River estuary was Hurricane Hurgo, which hit the coast of South Carolina near Charleston in 1989. This Category 4 hurricane produced a 20-foot-high storm surge in the Cape Romain-Bulls Bay area in South Carolina.

Regulatory Guide 1.59, Design Basis Floods for Nuclear Power Plant, Revision 2, August 1977 (RG 1.59), Appendix C provides the distribution of probable maximum surge levels from hurricanes along the Atlantic coast. It shows maximum surge heights of 28.2 ft mean low water (mlw) at Folly Island, South Carolina, and 33.9 ft mlw at Jekyll Island, Georgia, located northeast and southwest of the Savannah River estuary, respectively.

The probable maximum storm surge height at t he mouth of the Savannah River can be estimated from these values following the procedure described in RG 1.59 Appendix C, as shown in VEGP 3 & 4 UFSAR Table 2.4-231 (Attachment 1).

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 The high tide at the estuary with a 10 percent exceedence level is defined as 9.0 ft mlw, and the mlw at the entrance to Savannah River, Georgia is 1.2 ft below msl. Considering the coincidence of the probable maximum surge with a 10-percent-exceedence high tide at the river mouth, a probable maximum surge height of 32.3 ft mlw or 31 .1 ft msl may be obtained for the Savannah River estuary, as shown in VEGP 3 & 4 UFSAR Table 2.4-231 (Attachment 1 ) .

If it is assumed! that a storm surge of such a magnitude is generated in the Savannah River estuary moving inland, the surge height would dissipate before reaching the VEGP site (151 River Miles inland and at grade El. 220 ft msl), and the site would be free from any resultant flood. Also, because the VEGP site is not located on a large enclosed body of water, flooding due to seiche is precluded.

The probable maximum surge data from RG 1.59 have not included those from the hurricanes after 1975. The inclusion of the data from recent hurricanes, including Hurricane Hugo, may have changed the probable maximum surge data from RG 1.59 somewhat. However, because the VEGP site is 151 River Miles inland and at grade El.

220 ft msl, the effects of probable maximum surge at the estuary of Savannah River would be insignificant at the site, and would not cause flooding of the site.

2.5 Tsunami To represent reevaluation of flooding caused by Tsunami for VEGP 1 & 2, VEGP 3 & 4 UFSAR Section 2.4.6, Probable Maximum Tsunami Flooding (Reference 2) is provided below. The subsections are renumbered and references are removed for clarity. The VEGP 3 & 4 UFSAR integrates the VEGP 3 & 4 ESP, which was issued by the NRC on August 26, 2009, and the VEGP 3 & 4 COL, which was issued by the NRC on February 10, 2012.

2.5.1 Probable Maximum Tsunami Flooding (VEGP 3 & 4 UFSAR Section 2.4.6)

Since the VEGP site is not located on an open ocean coast or large body of water, tsunami-induced flooding will not produce the maximum water level at the site.

The Atlantic Ocean region is characterized by infrequent seismic and volcanic activities, resulting in few recorded tsunamis. The majority of tsunamis in the Atlantic Ocean and Caribbean Sea have been either triggered by seismic (earthquake) activity or the result of volcanic eruption. The most notable Atlantic tsunami was generated by the Great Lisbon Earthquake of 1755. The tsunami hit the coasts of Portugal, Spain, and northern Africa and traveled across the Atlantic Ocean with a 10-to-15-ft wave reportedly reaching the Caribbean coasts. Computer models suggested a wave height of 10 ft along the east coast of the US from this tsunami.

The effects of any tsunami w ith similar height approaching the Savannah River estuary would be dissipated before reaching the VEGP site (151 River Miles inland and at grade El. 220 ft msl), and the site would be free from any resultant flood.

2.6 Ice Induced Flooding To represent reevaluation of ice induced flooding for VEGP 1 & 2, VEGP 3 & 4 UFSAR Section 2.4.7, Ice Effects (Reference 2) is provided below. The subsections are renumbered and references are removed for clarity. The VEGP 3 & 4 UFSAR integrates the VEGP 3 & 4 ESP, which was issued by the NRC on August 26, 2009, and the VEGP 3 & 4 COL, which was issued by the NRC on February 10, 20 12.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 2.6.1 Ice Effects (VEGP 3 & 4 UFSAR Section 2.4.7)

Long-term air temperature records available at the National Weather Service (NWS) weather station at Augusta, Georgia (Bush Field), and seven other cooperative observation stations around the VEGP site are used to analyze historical extreme air temperature variations at the VEGP site. The analysis was also supported by onsite temperature data measured at the VEGP site. A detailed description of station locations and data availability is presented in VEGP 3 & 4 UFSAR Subsection 2.3.2.

The climate at the VEGP site is characterized by short, mild winters and long, humid summers. Local climatology data at Augusta, Georgia, for a period of 129 years show an average annual air temperature of 64.2°F (17.9°C). January is the coldest month, with an average temperature of 46.8°F (8.2°C). July is the warmest, with an average temperature of 81.3°F (27.4 °C). Based on temperature records at Augusta and seven surrounding stations, the lowest air temperature on record was observed to be -4.0°F (-20.0°C) at Aiken in January 1985 (VEGP 3 & 4 UFSAR Table 2.3-205, Attachment 1). The January 1985 event produced a minimum air temperature of-0.1°F (-17.8°C) at the VEGP site, with the air temperature remaining below freezing (32°F [0°C]) for only about 50 hours5.787037e-4 days 0.0139 hours 8.267196e-5 weeks 1.9025e-5 months (VEGP 3 & 4 UFSAR Figure 2.4-227, Attachment 2). VEGP temperature data from 1984 through 2002 show that the average daily air temperature has remained below freezing for a maximum of 3 consecutive days (VEGP 3 & 4 UFSAR Table 2.4-232, Attachment 1). In three instances, the average daily air temperature remained above freezing the entire year.

Historical water temperatures recorded at five USGS stations located on the Savannah River are presented in VEGP 3 & 4 UFSAR Table 2.4-233 (Attachment 1). These USGS stations include: No. 02187500 near Iva, South Carolina, at River Mile 280.4; No.

02189000 near Calhoun Falls, South Carolina, at River Mile 263.6; No. 02197000 at Augusta, Georgia, at River Mile 187.4; No. 02197500 at Burtons Ferry near Milhaven, Georgia, at River Mile 118.7; and No. 02198500 near Clyo, Georgia, at River Mile 60.9.

The data cover a river reach that includes the VEGP site. Within this river reach, the minimum water temperature is observed in February, which shows a variation between 39.2°F (4.0°C) and 42.8°F (6.0°C).

Based on the record of air and water temperatures, it is very unlikely that surface or frazil ice formation would occur in the Savannah River in the vicinity of the proposed VEGP Units 3 and 4 river intake location.

2.6.2 Ice Jam Events (VEGP 3 & 4 UFSAR Section 2.4.7.2)

There are no recorded ice jam events in the lower reach of the Savannah River based on a search of the Ice Jam Database of the US Army Corps of Engineers.

The large dams and reservoirs on the Savannah River located upstream of the VEGP site reduce the possibility of any surface ice or ice floes moving downstream. Since the water temperatures in the lower reach of the Savannah River remain consistently above freezing, as seen in VEGP 3 & 4 UFSAR Table 2.4-233 (Attachment 1), the formation of frazil ice or ice jams would be very unlikely at the proposed VEGP Units 3 and 4 intake location.

2.7 Channel Diversions To represent reevaluation of flooding caused by channel diversions for VEGP 1 & 2, VEGP 3 & 4 UFSAR Section 2.4.9, Channel Diversions, (Reference 2) is provided below. The subsections are renumbered and references are removed for clarity. The VEGP 3 & 4 UFSAR integrates the 36 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 VEGP 3 & 4 ESP, which was issued by the NRC on August 26, 2009, and the VEGP 3 & 4 COL, which was issued by the NRC on February 10, 2012.

2.7.1 Channel Diversions (VEGP 3 & 4 UFSAR Section 2.4.9)

The VEGP site area lies in the Upper Coastal Plain of the Atlantic Coastal Plain physiographic province and is bordered by the Savannah River to the east. The surrounding topography consists of gently rolling hills with surface topography elevation ranges from about 200 to nearly 300 ft msl. Local site drainage consists of a principally dendritic drainage pattern where all major streams are tributary to the Savannah River.

T he VEGP site and surrounding areas are shown in VEGP 3 & 4 UFSAR Figure 2.4-206 (Attachment 2) .

Near the site area, incision of the Savannah River has produced a deep valley with topographic relief of nearly 150 ft from the river surface and a valley width of over 4 mi.

The present-day river course is located at the western side of the valley, forming steep bluffs near the VEGP site. The river floodplain consists of a broad alluvial surface extended on the eastern side at heights of 5-10 ft above the riverbank.

Rivers in the Upper Coastal Plain are typically underlain by sands, clays, limestones, and gravels and exhibit gentle to moderate bed slopes, wide floodplain development, and increased sinuosity. Consequently, diversion of the river channel in this region cannot be completely discounted.

Historical development of the river plan-form, which is the shape on map of river bank-line, near the VEGP site is well-represented in the USGS 7 .5-minute series (topographic) maps. Oxbow lakes, meander cutoffs, abandoned meanders, low-lying swamps, and forested wetlands provide considerable evidence of historical channel plan-form development. A lthough meander river plan-form is present upstream and downstream of the site, the Savannah River near the site has a relatively straight and stable reach extending approximately from River Mile 143 to River Mile 152. A comparison o f river bank-lines between 1965 and 1989, obtained from USGS topographic maps and topographic maps used for VEGP Units 1 and 2, shows a nearly unchanged river plan-form within the reach during this period.

Since 1952, the Savannah River flow has been regulated by large federal multipurpose projects: Hartwell Dam, Richard B. Russell Dam, and J . Strom Thurmond (also known as Clarks Hill} Dam. A major impact of dam operation on river flow downstream of the J.

Strom Thurmond Dam is the modulation of the outflow hydrograph, with reduced peaks and increased low-flow rates, as can be seen from VEGP 3 & 4 UFSAR Figure 2.4-228 (Attachment 2). Such flow modulation results in much-reduced river morphological activity, and a sudden river plan-form change is unlikely.

It is, therefore, unlikely that the river at the VEGP site will be diverted from the river intake by natural causes. Furthermore, analysis for existing VEGP Units 1 and 2 indicate that any possible effect on water supply to the intake from river channel diversion should come from extremely slow changes, which can be remedied as they occur.

While it is unlikely that a diversion of the main river channel will occur, such a d iversion, either upstream or downstream of the proposed river intake, cannot be discounted. The river upstream and downstream from the proposed river intake has bluffs and steep slopes along the west bank. If it is assumed that a bluff slid into the river bed just upstream from the river intake structure, it may obstruct the flow of the main river channel, and river flow would divert over the floodplain on the eastern side of the river and away from the river intake. This could result in loss of the river intake due to river water starvation. Likewise, if a bluff slid into the river bed just downstream of the river 37 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 intake structure, it again may obstruct the flow of the main river channel, but could possibly flood the river intake structure before diverting river water over the floodplain on the eastern side of the river. In this case, the river intake structure would be lost due to flooding. However, all the safety-related cooling water systems for the AP1000 reactor plants do not use water from the river intake. Hence, the river intake is not classified as a safety-related structure and loss of the river intake for either of these described scenarios would have no adverse affect on plant safety.

2.8 Combined Effects Combined flooding effects (PMP, PMF, dam failure, and/or wind-driven waves) are discussed in Sections 2.2 and 2.3 of this report.

3.0 Comparison of Current Design Basis and Reevaluated Flood Causing Mechanisms Table 3-1 below summarizes the comparison of current design basis and reevaluated flood causing mechanisms, which include wind effect for flooding in streams and rivers and dam failures.

Table 3-1. Current Design Basis and Reevaluation Flood Elevations Current Reevaluation Reevaluation Flood Causing Design Basis Reevaluation Flood Delta Flood Delta from Mechanism Flood Flood Elevation From Design SSC Elevation Basis (219.6 ft msl)

Local Intense 219.1 ft msl 219.3 ft msl +0.2 ft -0.3ft Precipitation Flooding in 165 ft msl 151 ft msl -14 ft -68.6 ft Streams and Rivers (b)(3) 16 U SC § 8240-1 (d) (b)(4), (b)(r)(F)

Storm Surge and N/A N/A N/A N/A Seiche Tsunami N/A N/A N/A N/A Ice Induced N/A N/A N/A N/A FloodinQ Channel N/A N/A N/A N/A Diversion 3.1 Local Intense Precipitat ion The current design basis VEGP 1 & 2 flood elevation from the local intense precipitation is 219.1 ft msl.

The reevaluated flood elevation is 219.3 ft msl , 0.2 ft more than the current design basis flood elevation, and 0.3 ft below the elevation of safety-related SSCs of 219.6 ft msl.

3.2 Probable Maximum Flooding The current design basis VEGP 1 & 2 flood elevation from probable maximum flooding in streams and rivers is 138 ft msl or 165 ft msl with coincident wave run-up.

The reevaluated flood elevation is 139 ft msl or 151 ft msl with coincidental wave run-up. The flood elevation decreased by 14 ft from the current flood elevation and is 68.6 ft below the elevation of safety-related SSCs at 219.6 ft msl.

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluration Report Report Number: SNC453475 Rev. 0 3.3 (b)(3) 16 U.SC § 824o-1(d) (b)(4), (b)(?)(F) 3.4 Storm Surge and Seiche Storm surge and seiche are not expected to affect the site for the reasons listed above in Section 2.4.

3.5 Tsunami Tsunami induced flooding is not expected to affect the site for the reasons listed above in Section 2.5.

3.6 Ice Induced Flooding Ice induced flooding is not expected to affect the site for the reasons listed above in Section 2.6.

3.7 Channel Diversion Channel diversions are not expected to affect the site for the reasons listed above in Section 2.7.

3.8 Combined Effects Combined flooding effects are discussed within the Local Intense Precipitation, Probable Maximum Flood, and Dam Failure sections above.

4.0 Interim Evaluation and Actions Taken or Planned Because the VEGP site location is on top of a bluff overlooking the Savannah River, approximately 140 ft below the site elevation, the reevaluations of the Savannah River PMF and dam break, discussed in Section 3.0 of this FHRR did not produce significantly changed flood heights. The flood height for the reevaluated local PMP event during the construction phase of VEGP 3 & 4 is shown to increase 0.2 ft to elevation 219.3 ft msl, and the APM is reduced from 0.5 ft to 0.3 ft. This remaining margin indicates that no further evaluation of potential consequences is necessary and that no flood protection changes need to be considered.

5.0 Additional Actions The local PMP for VEGP Units 1, 2, 3, & 4 will be combined in a future analysis to reflect the final grading of the site and to include the common PA fence and final configuration of the VBS for the entire plant area. As part of the future analysis, the steady-state flooding model for the VEGP 3 & 4 area will be replaced with a dynamic, unsteady-state flooding model for the entire four-unit plant area using finished grade for as-built conditions.

The analysis of the local intense PMP flooding for the entire site will include the following changes (details are not presently available):

Updated sub-watershed areas with new flow paths, including two additional runoff discharge paths will be added to t he PMP model:

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Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0 o The drainage ditch to the north, located after flow crosses the new east-west railroad track to the VEGP Unit 2 turbine building discharging a portion of the VEGP 3 & 4 existing watershed away from the PMP flood zone will be added.

o A permanent yard drainage divide between the VEGP 3 & 4 cooling towers will be added to permanently divert a section of the cooling towers drainage area away from the PMP flood zone.

The final grading of the area north of the railroad at the site grid North 8350 (approximate) will be reflected to reduce the drainage area contributing to the flood depth.

As-built VBSs for the VEGP 3 & 4 perimeter will be included, along with changes to the existing VBS, such as the elimination of portions of the VBS between VEGP Unit 2 and VEGP Unit 3.

The entire 100-year drainage ditch will replace the temporary construction diversion ditch and the construction crossings used in the PMP model (Figure 1-6) will be removed.

This flood evaluation considers less onerous conditions (additional runoff flow paths from the exiting power block areas and grading that permanently directs portions of the runoff flow away from the modeled area); therefore, the results are expected to confirm a calculated flood height no greater than the current reevaluation flood elevation of 219.3 ft msl.

6.0 References

1. Southern Nuclear Operating Company, Vogtle Electric Generating Plant Units 1 & 2, Final Safety Analysis Report, Revision 18, September 12, 2012.

2. Southern Nuclear Operating Company, Vogtle Electric Generating Plant Units 3 & 4, Updated Final Safety Analysis Report, Revision 1, June 25, 2012.

3. Hager, Willi H., "Discharge Measurement Structures," Hydraulic Structures Design Manual #8, Discharge Characteristics, Chapter 2, D.S. Miller, Editor, International Association of Hydraulic Research (IAHR), 1994.

4. Kolkman, P.A., "Discharge Relationships and Component Head Losses for Hydraulic Structures,"

Hydraulic Structures Design Manual #8, Discharge Characteristics, Chapter 3, D.S. Miller, Editor, International Association of Hydraulic Research (IAHR), 1994.

5. Chow, Ven Te, Maidment, David R., Mays, Larry W., Applied Hydrology, McGraw Hill, 1988.

6. U.S. Army Corps of Engineers, HEC-RAS, River Analysis System, Version 4.0.0, Hydrologic Engineering Center, March 2008.

7. Chow, Ven Te, Open-Channel Hydraulics, McGraw Hill , 1959.

8. U.S. Army Corps of Engineers, HEC-HMS, Hydrologic Modeling System, Version 3.5.0, Hydrologic Engineering Center, August 2010.

9. Bechtel Power Corporation, VEGP Report on Settlement, Vogtle Nuclear Generating Plant Units 1 and 2, August 1986 40 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluiation Report Report Number: SNC453475 Rev. 0

10. Southern Nuclear Operating Company, Response to Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendation 2.3: Flooding of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, Report Number SNC429731 , Version 0, November 26, 2012.

11. Bechtel Power Corporation, Plant Vogtle Settlement Review, Vogtle Nuclear Generating Plant Units 1 and 2, September 1984.

12. Southern Nuclear Company, "Marker Measurements to Determine Floor Elevation of Selected Safety Related Structures," Vogtle Electric Generating Plant Units 1 & 2, Calculation X2CA79, Version 1.

13. Southern Nuclear Company, "PMP Generated Maximum Flood Water Surface Elevation in the Units 1&2 Power Block Area Based on the Design Basis Precipitation," Vogtle Electric Generating Plant Units 1 & 2, Calculation X2CA78, Version 1, 21 Nov. 2012.

14. Southern Nuclear Company, "PMP Generated Maximum Water Surface Elevation in the Units 1&2 Power Block Area in Response to NRG 50.54(f) Request for Information Letter," Vogtle Electric Generating Plant Units 1 & 2, Calculation X2CA77, Version 1, 21 Nov. 2012 41 of 41

Vogtle Electric Generating Plant, Units 1 & 2 50 .54(f) Enclosure 2 Flooding Hazard Reevaluation Report Report Number: SNC453475 Rev. 0 Attachment 1 VEGP 3 & 4 UFSAR Tables (12 pages not including this cover sheet)

V EGP 3&4 - UFSA R Table 2.3-205 Climatological Extremes at Selected NWS and Cooperative Observing Stations in the VEGP Site Area Waynesboro Augusta Midville Exp Louisville Newington Blackville Parameter 2NE wso Millen 4N Station 1E 2NE Aiken4NE 3W Springfield Bamberg Maximum 108 oF a, o 108 °Fa 109 °F 0 105 °Fa, 0 112 °Fa 110 °Fa 109 °F a 108 °Fa NA 0 109 °Fa Temperature (7/25/52); (8/21 /83) (7/24/52) (7/13/80); (7/24/52) (7/13/80) (8/22/83) (8/1/99) (7/24/52)

(7/14/80) (8/21/83)

(7/19/86);

(7/21/86)

Minimum _1 oF a, o -1 oF a 0 OF D -1 oF a -2 oF a -1 oF a -4 oF a -1 oF a NA 0 2 oF a Temperature (1/20/85); (1/21/85) (1/21/85) (1/21/85) (1/21/85) (1/21/85) (1/21/85) (1/21/85) (1/21/85)

(1/21/85)

Maximum 7.40 in. a 7.30 in. a 8.02 in. D 8.19in.a 8.60 in. a 5.50 in. a 9.68 in. a 7.53 in. a 7.10in.D,C 8.02 in. a, c 24-hr Rainfall (10/3/94) (9/3/98) (8/29/64) (10/12/90) (10/12/90) (10/10/90) (4/16/69) (9/30/59) (9/30/59) (9/23/00)

Maximum 16.99 in. a, b 14.82 in. a, b 13.45 in. b 15.97 in. b, C 14.76 in. b , C 15.29 in. a, b 14.45 in. a, b 14.67 in. a, b 17.32 in. b , C 15.26 in. a, b Monthly (10/94) (10/90) (8/64) (8/70) (8/91 ) (7/89) (3/80) (10/90) (6/73) (8/95)

Rainfall Maximum 16.0 in. a, 0 8.0 in. a, 0 14.0 in. b 14.0 in. b , C 14.8 in. a, 0 5.0 in. a, 0 15.0 in. a, 0 17.0 in. b, C 8.0 in. b, C 19.0 in. a, 0 24-hr Snowfall (2/10/73) (2/9/73) (2/10/73) (2/10/73) (2/10/73) (2/10/73) (2/10/73) (2/10/73) (2/11/73) (2/10/73)

Maximum 16.0 in. a, 0 14.0 in. a, 0 15.0in. 0 14.0 in. D, C 14.8 in. a, 0 8.0 in. a, 0 15.0 in. a, 0 17.0 in. D, C 15.0 in. D, C 22.0 in. a, 0 Monthly (2/73) (2/73) (2/68) (2/73) (2/73) (2/73) (2/73) (2/73) (2/73) (2/73)

Snowfall Sources:

a - NCDC 2005b b - SERCC 2006 c - NCDC 2002c d - NA = Measurements not made at this station 2 .3-42 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flood ing Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 2 of 12 Table 2.4-201 Rainfall Depths Used as Input for Frequency Storm HEC-HMS Module Duration, Minutes Depth, Inches 5 6.20a 15 9.aoa 60 19.20a 120 23.52° 180 25.95° 360 31.ooa a) Calculated with HMR51/52 b) Calculated by curve fitting 2.4-69 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 3 of 12 Table 2.4-208 Savannah River Subbasins and Drainage Areas above VEGP Site NWS Subbasin Drainage Area, mi~

upstream of downstream of No. I.D. NWS Subbasin Name site (1) site (2) 1 TIGG1 Burton Dam, GA 122.3 0.0 2 JCSS1 Jocassee Dam, SC 157.7 0.0 3 KEOS1 Keowee Dam, SC 288.0 0.0 4 HRTG1 Hartwell Dam, GA 1544.7 0.0 5 RBRS1 R.B. Russell Dam 738.2 0.0 6 CARG1 Carlton Bridge, GA 760.6 0.0 7 CHDS1UP Clark Hill - Thurmon Dam (upstream) 665.9 0.0 8 CHDS1 Clark Hill Dam 1847.7 0.0 9 MODS1 Modoc, S.C. 539.9 0.0 10 AGTG1 Steven Creek Dam, GA 454.8 0.0 11 AGSG1 Augusta 5th Street 77.1 0.0 12 AUGG1 Augusta/Butler Creek 273.6 0.0 13 JACS1 Jackson, S.C. 651.2 0.0 14 BFYG1 Burton's Ferry, GA 182.5 293.4 15 BRIG1 Millhaven, GA 0.0 646.2 16 CLYG1 Clyo, GA 0.0 634.7 Estimated Savannah River drainage area at site 8304.2

1) Based on data from Southeast River Flood Forecasting Center, Atlanta, GA. (NWS 2005)

2) As estimated from HUC-12 shapefiles 2.4-79 Revision 1

VEGP 3&4 - UFSAR Table 2.4-215 Inventory of Savannah River Watershed Water Control Structures 2.4-86 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 5 of 12 Table 2.4-220 Probable Maximum Precipitation Values for Point Rainfall at VEGP Site Watershed PMP depth Duration Area, mi2 Multiplier Applied to Source (inches) 6-hour 10 n/a n/a HMR-51, Fig 18 31 .0 1-hour 1 0.620 6-hr 10 mi" value HMR-52, Fig 23 19.2 30-minutes 1 0.736 1-hr 1 mi" value HMR-52, Fig 38 14.1 15-minutes 1 0.509 1-hr 1 mi" value HMR-52, Fig 37 9.8 5-minutes 1 0.323 1-hr 1 mi" value HMR-52, Fig 36 6.2 Table 2.4-221 Results of Previous PMF Modeling Efforts PMF and PMF Stage Flood Elevation Including Freeboard wrt Model Description Results Wave Action El. 220 ft msl HEC-1 Model with Ignoring Valley Storage 895,000 cfs, 163 ft msl 57 ft HMR 51 and 52 PMP 136 ft msl Valley Storage Modeled 540,000 cfs, 153 ft msl 67 ft in NWS DAMBRK 126 ft msl USAGE PMF with NWS DAMBRK Model 710,000 cfs, 165 ft msl 55 ft 138 ft msl Table 2.4-222 PMF Values for an Area-PMF Relationship at the VEGP Site Watershed Area, PMF in cfs from Supporting Figure sq. mi. isolines (RG 1.59) 100 110,000 B-2 500 250,000 B-3 1,000 330,000 B-4 5,000 750,000 B-5 10,000 1,050,000 B-6 20,000 1,300,000 B-7 2.4-92 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 6 of 12 Table 2.4-223 PMF Flood Stages for Cross-Section Nearest VEGP Site w.s. E.G. Vel Chnl, Flow Area, Top Froude Profile QTotal,cfs Elev, ft Elev, ft E.G.Slope fps sf Width, ft # Chi Avg Daily 13,669 88.22 88.25 0.000056 1.50 31 ,765 8,238 0.07 Max Avg Annual 28,734 92.37 92.39 0.000056 1.64 66,743 8,551 0.07 Peak Historic Max 360,000 118.55 118.63 0.000093 4.12 384,032 14,534 0.11 PMF 917,965 138.82 138.95 0.000102 5.66 680,627 14,681 0.13 2x PMF 1,835,930 160.50 160.71 0.000120 7.50 999,754 14,784 0.14 Table 2.4-224 Estimated Probable Maximum Flood Stage at VEGP Site PMF Stage: 138.82 ft msl - HEC-RAS WSL at River Mile 150.906 Wave run-up & wind set-up 11.31 ft - result for 2h: 1v slope w/ 50 mph wind from NE over an 11-mile fetch resulting from dam-break Total PMF Stage: 150.13 ft msl Minimum Slab elevation 220.00 ft msl Estimated Freeboard 69.87 feet Table 2.4-225 Normal Pool Storage Volumes Mode 1 Mode2 Reservoir Reservoir Storage Volume Storage Volume Dam (1,000 ac-ft) (1 ,000 ac-ft)

Jocassee 1,100 Keowee 940 Burton 108 Nacoochee 8.2 Mathis 31.4 Tallulah Falls 2.46 Tugalo 43.2 Yonah 10.2 Total 2,040 203 2.4-93 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 7 of 12 Table 2.4-226 Breach Parameter Estimation Formulas Number of Case Relations Proposed Reference Studies (S.I. units, meters, m 3/s, hours)

Johnson & Illes (1976) 0.5hcf-,8$3hd for earthfill dams Singh & Snorrason 20 2hd s; B $ 5hd (1982, 1984) 0.15 m $ dovtop 0.61 m 0.25 hr $ t1 $ 1.0 hr MacDonald & 42 Es;n!hfill gs,1ml!:

769 Langridge-Monopolis Ve,= 0.0261 (V0 ~

hwJ°- [best-fit]

4 (1984) Tr= 0.0179(Ve,l 6 (upper envelope)

Non-earthfill dams:

Ve,= 0.00348( Vout

hw)°*

852 [best-fit]

FERG (1987) 8 is normally 2-4 times hd B can range from 1-5 times hd Z = 0.25- 1.0 [engineered, compacted dams)

Z= 1 -2 [non-engineered, slag or refuse dams]

t,=0.1-1hr (engineered, compacted earth dams]

tr= 0.1 - 0.5 hr [non-engineered, poorly compacted earth dams]

Froehlich (1987) 43 B = 0.47Ko (S f*;.c, Ko= 1.4 overtopping; 1.0 otherwise Z = 0.75 Kc(h.w) 157(W*)o.73 Kc= 0.6 w ith corewall; 1.0 without corewall tr= 79(S)0.47 Reclamation (1988) 52 B =3hw t,= 0.0118 Von Thun & Gillette (1990) 57 B,Z,tr see guidance in USBR 1998 Froehlich ( 1995b) 63 B= 0.1803K0 V u:J'Lh u.i!I JJ*

tr= 0.00254 V 53h/o. 9oJ Ko= 1.4 for overtopping; 1.0 otherwise Source: USBR 1998 2.4-94 Revision 1

Vogtle Electrical Generating P lant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 8 of 12 Table 2.4-227 (b)(3) 16 USC § 8240-l (d) (b)(4) (b)(7)(F) l (b)(3) 16 U SC § 8240-1 (d) (b)(4) (b)(7)(F)

I Reference I B (m) I B (ft) I z I tf (hrs)

(b)(3) 16 U SC § 8240-1 (d) (b)(4) (b)(7)(F) 2.4-95 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 9 of 12 Table 2.4-229 l (b)(3) 16 USC § 8240-1 (d) (b)(4) (b)(7)(F)

Input Variable I English Units I SI Units (b)(;J) 1t:i U s G !:i llZ4o-1 (d) (b)(4), (b)(f )(I")

Table 2.4-230 l (b)(3) 16 USC § 824o-1(d) (b)(4), (b)(7)(F)

I Reference I B (m) I B (ft) I z I tf(hrs)

(b)(3) 16 U SC § 8240-1 (d) (b)(4) (b)(7)(F) 2.4-96 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 1Oof 12 Table 2.4-231 Estimated Probable Maximum Surge at the Savannah River Mouth Folly Jekyll Savannah Components Unit lslanda lslandb Estuaryc Comments Wind Setup ft mlwd 17.15 20.6 18.89 Taken as average of wind set-up from Folly 3 Island and Jekyll Island Pressure Set-up ft mlw 3.23 3.34 3.29 Taken as average of pressure .set-up from Folly Island and Jekyll Island Initial Water Level ft mlw 1.00 1.20 1.10 Taken as average of initial water level from Folly Island and Jekyll Island 10% Exceedence High ft mlw 6.80 8.70 9.00 Magnitude at the Savannah River estuary nde taken from ANSI/ANS-2.8-1992; others from NRC RG 1.59 1977 Total Surge Height ft mlw 28.2 33.9 32.3 Sum of wind and pressure set-up, initial water level, and 10% exceedence high tide mlw to msl conversione ft -1.2 Magnitude at the Savannah estuary obtained from ANSI/ANS-2.8-1992 Sea Surface Anomaly ft 0.0 Magnitude at the Savannah estuary obtained from ANSI/ANS-2.8-1992 ft msl 31.1 Total Surge Height a NRC RG 1.59 1977 b NRC RG 1.59 1977 C Wind and pressure set-up, and initial water level averaged from Folly Island and Jekyll Island, tidal data was obtained from ANSI/

ANS-2.8-1992 d Mean low water (mlw) e Mean sea level (msl) = (mlw +1 .2) ft at the Savannah estuary (ANSI/ANS-2.81992) 2.4-97 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 1, Page 11 of 12 Table 2.4-232 Variation in Lowest Average Daily Temperatures and Number of Days with Average Daily Temperature Below Freezing Date Lowest Lowest Average Daily Average Daily Maximum No. Total No. of Temp Temp of Consecutive Freezing Year OF (OC) Occurred Freezing Days Days 1984 25.7 -(3.5) 1217/1984 1 3 1985 11.9 -(11.2) 1/21/1985 3 5 1986 20.7 -(6.3) 1/28/1986 2 3 1987 31.2 -(0.4) 1/27/1987 1 1 1988 25.2 -(3.8) 1/8/1988 3 6 1989 19.0 -(7.2) 12/23/1989 3 6 1990 37.3 (2.9) 12/25/1990 0 0 1991 26.0 -(3.3) 2/16/1991 1 1 1992 33.4 (0.8) 1/16/1992 0 0 1993 30.4 -(0.9) 3/14/1993 1 1 1994 21.3 -(5.9) 1/19/1994 2 4 1995 29.2 -(1.6) 2/9/1995 2 4 1996 20.8 -(6.2) 1/8/1996 3 8 1997 28.9 -(1.7) 1/18/1997 2 2 1998 34.8 (1.6) 12/26/1998 0 0 1999 25.2 -(3.8) 1/14/1999 3 3 2000 26.5 -(3.1) 12/20/2000 2 4 2001 30.9 -(0.6) 1/3/2001 2 2 2002 29.7 -(1.3) 1/4/2002 2 2 Average days 1.7 2.9 2.4-98 Revision 1

VEGP 3&4 - UFSAR Table 2.4-233 Variation in the Minimum Water Temperatures at Five Locations on the Savannah River USGS Station River Data Observed Minimum Temperature, °F (°C)

No. Location Mile Period Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep.

02187500 Savannah River near Iva, 280.4 1958- 62.6 55.4 46.4 44.6 39.2 42.8 48.2 48.2 57.2 55.4 53.6 57.2 SC 1984 (17.0) (13.0) (8.0) (7.0) (4.0) (6.0) (9.0) (9.0) (14.0) (13.0) (12.0) (14.0) 02189000 Savannah River near 263.6 1957- 65.3 59 46.4 46.4 42.8 51.8 53.6 59.9 64.4 66.2 68 71.6 Calhoun Falls, SC 1974 (18.5) (15.0) (8.0) (8.0) (6.0) (1 1.0) (12.0) (15.5) (18.0) (19.0) (20.0) (22.0) 02197000 Savannah River at 207.0 1958- 64.4 59 51.8 42.8 42.8 50 57.2 59.9 66.2 66.2 64.4 69.8 Augusta, GA 1973 (18.0) (15.0) (11.0) (6.0) (6.0) (10.0) (14.0) (15.5) (19.0) (19.0) (18.0) (21.0) 02197500 Savannah River at Burtons 118.7 1957- 63.5 58.1 46.4 43.7 39.2 44.6 55.4 59 66.2 73.4 71.6 71.6 Ferry near Milhaven, GA 1979 (17.5) (14.5) (8.0) (6.5) (4.0) (7.0) (13.0) (15.0) (19.0) (23.0) (22.0) (22.0) 02198500 Savannah River near Clyo, 60.9 1938- 59.9 46.4 44.6 41 40.1 44.6 57.2 57.2 68 73.4 71.6 67.1 GA 1984 (15.5) (8.0) (7.0) (5.0) (4.5) (7.0) (14.0) (14.0) (20.0) (23.0) (22.0) (19.5)

Source: Dyer and Alhadeff 1997 2.4-99 Revision 1

Vogtle Electric Generating Plant, Units 1 & 2 50 .54(f) Enclosure 2 Flooding Hazard Reevaluation Report Report Number: SNC453475 Rev. 0 Attachment 2 VEGP 3 & 4 UFSAR Figures (21 pages not including this cover sheet)

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report Report Number: SNC453475, Attachment 2 Page 1 of 21 (b)(3) 16 USC § 824o-1(d) (b)(4) (b)(?)(F)

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02187252 - USGS Gage on the Savannah River beloW Hartwell l.al<e nea, Hartwell, Georgia

- - #02187500 - USGS Gage on the Savannah River near

~------

-- -- -- -- - -- - - - - - -._1v_~_Sou

__ ~_ea_r_o1_ina

------------ #02189000 - USGS Gage on the Savannah River near Calhoun Falls, South Carolina

02195000 - USG$ Gage on the Savannah River near Cta,1<'& Hin. South Carolina A ESP S ite NWS Sub-Watersheds

#02196484 - USG$ Gage on the Savannah River nea, NO<lh Augusta, Sou~ Ca,ollna 1 Burton Dam GA 2 Jocasse Dam. SC #02197000 -USGS Gage on the Savannah Rlvar 3 Keowee Dam SC Augusta, Geo<g,a 4 Hartwell Dam, GA 5 R B. Russell Dam 6

7 Carlton Brld e, GA Clark Hill - Thurmon Dam u stream

1 #02197320 - USGS Gage on the Savannah Rover near Jackson, SC 8 C lark Hill Dam 9 Modoc, SC 10 Steven Creek Dam GA ---- #02197500-USGS Gage on the Savannah River al 11 Au usta 5 Street Burton's Ferry near Millhaven, Georgia 12 Au usta/Butler Creek 13 Jackson, SC 14 Burton's Fe , GA 15 Millvaven, GA -

1 #002198500- USG$ Gauge on the Savannah River near Clyo, Georgia 16 C lo, GA Figure 2.4-204 Savannah River Watershed and HUCs (No Scale) 2.4-159 Revision 1

V EGP 3&4 - UFSAR ID Stream Identification Unnamed creek at Hancock 1

Landlno to the Savannah River Unnamed tributary to Daniels

- 2 3

Branch Red Branch to Daniels Branch

--- 4 Daniels Branch D/S of embankment dam to confluence with Red Branch Unnamed tributary to Beaverdam 5

Creek

- 6 7

Beaverdam Creek to Telfair Pond Beaverdam Creek, D/S ofTelfair

- Pond to Savannah River

. I , *

f "' t .._ f j Figure 2.4-206 Site Drainage 2.4-161 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 2 Page 4 of 21 4741 y = 12726x°"

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Appendix B map data, RG 1.59 Vogtle PM= per best-fit pow er c urve A Vogtle PM= reported in Table B.1 Figure 2.4-211 Area-PMF Plot for VEGP Site per Approximate Method from RG 1.59) 2.4-166 Revision 1

V EGP 3&4 - UFSAR V EGP RvF Stage Calculation RI VER-1 Reach- 1 2oof Le~end WS 2xf>Mc WS PM' 180 WS Historic Max W S Avg DaJy Max Ground 160 140

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~ 100 60 Q.

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100000 200000 300000 400000 500000 600000 Main Channel Distance (f t)

Figure 2.4-212 Longitudinal Profiles of the Savannah River from Steady-State HEC-RAS Model Run 2.4-167 Revision 1

V EGP 3&4 - UFSAR VEGP FM' Stage Calculation Appr<><imate locat ion of V EGP ooh extended x-section data

. 175 + 04+ . 175 250 Legend WS 2 X P'-f' W SPMF W S Historic Wax W S Avg Daily Max Ground 200 Levee Bank Sta

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.Q 150

~

iD 100 50 0 5000 10000 15000 20000 Station (f t)

Figure 2.4-213 HEC-RAS Model Section at VEGP Site (Looking Downstream) 2.4-168 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 2 Page 7 of 21 1 NEW SAVANNAH BLUFF LOCK m1lf DAM 2 I TIVtiNICREEK DAM ' '\

3 CLARK HILL DAM I

RICHAIID 8 . RUSSELL D AM C TROlTI IISSHOALS DAM )

5 HARTftUL DAM I YONAHOAM 7 TUGALDOAM 8 TALLULAH f ALLS DAM t MATHIIDAM 10 NACOOCHEE DAM 11 BURTON DAM 12 K&OWHDAM 0 5 10 20 13 LllTLI RIVl!II DAM 1* JOCAIIEE DAM MILES Figure 2.4-214 Savannah River Basin Dam Locations 2.4-169 Revision 1

Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report Report Number: SNC453475, Attachment 2 Page 8 of 21

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Vogtle Electrical Generating Plant, Units 1 & 2 50.54(f) Enclosure 2 Flooding Hazard Reevaluation Report VEGP 3&4 - UFSAR Report Number: SNC453475, Attachment 2 Page 21 of 21 EUSGS USGS 02197000 SAVANNAH RIVER AT AUGUSTA, GA

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NRC-2017-000688 (Formerly FOIA/PA-2017-0690) - Resp 5 - Final, Agency Records Subject to the Request Are Enclosed, Part 13 of 15
Person / Time
ML20366A021
Issue date:	12/29/2020
From:	NRC/OCIO
To:
Shared Package
ML20366A007	List: ML20366A008 ML20366A009 ML20366A010 ML20366A011 ML20366A012 ML20366A013 ML20366A014 ML20366A015 ML20366A016 ML20366A017 ML20366A018 ML20366A019 ML20366A020 ML20366A021 ML20366A022 ML20366A023
References
FOIA, FOIA/PA-2017-0690, NRC-2017-000688
Download: ML20366A021 (80)
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