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Calculation 32-9207374-000, Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitaion, Rev 18
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Site:	Arkansas Nuclear
Issue date:	04/30/2014
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Page 1 of 157 0402-01-F01 (Rev. 017, 11/19/12)

Assure that Proprietary statement here is deleted, if applicablePROPRIETARY CALCULATION

SUMMARY

SHEET (CSS)

Document No.

32 9207374 000 Safety Related:

Yes No Title Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation PURPOSE AND

SUMMARY

OF RESULTS:

The purpose of this calculation is to establish the Probable Maximum Precipitation (PMP) at Arkansas Nuclear One (ANO), both at the site and its upstream Arkansas River watershed. The PMP will be used as input to Calculation No. 32-9207382-000 ANO Local Intense Precipitation - Generated Flow and Elevations, and Calculation No. 32-9207375-000 ANO Probable Maximum Flood (PMF) on Arkansas River - Hydrology. This calculation supports the flood hazard re-evaluation of ANO Units 1 and 2.

The results of the re-evaluated PMP at ANO are as follows:

1. A site specific PMP study was performed for the 1-hour, 1-square-mile point PMP and the 6-hour, 10-square-mile PMP for application to the Local Intense Precipitation. The 1-hour, 1-square-mile point PMP and the 6-hour, 10-square-mile PMP were calculated as 16.3 inches and 22.0 inches, respectively.

2. A site specific PMP study was also conducted for the Arkansas River watershed near ANO because the watershed (over 150,000 square miles) exceeds the watershed area upper limit of 20,000 square miles in Hydrometeorological Reports 51 and 52.

3. The results on the site specific PMP study are that the maximum storm duration is 72-hours and the maximum storm size is 100,000 square miles.

4. The 100,000 square mile PMP does not cover the entire watershed due to meteorological limitations on storm duration, size, and orientation.

5. The average PMP rainfall depth for the portion of the watershed covered by the 100,000 square mile, 72-hour PMP centered at the watershed centroid is 7.7 inches. A higher average PMP rainfall depth of 10.2 inches for the 72-hour duration occurs with the PMP centered at the centroid of the Robert Kerr subwatershed. See Sections 6.4.1 and 6.4.2 for the results of other PMP storm center locations.

- Note, for this template Green bracketed text indicates text that should be replaced for the specific calculation, and yellow highlighted text is guidance and should be deleted prior to finalization of document Comments should also be deleted and are used to provide additional guidance where yellow text is not appropriate..

AREVA NP INC. PROPRIETARY Cant read text below red line.This document and any information contained herein is the property of AREVA NP Inc. (AREVA NP) and is to be considered proprietary and confidential and may not be reproduced or copied in whole or in part. This document shall not be furnished to others without the express written consent of AREVA NP and is not to be used in any way which is or may be detrimental to AREVA NP. This document and any copies that may have been made must be returned to AREVA NP upon request.

THE DOCUMENT CONTAINS ASSUMPTIONS THAT SHALL BE VERIFIED PRIOR TO USE THE FOLLOWING COMPUTER CODES HAVE BEEN USED IN THIS DOCUMENT:

CODE/VERSION/REV CODE/VERSION/REV YES NO 0402-01-F01 (Rev. 018, 01/30/2014)

A AREVA 0402-01-F01 (Rev. 018, 01/30/2014)

Document No. 32-920737 4-000 Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Review Method:~ Design Review (Detailed Check)

D Alternate Calculation Signature Block P/RIA Name and Title and Pages/Sections (printed or typed)

Signature LP/LR Date Prepared/Reviewed/ Approved David Leone, GZA LP

'1/1-Y j1.0 11 All except Appendix A Hydraulic Engineer William Kappel, Appendix A AWA Meteorologist

. Peter Baril, GZA All except Appendix A Hydrologist Ed Tomlinson, Appendix A AWA Meteorologist Daniel T. Brown All Scientist Ill,

~/zt/11 Environmental Health and Safety Ancdysis Mark A. Rinckel A

All Technical 1/loa/11-

Manager, 1?~

Radiological and Environmental Analysis Note: P/RIA designates Preparer (P), Reviewer (R), Approver (A);

LP/LR designates Lead Preparer (LP), Lead Reviewer (LR)

Project Manager Approval of Customer References (N/A if not applicable)

Name Title (printed or typed)

(printed or typed}

Signature N/A Date Page2

Document No. 32-9207374-000 0402-01-F01 (Rev. 017, 11/19/12)

PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Signatures (continued)

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Mentor to:

(P/R)

Signature Date N/A 0402-01-F01 (Rev. 018, 01/30/2014)

Document No. 32-9207374-000 0402-01-F01 (Rev. 017, 11/19/12)

PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 4 Record of Revision Revision No.

Pages/Sections/Paragraphs Changed Brief Description / Change Authorization 000 All Initial Issuance 0402-01-F01 (Rev. 018, 01/30/2014)

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 5 Table of Contents Page SIGNATURE BLOCK............................................................................................................................. 2 RECORD OF REVISION....................................................................................................................... 4 LIST OF TABLES.................................................................................................................................. 7 LIST OF FIGURES................................................................................................................................ 8

1.0 INTRODUCTION

........................................................................................................................ 9 1.1 Background.......................................................................................................................................9 1.2 Purpose.............................................................................................................................................9 2.0 METHODOLOGY....................................................................................................................... 9 2.1 Local Intense Precipitation PMP Calculation................................................................................. 10 2.2 Perform Site Specific Probable Maximum Precipitation Study...................................................... 10 2.2.1 Storm-Based Approach................................................................................................... 11 2.2.2 Storm maximization and transposition............................................................................. 11 2.2.3 PMP Depth-Area and Depth-Duration............................................................................. 11 2.3 Calculate Site Specific Depth-Area-Duration data for the watershed for various storm centers... 12 2.4 Calculate 6-hour Incremental 72-hour PMP Values....................................................................... 12 2.4.1 72-Hour PMP - Moving Storm.......................................................................................... 16 2.5 Cool-Season PMP.......................................................................................................................... 17 3.0 ASSUMPTIONS....................................................................................................................... 17 3.1 Unverified Assumptions.................................................................................................................. 17 3.2 Justified Assumptions..................................................................................................................... 18 3.3 Model Simplifications...................................................................................................................... 18 4.0 DESIGN INPUTS...................................................................................................................... 18 5.0 COMPUTER USAGE............................................................................................................... 18 5.1 Software......................................................................................................................................... 19 5.2 Computer Files............................................................................................................................... 19 6.0 CALCULATIONS AND RESULTS............................................................................................ 19 6.1 Local Intense Precipitation PMP Calculation................................................................................. 20 6.2 Perform Site Specific Probable Maximum Precipitation Study...................................................... 20 6.3 Calculate Site Specific Depth-Area-Duration data for various stationary PMP storm centers....... 22 6.4 Calculate 6-hour Incremental 72-hour PMP Values....................................................................... 22 6.4.1 72-Hour Stationary PMP.................................................................................................. 22 6.4.2 72-Hour Moving PMP...................................................................................................... 23

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Table of Contents (continued)

Page Page 6 6.5 Cool-Season PMP.......................................................................................................................... 24

7.0 CONCLUSION

S....................................................................................................................... 25

8.0 REFERENCES

......................................................................................................................... 25 APPENDIX A :

PROBABLE MAXIMUM PRECIPITATION AND LOCAL INTENSE PRECIPITATION ANALYSIS FOR ARKANSAS NUCLEAR ONE......................................................................... A-1 APPENDIX B :

FIGURES FROM HYDROMETEOROLOGICAL REPORT NO.52........................................... B-1 APPENDIX C :

AVERAGE 6-HOUR INCREMENTAL PMP SPREADSHEETS................................................ C-1 APPENDIX D :

EXTRAPOLATED PERCENTAGE OF 6-HOUR PMP INCREMENT HMR-52 BASED NOMOGRAPHS AND TABLES................................................................................................. D-1 APPENDIX E :

FIGURE 16, 18, 19 AND 20, TABLE 15, 16, 17 AND 18, HYDROMETEOROLOGICAL REPORT NO.52........................................................................................................................ E-1 APPENDIX F :

AVERAGE MONTHLY SNOW DEPTH..................................................................................... F-1 APPENDIX G :

SOFTWARE VERIFICATION................................................................................................... G-1 APPENDIX H :

SPAS SOFTWARE CERTIFICATION....................................................................................... H-1 APPENDIX I :

DEPTH-AREA-DURATION TABLES.......................................................................................... I-1 APPENDIX J :

PROBABLE MAXIMUM PRECIPITATION HYETOGRAPHS................................................... J-1

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 7 List of Tables Page Table 1: Point (1 square mile) Probable maximum Precipitation Depths............................................. 28 Table 2: Location of Stationary Storm Centers.................................................................................... 28 Table 3: Location of Moving Storm Centers........................................................................................ 28 Table 4: Isohyetal Pattern Orientations for Stationary PMP................................................................. 28 Table 5: Maximum 18-hour Precipitation Volume for Stationary PMP................................................. 28 Table 6: Isohyetal Pattern Orientations for Moving PMP..................................................................... 29 Table 7: Maximum 18-hour Precipitation Volume for Moving PMP...................................................... 29 Table 8: Cool Season Maximum Monthly Average Snow Depth for Arkansas..................................... 29 Table 9: Cool Season Maximum Monthly Average Snow Depth for Kansas........................................ 30 Table 10: Cool Season Maximum Monthly Average Snow Depth for Missouri.................................... 30 Table 11: Cool Season Maximum Monthly Average Snow Depth for Oklahoma................................. 30 Table 12: Cool Season Maximum Monthly Average Snow Depth for Texas........................................ 31 Table 13: Cool Season Maximum Monthly Average Snow Depth for Colorado................................... 31 Table 14: Cool Season Maximum Monthly Average Snow Depth for New Mexico.............................. 31

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 8 List of Figures Page Figure 1: Site Locus Map.................................................................................................................... 33 Figure 2: Subwatershed Map.............................................................................................................. 34 Figure 3: 1-hour PMP - Incremental Hyetograph................................................................................ 35 Figure 4: 6-hour PMP - Incremental Hyetograph................................................................................ 35 Figure 5: Stationary Storm Center Locations...................................................................................... 36 Figure 6: Moving Storm Center Locations........................................................................................... 37 Figure 7: Moving Storm Center Locations - 160 Miles per Day........................................................... 38 Figure 8: Moving Storm Center Locations - 80 Miles per Day............................................................. 39 Figure 9: Stationary PMP at Watershed Centroid Isohyet Orientation................................................. 40 Figure 10: Stationary PMP at John Martin Subwatershed Centroid Isohyet Orientation...................... 41 Figure 11: Stationary PMP at Robert Kerr Subwatershed Centroid Isohyet Orientation...................... 42 Figure 12: Moving PMP (160 miles per day) Isohyet Orientation......................................................... 43 Figure 13: Moving PMP (160 miles per day) Point 1 Centroid Isohyet Orientation.............................. 44 Figure 14: Moving PMP (160 miles per day) Point 2 Centroid Isohyet Orientation.............................. 45 Figure 15: Moving PMP (160 miles per day) Point 4 Centroid Isohyet Orientation.............................. 46 Figure 16: Moving PMP (160 miles per day) Point 6 Centroid Isohyet Orientation.............................. 47 Figure 17: Moving PMP (160 miles per day) Point 7 Centroid Isohyet Orientation.............................. 48 Figure 18: Moving PMP (80 miles per day) Isohyet Orientation........................................................... 49 Figure 19: Moving PMP (80 miles per day) Point 2 Centroid Isohyet Orientation................................ 50 Figure 20: Moving PMP (80 miles per day) Point 3 Centroid Isohyet Orientation................................ 51 Figure 21: Moving PMP (80 miles per day) Point 4 Centroid Isohyet Orientation................................ 52 Figure 22: Moving PMP (80 miles per day) Point 5 Centroid Isohyet Orientation................................ 53 Figure 23: Moving PMP (80 miles per day) Point 6 Centroid Isohyet Orientation................................ 54

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 9

1.0 INTRODUCTION

1.1 Background

Following the Fukushima Daiichi accident on March 11, 2011, which resulted from an earthquake and subsequent tsunami, the U.S. Nuclear Regulatory Commission (NRC) established the Near-Term Task Force (NTTF) to review the accident.

In response to the NTTF recommendations, and pursuant to Title 10 of the Code of Federal Regulations, Section 50.54 (f), the NRC has requested information from all operating power licensees (NRC, 2012). The purpose of the request is to gather information to re-evaluate seismic and flooding hazards at U.S. operating reactor sites.

Arkansas Nuclear One (ANO), located on a peninsula formed by Lake Dardanelle (Pool No. 10, Dardanelle Lock and Dam as owned and operated by the U.S. Army Corps of Engineers), is one of the sites required to submit information. ANO is located near river mile 210 of the Arkansas River, approximately 6 miles northwest of Russellville, Arkansas. A locus map of the site is included as Figure 1.

The NRC information request relating to flooding hazards requires licensees to re-evaluate their sites using updated flooding hazard information and present-day regulatory guidance and methodologies.

The results of the re-evaluation are then compared against the sites current licensing basis (CLB) for protection and mitigation from external flood events. Attachment 1 to Enclosure 2 of the NRC information request (NRC, 2012) identifies eight flood causing mechanisms to be considered, as well as a combined effect flood.

This document addresses one of the inputs, the Probable Maximum Precipitation (PMP) to flooding causing mechanisms at ANO, the Local Intense Precipitation (LIP) and the Probable Maximum Flood (PMF). The other mechanisms are addressed in other documents.

This calculation was prepared by GZA GeoEnvironmental, Inc. under subcontract to AREVA.

1.2 Purpose The purpose of this calculation is to establish the PMP on the local drainage area (for the LIP analysis) of ANO Units 1 and 2 and the separate larger watershed of the Arkansas River upstream of ANO.

As noted below, the watersheds locations, areas, boundaries and configurations, used as input in this calculation, were developed in another calculation (AREVA, 2014a).

This calculation also used a site specific Probable Maximum Precipitation Study provided by Applied Weather Associates, LLC (AWA), under subcontract to GZA, as technical input for the development of the site specific PMP (see Appendix A).

2.0 METHODOLOGY The calculation methodology is described below. Unless noted otherwise, the methodology used in the calculation is consistent with the following standards and guidance documents:

1. NRC Standard Review Plan, NUREG-0800, revised March 2007 (NRC, 2007);

2. NRC Office of Standards Development, Regulatory Guides:

RG 1.102 - Flood Protection for Nuclear Power Plants, Revision 1, dated September 1976 (NRC, 1976);

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 10 RG 1.59 - Design Basis Floods for Nuclear Power Plants, Revision 2, dated August 1977 (NRC, 1977);

3. NUREG/CR-7046 - Design Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, dated November 2011 (NRC, 2011); and

4. American National Standard for Determining Design Basis Flooding at Power Reactor Sites (ANSI, 1992).

The method of analysis for calculation of the Local Intense Precipitation PMP is summarized briefly below and in more detail as described in Sections 2.1 through 2.2. The calculation methodology includes the following steps:

Local Intense Precipitation PMP Calculation:

1. Perform site specific Probable Maximum Precipitation study to calculate the one-hour, one-square mile and six-hour, ten-square mile PMP.

The method of analysis for the PMP over the contributory Arkansas River watershed at ANO is summarized briefly below and in more detail as described in Sections 2.2 through 2.4. The calculation methodology includes the following steps:

1. Perform site specific PMP Study

2. Calculate site specific depth-area-duration data for various PMP storm centers.

3. Calculate 6-hour incremental 72-hour PMP values.

2.1 Local Intense Precipitation PMP Calculation The Local Intense Precipitation event is a distinct flooding mechanism that consists of locally heavy rainfall centered upon the plant site itself. As per guidance contained in NUREG/CR-7046 (NRC, 2011), the LIP is considered to include the one-square-mile, one-hour-duration PMP. The one-square-mile, one-hour-duration PMP depth was calculated based on the site specific Probable Maximum Precipitation study (see Section 2.2 below and also refer to Appendix A for a detailed summary of this work). Note that the cool season PMP with snow melt is typically not part of the LIP evaluation; therefore, the cool season with snow melt was not incorporated in the LIP analysis (see Section 2.5 for additional discussion). The sub-one hour divisions are calculated based on the methodology of Hydrometeorological Report No. 52 (HMR-52) (NOAA, 1982). The sub-hour divisions for the one-hour, one square-mile PMP are based on Figures 36, 37, and 38 in HMR-52 (NOAA, 1982, see Appendix B).

The LIP also included the ten-square-mile, six-hour duration PMP, which was calculated as part of the site specific study (see Appendix A). The sub divisions of the ten-square-mile six-hour PMP are calculated based on methodology of HMR-52 and are the same as the sub-divisions for the one-square-mile one-hour PMP for the first hour. The sub-divisions from the second hour to the sixth hour are based on recommendations in NUREG/CR-7046 (NRC, 2011).

2.2 Perform Site Specific Probable Maximum Precipitation Study The majority of the ANO watershed is within the domain of the National Weather Service / U.S. Army Corps of Engineers Hydrometeorological Report No. 51 (HMR-51) (NOAA, 1978), with areas west of 103° longitude covered by HMR 55A. However, the use of HMR-51 for developing PMP values was not applicable due to the watershed area size limitation of 20,000 square miles. The watershed contributory to the Arkansas River upstream of ANO is over 150,000 square miles.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 11 A site specific PMP study for ANO was therefore performed (Appendix A). The purpose of the study was to calculate PMP values specific to entire contributory watershed of the Arkansas River upstream of ANO. This included only an all season PMP analysis due to the climatology of the watershed, which would not support a cool-season PMF that would exceed the all-season PMF (see Section 2.5).

The understanding of meteorological processes, interactions, and storm patterns have advanced since the publication of HMR-51 and HMR-55A. Satellite and radar technology have added to the understanding of storm patterns over the last 40 years. The site specific study incorporated the current understanding and updated technology to analyze extreme rainfall events.

2.2.1 Storm-Based Approach The approach used in the study is storm-based using many of the procedures used by the National Weather Service (NWS) in the development of the HMRs. These same procedures are recommended by the World Meteorological Organization (WMO) for PMP determination (WMO, 1986 and WMO, 2009). The methods and procedures used to derive the site specific PMP values are similar to other site specific PMP studies conducted by AWA located within the HMR-51 domain. This approach identifies extreme rainfall events that have occurred in a region that has meteorological and topographical characteristics similar to extreme rain storms that could occur over the ANO watershed.

The largest of these historic rainfall events are selected for detailed analyses.

Extreme rainfall events were identified as storms having similar characteristics to extreme rainfall events that could potentially occur over the ANO watershed and could potentially influence PMP values at one or more of the area sizes and/or durations analyzed.

2.2.2 Storm maximization and transposition HMR procedures for maximization, transposition, and elevation moisture adjustments were used in the site specific study. Additional techniques, as described in Appendix A, were used in the study to increase accuracy and reliability, while adhering to the basic procedures depicted in the HMRs and in the WMO Manuals. For the storms previously analyzed as part of existing PMP studies (i.e., HMR-52, see reference list in Appendix A), existing storm maximization factors were used. For additional storms analyzed as part of this calculation using the Storm Precipitation Analysis System (SPAS),

maximization factors were calculated following standard HMR / WMO procedures and employing updated climatology and data.

2.2.3 PMP Depth-Area and Depth-Duration The storms of interest were maximized, transpositioned, and elevation-adjusted to the watershed centroid and to each of 22 grid points used to distribute the PMP over the large ANO watershed.

Depth-Area (DA) plots were made for durations of 6-, 12-, 24-, 48-, and 72-hour for area sizes of 10-, 200-, 1,000-, 5,000-, 10,000-, 20,000-, 50,000-, and 100,000-square miles. Enveloping curves were constructed for each set of curves at each grid point and at the watershed centroid.

Depth-Duration (DD) plots were then made and curves constructed. These final curves provide PMP values for each grid point and the watershed centroid. The resulting values were spatially interpolated using ArcMap 10.1 Geographic Information System (GIS) and manually adjusted to ensure continuity in space and time across the watershed. The results allow PMP values for standard duration and project specific and standard area sizes (up to 100,000-square miles), to be calculated from any point within the entire watershed.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 12 2.3 Calculate Site Specific Depth-Area-Duration data for the watershed for various storm centers Site specific depth-area-duration (DAD) data was calculated using the results of the site specific PMP study described above based on the location of the storm center being evaluated. Given the large watershed size, several storm centering locations were selected based on the shape/configuration of the watershed and evaluated to support the generation of candidate PMF inflow hydrographs.

Site-specific DAD data was generated in a graphical (i.e., GIS-based) and limited (watershed centroid only) tabular format (Appendix A). DAD information (resulting from the site-specific PMP study) was extracted for storm durations of 6, 12, 24, 48, and 72-hours for area sizes of 10, 200, 1,000, 5,000, 10,000, 20,000, 50,000, and 100,000 square miles. The site specific study findings indicated the 72-hour duration captures the significant storm rainfall and evaluation of longer durations was not necessary.

DAD values were extracted from the GIS maps for each storm center evaluated, except the storm evaluated at the watershed centroid. Subwatershed centroids were calculated using the calculated x-coordinate and y-coordinate feature in GIS. The DAD values for the location of the storm centers being evaluated are extracted from GIS using the information feature.

2.4 Calculate 6-hour Incremental 72-hour PMP Values The methodology of HMR-52 was applied to generate 6-hour incremental 72-hour PMP values.

In certain instances, noted below, the procedures used in HMR-52 were extrapolated to cover the ANO watershed area (i.e., 72-hour PMP values for storms larger than 20,000 square miles).

The methodology applied is as follows (NOAA, 1982):

1. Establish the center of the PMP storm to be evaluated based on the shape/configuration of the watershed. 6-hour Incremental PMP values were calculated from the site specific DAD data for various storm center locations. Site specific DAD data allowed isohyetal patterns greater than 20,000 square miles, (i.e. 50,000 and 100,000 square miles) to be used.

a) The DAD data was plotted on a semi-log scale, with the storm area size plotted on the log scale. For each storm duration (6, 12, 24, 48, and 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months ) the points are joined to create smooth curves. The curves are either parallel or slightly converging with increasing area size.

b) Depth-duration data was obtained from the DAD curves. For a set of standard isohyetal area sizes (provided in HMR-52 and extrapolated to cover the ANO watershed), DAD values are taken from the curves.

c) For each of the isohyetal area sizes selected, the depth-duration data was plotted.

Smooth curves are creating by connecting values of common isohyetal area sizes.

From these curves, the 18-hour duration was obtained for each of the isohyetal areas.

d) Incremental differences for the first three 6-hour periods were obtained through the depth-duration values for the 6, 12, and 18 hour2.083333e-4 days 0.005 hours 2.97619e-5 weeks 6.849e-6 months duration for the isohyetal areas of interest. The values are calculated by subtracting depth-duration values from each successive time step (i.e., the incremental value for the 2nd 6-hour period is equal to the 12-hour depth-duration value minus the 6-hour value). The 6-hour values were plotted on a semi-log scale and smoothed to create a consistent data set.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 13 a) Isohyetal Pattern a) Using GIS, standard isohyetal patterns (NOAA, 1982, Appendix B) and additional larger isohyetal patterns consistent with HMR-52 geometry (NOAA, 1982, Appendix B), were placed over the storm center of interest. Several storm centers were analyzed by placing the isohyetal pattern over the centroids of different subwatersheds. Visual inspection of the isohyetal pattern was used to encompass the greatest number of whole isohyets within the drainage area to maximize the extent of precipitation occurring in the watershed, by rotating the isohyetal pattern about the subwatershed centroid of interest within the range recommended by HMR-52.

b) The orientation of the pattern placed on the drainage was found relative to degrees clockwise from the north.

c) The preferred orientation of the PMP from HMR-52 (see Appendix B) was obtained for each of the storm centers. HMR-52 states that if the selected storm orientation differs from the preferred pattern by more than 40°; a reduction to the isohyetal values should be made.

b) Maximum Precipitation Value The maximum volume of precipitation for the three largest 6-hour increments were calculated based on the placement of the preferred isohyetal pattern over the drainage using the following steps (see spreadsheets in Appendix C).

a) The first area size from step 1b - (the isohyetal patterns smaller and larger than the drainage area) is inserted into the column labeled Area Size.

b) Column I (Iso) contains the list of standard isohyetal labels.

c) For the area size selected in step 3a, list the appropriate percentages (percent of 1st 6-hour PMP increment, see Appendix D, Figure D1) in column II for each of the isohyets selected in step 3b. The second, third and fourth through twelfth, 6-hour increment percentages (see Appendix D Figure D2, D3 and D4, respectively) will be used in subsequent steps.

The percentages of each 6-hour increment of the PMP were calculated from nomographs and/or tables. The figures included in Appendix D were calculated by extrapolating the nomographs and tables included within HMR-52 (Appendix E), which were limited to storm sizes up to 20,000 square miles. Due to the size of the watershed contributory to ANO, larger standard isohyetal area size and two non-standard isohyetal area sizes are necessary to generate a PMP over the drainage area. The larger standard sizes used are isohyets Q, R, and S corresponding to 25,000, 40,000 and 60,000 square miles. The additional non-standard sizes are the 50,000 and 100,000 square miles isohyets. The nomographs and tables included in HMR-52 were extrapolated to include the additional larger storm sizes (25,000, 40,000, 50,000, 60,000 and 100,000 square miles). The additional percentages were extrapolated from the nomographs and tables to maintain the continuity of the curves presented in the HMR-52 nomographs. The extrapolated nomographs and tabular data are included in Appendix D.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 14 d) The value from step 1c that corresponds to the area size specified in step 3a and increment of computation from step 3c is placed in column III, under the heading Amt.

The percentage in column II is multiplied by the amount (Amt.) specified in this step to fill in column III.

e) Column IV describes the average depth between adjacent isohyets. The average depth for isohyet A is equal to value from column III. For other isohyets fully encompassed by the drainage area, the average depth is the arithmetic average with the subsequent isohyet.

For isohyets which are not fully encompassed within the drainage area, weighted averages are used to describe the portion of the drainage area which extends beyond the isohyet of interest. The average depth for the portion of the drainage extending beyond the isohyet is taken to be between 0.5 to 1 times the difference between the enclosing isohyet plus the lower isohyet. The weighed relation between the isohyets is:

Where F is between 0.5 and 1, A and B are adjacent isohyet values.

When only a small portion of drainage extends beyond isohyet A then F would be closer

1. If the portion of the drainage area extends almost to isohyet B then the value of F is close to 0.5.

f) Column V (A) is the incremental area (in square miles) between adjacent isohyets.

See Appendix B for the incremental areas for isohyets that are fully enclosed by the drainage area. GIS was used to calculate the incremental areas between isohyets for isohyets that are not fully encompassed. The GIS calculate geometry for area command was used to calculate the area of the watershed between adjacent isohyets using the preferred isohyetal orientation placed over the storm center being evaluated.

g) Column VI lists the incremental volume (in inch-square miles) between adjacent isohyets and is calculated by multiplying the average depths from column IV by the incremental areas in column V. Summing the incremental volumes yields total volume of precipitation in the watershed for the specified pattern area size in a 6-hour period.

h) Steps 3a through 3g are repeated for other storm pattern area sizes selected in step 1b.

i) The 2nd and 3rd 6-hour incremental volumes are computed by repeating steps 3a through 3h.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 15 j) The incremental volumes for the 1st, 2nd and 3rd 6-hour increments, calculated in steps 3g and 3i are summed at corresponding storm area sizes and plotted against storm area size on a semi log plot. The storm area size for the precipitation pattern that yields the maximum 18-hour volume over the watershed is found from this plot.

c) Distribution of Storm-Area Averaged PMP over the Watershed:

a) For the storm area size for the PMP calculated in step 3j, the values are extracted from the smoothed curves on the depth-duration graphs for each 6 hour6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months increment up to 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months .

b) The incremental 6-hour amounts are calculated for the 4th through 12th 6-hour increment by sequentially subtracting the values from step 4a. Note that the values should decrease with increasing 6-hour increments (NOAA, 1982). If the values do not decrease, the order was rearranged.

c) Step 4b yields the incremental average depths for each of 12, 6-hour increments which make the 72 hour8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months PMP storm. The values of the isohyets that cover the drainage during the 72 hour8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months PMP was computed by multiplying the 1st 6-hour incremental depth by the 1st 6-hour percentages obtained in step 3c for the storm area size calculated in step 3j.

This was repeated for the 2nd, 3rd and 4th through 12th 6-hour increments using the appropriate nomographs as discussed in step 3c. The isohyetal matrix is compiled from these resulting values.

d) To calculate the incremental average depths for the watershed, spreadsheets similar to those explained in step 3 were set up. Tables were set up for the storm area size selected in step 3j and for all twelve, 6-hour increments. The inputs to column II and column III were explained in step 3c. The values for columns IV and V are the same from step 3e and 3f for the selected storm area size. Column VI yields the incremental volume between adjacent isohyets and was calculated by multiplying the average depths from column IV by the incremental areas in column V. Summing the incremental volumes yields the total volume of precipitation in the watershed for the specified pattern area size in a 6-hour period. Dividing this by the area of the drainage encompassed by the selected storm size yields the average depth for a given 6 hour6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months increment.

d) Temporal Distribution The temporal distribution for the PMP was based on recommendations from HMR-52, Section 2.3, Recommended Sequence for PMP Increments, as follows:

a) The 6-hour increments are arranged to decrease to either side of the greatest 6-hour increment.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 16 b) The 4 greatest 6-hour increments are placed at any portion within the sequences except during the first 24-hour period.

The hyetograph for the all season PMP and the moving all season PMP were constructed using 40 percent of the PMP depths during the first 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months (i.e., antecedent storm), followed by a dry 72-hour period, and finally followed by the full 72-hour PMP storm (NRC, 2011).

e) Spatial Distribution of PMP across Subwatersheds The spatial distribution of the PMP across the subwatersheds was calculated by the following steps:

a) For the storm area size for the PMP calculated in step 3j and with the isohyetal storm pattern placed on the subwatershed as in step 2a, the incremental areas contained between isohyets within each subwatershed were calculated using the same process as described in step 3f.

b) Following steps 3d to 3g, as outlined above, the incremental subwatershed volumes for the twelve, 6-hour increments was calculated.

c) For each 6-hour increment, the subwatershed volumes were divided by the subwatershed area covered by precipitation to yield the average depth for the subwatershed.

f) Effective Area Adjustment Factors The spatially distributed PMP values for each subwatershed was adjusted based on the percent of the subwatershed that is covered by the PMP, and was calculated as follow:

a) The area covered by the PMP for each subwatershed was calculated in GIS using the calculate geometry command.

b) The effective area adjustment factor was calculated as the area of the subwatershed covered by the PMP divided by the total area of the subwatershed.

c) The effective area adjustment factor was applied to the spatially distributed PMP values for each subwatershed.

2.4.1 72-Hour PMP - Moving Storm The large drainage area contributory to the Arkansas River upstream of ANO makes it meteorologically reasonable for the rainfall center to travel across the drainage with time during the storm. It is conceivable that such movement could result in a higher flood peak if the direction and speed of movement coincides with downstream progression of the flood crest. While HMR-52 states that for drainages larger than 10,000 square miles, moving PMPs may produce high flood peaks, neither HMR-52 (NOAA, 1982) or NUREG/CR-7046 (NRC, 2011) does not provide guidance or design methodology for calculating a moving PMP.

The approach to calculate a moving 72-hour PMP was to evaluate the movement as three distinct storm center locations. Three 24-hour periods of the 72-hour PMPs centered at different points in the watershed were generated to simulate the movement of a PMP. A 24-hour period of precipitation is taken from each of the three storm centers, resulting in the total storm duration of 72-hours.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 17 The speed that the PMP moves across the watershed and the storm track (direction the storm move through the watershed) were selected to maximize precipitation in the watershed and were selected with AWA consultation and limitations calculated in Appendix A. The minimum storm velocity is 20 miles per day and the maximum velocity is 200 miles per day (Appendix A).

Storm track is defined with a starting position and end position of the moving storm. The position is defined numerically with north being 360, east at position 90, south at position 180 and west being 270.

A storm moving from north to south would have a track from position 360 to 180. The minimum storm ordinates correspond to a storm moving from position 220 to 40, and the maximum is moving from position 320 to 140 (Appendix A).

The three 72-hour PMPs were generated following the procedure described above. The temporal distribution of the moving storm is the same distribution used in the stationary storms as described in Section 2.4, step 5. The moving storm hyetographs were constructed by taking a 24-hour period of precipitation from each of the three storm center locations resulting in the total storm duration of 72-hours. The first 24-hour period was selected from first four of the 6-hour precipitation increments calculated from the first storm centering. The second 24-hour period is selected from the second set of four 6-hour precipitation increments calculated from the second storm centering. The last 24-hour period was selected from the final four of the 6-hour precipitation increments calculated from the third storm centering.

The hyetographs for the moving PMPs were constructed using 40 percent of the PMP depths during the first 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months (antecedent storm), followed by a dry 72-hour period, and finally followed by the full 72-hour moving PMP storm (NRC, 2011). The antecedent storm was modeled as a separate moving storm following the methodology described above. Each of the final hyetograph for the two moving PMP consists of 5 storm centers, with one storm center shared by the final center for the antecedent storm and the initial center of the PMP.

2.5 Cool-Season PMP The cool-season PMP for ANO was not evaluated because the climatology of the watershed does not support a bounding or controlling cool-season PMF consisting of a cool-season PMP in combination with snowmelt. The time of year when the PMP occurs (April through October) has no significant snowpack would be available in the majority of the watershed. Representative National Oceanic and Atmospheric Administration (NOAA), National Climatic Data Center (NCDC) Global Historical Climatology Network (GHCN) weather stations were selected to calculate maximum monthly average snow depth throughout the watershed. Multiple stations were analyzed for each state within the watershed. Stations were selected based on having an elevation that was representative for the portion the watershed it was located in, as well as length and quality of snow depth data. The entire period record for the selected gages was obtained from NOAAs online climate data (NOAA, 2013).

For each cool season month (January through April and October through December) the average monthly snow depth was calculated, and subsequently the maximum of the cool season monthly average was calculated. The average monthly snow depth was calculated using a Pivot Table in an Excel spreadsheet which is included as Appendix F.

3.0 ASSUMPTIONS 3.1 Unverified Assumptions There are no assumptions that require verification in this calculation.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 18 3.2 Justified Assumptions The following justified assumptions were made:

1. Site specific PMP study was used because the watershed area size of the Arkansas River at ANO exceeds the limitation of 20,000 square miles of HMR-51 (see Section 2.2).

2. The site specific PMP included only an all season PMP analysis due to the climatology of the watershed, which would not support a cool-season PMF that would exceed the all-season PMF (see Section 2.5).

3. The depth-duration values were spatially interpolated using GIS and manually adjusted based on the guidance of HMR-52 (NOAA, 1982) to ensure continuity in space and time across the watershed.

4. Given the large watershed size (i.e. over 150,000 square miles), several storm center locations were selected based on the shape/configuration of the watershed to develop a number of candidate PMP storms. These will be evaluated in the PMF calculation.

5. The nomographs and tables included in HMR-52 used to calculate the percentages of each 6-hour increment of the PMP were extrapolated based on information in HMR-52 to include the additional larger storm sizes: 25,000, 40,000, 50,000, 60,000 and 100,000 square miles.

6. Three 24-hour periods of the 72-hour PMPs centered at different points in the watershed were generated to conservatively simulate the movement of a PMP, covering as much of the watershed as practicable based on the results of the site specific PMP study. PMP speed and the storm track for the moving storms were selected to maximize precipitation in the watershed and were selected with AWA consultation and limitations.

Other assumptions made as part of the site-specific PMP study are noted in Appendix A, Section 13.1.

3.3 Model Simplifications No model simplifications were used in this calculation.

4.0 DESIGN INPUTS Design inputs are identified and addressed throughout the applicable sections of this calculation.

5.0 COMPUTER USAGE The following GZA workstations were used for the calculation:

System Name:

Microsoft Windows 7 Ultimate Version:

Service Pack 1 Computer Name:

01-stonierc Processor Intel Xeon CPU Memory:

12.00 GB of RAM System Name:

Microsoft Windows 7 Ultimate Version:

Service Pack 1 Computer Name:

01-HergottMA Processor Intel Xeon CPU E5620 Memory:

12.00 GB of RAM

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 19 The following AWA workstation was used for the calculation:

System Name:

Community enterprise Operating System (CentOS)

Version:

6.3 Computer Name:

Irene Processor Intel(R) Xeon(R) CPU E5530 @ 2.40 GHz Memory:

12 GB 5.1 Software

1. ESRI ArcMap' 10.1, Service Pack 1 (Build 3143)

2. Microsoft Excel' 2010, Version 14.0.7106.5001 (64-bit)

3. Storm Precipitation Analysis (SPAS by Applied Weather Associates, LLC)

ArcMap 10.1 (GIS) was used to generate graphic outputs of the calculated results and is not subject to verification per AREVA Procedure 0902-30, Section 4.6.

Excel 2010 was used to create calculation spreadsheets and organize data and is not subject to verification per AREVA Procedure 0902-30, Section 4.6. Excel spreadsheets are presented with dual tables: one with the spreadsheet itself and another showing only the formulas used in the spreadsheet.

SPAS is used for storm precipitation analysis for historic events under Section 4.7 of AREVA Procedure 0902-30. Software validation is provided in Appendix G. SPAS was tested on the computer used for this document by Douglas Hultstrand on January 3, 2014. The inputs for the installation test were the same as those used in the software verification report Appendix G, and the outputs are documented in Appendix H. The results of the installation test were acceptable.

5.2 Computer Files The full results from the site specific PMP, including SPAS input/output, for ANO can be found in Appendix A. Figures from HMR-52 can be bound in Appendix B. ArcMAP and Excel input/output is provided in Appendix C. Average monthly snow depth calculation is provided in Appendix F.

Depth-Area-Duration tables and maps can be found in Appendix I.

The path to the file is: \\cold\\General-Access\\32\\32-9000000\\32-9207374-000\\official 6.0 CALCULATIONS AND RESULTS The Arkansas River watershed was subdivided, into 22 subwatersheds based on the location of major tributaries to the Arkansas River, USGS stream gages and selected dams (Figure 2).

The following watershed areas are in units of square miles were calculated:

Arkansas River watershed area upstream of ANO:

approximately 153,380 square miles,

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 20 The following subwatershed areas, in units of square miles (mi2), were calculated:

Name Point of Delineation Area (mi2)

Amarillo USGS Stream Gage 07227500 19,010 Arkansas City USGS Stream Gage 07146500 7,640 Bridgeport USGS Stream Gage 07228500 5,810 Calvin USGS Stream Gage 07231500 2,720 Commerce USGS Stream Gage 07185000 5,940 Eufaula USGS Stream Gage 07245000 5,280 Great Bend USGS Stream Gage 07141300 17,290 Hudson USGS Stream Gage 07191500 1,200 Las Animas USGS Stream Gage 07124000 14,410 Lenapah USGS Stream Gage 07171000 3,590 Pensacola USGS Stream Gage 07190500 4,460 Ramona USGS Stream Gage 07175500 1,940 Ripley USGS Stream Gage 07161450 4,720 Seiling USGS Stream Gage 07238000 12,480 Tenkiller USGS Stream Gage 07198000 1,620 Tulsa USGS Stream Gage 07164500 12,920 Waynoka USGS Stream Gage 07158000 11,570 Wetumka USGS Stream Gage 07242000 2,030 John Martin Ungaged - John Martin Reservoir 4,170 Walnut Ungaged - Walnut Creek 1,870 Robert Kerr Robert S. Kerr Lock and Dam 6,760 Dardanelle Dardanelle Lock and Dam (outlet of ANO entire watershed) 5,950 6.1 Local Intense Precipitation PMP Calculation The one-square-mile, one-hour duration PMP was calculated in the site specific LIP analysis (see Appendix A) as 16.3 inches. The one-hour PMP was further subdivided into shorter duration increments based on methodology of HMR-52 (NOAA, 1982) as shown in Table 1.

The one-square-mile, one-hour duration PMP hyetograph was developed using guidance from NUREG/CR-7046 (NRC, 2011) and is shown in Figure 3.

The ten-square-mile, six-hour duration PMP was calculated in the site specific PMP analysis (see Appendix A) as 22.0 inches. The ten-square-mile, six-hour duration PMP hyetograph is shown in Figure 4.

6.2 Perform Site Specific Probable Maximum Precipitation Study The full results from the Probable Maximum Precipitation Study for ANO are included in Appendix A.

The site specific PMP values were calculated following the overall methodology used in HMR 51 and previous site-specific PMP studies in the area, and incorporated updated changes in technology, meteorological understanding, and availability of updated data. Consideration was given to the size of the ANO watershed as it relates to the PMP design storm movement and shape. The results from the site specific study indicated that it is not meteorologically feasible for the PMP to be larger than 100,000 square miles to occur in the ANO watershed based on the available storm data. Consideration was also given to the 72-hour duration limit in HMR-51/52 and longer durations were evaluated; however, the available data indicated durations beyond 72-hours were of limited importance (i.e., most

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 21 rainfall occurs well before the 72 hour8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months duration is reached or exceeded). The final PMP maps are included in Appendix A.

The following list contains the main conclusions from the site specific study:

HMR-51 and HMR-55A PMP values are outdated. The study provided updated PMP values to replace HMR-51 and HMR-55A values.

HMR-52 PMP design storm parameters were based on a set of storms that were not specifically transpositionable to the ANO watershed. The study provided updated PMP design storm movement guidance based on storms transpositionable to the ANO watershed and took into consideration the large size of this watershed.

The most recent storm used to derive PMP values in HMR-51 occurred in 1972. The study updated the storm database to include storms through 2013.

HMR-51 and HMR-52 did not use computer based technologies in the storm analyses procedures. The study used computer technology and GIS to more accurately analyze storm rainfall patterns and implement the spatially distributed PMP values.

HMR-51, HMR-52, and HMR-55A did not have weather radar to help spatially distribute rainfall among rain gage locations. SPAS storm analyses incorporates this information when available to provide the most advanced spatial representation of rainfall storm patterns possible.

The understanding of meteorological processes, interactions, and storm patterns have advanced since the publication of HMR-51 and HMR-55A. Satellite and radar technology have added to the understanding of storm patterns over the last 40 years. The site specific study incorporated the current understanding and updated technology to analyze extreme rainfall events.

HMR-51 and HMR-52 provide generalized and smoothed LIP values over a large geographic domain that covers the United States east of the 105th meridian. This calculation considered characteristics specific to the site, and produced PMP values that explicitly considered the meteorology of the PMP storm type which would result in the 1-hour 1-square mile area size LIP values.

The transposition limits of the Smethport, PA July 1942 storm, which produced the 4-and 6-hour world record rainfall, were not allowed to influence the LIP values at the ANO site.

The refined transposition limits used in this calculation result in lower LIP values compared to HMR-52 for locations where the Smethport storm apparently influenced PMP values in HMR-51. Smoothing of the PMP/LIP isolines in HMR-51 and HMR-52 necessarily had to encompass the Smethport maximized in-place rainfall far beyond its explicit transposition limits.

Note that Section 3.2.4 of HMR-51 states that they "slightly undercut" the maximize d 6-, 12-,

and 24-hour values by up to 7% to avoid "excessive envelopment of all other data in a large region surrounding the Smethport location." This over envelopment effect extended well beyond the intended transposition limits of the Smethport storm because the PMP/LIP isolines required smoothing and fitting over surrounding regions.

Each storms inflow vector was re-evaluated and combined with an updated set of dew point climatologies and when necessary, updated storm representative dew point values were used for the in-place maximization and transposition factors. A trajectory model was used to evaluate moisture inflow vectors for storms on the short storm list. Trajectory models were not available in HMR studies. Use of a trajectory model allowed for a high degree of confidence when evaluating moisture inflow vectors and storm representative dew points.

Several new storms have been analyzed and included in the LIP analysis that were not included in HMR-51 and HMR-52. This provided a higher level of confidence in the final PMP values.

Further, this allowed for a refined set of values that better represent the LIP estimates at the

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 22 site. This expanded the data set used to derive LIP includes a large number of recent storms where weather radar data were available.

The study provided adjustments for storm elevation to the nearest 100 feet of elevation, whereas HMR-51 and HMR-52 made no explicit adjustment for elevation. This adjustment depends on the elevation of the historic storm's maximum rainfall location and therefore varies from storm to storm.

Storms analyzed by the NWS/ U.S. Army corps of Engineers (USACE which) occurred prior to 1948 and used 12-hour persisting dew points in the storm maximization process were adjusted so that the updated dew point climatology could be used consistently. For thunderstorms and MCC storm events 7°F was added to the NWS/USACE storm representative dew point.

This was done to adjust for using average dew point values for varying durations vs. 12-hour persisting dew point values. Recent evaluations of 12-hour persisting storm representative dew points showed those used in HMR-51 underestimated the storm representative dew point values.

6.3 Calculate Site Specific Depth-Area-Duration data for various stationary PMP storm centers See Appendix A for detailed description for the calculation of site specific DAD data. For storm centers not located at the watershed centroid, values were obtained from site specific DAD maps (see Appendix A) created in GIS. Three storm centers were used for stationary PMPs:

1. Watershed Centroid: represents a PMP centered over the centroid of the overall Arkansas River watershed contributory to ANO;

2. Robert Kerr subwatershed centroid: represents a PMP centered over the lower portion of the watershed, closer to ANO than the overall watershed centroid;

3. John Martin subwatershed centroid: represents a PMP centered over the upper portion of the watershed, further from ANO than the overall watershed centroid.

The location of the storm centers evaluated are summarized in Table 2 and illustrated in Figure 5.

The effects of the various storm centers on the PMF will be evaluated in a separate calculation.

In addition to the locations list in Table 2, Table 3 identifies the location of the moving PMP storm centers (see Figures 6, 7 and 8).

DAD tables for the PMP at various storm centers are included as Appendix I.

6.4 Calculate 6-hour Incremental 72-hour PMP Values 6.4.1 72-Hour Stationary PMP The spreadsheets used to calculate 72-hour PMP values on a watershed average basis and on a subwatershed basis are included in Appendix C, the results are summarized below:

1. From the DAD tables, DAD plots and subsequent curves are created and as included in Appendix C. Depth-Duration data was read from the DAD curves for 8 isohyetal pattern sizes.

The selected isohyetal pattern area sizes selected were: 6,500, 10,000, 15,000, 25,000, 40,000, 50,000, 60,000 and 100,000- square miles. HMR-52 recommends selecting 4 pattern area sizes larger and 4 smaller than the watershed area. However, it is not considered meteorologically feasible for the PMP to be larger than 100,000 square miles.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 23

2. Isohyetal Pattern The isohyetal orientations were measured relative to degrees from the north. Listed below is the range of isohyetal orientations based on HMR-52 at the watershed centroid. Keeping the isohyetal orientation within the HMR-52, 40° range of the preferred isohyetal orientation, allows for the retention of the full value of the isohyets without having to apply the reduction factor while allowing for maximization of the orientation over the watershed.

HMR-52 preferred isohyetal orientation at watershed centroid is 225°.

HMR-52 preferred isohyetal storm orientation range: 185° to 265° The selected isohyetal pattern orientation for the PMP at the various storm centers are included as Figures 9 to 11 and are summarized in Table 4.

3. Maximum Precipitation Value The values of 18-hour Maximum Precipitation for the various PMP scenarios are included in the spreadsheets in Appendix C and are summarized in Table 5.

4. Distribution of Storm-Area Averaged PMP over the Drainage Area The 72-hour total watershed averaged PMP values are summarized below (see Appendix C for isohyetal matrix):

The total 72-hour PMP o Centered at Watershed Centroid 7.7 inches o Centered at John Martin Subwatershed Centroid 4.7 inches o Centered at Robert Kerr Subwatershed Centroid 10.2 inches

5. Temporal Distribution The temporal distribution for the PMPs are based in HMR-52 guidance and are as follows: 12, 11, 10, 9, 7, 6, 5, 3, 1, 2, 4, and 8, where the numbers indicate the largest precipitation (1) to smallest (12). The hyetograph for PMPs were constructed using 40 percent of the PMP depths during the first 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months , followed by a dry 72-hour period, and finally followed by the full 72-hour PMP storm (NRC, 2011).

6. Subwatersheds The hyetographs graphically and in tabular format for the subwatersheds for each of the PMP storm centers and are included within Appendix J.

7. Effective Area Adjustment Factor The hyetographs for the subwatersheds for each of the PMP storm centers and are included graphically and in tabular format within Appendix J.

6.4.2 72-Hour Moving PMP The selected isohyetal pattern orientation for the PMP for each storm center are included as Figures 12 to 23 and are summarized in Table 6:

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 24 The storm velocities selected were based on maximizing the precipitation occurring over the watershed (i.e., maximizing the number of fully encompassed isohyets for the three storm movements) and moving a distance large enough to generate a difference compared to a stationary PMP. The selected storm velocities were 160 miles per day and 80 miles per day.

The storm orientation (direction the PMP moves across the watershed) was selected to maximize the precipitation occurring over the watershed (i.e. maximizing the number of fully encompassed isohyets for the three storm movements). The storm orientation ordinates move from 293 to 113.

The values of 18-hour Maximum Precipitation for the moving PMP scenarios are included in the spreadsheets in Appendix C and are summarized in Table 7.

The 72-hour total watershed averaged PMP values are summarized below (see Appendix C for isohyetal matrix):

The total 72-hour Moving PMP (160 miles per day) 7.1 inches The total 72-hour Moving PMP (80 miles per day) 8.1 inches The temporal distribution for the all season and cool season PMPs are based in HMR-52 guidance and are as follows: 12, 11, 10, 9, 7, 6, 5, 3, 1, 2, 4, and 8, where the numbers indicate the largest precipitation (1) to smallest (12). The moving storm hyetographs were constructed by taking a 24-hour period of precipitation from each of the three storm center locations resulting in the total storm duration of 72-hours. The hyetographs for the moving PMPs were constructed using 40 percent of the PMP depths during the first 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months (antecedent storm), followed by a dry 72-hour period, and finally followed by the full 72-hour moving PMP storm (NRC, 2011). The antecedent storm was modeled as a separate moving storm following the methodology described above. The final hyetograph for the moving PMP consists of 5 storm centers.

The hyetographs graphically and in tabular format for the subwatersheds for the moving PMPs are included in Appendix J.

6.5 Cool-Season PMP The cool-season PMP for ANO was not evaluated because the climatology of the watershed does not support a bounding or controlling cool-season PMF consisting of a cool-season PMP in combination with snowmelt. The time of year when the PMP occurs (April through October) has no significant snowpack would be available in the majority of the watershed. The results from the analysis of the maximum monthly average snow depth during the cool season (January through April and October through December) indicated that on average there is less than 1.5 inches of snow within the watershed. The maximum month average snow depths are summarized in Tables 8 through 14.

The raw data and Pivot Tables are included as Appendix F.

Additionally, the largest amount of moisture available for rainfall (and thus the PMP) over the region comes from the Gulf of Mexico. The major types of extreme rainfall events in the region are produced by Mesoscale Convective Systems (MCS) (short durations and small area sizes), synoptic events/fronts (large areas sizes and longer durations), and remnant moisture from tropical systems which have made landfall along the Gulf of Mexico coastline. MCSs primarily form during the warm season months (April through October) around the ANO region and tropical systems typically occur from June through November. Since snow depth is insignificant and based on the results of the site specific PMP study,

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 25 no explicit cool-season PMP values and/or rain-on-snow analyses were completed. See Appendix A for additional discussion.

7.0 CONCLUSION

S The following summarizes the results and conclusions:

1.

A site specific PMP study was performed for the 1-hour, 1-square-mile point PMP and the 6-hour, 10-square-mile PMP for application to the Local Intense Precipitation. The 1-hour, 1-square-mile point PMP and the 6-hour, 10-square-mile PMP were calculated as 16.3 inches and 22.0 inches, respectively.

2.

A site specific PMP study was also conducted for the Arkansas River watershed near ANO because the watershed (over 150,000 square miles) exceeds the watershed area upper limit of 20,000 square miles in Hydrometeorological Reports 51 and 52.

3.

The results on the site specific PMP study are that the maximum storm duration is 72-hours and the maximum storm size is 100,000 square miles.

4.

The 100,000 square mile PMP does not cover the entire watershed due to meteorological limitations on storm duration, size, and orientation.

5.

The average PMP rainfall depth for the portion of the watershed covered by the 100,000 square mile, 72-hour PMP centered at the watershed centroid is 7.7 inches. A higher average PMP rainfall depth of 10.2 inches for the 72-hour duration occurs with the PMP centered at the centroid of the Robert Kerr subwatershed. See Sections 6.4.1 and 6.4.2 for the results of other PMP storm center locations.

Comparison of the stationary and moving storm for all storm centers, indicates that the controlling PMP (i.e., resulting in the PMF) cannot be determined based on inspection of rainfall data alone and these scenarios will be evaluated to determine the controlling PMF at ANO.

8.0 REFERENCES

ANSI, 1992. American National Standard for Determining Design Basis Flooding at Power Reactor Sites (ANSI/ANS 2.8-1992).

AREVA, 2014a. ANO Probable Maximum Flood on Arkansas River - Hydrology, GZA GeoEnvironmental, Inc., April 2014. AREVA Document No. 32-9207375-000 NOAA, 1978. Probable Maximum Precipitation Estimates - United States East of the 105th Meridian, Hydrometeorological Report No.51 (HMR-51) by US Department of Commerce, National Oceanic and Atmospheric Administration & U.S. Army Corps of Engineers, June 1978.

NOAA, 1982. Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian, Hydrometeorological Report No.52 (HMR-52) by US Department of Commerce, National Oceanic and Atmospheric Administration & U.S. Army Corps of Engineers, August 1982.

NOAA, 1988. Probable Maximum Precipitation Estimates - United States Between the Continental Divide and the 103rd Meridian, Hydrometeorological Report No.55A (HMR-55A) by US Department of Commerce National Oceanic and Atmospheric Administration & U.S. Army Corps of Engineers, June 1988.

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 26 NOAA, 2013. Climate Data Online, National Oceanic and Atmospheric Administration, National Climatic Data Center, http://www.ncdc.noaa.gov/cdo-web/search, date modified:

December 5, 2013, date accessed: December 5, 2013. See Electronic References*.

NRC, 1976. United States Nuclear Regulatory Commission Office of Standards Development, Regulatory Guides, RG 1.102 - Flood Protection for Nuclear Power Plants, Revision 1, dated September 1976.

NRC, 1977. United States Nuclear Regulatory Commission Office of Standards Development, Regulatory Guides, RG 1.59 - Design Basis Floods for Nuclear Power Plants, Revision 2, dated August 1977.

NRC, 2007. United States Nuclear Regulatory Commission Standard Review Plan, NUREG-0800, revised March 2007.

NRC, 2011. Design Basis Flood Estimation for Site Characterization at Nuclear Power Plants -

NUREG/CR-7046, United States Nuclear Regulatory Commission, November 2011.

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Reference archived in the AREVA file management system, ColdStor.

The path to the file is: \\cold\\General-Access\\32\\32-9000000\\32-9207374-000\\official

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 27 TABLES

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 28 Table 1: Point (1 square mile) Probable maximum Precipitation Depths Time (min)

PMP Depth (in) 60 16.3 30 12.2 15 8.4 5

5.4 Table 2: Location of Stationary Storm Centers Storm Center Latitude Longitude Watershed Centroid 36.9533

-99.8638 John Martin Subwatershed Centroid 37.4133

-103.9165 Robert Kerr Subwatershed Centroid 35.9541

-95.5654 Table 3: Location of Moving Storm Centers Storm Center Latitude Longitude Moving Point 1 38.2869

-105.4485 Moving Point 2 37.6518

-102.6323 Moving Point 3 37.3104

-101.2420 Moving Point 4 (Watershed Centroid) 36.9533

-99.8638 Moving Point 5 36.5809

-98.4983 Moving Point 6 36.1932

-97.1459 Moving Point 7 35.3732

-94.4811 Table 4: Isohyetal Pattern Orientations for Stationary PMP Storm Center PMP Orientation Watershed Centroid 225° John Martin Subwatershed Centroid 209° Robert Kerr Subwatershed Centroid 235° Table 5: Maximum 18-hour Precipitation Volume for Stationary PMP Storm Centering Storm Area (square miles)

Maximum 18-hr Volume (inch-square miles)

Watershed Centroid 100,000 368,700 John Martin Subwatershed Centroid 60,000 155,200 Robert Kerr Subwatershed Centroid 100,000 300,600

Document No. 32-9207374-000 PROPRIETARY Arkansas Nuclear One Flooding Hazard Re-Evaluation - Probable Maximum Precipitation Page 29 Table 6: Isohyetal Pattern Orientations for Moving PMP Storm Center PMP Orientation Moving Point 1 206° Moving Point 2 216° Moving Point 3 221° Moving Point 4 (Watershed Centroid) 225° Moving Point 5 229° Moving Point 6 232° Moving Point 7 236° Table 7: Maximum 18-hour Precipitation Volume for Moving PMP Storm Centering Storm Area (square miles)

Maximum 18-hr Volume (inch-square miles)

Moving Point 1 100,000 53,200 Moving Point 2 60,000 193,600 Moving Point 3 60,000 273,300 Moving Point 4 (Watershed Centroid) 100,000 368,700 Moving Point 5 100,000 420,800 Moving Point 6 100,000 453,300 Moving Point 7 100,000 241,100 Table 8: Cool Season Maximum Monthly Average Snow Depth for Arkansas Arkansas Station Name Maximum Monthly Average Snow Depth (mm)

Maximum Monthly Average Snow Depth (inches)