ML20366A016

NRC-2017-000688 (Formerly FOIA/PA-2017-0690) - Resp 5 - Final, Agency Records Subject to the Request Are Enclosed, Part 8 of 15

ML20366A016
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Issue date:	12/29/2020
From:	NRC/OCIO
To:
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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: ML20366A016 (167)

Text

Enclosure 1 Peach Bottom Atomic Power Station Flood Hazard Reevaluation Report Revision o (165 pages)

FLOOD HAZARD REEVALUATION REPORT IN RESPONSE TO THE &0.64(f) INFOR~ATION REQUEST REGARDING NEAR-TERM TASK FORCE RECOM,MENDATION 2.1: FLOODING Pntpal8r.

Verifier:

Approver:

for the Peach Bottom Atomic Power Station 1848 Lay Road, Delta, Pennsylvania

• Exelon.

Exelon Generation Co., LLC 300 Exelon Way Kemett Square, PA 19348 Prepared by:

0 ENERCON l.allma-fw11p,e/<<l fwr)'do, Enercon Services, Inc.

1501 Ardmore Boulevard, Suite 200 Pittsburgh, PA 1S221 Revision 0 Submitted Date: July 10, 2015 fdntad~IDJI amua11on Km !S!!!!lll!l!!, e

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ENERCON

• Tom Q'Bell'V, P.E.

ENERCON l!gnatu[I l2ltl

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Lead Responsible Engineer:

Jelle LucU, P.I!.

Exelon

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Branch Manager FrwOl!RP Exelon

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Senior Manager Oealgn

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Engineering Rldlnl Bmwer Exelon Exelon Corporate JO!alh Belin!, P.E.

Exelon

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7/10/15

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015

• Contents

• 1 PURPOSE................................................................................................................................ 10 1.1 Background...................................................................................................................... 10 1.2 Requested Actions........................................................................................................... 10 1.3*

Requested Information..................................................................................................... 11 2

SITE INFORMATION................................... '............................................................................. 12 2.1 Datums and Projections...................... :-............................................................................ 12 2.2 Detailed Site Information.................................................................................................. 13 2.3 Current Licensing Baals Flooding Hazards....................................................................... 17 2.3.1 Current Licensing Basis - Effects of Local Intense Precipitation............................... 17 2.3.2 2.3.3

.2.3.4 2.3.5 2.3.6.

2.3.7 2.3.8 2.3.9 current Licensing Basis - Flooding In Streams and Rivera....................................... 18 Current Licensing Basis-Dam Breaches and Fallures............................................. 19 Current Licensing Baals-Storm Surge..................................................................... 19 Current Licensing Baals - Seiche.............................................................................. 19 I

Current Licensing Basia - Tsunami............ :............................................................... 19 Current Licensing 81111-Ice-Induced Fioodlng........................................................ 19 Current licensing Baals -.Channel Migration or Diversion........................................ 19 Current Licensing Baals - Combined Effect Flood..................................................... 20 2.3.10 Current Licensing Baals - Flood-Related Loading..................................................... 20 2:4 Flood-Related Changes to the Llcenalng Basis and any Flood Protection Changes.

(Including Mitigation) since License Issuance............................................................................. 21 2.5 Changes to the Watershed and Local Area since License l11uance............................... 21 2.6 Current Llcen1lng Basia Flood Protection and Pertinent Flood Mitigation Features......... 22 3

SUMMARY

OF FLOOD HAZARD REEVALUATION............................................................... 23 3.1 Effects of Local Intense Precipitation............................................................................... 24 3.1.1 lnputs.......................................................................................................................... 24 3.1.2 Methodology................................................................................................................ 25 3.1.3 Results....................................................................................................................... 30 3.1.4 Conclualons... _.......................,....................................................................,................. 32

• 3.2

• Probable Maximum Flood of Rivers and Streams............................................................ 33 3.2.1 Inputs - Rock R.un Creek........................................................................................... 34 3.2.2 Inputs - Susquehanna River...................................................................................,,.. ~2 3.2.3 Methodology~ Rock Run Crffk................................................................................ 83 3.2.4 Methodology - Susquehanna River........,.................................................................. 69 3.2.5 Results-Rock Run Creek...... :.................................................................................. 83 3.2.6 Results-Susquehanna Rlver............... i.................................................................... 84 Peach Bottom Atomic Power Station Page 2 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10. 2015 3.2.7 Conclusions - Rock Run Creek.................................................................................. 96 3.2.8 Conclusions - Susquehanna River............................................................................ 96 3.3 Dam Breaches and Failures................................................................... ;.......... :............,. 97 3.3.1 Inputs - Hydrologic (HEC-HMS) Model.................................................'..................... 97 3.3.2 Inputs - Hydraulic (HEC-RAS) Model..................................,...,................................ 101 3,3.3 Methodology-Hydrologlc (HEC~HMS) Model.,.........."............................................. 102 3.3.4 Methodology - Hydraulic (HEC-RAS) Model........................................................... 104 3.3.5 Results - Hydrologic (HEC-HMS) Model................................................................. 107 3.3.6 Results - Hydraulic (HEC-RAS) Model.................................................................... 109 3.3. 7 Conclusions............................................................................ *..................,............... 111 3.4 Probable Maximum Storm Surge and Selche...........................,...................................... 111 3.4.1 Inputs............................................................... :........................................................ 112 3.4.2 Methodology...........,.................................................. :***....................,.,..................... 113 3.4.3 Results.....................................................................................................................-118 3.4.4 Conclusions.............................................................................................. :............... 120 3.5 Tsunami........................................................................................................,.........,........ 120 3.5.1 lnputs......................................................................... :.................................. +***.. *****120 3.5.2 Methodology....... '.........,...........-................................................................................. 120 3.5.3 Results................................. :................................................................................... 121 3.5.4 Conclusions.,...............,........................... :***.. ******.. ;*****,*... :........................................ 121 3.6 Combined Events Flood............................................ :..................................................... 121 3.6.1 lnputs......................................................................................................................... 124 3.6.2 Methodology.........,................................................................................................... 124 3.6.3 Results....................................................................,..............................................., 125 3.6.4 Conclusions................................................................................ _................. *..... 1.......... 125

3. 7 Ice-Induced Flooding.....................................................................................,................ 125 3.7.1 lnputs............................................................,............... \\............................................. 125 3.7.2 Methodology............................................................................................................... 126 3.7.3 Results.............................................................................. 1.:......................... :.......... 128 3.7.4 Concluslons.......,.............................................................,,........................................ 128 3.8 Channel Mi!lration............................................................. '............................................. 128 3.8.1 1nputs.............................. ;......................,.. i.;....................... *.............................,............ 129 3.8.2 Methodology..............................................,,.....................................,............,.......... 129 3.8.3 Results...........................,..............................................-.-..........,................................. 129 3.8.4 Concluslons.............................,...............................................,................................ 130 3.9 Error/Uncertainty.................,........................................................................................ _... 130 3.9.1 tnputs.............................................................................................,...........,................ 131 Peach Bottom Atomic Power Station Page 3 of 165

NTTF Recommendation 2.. 1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 3.9.2 Methodology............................................................................................................. 131 3.9.3 Results................................................................................................................ :.... 132

3. 9.4 Conclusions.............................................................................................................. 134 3.10. Associated Effects and Flood Duration Parameters...........................,........................... 134 3.10.1 lnputs.............................................................. :......................................................... 135 3.10.2 Methodology............................................................................................................. 136 3.10.3 Results................................................................................,.................................... 138 3.10.4 Concluslons.............................................................................................................. 143 4

FLOOD PARAMETERS AND COMPARISON WITH CURRENT DESIGN BASIS................ 146 5

REFERENCES....................................................................................................................... 155 Peach Bottom Atomic Power Station

• Page 4 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

List of Tables*

Revision 0 July 10, 2015 Table 2.3.1.1 -PBAPS IPEEE LIP.................................................................................................... 17 Table 2.5.1 - Summary of Land Use Changes in the Susquehanna River Watershed.. ;................ 22 Table 3.1.1.1.1 Mln, 15-Mln, 30-Min, and 60-Mln 1 Sq. Mi. LIP for PBAPS................ *.............. 25 Table 3.1.3.1 -Worst Case Consequences of LIP.......................................................................... 32 Table 3.2.1.2.1 -All-Season PMP Estimate..................................................................................... 35 Table 3.2.1.3.1 - Ratio of Seasonal Rainfall to All-Season Rainfall.............................,............,..... 35 Table 3.2.1.4.1 - NOAA Atlas 14 Precipitation 100-and 500-Year Frequency Estimates.............,36 Table 3.2.2.2.1 -All Season Site Specific PMP Values............................................,................ ;*..,.43 Table 3.2.2.3.1 - Cool Season Site Specific PMP Values.........................................,.....................44 Table 3.2.2.6.1 - Storms Used In the Cool-Season Meteorological Time Series Development...... 50 Table 3.2.2.12.1 - Summary of Bathymetry and Topography Inputs.............................................. 55 Table 3.2.2.17.1 - Maximum Observed Water Levels - 2011 Lee................................................... 60 Table 3.2.2.17.2 - Maximum Observed Water Levels - Ivan 2004................................................. 60 Table 3.2.2.17.3-Maximum Observed Water Levels -Agnes 1972............................................... 61 Table 3.2.2.17.4-Maximum Observed Water Levels-1936..........................,.............................. 61 Table 3.2.3.5.1 - Constant Loss Estimate Summary....,.................................................................... 68 Table 3.2.3.6.1 - Summary of Snyder Unit Hydrograph Parameters.....,......................................... 68 Table 3.2.3.9.1 - Manning's n Value Description............................................................................. 69 Table 3.2.4.1.1 - Calibration Goals.............................................................................,...........,....... 70 Table 3.2.4.2.1.1 - Flood Arrival Time and Peak Flow at TMI and Marietta for Different Storm Center Locations and Center-Peaking 27,048 Sq. ML Storm Event (Scenario 1, Single PMP Event)........ 76 Table 3.2.4.2.1.2 - Flood Arrival Time and Peak Flow at TMI and Marietta for Different Hyetograph Temporal Distributions for a 27,048 Sq. Mi. Storm Centered at the TMI Watershed Centroid (Scenario 1, PMP Event Including Antecedent Event)..................................................................... 76

. Table 3.2.6.2, 1 - Summary of Scenario 2 Water Level Calibration Results.................................... 91 Table 3.2.6.4.. 1 - Precipitation-Driven PMF Scenario Peak Flows at TMI....................................... 94 Table 3.2.6.5.1 -Summary of Results at PBAPS...........................................,................................. 96 Table 3.3.1.2.1 - Susquehanna River Site-Specific PMP.................. ;............................................ 100 Table 3.3.1.4.1 - Watershed-Averaged Precipitation.................................................................... 100 Table 3.3.5.1.1 - Peak Flows for the Overtopping Dam Breach Scenarios..................................... 107 rable 3.3.6.1 - Results for Precipitation-Driven Hydrologk:-lnduced Dam Failures...................... 109 r able 3.3.6.2 - Results for Seismically-Induced* Dam Failures..................................................... 110 Table 3.4.2.1.1 - Fetch Lengths and Average Depths for Wind Directions Evaluated................... 115

,able 3.4.3.2.1 - Elgen Periods for Conowingo Reservoir Closest to Wind Forcing Periods........ 119

,able 3.4.3.2.2 - Elgen Periods for Length of Conowlngo Reservoir...............,..................... "..... 119

,able 3.4.3.2.3 - Elgen Periods for Width of Conowingo Reservoir.....,.......................... :............. 119 Peach Bottom Atomic Power Station Page 5 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 Table 3.4.3.2.4-Seiche Runup Totals..................,........................... :........................................... 120 Table 3. 7.2.2.1 - Largest Ice Jams Upstream of PBAPS............................................................... 127 Table 3.9.3.1.1 - Errors/Uncertainties for LIP f:

..., 133 Table 3.9.3.3.1 -WSEL Results at PBAPS for b)( ) 16 U C § 8240*1(d) (b)(4) (b)( )(F)

... 134 Table 3.10.2.4.1 - Coefficents of Roughness and Maximum Permissible Mean Velocities.......... 136 Table 3.10.2.10.1-Soil Erodibillty.Factor...............................................................................,..... 137 Table 3.10.3.1.1.- Hydrodynamic and Hydrostatic Loads on Safety-Related Buildings................ 138 Table 3.10.3.6.1 - Duration Exceeding Flood Protection Level for Door Locations...................... 139 Table 3.10.3.9.1 - Qualitative Comparison of Impact Force Parameters...................................... 140 Table 3.10.3.13.1 - Duration ofFlooding for Combination Flooding. Scenarios at PBAPS........... 141 Table 3.10.3.13.2 -

PMF Warning Time for an Incremental Rise in the Susquehanna River at PBAPS................................. :..................................................................................................... 143 Table 4.0.1 - Summary of Licensing Basis and External Flooding Study Parameters.................. 150 Table 4.0.2 - Local Intense Precipitation....... :........................ :....................................................... 153

• Table 4.0.3 - Combinations in Section H.1 of NUREG/CR-7046 for the Susquehanna River with Precipitation-Driven Hydrologic Dam Failure................................................................................. 154 List of Figures Figure 2.2.1 - Present-Day General Site Map and Topography...................................................... 14 Figure 2.2.2-Enlarged Present-Day General Site Map and Topography...................................... 15 Figure 2.2.3 - Present-Day Site Layout........................................................................................... 16 Figure 3.1.2.3.1 - FLO-2O Manning's n Value............................................................................... *.28 Figure 3.1.2.5.1 - Temporal Distribution Mass Curves for 1 Hr (60 Min), 1 Sq. Mi. LIP....,............. 29 Figure 3.1.3.1 - Doors Examined in FLO2D Model......................................................................... 31 Figure 3.2.1.5.1 - March Maximized Dew Point Temperature Profile.... :.,....................................... 36 Figure 3.2.1.5.2-March Maximized Wind Speed Profile................................................................ 37 Figure 3.2.1.5.3-March Maximized Temperature Profile.................................................. :.. **... :.... 37 Figure 3.2.1. 7.1 - USGS Stream Gage Locations........................................................................... 38 Figure 3.2.1.8.1 - SSURGO Soil Data for York County, Pennsylvania........................................... 39 Figure 3.2.1. 9. 1 - Rock Run Creek Land Cover.............................................................................. 40 Figure*3:2.1:9;2 -'-*Rock Run Creek Percent*lmpervious--:-:........................................,=="...-.-...,.,-...-.*...-..41.-

Figure 3.2.2.2.1 - Flow Chart Showing the Major Steps Involved in PMP Development......... :...... 43 Figure 3.2.2.4.1 - SPAS Analysis Flow Chart..........................................................................,....... 45 Figure 3.2.2.4.2 - Total Storm Rainfall for Tropical Storm Lee (9/5/2011 to 9/2011)...................... 46 Figure 3.2.2.4;3 -Total Storm Rainfall for Hurricane Ivan (9/17/2004 to 9/19/2004)......................47 Figure 3.2.2.4.4 - Total Storm Rainfall for Hurricane Agnes (6/18/1972 to 9/24/1972)...............,... 48 Peach Bottom Atomic Power Station Page 6 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Figure 3.2.2.6.1 - Maximized March PMP Temperature Profile...................................................... 50 Figure 3.2.2.6.2 - Maximized March PMP Dew Point Temperature Profile..................................... 51 Figure 3.2.2.6.3 - Maximized March PMP Wind Speed Profile................................... :................... 51 Figure 3.2.2.9.1 - USGS Gage Locations.................................................. :...................................... 53 Figure 3.2.2.10.1 - NRCS Hydrologic Soils Groups In the Susquehanna River Watershed........... 54 Figure 3.2.2.11.1 - Regions Characterized by Different Values of Percent Impervious Area in the Susquehanna River Watershed....... :**.. ********................................................................................... 55 Figure 3.2.2.12.1 -Area Used for Susquehanna River Model........................................................ 56 Figure 3.2.2.15.1 - Susquehanna River Hydraulic Model Manning's n Values............................... 59 Figure 3.2.2.20.1 - HEC-RAS Lateral Inflow Locatlons................................................................... 62 Figure 3.2.3.4.1 - Rock Run Creek Watershed Delineation............................................................ 66 Figure 3.2.3.5.1 - Rock Run Creek Hydrologic Soll Groups............................................................ 67 Figure 3.2.4.2.1.1 - Storm Center Locations..........................................................................,......... 73 Figure 3.2.4.2.1.2 - Susquehanna River Watershed All-Season PMP Temporal Distributions........74 Figure 3.2.4.2.1.3 - lsohyetals between 10 Sq. Mi. and 100,000 Sq. Mi. at an Orientation of 212°Centered at the TMI Watershed Centroid................................................................................. 75 Figure 3.2.4.2.3.1 - Storm Center Locations.............................................................................. :.... 79 Figure 3.2.4.2.3.2 -

Susquehanna River Watershed Cool-Season PMP (Snowmelt Excluded)

Temporal Distributions........................................................... '.......................................................... 80 Figure 3.2.4.2.3.3 - 100-Year Snow Water Equivalent Surface...............,...............,...................... 81 Figure 3.2.5.1 - PMF Inundation Map for Rock Run Creek........................ :.................................... 84 Figure 3.2.6.1.1 - Individual Model Calibration Results................................................................... 85 Figure 3.2.6.1.2 - Peak Flow versus Drainage Area....................................................................... 86 Figure 3.2.6.1.3 - Combined Model and Observed Hydrographs for Tropical Storm Lee............... 87 Figure 3.2.6.1.4 - Combined Model and Observed Hydrographs for Hurricane Agnes.................. 88 Figure 3.2.6.1.5 - Combined Model Observed Hydrographs for Hurricane Ivan..................... :....... 89 Figure 3.2.6.3.1.1 - Area-Averaged Hyetograph and Hydrograph at TMI for Scenario 1................ 92 Figure 3.2.6.3.2.1 - Area-Averaged Hyetograph and Hydrograph at TMI for Scenario 2................ 93 Figure 3.2.6.3.3.1 -Area-Averaged Hyetograph and Hydrograph at TMI for Scenario 3................ 94 Figure 3.2.6.5.1 - WSEL Results for Alternatives 1, 2, and 3 at PBAPS............ :............................ 95 Figure 3.2.6.5.2 - Flow Results for Alternatives 1, 2', and 3 at PBAPS...................... ;.................... 95 Figure 3. 3.1.1.1 - Method 3 Flow Chart.......................................................................................... 98 Figure 3.3.1.1.2 - Composite and Individually Modeled Dam Locations............................ :,............ 99 Figure 3.3.5.1.1 - Hydrologic Breach Timing Estimation for PBAPS............................................. 108 Figure 3.3.5.2.1 - Seismic Dam Breach Timing Estimation for PBAPS......................................... 109 Figure 3.3.6.1 -

Precipitation Driven Hydrologic Dam Failure -

Stage-Flow Hydrograph at PBAPS.................................................................. ;.............,........,............................................. 110 Peach Bottom Atomic Power Station.

Page 7 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Figure 3.3.6.2 -

1/2 PMF and Seismic Dam Failure (124 hours0.00144 days <br />0.0344 hours <br />2.050265e-4 weeks <br />4.7182e-5 months <br />) -

Stage-Flow Hydrograph at PBAPS....................................................................................................................................... 111

• Figure 3.4.2.1.1 - Cross Sections and Directional Cases Examined for Wave Runup.................. 114 Figure 3.4.2.1.2 -Wave Setup...................................................................................................... 116 Figure 3.4.2.1.3 - Wave Runup on Smooth, Impermeable Slopes When ds/Ho > 3.0.................. 117 Figure 3.7.2.1.1 - HUC Units and Ice Jam Locations within Lower Susquehanna Watershed..... 126 Figure 3.8.3.1 -Selected Historical USGS Maps near PBAPS from 1912 to 2013....................... 130 Figure 3.10.3.6.1 - Water Surface Elevation vs. Time for Various Door Locations....................... 139 Figure 3.10.3.13.1 - Flood Duration - Precipitation-Driven Hydrologic-lnduced Dam Failure....., 141 Figure 3.10.3.13.2-Flood Duration - Se!smically-lnduced Dam Failure s.cenarlo....................... 142 Figure 4.0.1 - Illustration of Flood Event Duration (Reference 131, Figure 6)............................... 147 Figure 4.0.2 - Effect of Unsteady Flow on Stage-Discharge Relationship.................................... 149 Peach Bottom Atomic Power Station Page 8 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Acronyms and Abbreviations ANS American Nuclear Society ANSI American National Standards Institute ASCE American Society of Civil Engineers CEM Coastal Engineering Manual cfs cubic feet per second CLB Current Licensing Basis DEM Digital Elevation Model FFT Fast Fourier Transform EM Engineer Manual ESP Early Site Permit ESRI Environmental Systems Research Institute ft foot. feet GIS Geographic Information Systems HEC-HMS Hydrologic Engineering Center Hydrologic Modeling System HEC-RAS Hydrologic Engineering Center River Analysis System HHA Hierarchical hazard assessment HMR Hydrometeorological Report hr hour(s) in.

inch (inches)

LiDAR Light Detection and Ranging LIP Local Intense Precipitation mi.

mile(s) min minute(s)

MSL mean sea level NAVO North American Vertical Datum NGOC National Geophysical Data Center NLCD National Land Cover Database NOAA National Oceanic and Atmospheric Administration NRC U.S. Nuclear Regulatory Commission NRCS Natural Resources Conservation Service NTTF Near-Term Task Force NWS National Weather Service PMF probable maximum flood PMP probable maximum precipitation PMS probable maximum seiche PMSS probable maximum storm surge PMWS probable maximum windstorm

. _.sq.. mi.------ -square.mile(s)-

SSCs structures, systems, and components UFSAR Updated Final Safety Analysis Report UHS Ultimate Heat Sink USAGE U.S. Army Corps of Engineers USGS U.S. Geological Survey Peach Bottom Atomic Power Station Revision 0 July 10, 2015 Page 9 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

1 PURPOSE Revision 0 July 10, 2015 This report provides the Exelon Generation Company, LLC (Exelon) Peach Bottom Atomic Power Stati.on (PBAPS) response to the U.S. Nuclear Regulatory Commission's (NRC) March 12, 2012 Request for Information (RFI) pursuant to the post-Fukushima Near-Term Task Force (NTTF)

Recommendation 2.1 flooding hazards reevaluation of PBAPS.

1.1 Background

In response to the nuclear fuel damage at the Fukushima Dai-ichl power plant due to the March 11, 2011 earthquake and subsequent tsunami, the U.S. NRC established the NTTF to conduct a systematic review of NRC processes and regulations, and to make recommendations to the NRC for Its policy direction. The NTTF reported a set of recommendations that were intended to clarify and strengthen the regulatory framework for protection against natural phenomena.

On March 12, 2012, the NRC issued an information request pursuant to Title 10 of the Code of Federal Regulations Section S0.54(f) (10 CFR 50.54(f) or 50.54(f) letter) (Reference 127) which included five enclosures:

1. [NTTF] Recommendation 2.1: Seismic

2. [NTTF] Recommendation 2.1: Flooding

3. [NTTF] Recommendation 2.3: Seismic

• 4. [NTTFJ Recommendation 2.3: Flooding

5. [NTTF] Recommendation 9.3: EP In Enclosure 2 of the NRC-issued information request (Reference 127), the NRC requested that licensees "reevaluate the flooding hazards at their sites against present-day regulatory guidance and methodologies being used for early site permits (ESP) and combined operating license reviews."

On behalf of Exelon for PBAPS, this Flood Hazard Reevaluation Report provides the information requested in the March 12, 2012, 50.54(f) letter; specifically, the information listed under the "Requested Information" section of Enclosure 2, paragraph 1 (a through e). The "Requested Information" section of Enclosure 2, paragraph 2 (a through d) of the integrated assessment report will be addressed separately if the current design basis flooc(s do not bound the reevaluated hazard for all flood-causing mechanisms.

1.2 Requested Actions Per Enclosure 2 of the NRC-issued information request 50.54(f) letter, Exelon is requested to perform a reevaluation of all appropriate external flooding sources for PBAPS, including the effects from Local Intense Precipitation (LIP) on the site, probable maximum flood (PMF) on streams and rivers,

• storm surges, seiches, tsunami, and dam failures. It is requested that the reevaluation apply present-day regulatory guidance and methodologies being used for ESP and combined operating licensing (COL)" reviews, lncluding*currenttechnlques, software, and methods used in present-day standard.

engineering practice to* develop the flood hazard. The requested information wlll be gathered in Phase 1 of the NRC staffs two-phase process to implement Recommendation 2.1, and will be used to identify potential "vulnerabilities" (see definition b~low).

For the sites where the reevaluated flood exceeds the design basis, addressees are requested to submit -an interim action plan with the Flood Hazard Reevaluation Report that documents actions planned or taken to address the reevaluated hazard.

Peacn Bottom Atomic Power Station Page 10 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Subsequently, addressees should perform an integrated assessment of the plant to identify vulnerabilities and actions to address them. A definition of vulnerability in the context of Enclosure 2 is as follows: Plant-specific vulnerabilities are those features important to safety that when subject to an increased demand due to the newly calculated hazard evaluation have not been shown to be capable of performing their intended functions.

The scope of the integrated assessment report will include full power operations and other plant configurations that could be susceptible due to the status of the flood protection features. The scope also includes those features of the Ultimate Heat Sinks (UHS) that could be adversely affected by the flood conditions and lead to degradation of the flood protection (the loss of UHS from non-flood associated causes is not included). It is also requested that the integrated assessment address the entire duration of the flood conditions.

1.3 Requested Information Per Enclosure 2 of the NRG-issued information request 50.54(f) letter, the report should provide documented results, as well as pertinent information and detailed analysis, and include the following:

a. Site information related to the flood hazard. Relevant structures, systems, and components (SSCs) important to safety and the UHS are included in the scope of this reevaluation, and pertinent data concerning these SSCs should be included. Other relevant site data include the following:

i.

Detailed site information (both designed and as-built), including present-day site layout, elevation of pertinent SSCs important to safety, site topography, as well as pertinent spatial and temporal datasets; ii.

Current design basis flood elevations for all flood-causing mechanisms; iii.

Flood-related changes to the licensing basis and any flood protection changes (including mitigation) since license issuance; iv.

Changes to the watershed and local area since license issuance;

v.

Current licensing basis (CLB) flood protection and pertinent flood mitigation features at the site; and vi.

Additional site details, as necessary, to assess the flood hazard (i.e., bathymetry, walkdown results, etc.).

b. Evaluation of the flood hazard for each flood-causing mechanism, based on present-day methodologies and regulatory guidance. Provide an analysis of each flood-causing mechanism that may impact the site, including LIP and site drainage, flooding in streams and rivers, dam breaches and failures, storm surge and selche, tsunami, channel migration or diversion, and combined effects. Mechanisms that are not applicable at the site may be screened out; however, a justification should be provided. Provide a basis for inputs and assumptions, methodologies and models used including input and output files, and other pertinent data.

c. Comparison of current and reevaluated flood-causing mechanisms at the site. Provide an assessment of the current design basis flood elevation to the reevaluated flood elevation for each flood-causing mechanism. Include how the findings from Enclosure 4 of the 50.54(f) letter (i.e., Recommendation 2.3 flooding walkdowns) support this determination. If the current design basis flood bounds the reevaluated hazard for all flood-causing mechanisms, Include how this finding was determined.

Peach Bottom Atomic Power Station Page 11 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015

d. Interim evaluation and actions taken or planned to address any higher flooding hazards relative to the design basis, prior to completion of the integrated assessment, If necessary.

e. Additional actions beyond Requested Information Item 1.3.d taken or planned to address flooding hazards, If any.

2 SITE INFORMATION Unless referenced otherwise, the PBAPS site information contained in the Section 2 subsections is from the PBAPS Updated Final Safety Analysis Report (UFSAR), Revision 24 (Reference 18).

Additional information from the PBAPS Units 2 and 3 Summary of Individual Plant Examination of External Events (IPEEE) Report (Reference 17), and the PBAPS Flooding Walkdown Report (Reference 40) is referenced as needed. Note that IPEEE information is added for completeness only. PBAPS does not have a regulatory commitment to the IPEEE, and IPEEE is not part of the site licensing basis. No changes were made*to the PBAPS CLB for the flood hazard reevaluation portion of the IPEEE.

2.. 1 Datums and Projections Various horizontal and vertical datums and

• mapping projections are referenced throughout this report. This section describes the horizontal and vertical datums and mapping projections used, their definitions and relationships, and the methods used to convert from one datum or projection to another.

A horizontal datum is a system which defines an idealized surface of the earth for positional referencing. The North American Datum of 1983 (NAD83) is the official horizontal datum for U.S.

surveying and mapping activities. Latitude and longitude are typically used to identify l~cation in spherical units.

A map projection is a mathematical transformation that converts a three-dimensional (spherical or ellipsoid) surface onto a planar surface. Different projections cause different types of distortions and, depending on their intended use, projections are chosen to preserve different relationships of characteristics between features. Projections In the United States are typically defined as State Plane coordinate systems with units of Northing and Easting. The United States is divided into many State Plane maps; large states can be defined by several maps. A site survey was performed in September 2013 (Reference 9). The PBAPS site* survey uses the NAD83 horizontal datum and is projected onto the State Plane Pennsylvania South coordinate system.

There are two types of vertical datums: tidal and fixed. Fixed datums are reference level surfaces that have a constant elevation over a large geographical area. Tidal datums are standard elevations that are used as references to measure local water levels. The following Is a list of tidal and fixed datums, as defined by the National Oceanic and Atmospheric Administration (NOAA)

(Reference 64):

Mean Sea Level (MSL) - The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch, where the National Tidal Datum Epoch is the specific 19-year period adopted by the National Ocean-Service as the official time segment over which tide observations are taken and reduced to obtain mean values for tidal datums.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 North American Vertical Datum of 1988 (NAVD88) - Fixed vertical control datum determined by geodetic leveling, referenced to the tide station and benchmark at Pointe-au-Pere (Father Point),

Rimouskl, Quebec, Canada.

National Geodetic Vertical Datum of 1929 (NGVD29) - Fixed vertical control datum, affixed to 21 tide stations in the United States and 5 in Canada.

The CLB and historical PBAPS site drawings are typically referenced to "Conowlngo Datum (C.D.)"

vertical datum, to which site benchmarks are referred.

The NRC has expressed a preference for flood level reporting in NAVD88. The updated PBAPS site survey (Reference 9) and reevaluation are in NAVD88 datum. Other datums are referenced or used where appropriate.

Where required, vertical transformations were performed using the site conversions as listed below.

MSL = NGVD29 (Reference 39)

NAVD88 = NGVD29 - 0.83 feet (ft) (Reference 87)

C.D. = NGVD29 - 0.7 ft (Reference 18)

NAVD88 = C.D. - 0.13 ft (determined from datum relationships above)

Note that these conversions only apply in the vicinity of PBAPS, and conversions would vary at other locations.

2.2 Detailed Site Information PBAPS is located on the west bank of the Conowingo Pond (the term "pond" is used locally, but it is actually a large run-of-river reservoir) of the Susquehanna River on the border of York and Lancaster Counties, Pennsylvania. The plant is about 38 miles (mi.) north-northeast of Baltimore, Maryland, and 63 mi. west-southwest of Philadelphia, Pennsylvania. The plant is located between Conowingo Pond and the foot of a low hill near the point at which Rock Run Creek discharges into the pond. Conowingo Pond is formed by the backwater of Conowingo Dam on the Susquehanna River; the dam is located about 9 mi. downstream. Holtwood Dam, located about 6 mi. upstream from PBAPS, forms the upper limit of Conowlngo Pond. The approximate coordinates of PBAPS are 39° 45' 30" North, 76° 16' 5" West.

The PBAPS site comprises approximately 620 acres and contains Units 1, 2, and 3. PBAPS contains two actrve General Electric boiling water reactor generating stations, Unit 2 and Unit 3, which are jointly owned by Exelon Corporation and Public Service Enterprise Group (PSEG) Power, and operated by Exelon Corporation. Unit 1 is inactive and ls located approximately 700 ft and 1,000 ft downstream (south) from PBAPS Units 2 and 3, respectively, and is included in their exclusion area.

Unit 1 is now in a SAFe STORage (SAFSTOR) status that allows it to be safely stored and

_s_up_sJ)_qu~.n.tly __ d..onJa,:ninated Jo_leY.els. that.permit.release_of _the.facility.for_unrestricted_use.** U nit 1 is not analyzed in this. report.

The major surficial drainage course within the property limits Is Rock Run, which flows in a generai easterly direction into Conowingo Pond further south of PBAPS Unit 1. Rock Run is a perennial stream dividing the property into two sections. The watershed area of the creek is approximately 4 square miles (sq. mi.). About two-thirds of the runoff within the property limits is diverted by means of perennial and intermittent streams into Rock Run and subsequently into Conowingo Pond. The remaining runoff flows directly into the pond.

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Revision 0 July 10, 2015 The major surface stream in the area Is the Susquehanna River, which originates at Lake Otsego in New York and terminates in the Chesapeake Bay. Most of the streams in the area drain into the Susquehanna River Basin. The Susquehanna River drains an area of 27,500 sq. mi. in New York, Pennsylvania, and Maryland. A total of 27,000 sq. mi. of drainage area is located upstream from PBAPS. The main stem of the Susquehanna River is formed near Sunbury, Pennsylvania by the confluence of its north and west branches. It flows southeastwardly 123 mi. to the Chesapeake Bay with a total fall of 420 ft. In the reach from Harrisburg, Pennsylvania to the mouth, a distance of 70 mi., there ls a total fall of 292 ft. Three major hydroelectric projects, Safe Harbor, Holtwood, and Conowingo, utilize 202 ft of this head. Peak flows in the upper Susquehanna River are controlled by 14 flood control dams.

The finished grade at the plant has been established at Elevation (EL) 116 ft-C.D. Grade rises abruptly In the area surrounding the reactor building to a grade of EL 135 ft-C.D., with the top of ground floor at EL 135 ft-C.D. On both sides of Conowingo Pond, steep sloping hills rise directly up to about 300 ft above plant grade. Condenser water is cooled by water from the Conowingo Pond.

Normal elevation ofConowingo Pond is between EL 104 ft-C.O. and EL 109.25 ft-C.D.

The plant's present-day general site map and topography are presented on Figure 2.2.1 and Figure 2.2.2. The detailed present-day site layout is presented on Figure 2.2.3.

L..,.S

• Peach Bottom Alomoo Power Stet.on 10 ft Conloura

&11klinet Figure 2.2.1 - Present-Day General Site Map and Topography Peach Bottom Atomic Power Station Page 14 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Figure 2.2.2 - Enlarged Present-Day General Site Map and Topography Peach Bottom Atomic Power Station Page 15 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations}: Flooding Exelon Generation Co.

o eo 120 2,0

ieo 480

--=-ic::J---====---FHI Figure 2.2.3 - Present-Day Site Layout Peach Bottom Atomic Power Station Revision 0 July 10, 2015 Page 16 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

2.3 Current Licensing Basis Flooding Hazards Revision 0 July 10, 2015 The following is a list of flood-causing mechanisms and their associated water surface elevations (WSELs) that are considered in the PBAPS CLB.

Unless otherwise referenced, any current PBAPS license basis information provided in this section was obtained from the PBAPS UFSAR, Revision 24 (Reference 18).

2.3.1 Current Licensing Basis - Effects of Local Intense Precipitation PBAPS does not define an LIP event in their CLB, but was considered in the IPEEE (Reference 17). The IPEEE states the following:

Estimates for short-duration rainfall intensities were established for the PBAPS site based on the procedures and criteria in NOAA/National Weather Service (NWS) Hydrometeorological Reports (HMR} 51, 52, and 53 (References 66, 67, and 68). The maximum point rainfall for a 1-hour (hr) duration and a 1 sq. mi. area was taken directly from HMR 52. For the PBAPS site, this corresponds to an accumulation of approximately 18.0 inches (in.). Point rainfall for 5, 15, and 30 minutes (min) is estimated from the 1-hr accumulation by extracting the shorter-duration ratios. The resulting short-duration rainfall intensities are provided in Table 2.3.1.1.

Table 2.3.1.1 - PBAPS IPEEE LIP Cumulative Incremental Time Increment Rainfall Intensity Time Rainfall Rainfall (in.)

(in.)

(min)

(in./hr) 5min 6.0 6.0 5

72.0 15 min 9.5 3.5 10 21.0 30min 13.6 4.1 15 16.4 1 hr 18.0 4.4 30 8.8 The effect of the increase in rainfall intensity on local site ponding and roof ponding was assessed as follows.

2.3. 1. 1 Site Ponding The new probable maximum precipitation (PMP) is judged to have no effect on the capability of the site drainage to divert runoff away from Class 1 SSCs. Local ponding may occur on the site In the vicinity of plant structures but would be limited because of the topography of the site and the surrounding area. Plant grade is established at EL 116 ft-C.D., more than 6 ft above the normal pool level in Conowingo Pond. The natural runoff will be away from the plant structures towards the pond. All of the structures housing safe shutdown equipment are protected from the effects of flooding. The protective measures are maintained In place (i.e., flood and secondary containment doors): therefore, in-leakage from any resulting local site ponding would not be expected.

Based on this assessment, LIP at the plant site would not cause ponding that would flood safety-related structures or render safe shutdown equipment inoperable. It is not credible that LIP could cause a water level as high as the PMF: therefore, a detailed runoff assessment of the site Is not necessary.

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2.3.1.2 Roof ponding Revision O July 10, 2015 Roof drainage is provided on all the critical structures at PBAPS. The design rainfall intensity for the roofs is the 25-year return rainfall from HMR 40. This corresponds to a rainfall intensity of 2.5 in. per hr. Roof drainage for all of the essential structures typically consists of a series of 6-in. drains and 3-in. overflow scuppers. All of the structures have parapet walls of various heights. The parapets would allow ponding on the roofs If the rainfall rate exceeds the overall drainage capacity. There are two effects which must be considered to assure that roof ponding has no detrimental effects on safe operation of the plant. They are:

The ability of the roof structure to carry the additional weight, locally and globally, resulting from the depth of ponded water; and Potential in-leakage through roof joints such as air intakes/exhausts which would not be submerged under normal rainfall but which may become submerged under intense rains such as the PMP.

It was concluded that the PBAPS structures would not be adversely affected by the higher rainfall intensities predicted in the NOAA/NWS publications.

2.3.2 Current Licensing Basis - Flooding In Streams and Rivers 2.3.2.1 Susquehanna River The PMF was determined by the U.S. Army Corps of Engineers (USACE), Baltimore District, utilizing the PMP estimated by the U.S. Weather Bureau over the Susquehanna River watershed above Harrisburg, Pennsylvania, and reported In their HMR 40. Tributary hydrographs were developed by the USACE using subbasin rainfall and appropriate unit hydrographs. Inflows from subbasins were combined and routed downstream. In developing tributary hydrographs, no allowance was made for storm travel across the basin, which has a long axis of about 300 mi. in a southwest-northeast direction and a short axis of about 150 mi. In a northwest-southeast direction. The assumption of simultaneous rainfall over such a large area (24,100 sq. mi.) constitutes an indeterminate but significant factor of safety.

In the flood studies performed by the firm of Tippetts-Abbett-McCarthy-Stratton (TAMS)

Hydro logic Engineers and Architects, it was assumed that the PMF hydrograph at Conowingo Dam is the same as that at Harrisburg, or a peak discharge of 1,750,000 cubic feet per second (cfs). In 1936, no flood control dams were in existence on this reach of river. The total drainage area of the eight flood control reservoirs constructed since then plus the controlled areas of additional projects authorized for construction total about 10 percent of the drainage area above Conowingo Dam. The effect of the reservoirs on a maximum flood would not necessarily be equivalent to the percentage of the controlled area but would be significant. Therefore, the PMF hydrograph devised for Harrisburg was considered to be applicable to Conowingo Dam.

Under this assumption, the PMF hydrograph at Conowingo Dam is the same as that at Harrisburg. The Conowingo Dam will pass the peak flood flow of 1,750,000 cfs with the headwater at EL 129.1 ft-C.D. The flood elevation at PBAPS is dependent upon the discharge capability of Conowingo Dam. Backwater computations between Conowingo Dam and PBAPS were made using channel characteristics computed from surveyed cross sections at intervals of 5,000 ft covering a distance of 60,000 ft upstream from the dam.

Available data from the flood of 1936, with a peak of EL 117.0 ft-MSL at a station located 11.7 mi. upstream of Conowingo Dam, were obtained from the U.S. Geological Survey Peach Bottom Atomic Power Station Page 18 of 165

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Revision 0 July 10, 2015

{USGS) Water Supply Paper 799, "The floods of March 1936, part 2, Hudson River to Susquehanna River region," to determine Manning's *n* coefficient. Several backwater curves were computed for calibration purposes, and a Manning's coefficient of 0.021 resulted in the best agreement between the computed and recorded WSELs. To be conservative, the coefficient used in the final design was increased 10 percent to 0.023. The resulting backwater computations estimate a maximum water level at PBAPS during the PMF to be EL 131.5 ft-C.O.

2.3.2.2 Rock Run Creek Rock Run Creek is a perennial stream, and the watershed area of the creek is approximately 4 sq. mi. Due to its limited drainage area and steep banks, Rock Run Creek presents no flood danger. About two-thirds of the runoff within the property limits is diverted by means of perennial and intermittent streams into Rock Run Creek and subsequently into Conowingo Pond. The remaining runoff flows directly into the pond.

2.3.3 Current licensing Basis - Dam Breaches and Failures Coincident with the PMF, Holtwood Dam was assumed to fail in a manner that would result in an instantaneous additional outflow of 200,000 cfs, and at the precise time that would produce a maximum water elevation at PBAPS. A Holtwood outflow hydrograph was prepared and used as inflow for a study of surge propagation in the Conowingo Reservoir. This study was performed assuming a flood on the river of 1,170,000 cfs, which was af e:rer estimate of the PMF. The transient wave produced by this failure is estimated to be

• • • • • • • • • ft at. PBAPS........ I he desJgn Qf (b)(3) 16 U s C 1,750,000 cfs will produce considerably higher WSELs,n e Holtwood and Conowingo * § B24o~1(d), (b)

Reservoirs. The transient water surface rise produced by the partial failure of Holtwood Dam at IA\\ 11,.,\\l'"l\\Jr\\

1,750,000 cfs will be smaller than at 1,170,000 cfs because incremental outflow produced by the da.. m

.. failure will be a smaller fracti?.?-.Qf.\\he total flow. Superimposing the height of the transient wave. conservatively estimat~ ~

ft, on the steady-state ~

a~er profile at a PMF of 1,750,000 cfs produces a max1mu er level at PBAPS of ELL.=JftC.O *.. ___.

..................... t~~~6~~):(~

Failure of other dams other than Holtwood Dam was not considered in the CLB.

2.3.4 Current Licensing Basis - Storm Surge PBAPS flooding due to surges was not considered in the CLB.

2.3.6 Current Licensing Basis - Seiche PBAPS flooding due to seiches was not considered in the CLB.

2.3.6 Current Licensing Basis - Tsunami PBAPS flooding due to a tsunami was not considered in the CLB.

2.3.7 Current Licensing Basis - Ice-Induced Flooding PBAPS flooding due to river ice was not considered in the CLB.

2.3.8 Current Licensing Basis - Channel Migration or Diversion Channel migration or diversion of the Susquehanna River was not considered in the CLB. The migration of the small creeks near the site was also not considered in the PBAPS CLB.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

2.3.9 Current Licensing Basis - Combined Effect Flood Revision 0 July 10, 2015 Critical equipment, systems, and structures essential to a safe shutdown of the reactor are flood protected to EL +135 ft-C.D. This design level provides protection against the most severe combination of the following conditions occurring simultaneously and still provides over 1 ft of freeboard protection:

1.

PMF - Peak flow 1,750,000 cfs

2.

Failure of Holtwood Dam

3.

Wind-generated waves.

The height of wind-generated waves was computed using the greatest weighted average fetch, 2.0 mi., that will produce the most severe effect at the plant site. From a consideration of the surrounding conditions, recorded wind velocities, wind direction, effect of topography, and time required for w9ves to develop, a 45 mi. per hr (mph) wind on a 2-mi. fetch was used In computing a wave height of 2. 7 ft, measured from trough to tip. The tip is assumed to be two-thirds of the wave height above still water. Superimposing an additional 1.8 ft of wind-gener~

es on the conditions assumed ~reviously yiel?s a peak ele~ation top of wave tip _of EL

• • • • • • • • • • • • • • • *:~*.D...... §(~~J~~~(~l~(~)

Compared to the protection level provided of Elevation +135.0 ft-C.D., this lea e-tj1t1onal "', *.,~.,n

... fr~l:>Q;:ird.. ot[Jt-This margin of freeboard, together with the conservative assumptions used in computing the water level under hypothesized PMF conditions, is considered more than adequate for the safety criteria of the plant.

The maximum wave of the spectrum analyzed is estimated to be approximately 1.67 times the significant wave height, or 4.5 ft high. Only a small percentage (1 percent) of all waves reach this maximum height. Wave runup is defined as the height above still water level to which a wave rises when it encounters an obstruction. At PBAPS, the obstructions encountered are the vertical walls of the various buildings. One of the parameters necessary to detennine the height of wave runup is the ratio of the depth of still water to the wave height. Previous wave studies indicate that as this ratio increases above about 3.0, the height of runup decreases. In estimating run up at this site, a ratio of depth of still water to height of wave of 3.0 was used although, under design conditions, the actual ratio was 4.0 for structures away from the shore and about 10.0 for the pump structure located at the shoreline. The wave runup heights estimated are, therefore, greater than the height that might occur. It Is estimated that the significant waves will run up

i;1

:~~~~a

~~-~~::u~t: j~* a r!:x~,::~:~~t;~;::iu;i~~ ~u:u~ri~p:1~b~~~.~~

2.3.10 Current Licensing Basis - Flood-Related Loading All safety-related SSCs which are below grade level are located within Class 1 structures. These structures have been designed to withstand the hydrostatic loading of the PMF. On this basis, groundwater hydrostatic loading is not the limiting condition for design. Class 1 structures at PBAPS will be partially submerged at the PMF water level. The structures required for safe shutdown were statically checked for hydrostatic pressures caused by the PMF with no loss of function.

(b)(3) 16 U SC.

§ff24o:1(d), (b)

The pump structure was further evaluated for the effects of floating objects (missiles). Impact of an object weighing 10,000 pounds (lb), 50 ft long by 2.5 ft in diameter, traveling at 5 ft per second (ft/s) was considered in the design of the structure and impinging upon it, and the bottom slab was.

checked for pressure differentials caused by the PMF.

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Revision 0 July 10, 2015 The PBAPS IPEEE (Reference 17) states the following additional Information about flood-related loading:

It is unclear whether or not the original design considered the buoyant effects of the PMF. It is also unclear whether or not the original design considered the effects of forces associated with wind-generated waves. The Standard Review Plan (SRP) specifies that the USACE Technical Report No. 4, Shore Protection Planning and Design, should be used to compute the effects of wave action. However, since the Class 1 structures have exterior concrete walls, a minimum of 2 ft thick to the flood elevation with sufficient strength to resist the hydrostatic pressures associated with the PMF, the effects of wave action are judged to be acceptable. During the initial progressive screening of PBAPS structures for flood protection, it was unclear whether or not original plant design considered the effects of buoyancy under PMF conditions. To resolve this item, each safety-related structure was evaluated to determine whether adequate protection is provided to preclude "floating." For each structure, the total weight of the structure was compared to uplift (buoyancy) force at maximum still water levels under PMF conditions. The uplift force was calculated by determining the volume of water displaced by the structure and multiplying by the specific weight of water. The total weight of each structure was typically extracted from the mass models used to develop the plant response spectra. In general, this estimate is conservative because it does not account for plant equipment such as valves, piping, small pumps, etc. Based on the calculations performed, all safety-related plant structures have a factor of safety of at least 1. 1.

Flood loading (flood level at 135 ft-C.O.) Is included in the loading combinations of Seismic Class 1 structures.

2.4 Flood-Related Changes to the Licensing Basis and any Flood Protection Changes (Including Mitigation) since License Issuance No flood-related changes or flooding protection-related changes, including mitigation, have been made since the latest license issuance. Any licensing-related changes are actively captured in the PBAPS USFAR living document.

2.5 Changes to the Watershed and Local Area since License Issuance Available studies of the watershed prior to 1974 were reviewed to evaluate the changes to the Susquehanna River watershed. The most significant change since the 1969 PMF study was the construction of three flood control dams in the upper portions of the watershed in 1979 (Tioga, Hammond, and Cowanesque Dams). Land use has also changed in the Susquehanna River watershed due to development, changes in land use, and planning practices, such as efforts to reforest agricultural areas. In addition, stormwater management practices such as Pennsylvania Stormwater Management Act 167 (enacted since 1978) plans and changes in agricultural practices have been implemented throughout the watershed to achieve peak flow reduction. Table 2.5.1 summarizes the land use changes in the watershed based on an evaluation of available land use data obtained from a 1970 study and 2011 study from the Susquehanna River Basin Commission.

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Revision O July 10, 2015 Table 2.5.1 - Summary of Land Use Changes in the Susquehanna River Watershed Land Use Coercent of land area)

Referenced Studv Forest Grass Cultivated Urban Water/Wetland Other 1970 Susquehanna River Basin 51%

20%

25%

4%

NA NA Study (Reference 83)1 2011 Nutrients and Suspended Sediment in the Susquehanna 69%

NA 21%

7%

2%

1%

River Basin (Reference 82)

'Estimated based on land use data provided in Reference 83.

NA - Land use category not reported in referenced study and therefore not availabl~ for comparison.

Land use of forested and urban areas increased by 2 and 3 percent, respectively, while cultivated land use decreased by 4 percent.

2.6 Currant Licensing Basis Flood Protection and Pertinent Flood Mitigation Features Watertight doors are provided at all structures; waterproofing is installed to EL 135.0 ft-C.D., and any penetration in the exterior walls is sealed to ensure leaktightness necessary to plant safety. The integrity of the waterproofing on the external surfaces of vertical walls below grade cannot be checked since such surfaces are Inaccessible. Accessible joints are visually inspected and caulked as required on a periodic basis as part of regular plant maintenance. Plastic waterstops are used at all construction joints to maintain the integrity of joints. Penetrations and conduits in exterior walls are sealed with approved, pre-tested seal details and material which assure leaktightness against ground or floodwater.

Penetration seals are installed in accordance with approved specifications and procedures and are Inspected to assure proper installation.

The following structures are required for safe shutdown of Units 2 and 3. In addition, the radwaste building Is flood protected to EL 135.0 ft-C.O., but Is not required for safe plant shutdown.

1. Reactor Building This structure has a minimum number of doors below EL 135.0 ft-C.D. They are watertight.

The structure is sealed to EL 135.0 ft-C.D. Reactor building doors above EL 135.0 ft-C.D. are weatherstripped for leaktightness as secondary containment and the doors are on the shoreside of the structures. Small amounts of water which might leak through the doors' weatherstripping would be handled by the building drainage system and pumped out.

2.

Main Control Room Complex The control room and cable spreading room are well above the flood level. The emergency switchgear room is at EL 135.0 ft-C.D., well above the maximum still water level of EL 132.0 ft-C.D. Since it is inside the turbine building, no wave runup effects are anticipated.

3.

Diesel Generator Building This structure has watertight doors to above EL 138.0 ft-C.D., or more than 1.1 ft above the maximum wave runup.

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4.

Pump Structure D

The parapet around the roof of the critical pump area is at EL 137.5 ft-C.D., o

.......... above

~~t=~im:~~ t~:Xi~~~rg:~~; r~~':1 °~;;~

t~~~~~~e~~~~en~tected 1

gu~~~f~

EL 137.5 ft-C.D. and are capable of continuous operation, so the diesels wilt supply power to the essential shutdown equipment and engineering safeguard equipment.

5.

Eme_rgency Heat Sink Facility, Including Cooling Tower This structure has one weathertight door below the maximum wave runup elevation and is sealed against flooding to above this height (except for the door). The IPEEE (Reference 17) states that, based on the standard details for the door, a gap of less than 1 /8 in. would be expected between the door and door jamb. This volume of in-leakage results in water accumulating on the floor of the valve pit. Based on the type and location of equipment in the valve pit, this depth of water was determined to be Insignificant and to have no adverse effect on safe operation/shutdown of the plant.

3

SUMMARY

OF FLOOD HAZARD REEVALUATION Flooding hazards from various flood-causing mechanisms are evaluated for PBAPS in accordance with Enclosure 2 of the NRC's March 12, 2012, 50.54(f) RFI letter (Reference 127).

Following the guidance outlined in NUREG/CR-7046, "Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America" (Reference 128), the Hierarchical Hazard Assessment (HHA) approach is utilized in the reevaluation study. The HHA approach is a progressively refined, stepwise estimation of site-specific hazards that evaluate the safety of SSCs with the most conservative plausible assumptions consistent with available data.

Consistent with the HHA approach, flooding mechanisms that are determined to be not applicable for the site are screened out using qualitative and quantitative assessments with conservative, simplified assumptions and/or physical reasoning based on physical, hydrological, and geological characteristics of the site. For the flooding mechanisms that can potentially affect the design basis, detailed analyses are performed based on present-day methodologies, standards, and available data.

This section describes in detail the reevaluation analysis performed for each plausible flooding mechanism: flooding due to LIP, flooding on streams and rivers, storm surge and seiche, ice-induced flooding, channel migration, and combined effects flood. Bases for screening out other flood-causing mechanisms are also provided.

The methodology used in the flooding reevaluation study performed for PBAPS is consistent with the following standards and guidance documents:

NRC's SRP, NUREG-0800, revised March 2007 (Reference 125)

NRC Office of Standards Development, Regulatory Guides, Regulatory Guide 1.102 - Flood Protection for Nuclear Power Plants, Revision 1, dated September 1976 (Reference 123)

Regulatory Guide 1.59, "Design Basis Floods for Nuclear Power Plants," Revision 2, dated August 1977 (Reference 124)

Peach Bottom Atomic Power Station Page 23 of 165 (b)(3).16 U SC

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 NUREG/CR-7046, "Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants In the United States of America," dated November 2011 (Reference 128)

NUREG/CR-6966, "Tsunami Hazard Assessment at Nuclear Power Plant Sites in the United States of America," dated March 2009 (Reference 126)

American National Standards Institute (ANSl)/American Nuclear Society (ANS)-2.8-1992, "American National Standard for Determining Design Basis Flooding at Power Reactor Sites,"

dated July 28, 1992 (Reference 2)

JLD-ISG-2012-06, "Guidance for Performing a Tsunami, Surge and Seiche Hazard Assessment Revision O," Japan Lessons-Learned Project Directorate Interim Staff Guidance, dated January 4, 2013 (Reference 129)

JLD-ISG-2013-01, "Guidance for Assessment of Flooding Hazards Due to Dam Failure," Japan Lessons-Learned Project Directorate Interim Staff Guidance, Revision 0, dated July 29, 2013 (Reference 130).

3.1 Effects of Local Intense Precipitation LIP is a measure of extreme precipitation (high intensity/short duration) at a given location.

Generally, for smaller basin areas (up to 10 sq. mi.), shorter storm durations produce the most critical runoff scenario. High intensity rainfall in a small area has a short time of concentration and therefore a high intensity runoff.

The LIP is equivalent to the 1 hr, 1 sq. mi. PMP as described in NUREG/CR-7046 (Reference 128).

Site bathymetry and topography used in the analysis are created in:

Calculation PEAS-FLOOD-01, "BDBEE -

Flood Re-Evaluation -

Topography and Bathymetry Data Processing* (Reference 20).

The LIP at PBAPS is computed in:

Calculation PEAS-FLOOD-02, "BDBEE -

Flood Re-Evaluation -

Rock Run Creek Probable Maximum Precipitation (PMP) and Local Intense Precipitation (LIP)"

(Reference 21 ).

The LIP runoff is modeled in:

Calculation PEAS-FLOOD-03, "BOBEE - Flood Re-Evaluation - FL0-2D Local Intense Precipitation (LIP) Flooding" (Reference 22).

3.1.1 Inputs The inputs for the analysis are described below.

3.1.1.1 Precipitation As prescribed in NUREG/CR-7046 (Reference 128), the LIP is the 1 hr, 1 sq. mi. PMP at the PBAPS site location.

Parameters to estimate the LIP are from USACE HMR 51 (Reference 66) and HMR 52 (Reference 67). Point rainfall (1 sq. mi.) LIP values for durations of 1 hr and less are determined using the charts provided In HMR 52. Using HMR 52 and the site location, the 1 hr, 1 sq. mi. precipitation depth estimate is 18.0 in. per hr. Note that the HMR 51 and HMR 52 studies do not differentiate between thunderstorms and tropical Peach Bottom Atomic Power Station Page 24 of 165

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Revision O July 10, 2015 storms for the causative mechanism for the LIP. The 5-min, 15-min, 30-min, and 60-min 1 sq. mi. LIP for PBAPS are summarized in Table 3.1.1.1.1.

Table 3.1.1.1.1 Mln, 15-Min, 30-Min, and 60-Mln 1 Sq. Ml. LIP for PBAPS Area Duration min)

(sq. ml.)

5 I

15 30 I

60 (1 hr) 1 6.03 in.

I 9.54 in.

13.59 in.

I 18.0 in.

3.1.1.2 Ground Surface Topography The surrounding PBAPS site topography data are obtained from the Pennsylvania Department of Conservation and Natural Resources (PA DCNR) (Reference 77). This dataset, produced by the PAMAP Program, consists of a raster Digital Elevation Model (DEM) with horizontal ground resolution of 3.2 ft. The model topographic data for modeling the LIP are constructed from the PAMAP Light Detection and Ranging (LiDAR) DEM data referenced to vertical datum (ft-NAVD88) and horizontal coordinate system State Plane Pennsylvania South (NAD83). Additional topographic data were obtained from digitized site topographic drawings.

The ground surface elevations, collected via LiDAR, and site topographic drawings were verified in September 2013 using a site survey (Reference 9). The data are processed to produce data in various required formats using the Environmental Systems Research Institute (ESRI) ArcGIS software (Reference 12).

3.1.1.3 Manning's Roughness Coefficients For surface roughness coefficients in the LIP model, the Manning's n values are used. The roughness coefficients are selected based on the land cover type identified using aerial topographical survey information and available aerial imagery. The Manning's n values are selected following the suggested range for the overland flow runoff provided in the FL0-2D Software, Inc. (FLO-2D) "FLO-2O Reference Manual" (Reference 48) and Open-Channel Hydraulics textbook (Reference 6).

3.1.2 Methodology The LIP analysis uses a two-dimensional hydrodynamic model, the FLO-2D PRO (Build 13.11.06) model (Reference 49). FLO~2O is a two-dimensional, physical process model that routes rainfall-runoff and flood hydrographs over unconfined flow surfaces or in channels using the continuity equation and a dynamic wave approximation to the momentum equation. A set of partial differential equations (St. Venant equations) is solved using the second-order Newton-Raphson tangent finite difference method. The solution domain in the FLO-2D model is discretized into uniform, square grid elements, where the discharge is computed in eight different flow directions across the grid element (Reference 48).

The full dynamic wave equation is a second-order, nonlinear, hyperbolic partial differential equation. To solve the equation for flow velocity at a grid element boundary, initially the flow velocity is calculated with the diffusive wave equation using the average water surface slope (bed slope plus pressure head gradient). This velocity is then used as a first estimate in the second-order Newton-Raphson tangent method to determine the roots of the full dynamic equation, and Manning's equation is applied to compute the friction slope. If the solution fails to converge after three iterations, the algorithm defaults to the diffusive wave solution (Reference 48).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 The model has a number of components to simulate sheet flow, buildings and obstructions, spatially variable rainfall and infiltration, floodways, and many other flooding details. Predicted flow depth and velocity between the grid elements represent average hydraulic flow conditions computed for a small time step (on the order of seconds). Typical applications have grid elements that range from 5 to 500 ft on a side, and the number of grid elements Is theoretically unlimited (Reference 48).

The resultant output flies yield surface water elevations, flow velocities, and other hydraulic parameters at individual grid elements.

The steps applied to model the LIP event in FLO-2O PRO are described as follows:

Create a grid system using ground surface topographical data Assign properties and pertinent conditions such as computational boundary and outflow elements Specify roughness coefficients (Manning's coefficients) corresponding to the site's land cover type (e.g., concrete, grass, water)

Identify obstructions completely blocking or diverting water flow (i.e., buildings and/or structures)

Assign precipitation inflow to the model Perform the FLO-2D PRO computation Analyze the results produced by FLO-2D PRO.

3.1.2.1 Grid System and Boundary Conditions Bathymetry and topography data points are imported into FLO-2D PRO. A 10-ft grid resolution (ox) is used to describe the surface topography of the PBAPS because the flow pathways on the site are of that width or wider. The boundary elements are prescribed as outflow points with no prescribed hydrograph; thus, the water can flow freely out of the domain.

3.1.2.2 Grid Properties and Model Characteristics Following the guidance outlined in NUREG/CR-7046 (Reference 128), the runoff losses are ignored. Because of the large percentage of impervious area over the model domain, the relatively short duration of the storm, and the extreme value nature of the simulation (18.0 in.

of rain in 1 hr), the antecedent conditions are of full ground saturation; thus, zero infiltration values (zero runoff losses) are used. The zero infiltration assumption is consistent with the prescribed methodology of NUREG/CR-7046 (Reference 128).

Direct rainfall onto roofs is assumed to be contributing to the overland runoff. Runoff from buildings is a hydrologic feature built into the model. The roof elevations are explicitly built into the FLO-2D model since building heights are captured relative to each other. Care is taken to guarantee the existing flow paths on the ground since water will only flow around elevated grid cells that represent buildings. As a result, the building rooftops are not credited for water storage, and water is not able to flow into the buildings in the model. Flows from rooftops are routed directly to the ground adjacent to the building. Similarly, the drainage system at the site (gutters, pipes, inlets, and culverts) is assumed to be nonfunctional at the time of the LIP event.

3.1.2.3 Surface Roughness The following Manning's n coefficients for overland flow are selected for the different topographical areas shown on Figure 3.1.2.3.1:

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0.4 - Forested areas (Reference 48)

Revision 0 July 10, 2015 0.2 - Areas with minimal grass cover over a rough surface or for open areas with debris (Reference 48) 0.1 - Open ground, no debris (Reference 48) 0.02 - Asphalt or concrete areas (Reference 48) 0.018 - Shot-concrete lining without smooth treatment (Reference 6).

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c::J Model Boundary PNcft lo11Dffl Atomle l'ON!' Stnon 0

t65 330 660 990 1.320 M

a:=iw-==---c:==::::i--* Feel Figure 3.1.2.3.1 - FL0-2D Manning's n Value Peach Bottom Atomic Power Station Revision 0 July 10, 2015 Page 28 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.1.2.4 Obstructions Revision 0 July 10, 2015 Obstructions and impediments to surface water flow include buildings and topographic features. Buildings are entered explicitly into the model elevation grid. Topographic features are captured in the DEM surface rendering.

3.1.2.5 Precipitation When applying LIP to determine the flood hydrograph, it is necessary to specify how the rain falls with time; that is, in what order various rain increments are arranged with time from the beginning of the storm. Such a rainfall sequence in an actual storm is given by a mass curve of rainfall, or the accumulated rainfall plotted against time from the storm beginning.

Therefore, once the depth-duration is determined, a critical temporal distribution is created for a synthetic storm hyetograph. HMR 52 (Reference 67) uses 12 ordered segments for an event to define a synthetic storm hyetograph. HMR 52 ranks the 12 segments based on the total rainfall and defaults to a center loaded distribution, which has the most intense rainfall in the middle of the storm duration. By comparison, a front loaded temporal distribution has the most intense rainfall at the beginning of the rainfall duration; likewise, an end loaded temporal distribution has the most intense rainfall at the end of the rainfall duration.

To determine the bounding LIP effects, three LIP hyetographs are evaluated for all the scenarios, using the following rainfall temporal distributions: front loaded, center loaded, and end loaded. The maximum rainfall intensity occurs at the beginning of the front, center, or end of the event, respectively. All hyetographs have a total rainfall of 18.0 in. for a 1-hr duration. Figure 3.1.2.5.1 shows the mass curve for each temporal distribution. All the scenarios are simulated for a time period of 1.5 hr to cover the recession and establish the duration of flooding. The duration of the rainfall event is always set equal to 1 hr. The postulated LIP event is independent of external flooding events, such as the Rock Run Creek and Susquehanna River flooding scenarios.

20 18 16 J 14

.i 12 j I 10 f *

~ 6

.I

=i 4

E a 2

0 0

10 20 JO 40 lime (mlnutas) 50 60

--Front Loaded

...... Centu loaded End Loaded Figure 3.1.2.5.1 -Temporal Distribution Mass Curves for 1 Hr (60 Min), 1 Sq. Mi. LIP Peach Bottom Atomic Power Station Page 29 of 165

• NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.1.2.6 fL0-2D Model Numencs The three keys to a successful application of the FLO-2D model are:

1.

Mass balance {volume conservation)

2.

Area of inundation

3.

Maximum velocities to ensure there is no numerical surging.

Revision o July 10, 2015 The maximum mass balance error (volume conservation error) for the LIP estimates at PBAPS is within the acceptable continuity error range {Reference 48).

The maximum area of inundation is examined as the maximum flow depth, temporally and spatially. Results show that the area of inundation is reasonable since the entire plant area lies within the 1 sq. mi. LIP of 18.0 in. in 1 hr.

The maximum velocity and numerical surging are related to the area of inundation. Numerical surging is the result of a mismatch between flow areas, slope, and roughness and can cause an oversteepening of the flood wave. To avoid numerical surging, the FLO-20 model is subject to the Courant-Friedrichs-Lewy (CFL) condition due to the explicit numerical scheme used for the solution. To preserve stability, e conservative maximum Cmax s 0:6 is imposed on the floodplain cell solution. This setting forces a reduction in time step if the stability threshold is approached as the solution progresses.

3.1.3 Results The FLO-2O program displays the results by storing attributes within each grid element.

Attributes such as flow depth, flow velocity, and flow direction can then be rendered and displayed as a map to give an _overview of the results, or an individual element can be selected to obtain the results at a particular grid cell.

Three different scenarios ere considered: front loaded, center loaded, and end loaded precipitation hyetograph distributions of the LIP. The maximum water depths and flow velocities are determined over the entire domain for the precipitation hyetographs considered. Results are presented for the doors that provide flood protection for plant areas containing safety-related equipment, as shown on Figure 3. 1.3. 1. The time series of both depth and velocity are obtained at these doors.

The resulting WSELs at the doors that provide protection to safety-related equipment are presented in Table 3.1.3.1.

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Pueh Bottom Atomic: Pow.r Station 0

100 200 400 600

--==---==----~====::i*FNI Figure 3.1.3.1 - Doors Examined in FL02D Model Peach Bottom Atomic Power Station Revision 0 July 10, 2015 Page 31 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Table 3.1.3.1 -Worst Case Consequences of LIP R.0-20 FL0-20Grld Revision 0 July 10, 2015 Protection Level Door Grid c,11 Cell Number Oro11nd VIIOOlty Mul'low WSl!L (134.87 ft*

Number for Cius 1 Struotllre Elevetlon Depth NAVD88, 136 ft.

Number Water for Water (ft-NAVD88)1 (ft/8)

(ft)

(ft-NAVD88)

C.D.)Dtptll Dentt.

Velocity Exctedance (ft)

Emergency T01 13856 13856 Cooling Tower 125.0 4.84 1.95 126.95 No E)(lllledance (Emergencf Heat Slnkl:

246 13611 13826 Unit 3 Reactor 134.57 1.42 0.60 135.17 0.30 Bulldina*

244 12327 12326 Unil 3 Reactor 134.61 1.72 0.82 135.-43 0.56 Bulldlna' 239 13799 14016 Unit 3 Recin: MG 134.30 4.50 1.61 135.91 1.04 SelRoom' 188 14882 15104 Unit 2 Recirc MG 134.30 4.47 1.55 135.85 0.98 Set Room' 183 16853 16853 Unit 2 Reactor 134.70 1.95 0.83 135.53 0.66 Buildina' 198 19407 19407 Unit 2 Reactor 134.57 1.78 0.86 135.23 0.38 Bulldiml 111 19858 19859 Turbine Bulldina*

115.S 0.95 1.65 117.15 N/A 22631 22827 129.00 (Svv-)

1.36 2.96 131.96 No Exceedance MuHipte 23993 23993 Diesel Generator 127.00 (SE')

1.55 0.46 127.46 No Exceedance 22646 22842 Buildingt 117.50 (NE')

1.03 0.12 117.62 No Exceedance 21450 21449 117.501N'N'l 14.05 3.33 120.83 No Exceedance Multiole 25379 25378 Pumo Structure 115.50 1.45 2.00 117.5 No Exceedance'*'

'Ground elevations are deterrmned rrom the DTM elevation shapefila.

"The emergency cooling tower has 01111 weather1tripped door (Reference 19).

'Reactor building doors below EL 135.0 ft-C.D. (134.87 ft-NAV088) are watertight.

The structure is sealed to EL 135.0 ft-C.O.

(134.87 ft-NAVD88). Reactor building doors above EL 136.0 ft-C.O. (134.87 ft-NAV088) are airtight as secondary containments (Reference 18).

"The turbine buMding Is a nonsafety-related structure and Is not flood protected. The water level outside the turbine buffding at the roll up door Is analyzed.

5No specific door location was analyzed. Therefore, flood depths were evaluated around the entire diesel generator building.

'The diesel generator building floor elevation was measured from the southwest (SW), southeast (SE). northeast (NE), and northwest (NW) comers of the building.

' The parapet around the roof of the critical pump area isat EL 137.5 ft-C.D. (137.3711-NAVOBS) (Reference 18) 1No specific door location was analyzed. Therefore, ftood depths were evaluated around the entire pump structure.

3.1.4 Conclusions LIP is not considered in the PBAPS CLB. The reevaluated maximum WSELs at many of the doors potentially leading to the safety-related areas of the plant exceed the flood protection elevations as shown in Table 3.1.3.1 and will be addressed in the integrated assessment.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.2 Probable Maximum Flood of Rivera and Streams Revision 0 July 10, 2015 The PMF is the hypothetical flood (peak discharge, volume, and hydrograph shape) that is considered to be the most severe reasonably possible, based on comprehensive hydrometeorological application of PMP and other hydrologic factors favorable for maximum flood runoff such as sequential storms and snowmelt.

As outlined in the guidance provided in ANSI/ANS-2.8-1992 (Reference 2) and in NUREG/CR-7046, Appendix H (Reference 128), the design basis from flood hazards should include several flood-causing mechanisms and combinations of these mechanisms. For the floods caused by precipitation events, the following should be examined:

Flooding in Riyers and Streams Alternative 1 - Combination of:

Mean monthly base flow Median soil moisture Antecedent or subsequent rain: the lesser of (1) rainfall equal to 40 percent PMP and (2) a 500-year rainfall The PMP Waves induced by two-year wind speed applied along the critical direction.

Alternative 2 - Combination of:

Mean monthly base flow Probable maximum snowpack A 100-year snow-season rainfall Waves induced by two-year wind speed applied along the critical direction.

Alternative 3 - Combination of:

Mean monthly base flow A 100-year snowpack Snow-season PMP Waves induced by two-year wind speed applied along the critical direction.

The PMF at PBAPS is computed for both Rock Run Creek and the Susquehanna River. The PMF flow hydrographs for Rock Run Creek and the Susquehanna River are determined using USACE Hydrologic Engineering Center Hydrologlc Modeling System (HEC-HMS) computer software. The maximum WSELs for Rock Run Creek and the Susquehanna River are determined using the USACE Hydrologic Engineering Center River Analysis System (HEC-RAS} computer software.

Rock Run Creek topography used in the analysis is compiled in:

Calculation PEAS-FLOOD-01, "BDBEE -

Flood Re-Evaluation -

Topography and Bathymetry Data Processing* (Reference 20).

The precipitation for Alternatives 1, 2, and 3 for Rock Run Creek is determined in:

Calculation PEAS-FLOOD-02, "BDBEE - Flood Re-Evaluation* - Rock Run Creek Probable Maximum Precipitation (PMP) and Local Intense Precipitation (LIP)" (Reference 21)

Calculation PEAS-FLOOD-11, "BDBEE - Flood Re-Evaluation - Site-Specific Probable Maximum Precipitation (PMP) and Climatology Calculation* (Reference 28).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 The hydrologic effects of the PMP over the Rock Run Creek Basin are modeled in:

Calculation PEAS-FLOOD-07, "BDBEE - Flood Re-Evaluation - HEC-HMS Rock Run Creek Hydrologic Calculation" (Reference 24).

The hydraulic effects of Rock Run Creek are modeled in:

Calculation PEAS-FLOO0-08, "BOBEE - Flood Re-Evaluation - HEC-RAS Rock Run Creek Probable Maximum Flood (PMF) Hydraulic Calculation" (Reference 25).

For the Susquehanna River PMF evaluation, the bathymetry and topography used in the analysis are compiled in:

Calculation PEAS-FLOOD-01, "BDBEE -

Flood Re-Evaluation -

Topography and Bathymetry Data Processing" (Reference 20).

The precipitation events for Alternatives 1, 2, and 3 are determined In:

Calculation PEAS-FLOOD-11, "BDBEE -

Flood Re-Evaluation -

Site-Specific Probable Maximum Precipitation (PMP) and Climatology Calculation" (Reference 28).

The hydrologic effects of the PMP over the Susquehanna River Basin are performed in:

Three Mile Island Nuclear Generating Station (TMI) Calculation C-1101*122-E410-011, "Precipitation-Driven Discharge Calculation Package" (Reference 37)

TMI Calculation C-1101-122-E410-010, "HEC-HMS Model Calculation Package-Three Mile Island and Peach Bottom Riverine Hydrology Calibration" (Reference 36).

The hydraulic effects and the hydraulic model calibration of the Susquehanna River are performed in:

Calculation PEAS-FLOOD-16, "BDBEE -

Flood Re-Evaluation -

HEC-RAS Probable Maximum Flood (PMF) Water Level" (Reference 29)

Calculation PEAS-FLOOD-06, "BDBEE -

Flood Re-Evaluation -

HEC-RAS Model of Susquehanna River Development and Calibration" (Reference 23).

3.2.1 Inputs - Rock Run Creek The inputs for the analysis are described below.

3.2.1.1 Hvdrologic CHEC-HMS) Model: Ground Surface Topography The ground surface elevations of the Rock Run Creek watershed are obtained from UOAR-produced topography from the PAMAP DEM (Reference 77), which has a horizontal resolution of approximately 3.2 ft referenced to the NAVD88 vertical datum. Additional topographic data were obtained from the Rock Run Creek survey performed In September 2013 (Reference 9).

3.2.1.2 Hvdrologjc (HEC-HMS) Model: AU-Season PMP The all-season depth-area-duration (DAD) PMP estimates for Rock Run Creek are derived from the charts presented in the generalized HMR 51 (Reference 66) and HMR 52 (Reference 67).

The PMP estimates are derived based on the site location and site watershed area.

The all-season DAD estimate for Rock Run Creek Is given In Table 3.2.1.2.1.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Table 3.2.1.2.1 - All-Season PMP Estimate Area Duration (hr (sq. ml.)

8 12 24 48 10 27.11 31.56 34.96 38.68 200 18.72 22.23 26.19 29.72 1000 13.45 17.01 20.91 23.94 5000 8.20 11.51 14.48 17.83 10000 6.32 9.50 11.98 14.95 20000 4.44 7.50 9.87 13.07 3.2.1.3 Hydrologic lHEC-HMSl Model: Cool-Season PMP 72 Revision 0 July 10, 2015 40.35 30.92 24.72 18.96 16.20 13.97 The cool-season DAD PMP estimates for Rock Run Creek are derived from the seasonal variation charts presented in the generalized HMR 53 (Reference 68). The PMP estimates are derived based on the site location and site watershed area. The cool-season DAD estimate for Rock Run Creek is given in Table 3.2.1.3.1.

Table 3.2.1.3.1 - Ratio of Seasonal Ralnfall to AH-Season Rainfall 10 Sa. Ml. Seasonal PMP tin. - Ratio to All Season)

Month 6 Hr 24 Hr 72 Hr In.

Ratio In.

Ratio In.

Ratio Januarv/Februarv 8.99 0.33 13.75 0.39 17.63 0.44 March 9.39 0.35 15.11 0.43 18.67 0.46 April 11.76 0.43 17.27 0.49 21.41 0.53 May 17.02 0.63 22.59 0.65 27.26 0.68 June 23.95 0.88 31.53 0.90 36.98 0.92 Julv/Auaust 27.11 1.00 34.96 1.00 40,35 1.00 September 25.98 0.96 34.58 0.99 40.05 0.99 October 19.85 0.73 28.03 0.80 34.40 0.85 November 13.9 0.51 21.73 0.62 27.08 0.67 December 10.9 0.40 16.37 0.47 20.79 0.52 3.2.1.4 Hydrologlc CHEC-HMS) Model: 100-Year and 500-Year Rainfall The 100-year and 500-year rainfall estimates for a 72-hr storm were obtained from the NOAA Precipitation Frequency Data Server (Reference 65) for the PBAPS location. The 100-year and 500-year point precipitation depth-duration values are determined using the NOAA Atlas 14 (Reference 65) website. Input obtained from the NOAA Atlas 14 website included annual exceedance precipitation frequency estimates for the 100- and 500-year storm for durations ranging from 5 min up to 60 days. The precipitation frequency estimates are given in Table 3.2.1.4.1.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Table 3.2.1.4.1 - NOAA Atlas 14 Precipitation 100- and 500-Year Frequency Estimates 100-Year 500-Year Duration Precipitation Precipitation (In.)

(In.)

5min 0.652 0.735 10 m,n 1.04 1.16 15 min 1.31 1.46 30 min 2.01 2.33 60min 2.76 3.34 2 hr 3.63 465 3 hr 3.95 5.06 6 hr 5.04 6.66 12 hr 6.34 8.69 24 hr 7.52 10.5 2 day 8.32 11.4 3 day 8.81 12.0 3.2.1.5 Hydrologlc CHEC-HMSl Model: Maximized Temperature, Dew Point Temperature, and Wind Speed The dew point temperatures are used as an Input to the energy budget equation to calculate snowmelt rate. Tlme series data for dew point temperatures are derived in the site-specific climatology study, as described in Section 3.2.2.6 and shown on Figure 3.2.1.5.1. The 72-hr period with the highest dew point temperatures is determined from a 120-hr range of values for elevations ranging from 0 to 4,000 ft in March. Based on LIDAR-derived DEM, Rock Run Creek Basin ranges from EL 105.56 ft-NAVD88 to 596.65 ft-NAVD88. Thus, dew point temperature values from EL 500 ft-NAVD88 are used.

0..----Maximized Dew Point Temperature Profile 10 lO

!O fiD --------

---..--~----

II 21 n

41 St 61 11 11 91 IOI Ul IAOff U,00 uoae WN

-uooo

-ui.o,

-It --

Figure 3.2.1.5.1 - March Maximized Dew Point Temperature Profile The wind speeds are used as an input to the energy budget equation to calculate snowmelt rate. The maximum three-day wind speed (mph) is determined in a similar manner to the dew point temperature as described previously for the month of March. Tlme series data for wind Peach Bottom Atomic Power Station Page 36 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 speed are derived in the site-specific climatology study wind speed profile as described in Section 3.2.2.6 and shown on Figure 3.2.1.5.2. Wind speed values from EL 500 ft are used.

liO..------ --Wind Speed Profile- ------,

11 21 31 O

S1 6\\

71 11 91 101 tit lftdaHour

• • aao usoo uooo moo 0000

-nsoo uooo uoo

,o Figure 3.2.1.5.2 - March Maximized Wind Speed Profile March time series data for temperature are derived in the site-specific climatology study wind speed profile as described in Section 3.2.2.6 and shown on Figure 3.2.1.5.3. Wind speed values from EL 500 ft are used.

o..-------Maximized Temperature Profile- --- -

10 20

!' ! 30 f.

... <10 so l-.

3.2.1.6 11 n

31 o

51 es 11 11 n

101 111 lnffaHour Figure 3.2.1.5.3 - March Maximized Temperature Profile 14000 usoo uooo usoo

-uooo 11500 uooo uoo

-10

__J Hydrologic CHEC-HMS) Model: 100-Year Snowpack and Probable Maximum Snowpack Snow water equivalent (SWE) is the amount of liquid contained in a snowpack. The SWE is used as a limiting factor for the total snowmelt available from the 100-year snowpack. The Peach Bottom Atomic Power Station Page 37 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 100-year snowpack estimate for Rock Run Creek is determined using the "Atlas of Extreme Snow Water-Equivalent for the Northeastern United States* (Reference 134). The 100-year SWE estimate for Rock Run Creek Is determined to be 4 in.

For the probable maximum snowpack, snowpack is assumed to cover the whole watershed with no significant variation of temperature or snow depth for the duration of the storm.

Therefore, unlimited snowpack depth exists for the duration of the rainfall.

3.2.1.7 Hydrologic (HEC-HMS) Model: Mean Monthly Base Flow The Rock Run Creek watershed does not have streamflow gages that could be used to determine the base flows. Mean monthly base flow is estimated using available mean monthly discharge data at USGS gages in the area, in the absence of a gage located on Rock Run Creek. All gages located within the Lower Susquehanna watershed, south of Harrisburg, Pennsylvania, having a drainage area of 25 sq. mi. or less are selected for analysis. Mean monthly data are obtained from the USGS Surface Water Data website (Reference 112). Stream gages are shown on Figure 3.2.1.7.1.

S11qvtbu1 Rinr \\\\ 11,n*NI Bovad1r) f ". -*

..,.,. ** IPaJ,ft...,,.

Figure 3.2.1.7.1 - USGS Stream Gage Locations Peach Bottom Atomic Power Station Page 38 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.2.1.6 Hydro!ogjc (HEC-HMS) Model: Soil Data Revision 0 July 10, 2015 The Soil Survey Geographic Database (SSURGO) for York County, Pennsylvania is obtained from the Natural Resources Conservation Service (NRCS) (Reference 74). The soil survey map of York County is shown on Figure 3.2.1.6.1.

PB \\.P~

Legend

~Soll Clloowlllcnww,o,y*- ]IOI.,.,._._.

... Ntyro--25D OO-- l1'l'lfflllyoll>>)

-Gln!Qef..-..y.. _ll015_tl0Pn_

l.t.l>llyN-dl.ltol--

~llllklom

- ~*-ODJ--

.._Nty ___ llOIS--

a-...lill-ll08--

- --- JIOI--

=l,t. Nrylnl-..... ll02S-- V-,ttony o-*"'""'-1io1s.,......,

Co:llr\\lJ-lcom

.._NryN MOrler.,_ 1SI025-- "'--Nry~ID IS-*-

_OllftolQ ____ 1Slo2S ___

Ml qynl-lDill ZSIOJ$--

Figure 3.2.1.8.1 - SSURGO Soll Data for York County, Pennsylvania Peach Bottom Atomic Power Station Page 39 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 3.2.1.9 Hydro!oaic lHEC-HMS} Model: Land Use Land Cover Data and Impervious Data Land coverage is obtained from the 2006 National Land Cover Database (NLCD)

(Reference 52). The land coverage data for the Rock Run Creek Basin are available at a resolution of 30 meters (98.42 ft).

Land cover for Rock Run Creek is shown on Figure 3.2.1.9.1.

N A

NLCD und Cover Clanlflcatlon L09end

-II*~-*

21

• Do-Opoo Spac.

22

• 0.voio,od.LOW"'I-

2)

• Do"'1opod Mtcl""' IIMn,lly 2* *0.........,H<Qlll.,_,ty 31

• a.,,.,, Land (Aod<IS1nd/Cllyl

- ***Dldclwutfof1tl

'2. (--*,.,..,

• 3

• Mt!IO f OIHI 52

• SN\\Jll/Scnib a, a Pasture/Hay 12

• Cullivotod C10Po 80* Wtllond*

85

• Emfl'9"'l Ho..,...oul Wolo,_,.

Figure 3.2.1.9.1 - Rock Run Creek Land Cover Peach Bottom Atomic Power Station Page 40 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 The percent developed imperviousness land cover Is obtained from the NLCD (Reference 63). The percent impervious coverage data are available at a resolution of 30 meters (98.42 ft).

The percent impervious for Rock Run Creek is shown on Figure 3.2.1.9.2.

Leftnd

• PNdl Bottom Alcm,c Power Sta1ion O

Subbaaint o

o>>

o s

--===----Mm Figure 3.2.1.9.2 - Rock Run Creek Percent Impervious 3.2.1.10 Hydraulic (HEC-RAS) Model: Ground Surface Topography N

A The ground surface elevations of the Rock Run Creek watershed are obtained from the PAMAP DEM (Reference 77), which has a horizontal resolution of approximately 3.2 ft.

Supplemental topographic data of Rock Run Creek are from the survey performed In September 2013 (Reference 9).

3.2.1.11 Hydraulic (HEC-RAS) Model: Bridges Dimensions for the four bridges are determined from a survey of Rock Run Creek and bridge crossings completed in September 2013 (Reference 9).

3.2.1.12 Hydraulic (HEC-RAS) Model: Surface Roughness Coefficients Manning's roughness coefficients are obtained based on the topographical survey and available aerial imagery for PBAPS using the guidelines outlined in the HEC-RAS Hydraulic Reference Manual (Reference 86).

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3.2.2 Inputs - Susquehanna River The inputs for the analysis are described below.

3.2.2.1 Hydrotoqic CHEC-HMS} Model: Ground Surface Topography Revision 0 July 10, 2015 For delineating the watersheds of the Susquehanna River Basin, a DEM of the area was obtained online from the National Elevation Dataset (NED) (Reference 117). All NED data are distributed in geographic coordinates in units of decimal degrees and in conformance with NAD83. All elevation values are provided in units of meters and are referenced to the NAVO88 at a resolution of 10 meters (32.8 ft) over the conterminous United States.

3.2.2.2 Hydro!ogjc (HEC-HMS) Model: All-Season PMP A site-specific (or basin-specific) PMP analysis is performed to estimate the Susquehanna River watershed PMP as an alternative to the more conservative HMR 51/52 methodology.

The site-specific PMP evaluation utilizes the most recent data available and provides significant improvements to the HMRs relevant for the site, as the HMRs are relatively more general in their approach for enveloping much wider application areas. Parameters to estimate the Susquehanna River PMP are derived based on past extreme rainfall events that have occurred in and around the northeasterm United States after appropriate adjustments, normalization, storm maximizations, and transpositioning techniques are applied following a recommended storm-based approach (Reference 2, Reference 135, and Reference 128).

The storm-based approach is detailed in HMR 33 (Reference 80) and HMR 51 (Reference 66). The storm-based approach uses historical, regional rainfall data which are maximized and transpositioned to occur at PBAPS. A complete description of the site-specific PMP development is in Calculation PEAS-FLOOD-11, "BDBEE -

Flood Re-Evaluation -

Site-Specific Probable Maximum Precipitation (PMP) and Climatology Calculation" (Reference 28). The flow chart shown on Figure 3.2.2.2.1 is the method used to perform the site-specific PMP estimates.

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Probable Maximtm:1 Precipitation Determination Flowchart

~,.-.-,-*)

.,.,,~,IC'Wt s

[ ----=---- J

(

[ ~""-::..--J

\\

• lf**,.,.~*:,"---~

~

~.-~,,~

c-._...,

Revision 0 July 10, 2015 Figure 3.2.2.2.1 - Flow Chart Showing the Major Steps Involved in PMP Development The final product of the site-specific PMP is a DAD table for precipitation. These include durations of 6, 12, 18, 24, 48, 72, 96, and 120 hr and area sizes from 10 sq. mi. to 100,000 sq. mi. The all-season DAD table is shown in Table 3.2.2.2.1.

Table 3.2.2.2.1 - All Season Site Specific PMP Values 111, b I I *r~--~-

6-How

/ J.ffow II-How 24-Jlow

" *How 1J.Ho,u 96-How 110-How 14 S 21J 224 23 0 233 234 23.S 23 6 II 6 16 0 172 11 l 200 20 a 21 0 21.l 10 7 14.3 IS 8 169 19 I 198 20 I 204 96 12.3 140 IS 2 179 116 190 193 16 101 126 14 0 169 17..$

18 0 Ill 15 9S 113 129 IS6 l6J 16 7 170 80 96 10 a 13 7 14 S ISO us 49 69 81 9.3 120 130 13 7 141 31 57 68 78 104 II 3 12 2 126 34 52 61 73 99 10 a 11 6 12 I 23 31 4 6 5S 12 9 1 98 104 14 24 30 H

60 69 76 13 Note that individual thunderstorms were not included in the all-season PMP development.

Therefore, the site-specific PMP values for area sizes less than 500 sq. mi. and for durations less than 12 hr may be lower than if those storm types were included.

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3.2.2.3 Hydrologic IHEC-HMS) Model: Cool-Season PMP Revision 0 July 10, 2015 A complete description of the site-specific PMP development is presented in Calculation PEAS-FLOOD-11, "BDBEE -

Flood Re-Evaluation -

Site-Specific Probable Maximum Precipitation (PMP) and Climatology Calculation" (Reference 28). The final product of the site-specific PMP is a DAD table for precipitation. These include durations of 6, 12, 18, 24, 48, 72, 96, and 120 hr and area sizes from 10 sq. mi. to 100,000 sq. mi. The cool-season DAD table is shown in Table 3.2.2.3.1.

Table 3.2.2.3.1 - Cool Season Site Specific PMP Values

---cw:e*~-..-..

11.Jlo,u,,_,,,_ 1'-H*r 46.Hot,, 72.&.,,,,_Ha,,, 110-Ho<<,

S6 79 101 110 Ill 136 tl8 14 0 48 67 8 7 9S 12 2 12 12S 126 S

6J 12 90 II 1 II 9 120 121 0

SI 16 109 112 11 3 II 4 36 SJ 71 19 103 10 S 106 107 32 48 6S 73 96 91 99 100 26 4 I S7 6S SJ 87 19 90 22 36 so S8 73 77 78 79 II 30 4J SI 64 66 68 70 I 7 30 4 I 49 61 63 6S 67 I 2 22 32 39 so SJ ss S8 oa I S 22 a

36 4 I 43 4S 3.2.2.4 Hydroloqic lHEC-HMS} Model: Historical Storm Precipitation The precipitation inputs for the historical storm precipitation utilized for calibration and validation of the hydrologic model are developed from the Storm Precipitation Analysis System (SPAS) by Applied Weather Associates (AWA). The database is largely comprised of data from NOAA's National Climatic Data Center (NCDC) TD-3240, but also precipitation data from other mesonets and meteorological networks that have been collected and archived as part of previous studies.

SPAS utilizes Level II Next-Generation Radar (NEXRAD) reflectivity data in units of decibel relative to Z (dBZ), available every 5 min in the United States and 10 min in Canada. The primary vendor of NEXRAD weather radar data for SPAS is Weather Decision Technologies, Inc. (WOT}. who accesses, mosaics, archives, and quality controls NEXRAD radar data from NOAA and Environment Canada. A complete description of the historical precipitation development is in Calculation PEAS-FLOOD-23, "BDBEE -

Precipitation Data Processing" (Reference 35}. The flow chart shown on Figure 3.2.2.4.1 outlines the method used to process the historical storm precipitation. The final product is hourty ASCII Gridded Rainfall Data over the Susquehanna River Basin. This analysis provides actual spatially and temporally distributed precipitation data for three historical storms: Tropical Storm Lee, Hurricane Ivan, and Hurricane Agnes. Precipitation for the three storms is shown on Figures 3.2.2.4.2, 3.2.2.4.3, and 3.2.2.4.4, respectively.

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...........1-.f.d __ _

oc..,-= 1-------, ------

Figure 3.2.2.4.1 - SPAS Analysis Flow Chart Peach Bottom Atomic Power Station Revision 0 July 10, 2015 Page 45 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

N A

Precipitation ( Inches)

Total 96-hour Precipitation 9/5/2011 01 OOZ - 09/09/2011 ooooz SPAS#1298 Stations 0.18 -0.SO O 2.51

• 3.00

• 5.01

• 8.00 10.01

• 12.00

• Dally 0,51 - 1.00 D 3.01

• 3.50 8.01 - 7.00 12.01

• 14.00

• Hourly 1.01

• 1.50 3.51. 4.00 7.01 -8.00 14,01

• 1&,00

• Hourly Esl 1.51

• 2.00 4.01 *,.so 8.01

• 9.00 0 te.ot - 18.00

• Hourly Pseudo E1) 2.01

• 2.50

'-51

• 5.00 11.01

• 10.00 0 18.01

• 20.00

• Supplffllental

• SUppl*m enta_i Est.

Revision 0 July 10, 2015

-c:::..ic:::==:::iMIH 0

l7.5 74 160 Figure 3.2.2.4.2 - Total Storm Rainfall for Tropical Storm Lee (9/5/2011 to 9/2011)

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l)'W l1'W tl'W IO'W 1PN

.. 'W ll'W rrw ll'W 1rw 1rw TT'W 7W rrw Total Storm (48-hr) Precipitation (Inches)

September 17 (0100 UTC) -19 (0000 UTC), 2004 SPAS-NEXRAD 1275 GIIUgM

• °'"". -~

a -~"'-

Pt~on (ftc:ha) ooo., 00Q 201 -3008401 -11 oo so,. 1 ooO11 01 -1100 0

101 -2000301. 400 501 -600 101 -1100 7S'W U'W Revision 0 July 10, 2015 WW Figure 3.2.2.4.3 - Total Storm Rainfall for Hurricane Ivan (9/17/2004 to 9/19/2004)

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Total Storm (168-hr) Precipitation (Inches)

June 18-24, 1972 Q.augaa

• °"'. -

SPAS 1276

--====-------

0 tO II 1'4

" -~

11,sd_..ruf

---===------oe.-....

IN Pl'aclpbtion (MMII 0 511 -1 00 04 01 -500Q 1101 -1100 12 01 -13000

• 1600 CJ, 01 -2 ooO 501.sooO 1101 -10 oo,301. 1* oo 2.01 -3000 601. 7 coo 10 01. 11 ooo 14 01-15 00 0

301 - 4 oo 101 -soo 11 01-12 ooO 1501.,s oo CllO Revision 0 July 10, 2015 IVlll/1011 Figure 3.2.2.4.4 - Total Stonn Rainfall for Hurricane Agnes (6/18/1972 to 9/24/1972)

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Revision 0 July 10, 2015 3.2.2.5 Hydrologic CH EC-HMS) Model: 100-Year coo1-season and soo-Year All-Season Rainfall Daily precipitation data are obtained from NOAA's NCDC onllne archive from the Global Historical Climatology Network (GHCN) (Reference 54). Precipitation values for the 100-year cool-season and 500-year all-season rainfall are derived through statistical analysis of the climate gage data. The data are extracted for a limited number of sites In and around the Susquehanna River watershed for the period of January 1, 1948 to the present. This period corresponds to the time frame when many of the gage stations were installed. These sites reside either within the Susquehanna River watershed or just outside the watershed to extend interpolations to the edge of the basin where possible. A complete description of the 100-year cool-season and 500-year all-season precipitation Inputs is provided in TMI Calculation C-1101-122-E410-011, *Precipitation-Driven Discharge Calculation Package" (Reference 37).

3.2.2.6 Hydrojoajc <HEC-HMS) Model: Cool-Season PMP Maximized Temperature, Dew Point Temperature, and Wind Speed Because there are no concurrent HMR documents providing site-specific, storm based cool-season PMP values; and most data associated with cool-season PMF is outdated, the average temperature, average dew point temperature, and average wind speed for the Susquehanna River Basin for five historical rainfall/flood events (Table 3.2.2.6.1) were explicitly derived. These five storm events are chosen because they best represent an expected cool-season PMP rainfall scenario and encompass a range of months within the season that could produce a cool-season PMF. These five flood events are analyzed in order to provide storm-based, site-specific input parameters for the snowmelt equations.

Meteorological time series of daily average values of the parameters are created at 500 ft increments starting at Oft and extending to 4,000 ft. A 20-day time series of daily average values is recreated for each of the five events. The period analyzes meteorological data approximately one week prior to the rainfall event to one week after the storm event. In addition, hourly time series are created for a 120-hr period. The hourly temperature and dew point values are maximized to ensure continuity with the similarly maximized PMP rainfall amounts using the average in-place maximization temperature difference of the cool-season storms used to derive PMP. This represented the average temperature difference between the storm-representative dew point/sea surface temperature (SST} and the climatological maximum dew point/SST. This is done to best represent what the meteorological parameters would look like during a cool-season PMP rainfall event and to be consistent with the maximizations of the storm events used to derive the PMP values (Reference 28). The final product is an hourly time series of maximized average temperature, maximized average dew point temperature, and maximized average wind speed data. The three time series for all the events in Table 3.2.2.6.1 are shown on Figures 3.2.2.6.1, 3.2.2.6.2, and 3.2.2.6.3, respectively. The March meteorological data are used to compute snowmelt because they result in a higher melt rate than the December through February data.

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Revision 0 July 10, 2015 Table 3.2.2.6.1 - Storms Used In the Cool-Season Meteorologlcal Time Serles Development 0

10 20 s 30 i 40 I 50 60 70 ao 0

Storm Spas/SPAS-Ute#

Daily Data Ranae March 1936 1195 3/8/ 1936 - 3/27/1936 January 1996 1291 1/8/1996

• 1/27/1996 March 1979 5723 2/24/1979

• 3/15/1979 February 1984 5724 2/5/l!J84

• 2/24/1984 March 1993 5725 3/23/1993

• 4/11/1993 Maximized March PMP Temperature Profile March Events JIMJ> f llrllf I

24-hr O\\l-"l't'IP I

I 24 48 72

'6 1:20 144 168 192 216 Index How Figure 3.2.2.6.1 - Maximized March PMP Temperature Profile t4000 tJSOO t 3000 t2500 t2000

-11500 tlOOD tSOO tD Peach Bottom Atomic Power Station Page 50 of 165

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Revision 0 July 10, 2015 M aximized March PMP Dew Point Temperature Profile M1n:h E~b 0

t400O

~r 14-hr 10 PMP ['tent

~l-4'MP usoo 20 tlOO0

~ t 30 t2500 40 t2DDO E

~ 50

-11500 I

I 11000 I

60 I

I tSOO I

70 tO 80 0

24 48 72 120 144 161 192 216 240 Index Hour Figure 3.2.2.6.2 - Maximized March PMP Dew Point Temperature Profile March PMP Wind Speed Profile March Event 60..---- ----------------- -------.,----.

50 I 40 120-hr

---.J!t&' &mt I

14 hr

~

ti!Mf_

I I

I l 30 --r------~-----=-------

• I 20 w,~~~~~~~~~~..L/1.=-=~~~n""~~~d/~

24 48 72 120 Index Hour 144 168 1'2 216 Figure 3.2.2.6.3 - Maximized March PMP Wind Speed Profile 240 t4000 t3500 t3000 t2500 t:ZOOO

-tl.500 11000 tSOO to Peach Bottom Atomic Power Station Page 51 of 165

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Revision O July 10, 2015 3.2.2.7 Hydrologlc CHEC-HMS) Model: 100-Year Snowpack and probable Maximum Snowpack The snowpack depth data are obtained from the NCDC online archive from the GHCN.

Probable maximum snowpack is derived through statistical analysis of climate gage data obtained from NOAA. A cold/snow "season" is defined to be the period between October 1 and April 30, which Is consistent with HMR 42 (Reference 132).

The spatial distribution of the 100-year SWE over the Susquehanna River watershed is obtained from Cornell University's "Atlas of Extreme Snow Water-Equivalent for the Northeastern United States" (Reference 134).

3.2.2.8 Hydrologic {HEC-HMS) Model: Mean Monthly Base Flow Base flow was estimated by taking the yearly average of the discharges obtained from USGS.

(Reference 112) for each month of the year and dividing by the drainage area to obtain values of base flow per unit drainage area.

3.2.2.9 Hydrologic (HEC-HMS) Model: Streamflow Data Stream flow data throughout the watershed was obtained from USGS water data website (Reference 113) for calibration purposes. Where available, observed hydrographs from USGS stream flow gages were used to represent the outflows from the reservoirs in the watershed.

Figure 3.2.2.9.1 shows the location of the stream flow gages used for calibration of the HEC-HMS model, the stream flow gages used to represent outflows from reservoirs, and the remaining stream flow gages in the Susquehanna River watershed.

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Legend o USG$ gages ~r callbrallon Other USG$ gages HEC-HMS $Ubbasln 1

NE W Y O RK

~)'=~=~n,.J M A RYLAND Revision 0 July 10, 2015 k "--t:.

11°':J tomtctr

• NEW JERSEY Figure 3.2.2.9.1 - USGS Gage Locations 3.2.2.10 Hydrologic{HEC-HMS) Model: Soll Data Soil data (Figure 3.2.2.10.1) are obtained from the National Resource Conservation Service (NRCS) (Reference 107). Hydrologic Soils Groups (HSGs) for each soil type are Identified for each sub basin using ArcGIS. Identification of HSGs is necessary to derive hydraulic conductivity values. The soil data map is used to establish a range of values based on HSGs In which the calibrated infiltration loss rates could be adjusted.

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Legend HECIIIIS __,.

ydrologlc loll Group HSGT,oeA H&O tr111M>

HSG T,oeB HSOTflltlllO HSO 1,.,. C HSO Tn,e C'll H&OTn,eD

~:.::~:~nw,J Revision 0 July 10, 2015

~,!e;c;:.~hlJ N E W JERSEY OSGI 0157131 o uehanQA.fJ~_a~o J

Figure 3.2.2.10.1 - NRCS Hydrologlc Soils Groups In the Susquehanna River Watershed 3.2.2.11 Hydrologic IHEC-HMS) Model: Percent Impervious Data The average percent Impervious area for each subbasln is calculated from the USGS National Land Cover Database (Reference 137). Figure 3.2.2.11.1 shows the regions characterized by different values of percent impervious area In the Susquehanna River watershed.

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N E,W YORK 11 r

Legend o

Cities HEC-HMS subbaaln _

Low : O Revision 0 July 10, 2015

~!*:t~::~s ** oJ bM;Y*!ll~~~~~~~u,.,.J MARYLAND NEW JERSEY Figure 3.2.2.11.1 - Regions Characterized by Different Values of Percent Impervious Area In the Susquehanna River Watershed 3.2.2.12 Hydraulic lHEC-RAS) Model: Ground Surface Topography and Bathymetry Table 3.2.2.12.1 shows a summary of the topographic and bathymetric data used as input.

All data used for the model are re-projected, converted, and merged to a single dataset referenced to the NAVD68 vertical datum. The domain area for the bathymetry and topography is shown on Figure 3.2.2.12.1 (Reference 60).

Table 3.2.2.12.1 - Summary of Bathymetry and Topography Inputs Data Source Lake Clarke and Lake Aldred Bathymetry from USGS Survey Reference 59 Conowingo Pond Bathymetry from 2012 Survey Reference 55 Overbank and Island Topography from LiDAR-Derived 2-Ft Reference 76 Contours Peach Bottom Atomic Power Station Page 55 of 165

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76°30 76' 15' Revision 0 July 10, 2015 Su1Q._ue_

ft.,-n-

/- ~

A,ms~.,-

40' 08' 39° 45' Saft Harbor,.-

Dim 1 I PENNSY A L.__ _ _:::::;._..:.....~,+-(""

,./'-_

I MARYlANO

..~,-

a......,.s-M"h Rw, IWlm r.,n,1 0

I 0

I 5

5 I

I 10 MILES I

10 KILOMETERS Figure 3.2.2.12.1 -Area Used for Susquehanna River Model 3.2.2.13 Hydraulic {HEC-RASl Model: Bridges Bridge geometry data are obtained from the design drawings of the Department of Highways Bridge Division of the Commonwealth of Pennsylvania for the following bridges:

SR-372/Norman Wood Bridge (Reference 7)

US-JO/Wrights Ferry Bridge (Reference 8)

EC-155/Bridge between Columbia and Wrightsville (Reference 58).

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3.2.2.14 Hvdrauuc CHEC-BASl Model: laliae Structure Geometry Geometry data are obtained for (b)(3)*16 USC § 824o-1(d), (b)(4), (b)(7)(F) combination of as-built drawings, publishe papers, an reports.

(b)(3) 16 U S.C § 8240-1 (d), (b)(4) (b)(7)(F)

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(b)(3) 16 USC § 824o-1(d) (b)(4) (b)(7)(F) 3.2.2.15 Hydraulic (HEC-RAS) Model: Surface Roughness Coefficients Revision 0 July 10, 2015 Estimates of the Manning's roughness coefficients for the Susquehanna River are initially selected from the HEC-RAS Hydraulic Reference Manual (Reference 93) and refined through Peach Bottom Atomic Power Station Page 58 of 165

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Revision 0 July 10, 2015 calibration. The final Manning's n roughness for the reach of river modeled is shown on Figure 3.2.2.15.1.

Figure 3.2.2.15.1 - Susquehanna River Hydraulic Model Manning's n Values 3.2.2.16 Hydraulic <HEC-RAS) Model: Calibration and Validation Observed Streamflow

~

Observed streamflow data along the Susquehanna River for the HEC-RAS model calibration are obtained for three locations on the Susquehanna River between Marietta, Pennsylvania and Conowingo, Maryland. The three locations are:

USGS Gage 01576000 at Marietta, Pennsylvania (Reference 116)

Conowingo Dam (Reference 1)

USGS Gage 01578310 at Conowingo, Maryland (Reference 115).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Additional observed streemflow data are obtained for the tributaries of the Susquehanna River from the USGS website (Reference 113).

Observed streamflow data are obtained for the following time frames for the following events:

September 1, 2011 to October 1, 2011 - Tropical Storm Lee September 14, 2004 to October 10, 2004 - Hurricane Ivan June 11, 1972toJuly 11, 1972-HurricaneAgnes March 9, 1936 to March 26, 1936 - Flood of March 1936.

3.2.2.17 Hydraulic CHEC-RAS} Model: Calibration and Validation Observed Water Level Data Maximum observed water level data are obtained for the following locations for the events listed in Section 3.2.2.16. The observed data (Tables 3.2.2.17.1 through 3.2.2.17.4) are not directly input to the HEC-RAS model; however, the data are compared against the corresponding HEC-RAS outputs to evaluate the performance of the model.

Table 3.2.2.17.1-Maximum Observed Water Levels-2011 Lee Location Name Historical WSEL Reference (ft-NAVD88)

USGS 01576000 SusQuehanna River at Marietta, PA 257.8 Reference 69 PBAPS at ll-2278C (on 9/7/2011 at 4:20:07 PM)

>110.37 Reference 41 Conowingo Dam Forebey (at time of maximum flow) 108.08 Reference 1 USGS 01578310 SusQuehenna River at Conowinao, MD 36.57 Reference 70 Table 3.2.2.17.2 - Maximum Observed Water Levels - Ivan 2004 Location Name Historical WSEL Reference (ft-NAVD88)

USGS 01576000 Susauehanna River at Marietta PA 256.01 Reference 69 PBAPS at U3 Ll-3278 A/B/C (at time 9/20/04 at 109.67 Reference 41 8:34:55AM)

Conowinao Dam Forebav (at time of maximum flow) 108.24 Reference 1 USGS 01578310 Susauehanna River at Conowingo MD 34.23 Reference 70 Peach Bottom Atomic Power Station Page 60 of 165

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Revision 0 July 10, 2015 Table 3.2.2.17.3-Maximum Observed Water Levels-Agnes 1972 Location Historical WSEL Reference (ft-NAVD88)

USGS 01576000 Susauehanna River at Marietta, PA 264.27 Reference 69 Safe Harbor Dam Tailrace 196.07 Reference 122 Conestoaa River Outlet 195.07 Reference 122 Peauea Creek Outlet 194.37 Reference 122 Holtwood Dam (upstream) 185.07 Reference 57 PBAPS 200 Ft Upstream of Screen House 113.97 Reference 122 PBAPS, Inside Basin at South End of Screen House 112.97 Reference 122 Conowinao Dam Forebay (at time of maximum flow) 111.27 Reference 1 USGS 01578310 Susauehanna River at Conowlngo MD 40.99 Reference 70 Table 3.2.2.17.4-Maximum Observed Water Levels-1936 Location Historical WSEL Reference (ft-NAVD88)

USGS 01576000 Susauehanna River at Marietta, Pa 257.89 Reference 11 0 Safe Harbor Dam Forebay 226.37 Reference 11 0 Safe Harbor Dam Tailrace.

192.80 Reference 11 O Peouea Creek Outlet 190.57 Reference 110 Holtwood Dam Forebay 183.27 Reference 110 Holtwood Dam Tailrace 147.77 Reference 11 O PBAPS Vicinitv 112.9 Reference 85 Conowinao Dam Forebav 107.91 Reference 11 O USGS 01578310 Susauehanna River at Conowinao MD 38.74 Reference 11 o 3.2.2.18 Hydraullc tHEC-RAS> Model: Dam and Spillway Rating Curves The Safe Harbor Dam rating curve Is obtained from the Safe Harbor Water Power Corporation (Reference 81) to aid in determining the performance of the model.

The Holtwood Dam rating curve is obtained from the paper Dam Breach Analysis Simulation on the Lower Susquehanna River (Reference 57) to aid in determining the performance of the model.

The Conowingo Dam rating curve is obtained from Exelon (Reference 15) to aid in determining the performance of the model.

3.2.2.19 Hydraulic <HEC-RAS) Model: Calibration and Validation Inflow and Outflow Boundary Conditions For the callbratlon and validation of the hydraulic model, two scenarios are used as the inflow boundary condition to the model.

For Scenario 1, the observed streamflow at U'SGS Gage 01576000- Susquehanna River at Marietta, Pennsylvania (Marietta) (Reference 116) is used. For Scenario 2 of the model, the observed streamflow at the Conowingo Dam is obtained from the Conowingo Dam operator l_ogs (Reference 1). The data are used as the inflow boundary condition.to the upstream cross section of the model.

The outflow boundary condition of the Susquehanna River reach (below the Conowingo Dam) is set to normal depth. The choice of the normal depth as the outflow boundary condition has a negligible effect at PBAPS due to the presence of the Conowingo Dam.

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Revision 0 July 10, 2015 3.2.2.20 Hydraulic <HEC-RAS} Model: Probable Maximum Flood Lateral Inflow Boundary Conditions Numerous small tributaries enter the Susquehanna River between Marietta, Pennsylvania and Conowingo, Maryland, but in general they have small contributing watershed areas. For the calibration and validation of the HEC-RAS hydraulic model, the flow from the small tributaries and rivers is input to the HEC-RAS model using data from the USGS website (Reference 113) and a ratio of watershed areas.

The lateral Inflow boundary points are shown on Figure 3.2.2.20.1.

Figure 3.2.2.20.1 - HEC-RAS Lateral Inflow Locations Peach Bottom Atomic Power Statlon Page 62 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Gen_eration Co.

Revision 0 July 10, 2015 3.2.2.21 Hydraulic CHEC-RAS} Model: tnune Structure Boundary conditions (b)(J) 10 u :;_L; ~ oL<;0-1(d) (D)(4) (bJ(IJ(f-)

3.2.3 Methodology - Rock Run Creek The application of the PMP over the Rock Run Creek watershed is performed using the HMR 52 computer software (Reference 88). The PMF inflow hydrograph for the Rock Run Creek watershed runoff Is computed using the USACE HEC-HMS Version 3.5 computer software (Reference 89). The maximum WSEL in Rock Run Creek is determined using the HEC-RAS Version 4.1.0 computer software (Refer.ence 90). The HEC-HMS and the HEC-RAS models for Rock Run Creek are not calibrated due to a lack of stream gages or historical information.

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3.2.3.1 Hydrologjc lHEC-HMS} Model: Alternative 1 Precipitation Input Revision O July 10, 2015 As described above, the precipitation input for the Alternative 1 PMF (the all-season PMF) consists of the all-season PMP and an antecedent storm (lesser of 40 percent PMP or 500-year rainfall).

The all-season PMP estimates for Rock Run Creek are derived from the charts presented In the generalized HMR 51 (Reference 66) and HMR 52 (Reference 67). The PMP estimates are derived based on the site location and site watershed area. Five temporal distributions of rainfall (front loaded, one-third loaded, center loaded, two-thirds loaded, and end loaded) are applied over the basin to determine the critical distribution of rainfall.

The 500-year rainfall estimates are obtained from the NOAA Precipitation Frequency Data Server (Reference 65) for the PBAPS location. The estimates of 40 percent PMP and 500-year rainfall are compared, and the 500-year rainfall is a smaller rainfall event and is used for the Alternative 1 PMF.

As a summary, the Alternative 1 precipitation event consists of a 72-hr, 500-year rainfall followed by three rainless days and then the 72-hr all-season PMP. The various precipitation estimates are applied to the basin models in HEC-HMS using time series precipitation gages.

3.2.3.2 Hydrologic lHEC-HMS} Model: Alternative 2 Precipitation Input The precipitation input for the Alternative 2 PMF consists of 100-year snow-season rainfall and coincident snowmelt from the probable maximum snowpack.

The 100-year rainfall estimates are obtained from the NOAA Precipitation Frequency Data Server (Reference 65) for Rock Run Creek. The all-season rainfall values are not adjusted to represent the cold-season rainfall. Five temporal distributions of rainfall (front loaded, one-third loaded, center loaded, two-thirds loaded, and end loaded) are applied over the basin to determine the critical distribution of rainfall.

The snowmelt rate is calculated using the energy budget equation for the rain-on-snow condition following the guidance outlined in USACE Engineer Manual (EM) 1110-2-1406, "Runoff from Snowmelt" (Reference 94 ). The historical hourly dew point temperatures and wind velocities are in Calculation PEAS-FLOOD-11 "BDBEE - Flood Re-Evaluation - Site Specific Probable Maximum Precipitation (PMP) and Climatology Calculation (Reference 28).

Dew point temperatures and wind velocities for the month of March are used as input to the energy budget equation. The highest dew point temperatures and wind velocities for March are rearranged to follow the pattern of the cool-season PMP temporal distributions. The wind speed typical for the region during the cool-season period is used as an Input for the energy budget equation to determine the snowmelt rates from the probable maximum snowpack.

The wind velocity may not match the same days with the highest dew point temperature, although using the maximum three days is more conservative than using the wind data on the same days as the maximum dew point temperatures.

The probable maximum snowpack is assumed to be equal to an unlimited snowpack depth during the entire coincident 72-hr rainfall. While the snowpack can be determined directly from the snow depth, there are not adequate data to reliably extrapolate from the historical observations to the magnitude of the probable maximum event. Any estimated probable Peach Bottom Atomic Power Station Page 64 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 maximum snowpack would have an associated physical limit, i.e., maximum snow depth; therefore, an unlimited snow depth is a conservative assumption.

As a summary, the Alternative 2 precipitation event consists of a 72-hr, 100-year cool-season rainfall coincident with the snowmelt from the probable maximum snowpack. The various precipitation estimates are applied to the HEC-HMS basin models using time series precipitation gages.

3.2.3.3 Hydrologic {HEC-HMS} Model: Alternative 3 Precipitation Input The precipitation input for the Alternative 3 PMF consists of the cool-season PMP coincident with the snowmelt from a 100-year snowpack.

The cool-season PMP estimates for Rock Run Creek are determined using the charts provided in HMR 53 (Reference 68) for each cool-season month (October to April).

The snowmelt rate Is also determined for each cool-season month using the energy budget equation for the rain-on-snow condition (Reference 94). The historical hourly dew point temperatures and wind velocities are obtained from Calculation PEAS-FLOOO-11 "BOBEE -

Flood Re-Evaluation - Site Specific Probable Maximum Precipitation (PMP) and Climatology Calculation (Reference 28). Dew point temperatures and wind velocities for the month of March are used as input to the energy budget equation. The highest dew point temperatures and wind velocities for March are rearranged to follow the pattern of the cool-season PMP temporal distributions as an input to the energy budget to determine the snowmelt rates from a 100-year snowpack.

The snowmelt for the Alternative 3 PMF Is limited by a 100-year snowpack. The 100-year snow depth is determined using the "Atlas of Extreme Snow Water-Equivalent for the Northeastern United States" (Reference 134).

As a summary, the Alternative 3 precipitation event consists of a 72-hr cool-season PMP coincident with the snowmelt from a 100-year snowpack. The various precipitation estimates are applied to the HEC-HMS basin models using time series precipitation gages.

3.2.3.4 Hvdrologic <HEC-HMS} Model: Watershed Deljneation The Rock Run Creek basin is delineated using LiDAR data using the HEC-Geospatial River Analysis System (GeoRAS) (Reference 91) computer program in the ArcGIS environment.

The delineated watershed is shown on Figure 3.2.3.4.1.

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Figure 3.2.3.4.1 - Rock Run Creek Watershed Delineation 3.2.3.5 Hydroloqic IHEC-HMS} Model: Infiltration Loss Rate Revision 0 July 10, 2015 The Initial and Constant Loss Method described by the HEC-HMS Technical Reference Manual (Reference 92) is used to estimate the losses due to lnfiltratlon. Evapotransplration during the runoff event is set to zero. The land use/land cover and hydrologlc soil group data are used to develop the area-weighted infiltration loss rate values for each subbasin. The initial loss infiltration Is applied only to the all-season PMF and the inltial loss is ignored for the cool-season PMF alternatives due to an assumption of frozen ground. The loss rates for the Rock Run Creek subbaslns are given in Table 3.2.3.5.1. The hydrologic soil group map is shown on Figure 3.2.3.5.1.

The runoff must be carried, or "routed," by existing channels or "reaches" in the topography.

However, both of the subbasin outlets are essentially at the outlet of Rock Run Creek. Thus, routing from subbasins is assumed to be instantaneous; therefore, transmission loss Is assumed to be zero.

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Figure 3.2.3.5.1 - Rock Run Creek Hydrologlc Soll Groups Peach Bottom Atomic Power Station Revision O July 10, 2015 Page 67 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Table 3.2.3.5.1 -Constant Lo88 Estimate Summary Total Estlmatad Area of Constant Subbasin Subbasln Soil Group Soll Type Loss Constant Area Losa Rate Impervious (sq. ml.)

(sq. ml.)

(inJhr)

(in.lhr)

A 0.62 0.3 B

1.65 0.225 Tributary to C

0.13 0.1 Rock Run 2.68 0.21 8.9 Creek (RCB1)

D 0.04 0.025 Quarry/Pit 0.20 0.025 Water 0.04

.o A

0.44 0.3 Rock Run 1.14 B

0.64 0.225 0.25 2.44 Creek (RCB2)

C 0.05 0.1 Quarry/Pit 0.01 0.025 3.2.3.6 Hydrologic CHEC-HMS) Model: Unit Hydrograph Revision O July 10, 2015 Initial Initial Loss Loss (In.)-

(In.)-

All Cool Season Season 1

0 1

0 The Snyder unit hydrograph defined in the HEC-HMS Technical Reference Manual (Reference 92) is used as the basis to transform rainfall to runoff. The lag and peaking coefficients, C, and Cp, are obtained from Reference 61. The physical data are extracted from subbasln topography. The longest main stream length, L, and the main stream length from the outlet to a point nearest the watershed centroid, Lea, are determined using ArcGIS Desktop computer software (Reference 12). Due to the small size of the subbasin, the 5-min unit hydrographs are developed to ensure that the peak runoff is properly represented. For the same reason, the precipitation input is also derived with the 5-min time step interval to capture the peak runoff. The derived unit hydrographs are adjusted to account for the nonlinear basin response by increasing the peak discharge of the unit hydrograph by one-fifth and decreasing the time to peak by one-third following the guidance outlined in NUREG/CR-7046 (Reference 128).

Table 3.2.3.6.1 lists the Snyder unit hydrograph parameters prior to the nonlinear adjustment.

Table 3.2.3.6.1 - Summary of Snyder Unit Hydrograph Parametena Total Peaking Lag Total Length, Length to Coefficient, Coefficient, Lag Subbasln L

Centroid, Cp C1 (hr),

(mi.)

Loa T,

(ml.)

Tributary to Rock Run Creek (RCB1) 3.91 2.71 0.41 1.26 2.55 Rock Run Creek (RCB2) 2.40 2.09 0.41 1.26 2.04 3.2.3. 7 Hydrologic (HEC-HMS) Model: Rock Run Creek Base Flow The mean monthly base flow for Rock Run Creek Basin is estimated by averaging the mean monthly discharge data at USGS gages in the area, in the absence of a gage located on Rock Run Creek. A mean monthly base flow of 4.33 cfs is applied to the basin; however, it is Peach Bottom Atomic Power Station Page 68 of 165

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Revision 0 July 10, 2015 assumed frozen ground conditions exist during the winter months (November to March) and base flow will be zero because the overall base flow will be minimal. The base flow is negligible compared to the flood flows and has little effect on the peak PMF values.

3.2.3.8 Hydraulic CHEC-RAS) Model: Cross Sections. Structures. and Levees Cross sections along Rock Run Creek and the channel geometry are obtained from the LiDAR-derived ground surface topography and from the survey conducted in September 2013 (Reference 9) using USACE HEC-GeoRAS computer software. The DEM file Is used to extract channel geometry, bridges, and elevation data and determine the stream banks.

Background imagery for figures and location positioning is obtained from the ESRI World Imagery Service (Reference 13). The cross sections are created at locations that define geometric characteristics of the stream valley and overbanks. The cross sections are referenced to the NAVD88.

3.2.3.9 Hydraulic CHEC-RAS) Model: Manning's n Roughness Manning's roughness coefficients for the streambed are selected from the HEC-RAS Hydraulic Reference Manual (Reference 93). The values shown in Table 3.2.3.9.1 are selected as representative of the conditions present along Rock Run Creek.

Table 3.2.3.9.1 - Manning's n Value Description Manning's n Value Description of Land Mass 0.035 Water bodies and channels 0.045 Pasture, cultivated area, or urban areas with light development having few flow obstructions and conslstina of mosllv imoervious surfaces 0.100 Heavily forested area 3.2.3.10 Hydraulic (HEC-RAS) Model: Boundary Conditions The peak flow calculated from the hydrologic model of Rock Run Creek is used on the Rock Run Creek reach as the upstream boundary condition to the hydraulic model.

The normal depth method is used for the downstream boundary condition on the Rock Run Creek reach. A sensitivity analysis of the gradient is conducted using three different slope values. The three slope values are 0.05 ft/ft, 0.005 ft/ft, and 0.5 ft/ft.

3.2.4 Methodology - Susquehanna River 3.2.4.1 Hydroloaic CHEC-HMS> Model: Calibration and verification This analysis utilizes ArcGIS 10.0 (Reference 14), HEC-Geospatial Hydrologic Modeling Extension (GeoHMS) 10.0 (Reference 103), and HEC-HMS 3.5 (Reference 89) computer programs to develop a calibrated hydrologic model of the Susquehanna River watershed for the PBAPS site.

Creating a calibrated HEC-HMS model for estimating surface runoff requires the refinement of an initial set of parameters used to run the model and produce preliminary results. Some of the parameters can be measured, such as rainfall and precipitation. Other parameters, such as initial and constant infiltration loss rates, cannot be adequately measured or Peach Bottom Atomic Power Station Page 69 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 calculated, and are therefore initially estimated and then refined through calibration to a measured flow event.

The refinement of the initial parameters is required to adequately represent the overall watershed response from the combined individual subbasins, which may produce different results from an individual subbasin calibration. This variation in calibration from a single subbasin to a multiple subbasin model could be caused by the spatial variation in precipitation and corresponding subbasln runoff, or the nonlinear relationship in river routing.

The HEC-HMS model is initially calibrated to Tropical Storm Lee (September 2011),

Hurricane Agnes (June 1972), and Hurricane Ivan (September 2004). The subbasin response to each storm event varies due to the different rainfall intensities, rainfall spatial distribution, and antecedent conditions corresponding to each storm. Once the Individual storm calibrations are developed, the calibration parameters are reevaluated to develop the combined model to represent a watershed-wide response to a significant flood {such as the

.PMP).

Initial and constant infiltration loss rates, times of concentration (Tc), storage coefficients (R),

and Muskingum routing parameters (K and X) are refined through calibration against observed flow rates for each of the three storm events considered individually to produce three calibrated HEC-HMS models.

The calibration to each individual storm event initially focuses on calibrating the Clark instantaneous unit hydrograph parameters and the Muskingum routing parameters to match the timing of the hydrographs at the USGS streamflow gages. Once the timing is initially calibrated, the volume and peak flows are calibrated by adjusting the initial and constant infiltration losses. The soil map in Figure 3.2.2.10.1 is used as a supporting reference for the calibration of the constant infiltration losses.

The calibration of the individual storm event models is performed to reproduce the observed USGS streamflow hydrographs as close as the model parameter ranges allow. The calibration acceptance criteria presented by Donigian et al. ( 1984) (Reference 11) are selected for the individual calibrations.

Table 3.2.4.1.1 shows the calibration goals used as general guidance to evaluate the calibration performance. However, assessment of the calibration also considers external factors that may impact observed data such as gage failure, additional rainfall not associated with the storm of interest, or irregular dam operations during an event.

Table 3.2.4.1.1 - Calibration Goals Calibration Location Individual Calibration Goal Tributaries Off of Main Branches of Rivers

+/-10% Peak Flow and Volume 01570500 Susquehanna River at Harrisburg, PA 01576000 Susquehanna River at Marietta, PA 0% to 10% Peak Flow and Volume 01578310 Susquehanna River at Conowlnao MD At some locations, such as the headwaters, subsequent rainfall that is not part of the storm of interest is not included in the provided gridded precipitation dataset. The observed hydrograph in these areas would show additional runoff volume during the recession limb because the additional precipitation occurred after the storm was considered for calibration.

Since this subsequent rainfall is not included in the gridded precipitation dataset, the model Peach Bottom Atomic Power Station Page 70 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 cannot calculate the corresponding volume. In this case, the modeled volume may be off by greater than 10 percent when compared to the observed volume. In order to ensure the goodness of fit at these locations, the modeled hydrograph is visually inspected and calibrated based on the observed portion of the hydrograph corresponding to the calibration storm only.

Once the HEC-HMS model is calibrated to each individual storm event, the calibrated parameters are reviewed and compared to identify spatial and temporal trends and to make judgments regarding the establishment of parameters for a single composite model, such that a watershed-wide response to a significant flood (such as the PMP) would be represented. Following the development of the final calibrated model, each storm event Is reevaluated in order to verify the calibrated parameters.

In addition to evaluating the goodness of fit for the combined model by comparing the modeled peak flow, volume, and timing to the observed hydrograph counterparts, the Nash-Sutcliffe efficiency (NSE) is used to evaluate prediction of the overall hydrographs.

Since the parameters of the HEC-HMS combined model were based on sufficiently large storm events relative to the PMF, adjustments to the unit hydrographs to account for the non-linear watershed response were not deemed necessary.

The January 1996 rain-on-snow event is examined to evaluate if constant infiltration loss rates calibrated for the combined model provide an acceptable estimate of peak flow discharge in presence of snowmelt. The evaluation, carried out for the Loyalsock and Lycoming subbasins, shows that the combined model infiltration rates are representative of the basin response during the January 1996 rain-on-snow event. The combined model infiltration rates are considered representative of the watershed to evaluate rain-on-snow scenarios In the Susquehanna River watershed.

The combined calibrated model is then used to evaluate the PMF scenarios for the Susquehanna River watershed to PBAPS.

3.2.4.2 Hydrologic CHEC-HMS) Model: Probable Maximum Flood Scenarios Examined 3.2.4.2.1 Probable Maximum Flood Scenario 1 PMF Scenario 1 evaluates the combination of the hydrologic events from Alternative 1 as mean monthly base flow; median soil moisture; antecedent rain equal to a 500-year rainfall; and the PMP.

The combined calibrated model is used to evaluate PMF Scenario 1 for the Susquehanna River watershed to PBAPS.

Base flow is estimated by taking the yearly average of the discharges obtained from USGS (Reference 112) for each month of the year and dividing by the drainage area to obtain values of base flow per unit drainage area for several stream gages in the Susquehanna River watershed. The April average flow is the highest base flow throughout the year and is used to represent the average monthly base flow.

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Revision O July 10, 2015 The all-season PMP is developed according to the methodology outlined in HMR 52 (Reference 67) and the USACE HMR 52 User's Manual (Reference 105) using Matlab software. The Matlab software is utilized due to the USACE HMR 52 software being limited to evaluation of storm areas of up to 20,000 sq. mi. and 72 hr in duration. The intervals of the site-specific DAD values generated for the site-specific PMP are more refined and include the 27,078 sq. mi., 50,000 sq. mi., and 100,000 sq. mi. storm sizes. The time intervals of the site-specific DAD included three additional points (18 hr, 96 hr, and 120 hr).

The storm event is represented by elliptical isohyets as per HMR 52 (Reference 67).

Standard ellipses {A to S) are obtained using the HMR 52 procedures, and additional elliptical isohyetals are extrapolated up to 100,000 sq. ml. to envelope the watershed.

The basin-weighted precipitation for each subbasin is estimated following Steps 1 through 5 outlined in Section 5 of the USACE HMR 52 User's Manual (Reference 105). Various storm orientations and sizes are evaluated to determine the maximum rainfall depth over the watershed. Optimal combinations of storm orientation and size for input into HEC-HMS are determined by Identifying the peak 18-hr volume and maximum rainfall depth over the entire 120-hr storm.

The output from the storm center with the greatest watershed-averaged rainfall Is modeled in the calibrated HEC-HMS model to determine the storm that produces the highest runoff.

The precipitation input files are generated for use in HEC-HMS for each storm center location with multiple storm orientations. Figure 3.2.4.2.1.1 shows the storm centers selected for HMR 52 analysis. The storm center at Marietta, Pennsylvania Is selected to represent a location in close proximity to PBAPS. The storm with the highest watershed-averaged rainfall is centered at the centroid of the watershed.

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Figure 3.2.4.2.1.1 - Storm Center Locations Revision 0 July 10, 2015 HMR 52 prescribes the PMP to be temporally distributed by 6-hr increments, with the only limitation in the temporal arrangement being that the peak 6-hr Interval cannot occur within the first 24 hr of the storm. To evaluate the sensitivity of the watershed to the temporal distribution, the peak rainfall increment is set at 30 hr (front distribution), 48 hr (one-third distribution), 66 hr (center distribution), 90 hr (two-thirds distribution), and 120 hr (back distribution) following the start of the PMP precipitation. The temporal distributions are presented In Figure 3.2.4.2.1.2.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

.. i 0.9

~ 08

...... i 0.7

~

ta 0.6

-5 Q..,

C 0.5

.i=

0.4 g 0.3 l!

]. 0 2

~

t.

ii O 1

! o o

Revision 0 July 10, 2015 0

6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 Time (hr)

Pr.ik @ HoLJr 30 P1rak @ Hour 48

• Pt'oik Ii> Hour 66 Peak @ Hour 90 Peak @ Hour 120 Figure 3.2.4.2.1.2 - Susquehanna River Wat.ershed All-Season PMP Temporal Distributions NUREG/CR-7046 (Reference 128) recommends a configuration for the Scenario 1 precipitation event consisting of 120 hr of antecedent storm conditions, followed by the 120-hr PMP event with an unspecified dry period between the antecedent and PMP events.

ANSI/ANSl-2.8-1992 (Reference 2) states that a sequential storm could precede the PMP with a three-to five-day period between storms. A 72-hr dry period was selected to be consistent with the minimum recommended amount of time between sequential storms.

HMR 52 (Reference 67) and the USACE HMR 52 User's Manual (Reference 105) methodologies are used to estimate the average precipitation for the entire watershed above PBAPS and for each HEC-HMS subbasin for twelve 6-hr increments using the site-specific DAD values.

Following the procedures outlined in Section 7.1 of HMR 52 (Reference 67) and Section 5.0, "Computer Program Procedures,* of USACE HMR 52 User's Manual (Reference 105), the basin's average precipitation is estimated. Using the DAD values, a cubic spline curve fit is applied through the DAD values and the values for each area-time are estimated.

Based on HMR 52 (Reference 67), the preferred storm orientation is 200 degrees. This is further adjusted to the optimal orientation of212 degrees, which generates the highest basin-averaged rainfall. A representation of the watershed and the 100,000 sq. mi. isohyetal rotated to an angle of 212 degrees is shown in Figure 3.2.4.2.1.3.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

legend 212 deg Rotatff INhyeta l.eft*,lqMI A. 10 en c.ao 0100 E 115

,, JOO

- C.,O

-H.100 1 1000

,. uoo IC.2150 l lOOO M4500 H l)OO 0, 10000 o.nooo

".*OOCO R1.,oooo

- s-Revision 0 July 10, 2015 Figure 3.2.4.2.1.3 - laohyetals betwHn 10 Sq. Ml. and 100,000 Sq. Ml. at an Orientation of 212°Centered at the TMI Watershed Centroid The PMP centers considered correspond to the centroids of the following subwatersheds:

upper Susquehanna River, Chemung River, west branch of the Susquehanna River, Juniata River, Susquehanna River to TMI, and Susquehanna River to PBAPS. The location of the USGS streamflow gage at Marietta, Pennsylvania is also considered as a possible PMP center.

The flood arrival time and peak flow values associated with each storm center evaluated for the center-peaking storm event are reported in Table 3.2.4.2.1 1.

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Revision O July 10, 2015 Table 3.2.4.2.1.1 - Flood Arrival Time and Peak Flow at TMI and Marietta for Different Storm Center Locations and Center-Peaking 27,048 Sq. Ml. Storm Event (Scenario 1, Single PMP Event)

Area-TMI Marietta Weighted Precipitation Flood FIOod Storm Center on the Watershed Arrlval Peak Flow (cfs Arrlval Peak Flow (cfs)

Upstream of Time (hr)

Tlme(hr)

TMI (In.)

Centroid of Upper Susquehanna 5.47 128.0 666,517.2 129.0 669,844.0 River watershed Centroid of Chemung River 7.64 133.5 1,152,182.5 134.5 1,155,317.9 watershed Centroid of West Branch 8.40 119.5 1,239,665.0 120.5 1,258,929.2 SuSQuehanna River watershed Centroid of Juniata River watershed 8.28 118.5 1,206,883.9 119.5 1,226,479.3 Centroid of TMI watershed 11.15 121.0 1,607,438.4 122.0 1,529,482.5 Marietta, PA 4.55 96.0 775,256.3 97.5 812,270.7 Centroid of PBAPS watershed 10.82 119.0 1,418,951.8 120.0 1,451,510.6 Five different temporal distributions for rainfall hyetographs are considered: front, one-third, center, two-thirds, and back (Reference 67 and Figure 3.2.4.2.1.2). Table 3.2.4.2.1.2 reports flood arrival time and peak flow values for different hyetograph temporal distributions for a storm centered at the TMI watershed centroid, which is the storm center resulting In the largest peak discharge at TMI. The table also reports the results at Marietta.

Table 3.2.4.2.1.2 - Flood Arrival Time and Peak Flow at TMI and Marietta for Different Hyetograph Temporal Distributions for a 27,048 Sq. Ml. Stonn Centered at the TMI Watershed Centroid (Scenario 1, PMP Event Including Antecedent Event)

TMI Marietta Hyetograph Temporal Distribution FIOod Arrival Time Peak Flow (cfs) Flood Arrival Time Peak Flow (cfs)

(hr)

(hr)

Front 91.5 1,429,595.1 92.5 1,450,589.2 1/3 103.0 1,527,980.6 104.0 1,550,082.5 Center 120.6 1,529,663.8 121.5 1,561,721.0 2/3 144.5 1,528,141.3 145.5 1,550,186.6 Back 167.0 1,453,380.6 168.0 1,475,897.6 The 40 percent PMP is less than the 500-year all-season precipitation; therefore, the 40 percent PMP is used as the antecedent precipitation for the evaluation of Scenario 1.

3.2.4.2.2 Probable Maximum Flood Scenario 2 PMF Scenario 2 evaluates the combination of the hydrologic events from Alternative 2 as mean monthly base flow, Probable Maximum Snowpack, and 100-year snow-season rainfall.

Base flow is estimated by taking the yearly average of the discharges obtained from USGS (Reference 112) for each month of the year and dividing by the drainage area to obtain values of base flow per unit drainage area for several stream gages in the Susquehanna River Peach Bottom Atomic Power Station Page 76 of 165

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Revision O July 10, 2015 watershed. The April average flow is the highest base flow throughout the year and is used to represent the average monthly base flow.

For the 100-year cool-season precipitation, liquid equivalent data from valid sites are collated regardless of the actual phase of the precipitation. This information is then parameterized using Gumbel, Generalized Extreme Value (GEV), and Log Pearson Type Ill distributions fitted to seasonal maximums, according to NUREG/CR-7046 guidance for computing multiple types of distributions (Reference 128). The 100-year return period values are calculated by inputting the distribution parameters into the Cumulative Density Function (CDF) for the particular distribution, computing the 0.99 CDF value.

This analysis yields a 100-year probability point rainfall at each gage site analyzed. The values used in this analysis are calculated from the Log Pearson Type Ill distribution, determined to be the most conservative distribution. The average 100-year cool-season precipitation for the entire Susquehanna River watershed is distributed throughout the basin using the spatial distribution available in HMR 52 (Reference 67) through cubic spline interpolation. Area-weighted average 100-year rainfall is computed for each subbasin, and the five temporal distributions are estimated using Matlab software.

The calculation of the probable maximum snowpack is based on the techniques applied in HMR 42 and Buckler (References 132 and 5, respectively). At each valid site within the Susquehanna River watershed, the average seasonal snowfall and the maximum historical snowfall are calculated. The ratio of maximum to average seasonal snowfall is calculated and plotted against the average seasonal snowfall. An enveloping (straight line) relationship is developed from the data, such that every data point lies beneath the line. Using the average seasonal snowfall for the Susquehanna River watershed as a whole, the corresponding ratio is found from the enveloping relationship. Using this ratio and the average seasonal snowfall for the Susquehanna River watershed, the probable maximum snowpack is calculated at each site. A single basin-weighted average is calculated using the data from each site.

The equations reported in Tables 5-2 and 5-3 of EM 1110-2-1406 (Reference 94) are considered to calculate the snowmelt rates for rain-on-snow and rain-free scenarios throughout the duration of the simulated storm events. The Susquehanna River watershed is 62 percent forested upstream of PBAPS; appropriate equations and associated assumptions from EM 1110-2-1406 (Reference 94) are adopted. Snowmelt contribution is calculated with adequate snowpack to melt based on the assumption of ripe snow, as described in EM 1110-2-1406, being mostly saturated and ready to melt (assumed for the probable maximum snowpack).

The formulation for rain-on-snow snowmelt reported by EM 1110-2-1406 (Reference 94) is modified to consider the general situation of nonsaturated atmosphere (Reference 37).

For each time step in the simulation, the sum of snowmelt water depths is computed and compared with the SWE for each subbasin. If the sum (including the calculated melt for the current time step) is less than the total SWE, the full calculated snowmelt water depth computed using the appropriate equation (rain-on-snow or rain-free) is used for the current time step. Otherwise, the snowmelt water depth for the current time step is set equal to the difference between the SWE and the sum of snowmelt water depths computed up to the current time step. All subsequent time steps ere assumed to have no snowmelt contributions, due to the SWE supply being depleted for the event. Due to the magnitude of the probable maximum snowpack depth, the conservative value used for snowpack density, and range of Peach Bottom Atomic Power Station Page 77 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 calculated melt rates during the meteorological time series data, the supply of snow for melting does not run out during the simulation period adopted for Scenario 2.

3.2.4.2.3 Probable Maximum Flood Scenario 3 PMF Scenario 3 evaluates the combination of the hydrologic events from Alternative 3 as mean monthly base flow, 100-year snowpack, and snow-season PMP.

Base flow is estimated by taking the yearly average of the discharges obtained from USGS (Reference 112) for each month of the year and dividing by the drainage area to obtain values of base flow per unit drainage area for several stream gages in the Susquehanna River watershed. The April average flow is the highest base flow throughout the year and is used to represent the average monthly base flow.

The cool-season PMP is developed according to the methodology outlined in HMR 52 (Reference 67) and the USACE HMR 52 User's Manual (Reference 105) using Matlab software.

The basin-weighted precipitation is estimated following Steps 1 through 5 outlined in Section 7.1C of HMR 52 (Reference 67) and Section 5 of the USACE HMR 52 User's Manual (Reference 105). Various storm orientations and sizes are evaluated to determine the maximum rainfall depth over the watershed. Optimal combinations of storm orientation and size for input into HEC-HMS are determined by identifying the peak 18-hr volume and maximum rainfall depth over the entire 120-hr storm.

The output from the storm center with the greatest watershed-averaged rainfall Is modeled in the calibrated HEC-HMS model to determine the storm that produces the highest runoff.

The precipitation input tiles are generated for use in HEC-HMS for each storm center location with multiple storm orientations. Figure 3.2.4.2.3.1 shows the storm centers selected for HMR 52 analysis. The storm with the highest watershed-averaged rainfall is centered at the centroid of the watershed upstream of TMI.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015

/u~:=f:]

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,,..-!:,~ -:~

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..J 1--1 Figure 3.2.4.2.3.1 - Storm Center Locations As regards the precipitation temporal distributions, HMR 52 (Reference 67) prescribes the PMP to be temporally distributed by 6-hr increments, with the only limitation in the temporal arrangement being that the peak 6-hr interval cannot occur within the first 24 hr of the storm.

To evaluate the sensitivity of the watershed to temporal distribution, the peak rainfall increment is set at 30 hr (front distribution), 48 hr (one-third distribution), 66 hr (center distribution), 90 hr (two-thirds distribution), and 120 hr (back distribution) following the start of the PMP precipitation. The temporal distributions from the TMI watershed centroid orientation are presented in Figure 3.2.4.2.3.2.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

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Peak @ Hour 30 Peak @ Hour 48 Peak @ Hour G6 Peak @ Hour 90 Peak @ Hour 120 Figure 3.2.4.2.3.2 - Susquehanna River Watershed Cool-Season PMP (Snowmelt Excluded) Temporal Distributions The spatial distribution of the 100-year SWE over the Susquehanna River watershed Is obtained from Cornell University's "Atlas of Extreme Snow Water-Equivalent for the Northeastern United States" (Reference 134). A representation of the SWE surface and range values assigned to the subbasins Is shown in Figure 3.2.4.2.3.3.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

gend Revision 0 July 10, 2015 SWE Contou11 (11)

Area-Averaged SWE (In)*

• 1.0 - 5.0 5.1 - 7.0 7.1-9.0 9 1* 120 N

A Figure 3.2.4.2.3.3 -100-Year Snow Water Equivalent Surface For snowmelt, the same equations adopted for PMF Scenario 2 are used to calculate the snowmelt rates for rain-on-snow and rain-free scenarios throughout the duration of the simulated storm events. SWE is the upper limit for the total amount of snowmelt.

3.2.4.3 Hydraulic (HEC-RAS} Model: Calibration and Validation This analysis utilizes ArcGIS 10.0 (Reference 12), HEC-GeoRAS 10 (Reference 91), and HEC-RAS 4.1 (Reference 90) computer programs to create a calibrated hydraulic model of the Susquehanna River for the PBAPS site. The hydraulic mode! extends from Marietta, Peach Bottom Atomic Power Station Page 81 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Pennsylvania to Conowingo, Maryland, covering approximately 36 mi. of river. HEC-RAS 4.1 is a hydraulic modeling program used for simulating one-dimensional steady and unsteady flows in river channels. The modeler must supply geometric information to describe the channel, floodplain, and major obstructions (such as bridges and weirs). The model geometry used, along with discharge, boundary conditions, cross section roughness coefficients, expansion and contraction coefficients, and other characteristics. The hydraulic model and its associated parameters are validated by reproducing one or more large floods of particular Importance using the largest recorded historical floods near the site.

The following steps are taken to create the final calibrated hydraulic model:

1) The HEC-GeoRAS computer program is used to create cross sections containing data for the reach length, river stations, and bank stations from the Susquehanna bathymetric and topographic data. The DEM of the study region is used for creating the model. Bathymetric survey data are available for a significant portion of the Susquehanna River, both upstream and downstream of PBAPS. The DEM and bathymetry are imported into ArcMap and combined into a single file. The composite file is used to extract channel geometry, structures, and elevation data using the HEC-GeoRAS interface in ArcMap. Background imagery for figures and location positioning is obtained from the ESRI World Imagery Service (Reference 13). This process created a geometry file in the HEC-GeoRAS GIS interface that included a gee-referenced model with an efficient hydraulic model geometry. The files are exported from HEC-GeoRAS software to the HEC-RAS program and incorporated as part of the HEC-RAS model. The exported file included the cross section location stations and elevations, downstream reach lengths and bank stations, inllne structures, obstructions, bridges, levees, and ineffective flow areas. Channel and floodplain geometry for the Susquehanna River is modeled by developing cross sections of the streams. The cross sections are created at locations that define geometric characteristics of the river valley and overbanks. Cross sections are required at representative locations where changes occur in discharge, slope, shape, roughness, and at hydraulic structures. Floodplain areas outside of the streams' effective flow areas within the cross sections are modeled as ineffective flow areas (USACE, 2010c).

2)

Initial Manning's roughness coefficients, levee point locations, ineffective flow areas, and blocked obstructions are determined from examining ESRI World Imagery aerial photographs. Manning's roughness coefficients, dam parameters, and ineffective areas were reevaluated until the model flows and WSELs matched or were close to

(:t0.1 ft) the historical values.

3)

Bridges and dam structures are created based on available as-built drawings and descriptions from highway and dam design documents and publicly available information.

4)

Lateral inflows are entered into the HEC-RAS model to represent the influence of additional streamflow into the Susquehanna River between Marietta, Pennsylvania and Conowingo, Maryland.

5)

Boundary conditions are input for the upstream and downstream portions of the model. The upstream portion used flow hydrographs, while the downstream portion used the normal depth condition.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015

6)

Boundary conditions for the operations of the Safe Harbor Dam and the Conowingo Dam are input to the model.

7)

Two scenarios are run:

1) The model is run using the historical flow hydrograph for Tropical Storm Lee in 2011 using data from USGS streamflow Gage 01576000 at Marietta as the inflow boundary condition. Lateral inflow is added at select stream locations to account for the additional watershed area between Marietta and Conowingo.

2) The model is run using the historical flow hydrograph for Tropical Storm Lee in 2011 using data from the observed streamflow from the Conowingo Dam as the inflow boundary condition to the model.

Unsteady flow data and observed water level data along the Susquehanna River are used to calibrate the model and evaluate the results from Scenario 1 and Scenario 2.

Manning's roughness coefficients, dam parameters, and Ineffective areas are reevaluated until the model flows and WSELs match or are close to the historical values. The characteristics used to measure the accuracy of the HEC-RAS model are:

Peak discharge WSEL Rating curves for the dams.

8)

Unsteady flow data for storm events Ivan 2004, Agnes 1972, and the flood of March 1936 are run to validate the model for both Scenario 1 and Scenario 2. Unsteady flow data and observed water level data along the Susquehanna River are used to validate the model and evaluate the results. Additional data from each dam rating curve are used to evaluate the performance of each individual dam.

3.2.4.4 Hydraulic CH EC-RAS) Model: Probable Maxjmum Flood Scenarios Examined The precipitation-driven discharge hydrographs to the Susquehanna River downstream of Marietta, Pennsylvania are determined from the evaluation of three combined effect flood scenarios as defined by NUREG/CR-7046, Appendix H.1, Floods Caused by Precipitation Events (Reference 128) from the HEC-HMS hydrologic model. The hydrographs are input to the calibrated and validated HEC-RAS hydraulic model as upstream boundary conditions and lateral inflow boundary conditions. The HEC-RAS model is then used to route the flood to determine the maximum WSEL at PBAPS.

3.2.5 Results - Rock Run Creek The three PMF alternatives (all season and two cool season alternatives) are analyzed.

Alternative 1, the all-season PMF, results in the PMF peak flow rate for Rock Run Creek Basin at PBAPS as 10,893 cfs with the end temporal-distributed 72-hr storm. After comparing results from all five temporal distributions, it can be concluded that a general storm over Rock Run Creek Is Insensitive to temporal distribution. The antecedent rain will not affect the peak outflow from Rock Run Creek Basin due to the small size of the basin. The PMF peak flow rate for Alternative 2 is 1,203 cfs. The PMF peak flow rate for Alternative 3 is 7,807 cfs.

Peach Bottom Atomic Power Station Page 83 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 The maximum WSEL in Rock Run Creek near the site is approximately EL 118 ft-NAVD88.

A floodplain boundary map of Rock Run Creek at PBAPS is shown on Figure 3.2.5.1. The sensitivity analysis concluded that the flooding in the area is driven by the bridge structures and not the slope of the reach.

Figure 3.2.5.1 - PMF Inundation Map for Rock Run Creek 3.2.6 Results - Susquehanna River 3.2.6.1 Hydrologic CHEC-HMS} Model: Calibration and Validation Results A single combined model is created using the information gained during the calibration of the three individual storm events and based on the identified trends for the calibrated parameters.

Figure 3.2.6.1.1 shows the peak flow results for all three calibrated models compared to the observed peak flows and that the individual calibrations result in models that are able to reproduce the observed flows throughout the watershed.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 I

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Figure 3.2.6.1.1 - lndlvldual Model Calibration Results 1,250,000 The combined model is capable to appropriately reproduce the observed peak flows, timing, and volume for Tropical Storm Lee and Hurricane Agnes at Harrisburg and downstream. The combined model balances the volume and peak flow estimations for Tropical Storm Lee and Hurricane Agnes, which are two of the largest precipitation events that have occurred in the basin. The combined model constant loss rates when compared to the individually calibrated constant loss rates indicate that, to reasonably reproduce each of the storms In a single combined model, a weighted balance In parameters is needed. The combined model reproduced Tropical Storm Lee and Hurricane Agnes in the lower basin within 1 O percent.

The combined model conservatively overpredicts Hurricane Ivan since individually calibrated infiltration losses of Hurricane Ivan are greater than those of the combined model, as well as the Tropical Storm Lee and Hurricane Agnes Individual calibrated parameters.

Figure 3.2.6.1.2 compares the observed peak flows and the peak flows modeled with the combined model versus the drainage area for the three storm events considered.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

12 0

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5,000 10.000 lS.000 Drainage Area (mil) 20,000 Figure 3.2.6.1.2 - Peak Flow versus Drainage Area 15,000 30,000 Figure 3.2.6.1.3 presents the modeled Tropical Storm Lee hydrographs at Harrisburg, Pennsylvania; Marietta, Pennsylvania; and Conowingo, Maryland versus the observed hydrographs.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Marietta, PA Modeled Outflow 8 -------------+---+- --- Marietta, PA Observed 7

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9/4/2011 0:00 9/ 11/20110:00 Date&Tlme 9/18/20110:00 Figure 3.2.6.1.3 - Combined Model and Observed Hydrographs for Tropical Storm Lee Figure 3.2.6.1.4 presents the modeled Hurricane Agnes hydrographs at Harrisburg, Pennsylvania; Marietta, Pennsylvania; and Conowingo, Maryland versus the observed hydrographs. A 0.04 In/hr calibrated constant loss rate was used in both the Tropical Storm Lee and Tropical Storm Ivan Individual models.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 14 ~---------------------------~

12 8

4 Hartbbur1, PA Modeled Outflows Hart1sbur1, PA Obsen,ed Outftows

-- Mlrletta, PA Modeled Outftows M¥le111, PA Obsen1ed Outflows

-- Conowtneo MO Modeled Outflows Conowfn&o, MO Obsen11d Outflows 0

6/17/1972 0:00 I

6/24/1972 0:00 Date &Time 7/1/19720'JJ0 Figure 3.2.6.1.4 - Combined Model and Observed Hydrographs for Hurricane Agnes Figure 3.2.6.1.5 present the modeled Hurricane Ivan hydrographs at Harrisburg, Pennsylvania; Marietta, Pennsylvania; and Cor,owingo, Maryland versus the observed hydrographs.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

7 3

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-- M1rietta, PA Modeltd Outflows

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Marlena, PA Obierved Outflows Conowlnso, MD Observed Outflows Note: Thei USGS gage malfunctioned at Marietta, PA during the storm event re suiting In underestimation of the peak flow and volume.

Revision 0 July 10, 2015 0

- I 9/ 17/20040:00 9/19/2004 0:00

-4 9/21/2004 0:00 Date& Time

-L 9/23/2004 0:00 9/25/2004 0:00 Figure 3.2.6.1.5-Combined Model Observed Hydrographs for Hurricane Ivan 3.2.6.2 Hydraulic <HEC-RAS) Model: Calibration and validation Results Tropical Storm Lee 2011 is used as the calibration event and March 1936, Ivan 2004, and Agnes 1972 are used as validation events for the HEC-RAS hydraulic model. The Lee 2011 event is used as the calibration event because the bathymetry data used were collected in 2011 (Reference 55). The most recent data for bathymetry will best reflect the current bathymetry in the Conowlngo Reservoir.

The parameters used to calibrate the model are the Manning's n value for the stream channel, gate weir coefficients at the inline structures, and the bridge weir coefficients. This process involved several model run iterations, adjusting the parameters higher or lower to attain the best-fit results for the flow values at the observed gage locations.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Two scenarios are examined for the calibration of the HEC-RAS model:

Revision 0 July 10, 2015

1) USGS streamflow Gage 01576000 at Marietta, Pennsylvania is used as the inflow boundary condition to the model, and lateral inflows are added to the river reach between Marietta, Pennsylvania and Conowingo, Maryland to account for additional inflow from the watersheds in this reach of river.

2) The recorded streamflow at the Conowingo Dam is input to the model as the inflow boundary condition to the model. The goal is to examine the reported WSELs in the river reach between Marietta, Pennsylvania and Conowingo, Maryland and determine if the flow measured at the Conowingo Dam is more correct than the reported flow at the USGS gages. Note that inputting the Conowingo Dam observed flow data as the inflow boundary condition "double routes" the flow data. However, the double-routed flow did not reduce the observed peak flow of the flood at the Conowingo Dam by more than a few thousand cfs.

This reduction is negligible in comparison to the flood magnitudes over 500,000 cfs.

Since the only lateral inflow of significance between PBAPS and the Conowingo Dam is Broad Creek and the computed lateral inflow from Broad Creek is insignificant in comparison to the peak of the flood hydrograph, the flow at the Conowingo Dam Is considered a representative flow also observed at PBAPS. Transferring the observed streamflow hydrograph at the Conowingo Dam to Marietta will not significantly influence the results at PBAPS.

Results Indicated that Scenario 2 yielded the most appropriate and correct flow calibration.

Scenario 2 yielded the closest results between observed water levels and computed water levels by HEC-RAS. During the calibration, it was determined that the peak flow estimated by USGS streamftow Gages 01578310 at Conowingo and 01576000 at Marietta consistently overestimated the flood flows. The following are the reasons for conclusion of overestimation of the flood flows at the USGS streamflow gages:

Records of flow at the Conowingo Dam are significantly less than the observed flow at USGS streamflow Gage 01578310 directly below the dam (comparing flows from Reference 1 and Reference 115).

Examining USGS streamflow Gage 01578310 indicates that the current rating curve (Revision 4.2, established September 12, 2011) is only suitable for flows that are 245,000 cfs and less since that is the largest field measurement establishing the current streamftow rating curve (Reference 115).

Observed water levels at the Conowingo Dam and other points of interest (POis) during the flooding events are inconsistent with the established rating curve for the dam. The Conowingo Dam and gate dimensions have a fixed cross-sectional area that is not subject to change, whereas the cross-sectional area at the location of USGS Gage 01578310 is subject to change over time. Reported flows at the Conowingo Dam are consistent with the expected (and reported) WSELs at the dam for particular flow rates. The USGS Gage 01578310 flow rates would yield inconsistent WSELs at the dam for known flow rates. Furthermore, another study (Reference 85) indicates that at a flow of 1,130,000 cfs, as given by USGS Gage 01578310 during Hurricane Agnes, the flood profile of the Conowingo Pond would be much higher than was observed at the Conowingo Dam and other points of interest.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 Another study before the Federal Energy Regulatory Commission (FERC)

(Reference 39) has also concluded that the flows at USGS streamflow Gage 01578310 have overestimated flood flows for large flooding events {Agnes and Ivan).

For several of the modeled events, the total lateral inflow from streams between Marietta, Pennsylvania and Conowingo, Maryland did not equal the difference in streamflow between Marietta, Pennsylvania end Conowingo, Maryland, even using the conservative assumption that peak flow from each lateral inflow coincided with the peak flow of the river. A summary of the results from error functions indicates the results generated from the model are representative of the observed data; however, the deviation of peak flows varies enough to affect the WSELs as demonstrated by comparing the WSEL results from Scenario 1.

The characteristics used to measure the accuracy of fit in the HEC-RAS model of this analysis are peak discharge, WSEL, and the rating curves of the* three dams. Of those listed above, the WSEL and peak discharge are the major concerns since these are the primary metrics that influence the PMF level at PBAPS. The residual values for flow are used to judge the differences between the modeled and observed streamflow, with residuals being measures of the deviation model results to the observed value. For the HEC-RAS model to be acceptable, the calibration event (Lee) is limited to be within t0.1 ft of the observed value at PBAPS, and all three events (Agnes, Ivan, and Lee) not overpredicting or underpredicting in the HEC-RAS model together.

Scenario 2 yielded the closest results between observed water levels and computed water levels by HEC-RAS. For Scenario 1, the HEC-RAS model consistently overpredicted the observed water levels. The results are shown in Table 3.2.6.2.1. The Scenario 2 flows were adopted for model calibration.

Table 3.2.6.2.1 - Summary of Scenario 2 Water Level Callbratlon Results Hlstorlcal Model Difference In Event Location River WSEL Historical WSEL WSEL(ft)

Name Station (ft-NAVD88)

Reference (ft-NAVD88)

(Model-Observed)

Lee 2011 PBAPS 47929.89

>110.37 Reference 41 110.32

-0.05 Lee 2011 Conowingo Dam 587.93711S 108.08 Reference 1 108.19 0.11 Ivan 2004 PBAPS 47929,89 109.67 Reference 41 109.73 0.06' Ivan 2004 Conowlngo Dam 587.9371 IS 108.24 Reference 1 108.13

-0.11 Agnes PBAPS 47929.89 113.97 Reference 122 114.01 0,04 1972 Agnes Conowlngo Dam 587.93711S 111.27 Reference 1 111.21

-0.06 1972 March Vicinity of PBAPS 47929.89 112.9 Reference 85 110.99

-1.91 19361 March Conowingo Dam 637.1898 107,91 Reference 110 108.24 0.33 19361

'Limited hlstoffcal discharge data records eiaat for the March 1936 event.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 3.2.6.3 Hydrologic (HEC-HMS) Model: Probable Maximum Flood Results Three stream flow hydrograph datasets, which represent three PMF scenarios, are entered Into a hydraulic model at the upstream model boundary (Marietta, PA) and at lateral Inflow points. The inflow hydrograph locations are shown on Figure 3.2.2.20.1.

3.2.6.3.1 Results for Probable Maximum Flood Scenario 1 The Scenario 1 peak discharge at TMI, obtained for the PMP centered over the centroid of the Susquehanna River subwatershed (upstream of TMI) and center temporal distribution of rainfall hyetograph, is 1,529,654 cfs, with flood arrival time of 120.5 hr. At Marietta, the peak discharge is 1,551,721 cfs, with flood arrival time of 121.5 hr. Figure 3.2.6.3.1.1 shows the watershed-wide area-averaged hyetograph and the hydrograph at TMI. In Figure 3.2.6.3.1. 1, time zero is set at the start of the full all-season PMP rainfall event. The hydrograph has two peaks corresponding to the antecedent 40 percent PMP and the main PMP.

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Hours Figure 3.2.6.3.1.1 -Area-Averaged Hyetograph and Hydrograph at TMI for Scenario 1 3.2.6.3.2 Results for Probable Maximum Flood Scenario 2 The resulting Scenario 2 peak discharge at TMI is 1,025,866 cfs, with a flood arrival time of 120.5 hr. At Marietta, the peak discharge is 1,052,171 cfs, with flood arrival time of 121 hr.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Figure 3.2.6.3.2.1 shows the watershed-wide area-averaged hyetograph and the hydrograph at TMI. In Figure 3.2.6.3.2.1, time zero is set at the start of the 100-year rainfall event.

1,200,000 1,000,000

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--Hydrograph at Three Mile Island Nudear Generating Station I I 800,000 +-"'l"=iF=T=i==~;=="=f=r""r"+-+-ih-1-1..._+-+-IH -+-++-+-H-+-+

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2 Figure 3.2.6.3.2.1 - Area-Averaged Hyetograph and Hydrograph at TMI for Scenario 2 3.2.6.3.3 Results for Probable Maximum Flood Scenario 3 The Scenario 3 peak discharge Is 1,296,300.5 cfs at TMI, with a flood arrival time of 120.5 hr.

At Marietta, the peak discharge is 1,314,649.6 cfs, with flood arrival time of 121 hr. Figure 3.2.6.3.3.1 shows the watershed-wide area-averaged hyetograph and the hydrograph at TMI.

In Figure 3.2.6.3.3.1, time zero is set at the start of the cool-season PMP rainfall event.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 1,400,000 1,200,000 n

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3.5 Figure 3.2.6.3.3.1 -Area-Averaged Hyetograph and Hydrograph at TMI for Scenario 3 3.2.6.4 Precipitation-Driven Probable Maximum Flood Scenario Results The values of peak flow at TMI for the precipitation-driven scenarios simulated in this calculation package are provided in Table 3.2.6.4.1.

Table 3.2.6.4.1 - Precipitation-Driven PMF Scenario Peak Flows at TMI Scenario Description Peak Flow at TMI lcfs)

Scenario 1 0.4 All-Season Site-Specific PMP 1,529,654

+ Alt-Season Site-Soeclflc PMP Scenario 2 Probable Maximum Snowpack +

1,025,866 100-vr Cool-Season Rainfall Scenario 3 100-yr Snowpack + Cool-Season 1,296,301 Site-Specific PMP Scenario 1 (0.4 antecedent PMP + all-season site-specific PMP) is the governing precipitation-driven PMF scenario, with a peak discharge of 1,529,654 cfs at TMJ.

The inflow hydrographs downstream of TMI are run in the unsteady HEC-RAS model to evaluate the PMF scenarios at PBAPS. See Section 3.2.6.5 for PMF scenario results at PBAPS.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 c :-

3.2.6.5 Hydraulic <HEC-RASl Model: Probable Maximum f lood Results Excluding the effects of wind-generated waves (Section 3.6), the NUREG/CR-7046, Appendix H.1 Alternatives 1, 2 and 3 yielded the results in Figures 3.2.6.5.1 and 3.2.6.5.2:

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Figure 3.2.6.5.1 - WSEL Results for Alternatives 1, 2, and 3 at PBAPS

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"e." "

It II l>

JI Figure 3.2.6.5.2-Flow Results for Alternatives 1, 2, and 3 at PBAPS Peach Bottom Atomic Power Station Page 95 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Table 3.2.6.5.1 shows that Alternative 1 yields the highest WSEL at the PBAPS site.

Table 3.2.6.5.1 -Summary of Results at PBAPS Scenario HEC-RAS Plan Name Om.. Total WSEL Designation (cfs)

(feet NAV088) 1 - Warm Season PMF_SCN1 1,541,395 126.04 PMF 2 - Cool Season PMF

1. 1 (Max Snowpack +

PMF_SCN2 1,074,717 115.33 100-yr Seasonal Precio) 3 - Cool Season PMF

1. 2 PMF_SCN3 1,310,700 119.59 (100-yr Snowpack +

Seasonal PMP) 3.2.7 Conclusions-Rock Run Creek The maximum WSEL in Rock Run Creek (EL 118 ft-NAVD88) Is well below the PBAPS CLB protection level of EL 134.87 ft-NAVD88 (EL 135 ft-C.D.). The floodplain boundary of Rock Run Creek during the PMF is shown on Figure 3.2.5.1, and is approximately 300 ft (horizontally) from the PBAPS powerblock as Rock Run Creek drains into the Susquehanna River. PBAPS Units 2 and 3 are not affected by a PMF in Rock Run Creek.

3.2.8 Conclusions - Susquehanna River Excluding the effects of wind-generated waves (Section 3.6), the maximum WSEL of NUREG/CR-7046, Appendix H.1 PMF flooding alternatives (126.04 ft-NAVD88) is below the PBAPS CLB protection level of EL 134.87 ft-NAVD88 {EL 135 ft-C.D.).

Peach Bottom Atomic Power Station Page 96 of 165

(b)(3) 16 USC

§ 824o-1(d) (b)

(4) (b)(7)(F)

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.3 Dam Breaches and Fallures Revision 0 July 10, 2015 A dam assessment and dam failure evaluation are performed following the guidance outlined in JLD-ISG-2013-01 (Reference 130) and NUREG/CR-7046 (Reference 128).

NUREG/CR-7046 (Reference 128) specifies three types of dam failures to be evaluated:

1. Hydrologic Failure - Dam failure induced by an extreme precipitation/snowmelt event within the dam's upstream watershed.

2. Seismically-Induced Failure -

Dam failure induced by an earthquake that causes weakening of the dam's structural components, embankment, foundation, and/or abutments.

3. Sunny-Day Failure - A "sunny-day* dam failure is not associated or concurrent with an Initiating event (such as extreme flood or earthquake) and may result from a structural, geotechnical, or operational deficiency.

The criteria for flooding from dam breaches and failures are provided in NUREG/CR-7046, Appendix D. Two scenarios of dam failures are recommended and discussed in NU.REG/CR-7046, Appendix D, including:

1. Failure of individual dams

2. Cascading or domino-like failures of dams upstream of the site.

The PMF flow hydrographs for the Susquehanna River and tributaries are determined using USACE HEC-HMS computer software. The maximum WSELs for the Susquehanna River are determined using the USACE HEC-RAS computer software.

The hydrologic analyses for dam breaches and failures are performed in:

TMI Calculation C-1101-122-E410-012, "Dam Failure Peak Discharge Calculation Package" (Reference 38).

TMi Calculation C-1101-122-E410-011, "Precipitation Driven Discharge Calculation Package" (Reference 37).

The hydraulic analyses of dam breaches and failures are performed in:

Calculation PEAS-FLOOD-20, "BDBEE - Flood Re-Evaluation - Combination Flooding" (Reference 33).

3.3.1 Inputs - Hydrologlc (HEC-HMS) Model 3.3.1.1 Upstream Dam Information The USACE National Inventory of Dams (NID) database (Reference 106) reports that there are 708 dams in the Susquehanna River watershed upstream of TMI and 762 dams in the Susquehanna River watershed upstream of PBAPS. These dams are screened per JLD-ISG-2013-001 guidance (Reference 130). Inconsequential dams, which are defined as dams having minimal or no adverse failure consequences beyond the owner's property, are removed from consideration, as allowed by Section 3.1 of NRC's "Guidance for Assessment of Flooding Hazards Due to Dam Failure" (Reference 130). As a result, 279 dams are identified as inconsequential upstream of TMI, and 290 dams are identified as

• • • • • • • • • • • ~6ri~~f :f

~:~su:r:r:i~r:;,f~~;i:~~~~ ***;*~****h~**susquera~~=~~~~~8~~~~~~~11y critical or Peach Bottom Atomic Power Station Page 97 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 The dams that do not meet the definition of inconsequential dams are screened sequentially using the volume method, the peak outflow without attenuation method, and the peak outflow with attenuation method described in JLD-ISG-2013-01 to Identify potentially critical dams to be modeled individually in HEC-HMS for the watershed above TMI and above PBAPS, respectively.

Figure 3.3.1.1.1 resents the Method 3 flow chart Fl ure 14 from JLD-ISG-2013-01). l J 1t v

~ 240*1 d lL 1-. lbl1 dams are grouped into 34 "hyp e

- 410-012 (Reference 38).

Method, OamSl4 f 'IOlclme )

U.e More Refined Method See (dl Mathod 2 Oam691 ro.w,oAlt *,

Figure 3.3.1.1.1 - Method 3 Flow Chart Figure 3.3.1.1.2 shows the location of the individual and composite dams considered for the HEC-HMS modeling. Locations of dams and drainage areas for composite dams are also shown.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

(b)(;j) 1ti Us C § tl24o-1(d) (b)(4) (b)(f)(f-)

Revision 0 July 10, 2015 Figure 3.3.1.1.2 - Composite and lndlvldually Modeled Dam Locations 3.3.1.2 Probable Maximum Precipitation Event The all-season site-specific PMP DAD precipitation values are provided by Exelon (Reference 43). Table 3.3.1.2.1 presents the all-season site-specific DAD precipitation values for the Susquehanna River watershed to PBAPS.

Peach Bottom Atomic Power Station Page 99 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Table 3.3.1.2.1 - Susquehanna River Site-Specific PMP Area Size 6 Hr 12 Hr 24Hr 48Hr 72 Hr 96Hr 10mi2 14.5 21.3 22.4 23.0 23.3 23.4 100 mi2 11.6 16.0 17.2 18.2 20.0 20.8 200 mi2 10.7 14.3 15.8.

16.9 19.1 19.8 500mi2 9.6 12.3 14.0 15.2 17.9 18.6 1,000 ml2

• 8.6 10.8 12.6 14.0 16.9 17.5 2,000 ml2 7.5 9.5 11.3 12.9 15.6 16.3 5,000 ml2 6.1 8.0 9.6 10.8 13.7 14.5 10,000 ml2 4.9 6.9 8.1 9,3 12.0 13.0 20,000 mi2 3.8 5.7 6.8 7.8 10.4 11.3 27,078 ml2 3.4 5.2 6.1 7.3 9.9 10.8 50,000 mt2 2.3 3.8 4.6 5.5 8.2 9.1 100,000mi2 1.4 2.4 3.0 3.5 6.0 6.9 3.3.1.3 0.5 Probable Maximum Precipitation Event Revision 0 July 10, 2015 120 Hr 23.5 21.0 20.1 19.0 18.0 16.7 15.0 13.7 12.2 11.6 9.8 7.6 Precipitation depths for each basin for the 0.5 PMP event are calculated using the spatial distribution of the PMP as determined In TMI Calculation C-1101-122-E410-011 (Reference 37). To determine the 0.5 PMP, the rainfall values derived in the calculation (Reference 37) for the all-season PMP are divided in half.

3.3.1.4 soo-Year Rainfall Table 3.3.1.4.1 presents the watershed-averaged precipitation for the all-season PMP, 50 percent all-season PMP, and 500-year precipitation event.

Table 3.3.1.4.1 - Watershed-Averaged Precipitation 100% PMP 50% PMP 500-Year Total Watershed-Averaged Precipitation 11.15 5.58 (in.) Upstream ofTMI 8.35 Total Watershed-Averaged Precipitation 10.57 5.29 (in.) Upstream of PBAPS From Table 3.3.1.4.1, the 50 percent all-season PMP is lesser than the 500-year precipitation. Therefore, per NUREG/CR-7046 (Reference 128), the 50 percent all-season PMP is used as precipitation in the seismic dam failure evaluation.

3.3.1.5 25-Year Rainfall Precipitation values for the 25-year rainfall are derived through statistical analysis of the climate gage data. The data are extracted for a limited number of sites in and around the Susquehanna River watershed for the period of January 1, 1948 to the present. This period corresponds to the time frame when many of the gage stations were installed. These sites resid~ either within the Susquehanna River watershed or just outside the watershed to extend interpolations to the edge of the basin where possible. The gage sites used in this evaluation are listed In Table 4-1 ofTMI Calculation C-1101-122-E410-011 (Reference 37).

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NTTF Recommendation 2.1 (Hazard Reevaluations}: Flooding Exelon Generation Co.

3.3.1.6 Hydrologic Model for the Susquehanna River Watershed Revision O July 10, 2015 The USACE HEC-HMS model, calibrated in TMI Calculation C-1101-122-E410-010 (Reference 36), is used to simulate the hydrologic processes in the Susquehanna River watershed. Dam breach parameters, used as input to the HEC-HMS model, are estimated using the methodologies presented by Froehlich (Reference 51) and Xu and Zhang (Reference 136).

3.3.2 Inputs - Hydraulic (HEC-RAS) Model The hydrographs for the HEC-RAS hydraulic model are developed as part of the hydrologic evaluation and used as input to the Susquehanna River hydraulic model. The calibrated and validated one-dimensional, unsteady flow, HEC-RAS hydraulic model covers the lower portion of the Susquehanna River between Marietta, Pennsylvania and Conowingo, Maryland (refer to Section 3.2.6.2 for the calibration and validation model results}. The same hydraulic model is used to determine the water level at PBAPS due to dam failures and breaches.

3.3.2.1

• Precipitation-Driven Hydrologjc Dam Failure Hydrographs The precipitation-driven discharge hydrographs to the Susquehanna River downstream of Marietta, Pennsylvania are determined from the evaluation of three combined effect flood scenarios as defined by NUREG/CR-7046, Appendix H.1, Floods Caused by Precipitation Events (Reference 128).

Of the three combined effect alternatives specified in NUREG/CR-7046, Appendix H.1, only the critical combined effect precipitation scenario is evaluated for dam failure. The critical scenario is defined as the combination of mean monthly a

3.3.2.2 Seismically-Induced Dam Failure Hydrographs The seismically-induced dam failure discharge hydrographs to the Susquehanna River downstream of Marietta, Pennsylvania are determined from the evaluation of two combined effect flood scenarios as defined by NUREG/CR-7046 A endix H.2 Floods Caused b

• mic II -Induced Dam Failures (Reference 128).

(b)(3) 16 U SC § 8240-1 (d) (b)(4) (b)(7)(F) 3.3.2.3 Sunny-Day Dam Failure Hydrographs Sunny-day dam failure is not analyzed. Both hydrologic-and seismically-induced failure mechanisms are coincident with precipitation; therefore, these failures produce higher peak flow rates and peak flood levels compared to a sunny-day failure. The sunny-day failure mode may produce the shortest warning time. However, this analysis utilizes the most critical dam failure development time for both hydrologic-and seismically-induced failure modes, which results In the same warning time as the sunny-day failure. Additionally, for hydrologic-and seismically-induced failure modes, multiple dams fail simultaneously, which is not a reasonable assumption for a sunny-day failure, Based on the above discussion, a sunny-day failure Is bounded In this analysis.

Peach Bottom Atomic Power Station Page 101 of 165

(b)(3) 16 U SC

§ 824o-1(d}:(b)"

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.3.3 Methodology - Hydrofoglc (HEC0HMS) Model 3.3.3.1 Computation of Dam Breach parameters Revision 0 July 10, 2015 The dam breach parameters used as input to the HEC-HMS model for individual and composite dams are estimated using the applicable methodologies of Froehlich (Reference 51) and Xu and Zhang (Reference 136). The methods are compared for each dam and the more conservative of the results are used in the HEC-HMS modeling.

(b)(3).16 U S.C § Fo a24o-1(d). (bJ(4)

Dam, which is a concrete dam, a different methodology is used, based on Gees omparison of Dam Breach Parameter Estimators" (Reference 53).

Xu and Zhang's study considered data from 75 dam failure cases to develop regression

• equations for prediction of the following geometric and hydrographic parameters: breach depth, breach top width, average breach width, breach bottom width, side slope {horizontal to vertical) of breach, and breach formation time in hours. The equations require the following input data: dam height, depth of water above the final breach invert, and volu~ of water above the final breach invert. Details on the equations are reported in TMI Calculations C-1101-122-E410-011

{Reference

37) and C-1101-122-E410.-012

{Reference 38). When applying Xu and Zhang's methodology for the composite dams, the volume and the depth of water above the final breach invert, respectively, are assumed equal to the maximum reservoir storage and the height of dam as provided by NID; for the Individual dams, the actual maximum values of water volume and depth reached in the reservoir are used. Furthermore, when Xu and Zhang's formulation for breach depth, derived through data regression and not through physically based derivation, predicts a breach depth larger than the dam height, it Is set equal to the dam height for physical consistency.

Similarly to Xu and Zhang's method, Froehlich's method (Reference 51) depends on the storage volume of the reservoir. This method distinguishes between piping and overtopping failures through a coefficient, the failure mode factor, and, differently from Xu and Zhang's method, does not consider dam description and soil erodibility. The.input data for Froehlich's.

method are breach depth {assumed equal to dam height), volume of water above the final breach invert, failure mode factor (1.0 for piping failure mode and 1.3 for overtopping failure mode), and side slope {horizontal to vertical) of breach (0.7 for piping failure mode and 1.0 for overtopping failure mode). Froehlich's method provides average breach width, breach bottom and top width, and breach formation time. Details on the equations are reported in TMI Calculations C-1101-122-E410-011 and C-1101-122-E410-012. As done for Xu and Zhang's method, for the composite dams, the volume above the final breach invert is assumed equal to the maximum reservoir storage provided by the NID; for the individual dams, the actual maximum value of water volume reached in the reservoir is used.

Gee's paper, "Comparison of Dam Breach Parameter Estimators" (Reference 53), evaluates and compares different techniques for estimating dam breach parameters. It provides typical ranges of breach parameters for different dam types. Values for concrete dams are used to

_determineJti~ breach parameters Jori*

IDam. Breach width is conservatively assumed equal to half of the length dam as prov,dedby the NID, breach sides are assumed vertical, failure time is conservatively assumed equal to 0.1 hr, and breach height is conservatively assumed equal to the height of the dam.

The dam breach parameters are calculated for both Froehlich's and Xu and Zhang's methodologies for seismic failure mode. For Xu and Zhang's formulations, the Peach Bottom Atomic Power Station Page 102 of 165

(b)(3) 16 U SC

§ 824o-1(d):(6f.

(b)(3) 16 USC

§ 824o-1(d)::(bf NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 "Homogeneous/Zoned-Fill" dam type and conservative "High" erodlbility coefficients are used for all dams.

3.3.3.2 Dam Breach Analysis for Oyertopping fajlyre Mode The dam breach parameters are calculated for both Froehlich's (Reference 51) and Xu and Zhang's methodologies (Reference 136) for overtopping failure mode. For Xu and Zhang's formulations (Reference 136}, the "Homogeneous/Zoned-Fill" dam type and conservative "High" erodibility coefficients are used for all dams.

In accordance with the Interim Staff Guidance (ISG}, Froehlich's and Xu and Zhang's breach parameters are compared and the breach parameters used in the HEC-HMS model are based on the methodology producing the more conservative (shorter} breach formation time (Tf}. TMI Calculation C-1101-122-E410-011 presents a comparison summary of the calculated breach parameters obtained u5lng Froehllch'5 and Xu and Zhan9'5 methodologies. The comparison shows that both methodologies.result in similar values for breach formation time and the remaining breach parameters.

The initial WSEL in the reservoir

• the spillway crest (all individual. dams except (bl(3l 16 s c

. This value is selected because it is below the uncontro ed sp1 way crest e eva 10n an erefore does not cause outflow over the spillway immediately after the start of precipitation. It is an estimate of the maximum normal pool elevation and is higher than the conservation pool elevation; therefore, it conservatively allows for less storage available in the reservoir, making breach

~:. °:~~~~'";°':~~u~ :".':,':" :~~°ru~/-:.,Oj eleval~;;-.;;*52rnar :0~.*.*.*.*.1.:.*.**.*.;. iJ :~ -\\"1/i;l~~<~l 5

<~I

.---lil~:ftlll~

l.jj and equal to the top of dam elevation for composite dams and

......... 1111d f6l(1~%l~)s-c

\\%(~\\!~4~. ~b~7JM24o-Dams_(which are located along the Susquehanna River).

• *** I~~1?J~: (b)

Two scenarios are considered for the dam breach triggering criteria. In the first scenario (Scenario A), the triggering WSEL for dam breach by overtopping is set equal to the elevation of the top of the dam for the composite dams, whereas Individual dams are set not to breach.

This allowed for evaluating the time evolution of WSEL and water volume in the reservoirs of the Individual dams, to notice that they are not overtopped. In Scenario B, all individual dams

........ Ji!xceptJorl..

!Dam are set to breach at the time of maximum volume in the reservoir (determined m Scenario A), computing the breach parameters for the actual values of water volume and depth in the reservoir.

{

loam is not breached due to the lack of runoff inflow during the Scenario 1 PMP event I he bnly inflow from the contributing subbasin into the reservoir from the governing Scenario 1 PMP event is ~

ase flow of 25.4 cfs. Due to the distance from the

(§b~;I~~~ls,

..... (*~.------ *entroidofthe.PMP.tothe ****************************

Dam's subbasin, there is no precipitation roducin 1*

runoff into the impoundme n increase in water levels. In addition, ( (3 16 u

§ 0-

'Dam is a pump storage reservoir designed to maintain more than 5 ft of freeboar eservoir EL. 528.4 ft) during a local PMP of 26.6 'in., assuming the current uncontrolled emergency spillway configuration and all cylinder gates are closed (Reference 43}.

For individual dams, relationships for elevation-storage and elevation-discharge (for spillway and gates} provided by the USACE Baltimore District were used. During the simulations, all dam gates are assumed to be open. It is verified that gate operations do not have a significant effect on results.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Revision 0 Exelon Generation Co.

July 10, 2015 (b)(3):16 USC § Relationships for elevation-storage and elevation-discharge for 8240 1(d), (b)(4), (b) Dam were obtained from Reference 45.

(7)(F)

For hypOthetical cluster dams, outflow is modeled through a spillway formulation with spillway elevation equal to top of the dam, length of the dam within the hypothetical cluster with the maximum height, and weir coefficient equal to 2.8.

3.3.3.3 Dam Breach Analysis for Seismic Failure Mode For this scenario, the peak discharges at TMI and PBAPS are computed with HEC-HMS for simultaneous failure of all upstream dams due to a seismic event in conjunction with the lower of the 50 percent PMP or the 500-year rainfall, In accordance with NUREG/CR-7046, Section 3.9 and Appendix H.2.

In the HEC-HMS model, the "outflow control structure" reservoir routing method Is used to model dam breach. The "top of dam" is set as the only outflow structure for each of the dams.

The piping elevation is set to one-third of the height of the darn, with a piping coefficient of 0.8 for all dams. Computed dam breach inputs are entered in the HEC-HMS model for each dam. Dams are set to breach simultaneously to reflect a potential seismic event scenario.

Breach times, beginning with the earliest peak inflow to a reservoir and ending once a maximum flow is established, are evaluated.

In accordance with the ISG, Froehlich's and Xu and Zhang's breach parameters are compared and the breach parameters used in the HEC-HMS model are based on the methodology producing the more conservative (shorter) breach formation time (Tf). TMI Calculation C-1101-122-E410*012 presents a comparison summary of the calculated breach parameters obtained using Froehlich's and Xu and Zhang's methodologies. The comparison shows that both methodologies result in similar values for breach formation time.

In order to determine the peak flow based on a seismic dam failure, 19 different breach times are evaluated to determine the critical peak flow rate associated with the limiting dam failure event.

Multiple model dam breach scenario runs are performed, with llll dams failing simultaneously at various time steps beginning at 0 hr of the rainfall up to 125 hr after the start of the rainfall.

3.3.4 Methodology - Hydraulic (HEC-RAS) Model For the hydraulic portion of the dam breach and failure assessment, the following steps were taken to determine the critical combination flooding dam failure event:

1) The HEC-RAS model from Section 3.2 is used to determine the water level at PBAPS due to dam failures and breaches. No change Is made to. the model with the exception of the unsteady flow inflow boundary conditions and the addition of dam breach parameters for Safe Harbor and Holtwood Dams.

2) Unsteady flow upstream boundary conditions for the precipitation-driven hydrologlc dam failures and seismically-induced dam failures are input into the hydraulic model. These upstream bol.(ndary inflows account for all of the flow contributing to the drainage area of the Susquehanna River above Marietta, Pennsylvania.

For the precipitation-<lriven hydrologic dam failures, the timing of the breach was set such that a domino-type or cascading failure would occur; that is, each dam failed at the peak of Peach Bottom Atomic Power Station Page 104 of 165

(b)(3) 16 USC

§ s24a 1(ctr:wr IA\\ 1&..\\1-,\\II\\

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

the flood.wave. Note thatl(b)(3) 16 use, § 8240-,(d). (b)(4). (b)(?)(F)

Revision 0 July 10, 2015 line with the Susqueh,p,w.uLUU!. J;l,L.J;W,I~

• U.l!!!!dU'5i!.L:s:JJil.!Ui!~l!SZ..=~

W,M.~:.u&..ll:l!£5!;~.!!J.£!.!.!..

100 ft). Therefore, the (b)(3) 16 USC § 8240 l(d). (b)(4) (b)(7)(F)

For the seismically-induced dam failures, the time of failure is set such that the flood (0.5 PMF) would coincide with the peak of the seismically-induced dam failure flood wave.

The U.S. NRC staff position In Section 1.4.2 of JLD-ISG-2013-01 (Reference 130) states "It is acceptable to use the 1x10*4 annual frequency ground motions, at spectral frequencies Important to the dam, for seismic evaluation of dams, instead of 1x1 o-e. However, appropriate engineering justification must be provided to show that the dam has sufficient seismic margin.

Otherwise the 1x10-e ground motions should be used."

Information to demonstrate that dams in the Susquehanna River watershed have sufficient seismic margin Is not available to provide the required engineering justification for dams to remain stable during a 1x10*4 seismic event or a 1x10*11 seismic event. Therefore, ail dams fall simultaneously during a seismic event to provide the bounding condition. Since ail dams fail, Alternative 2 of the NUREG/CR-7046, Appendix H.2 combination, which considers the 0.5 PMF, bounds the 25-year flood. The 25-year flood is much less than the 0.5 PMF. Thus, the Alternative 1 combination of NUREG/CR-7046, Appendix H.2 Is not examined further.

To determine the critical timing of fallure, multiple model dam breach scenario runs were performed with all dams falling simultaneously at various time steps beginning at 96 hr after the onset of rainfall to 125 hr after the onset of rainfall. The purpose of different failure times was to determine the critical peak flow rate and WSEL at PBAPS associated with the llmltlng dam failure event.

3) Unsteady flow laterai boundary conditions for the precipitation-driven hydrologic dam failures and selsmlcally-induced dam failures are Input into the hydraulic model. These lateral Inflow boundary inflows account for all. of the flow contributing to the drainage area of the Susquehanna River between Marietta, Pennsylvania and Conowingo, Maryland. The flows Include the ~ffects of dam breach on the flow for dams upstream of PBAPS.

4) The HEC-RAS model Is run using Inflow Items 1 and 2 without dam break. The purpose was

~a:~o~~~t~~ed!:'b~:a~~~:~~~o~~t~~ ~~~~;e~~~i~~!h da~ :************************IDam... ana.1

• • • • • • • • • • • • • • • • • • 1***~ 4J.~

(~)~(~

/j\\ II.II-II/Fl

...... S)Thedambreakcrlterlaofl loam andr

!Damareinputtothe model ~nd (b)(3) 16 USC specified to breach at the time of maximum waler volume behind the dam. The breach

§B240:1(d), (bl IA\\ /k\\17\\/r\\

parameter criteria specify the breach width, horizontal component of breach side slope, and failure time.

(b)(3) 16 USC § 8240 1(d) {b)(4) (b)(7)(F)

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

(b)(3) 16 USC§ 824o-1(d) (b)(4) (b)(l){F)

The Conowingo Dam failure characteristics were as follows:

Revision o*

July 10, 2015 Per the guidelines In JLD-ISG-2013-01, the Conowingo Dam was not to be breached as part of the combination flooding events. Failure of the Conowing~ Dam was not Included in the model because It would have a beneficial effect of reducing the flood level at PBAPS from the combination flooding events.

6) The HEC-RAS model is run at a time step of one second, with output printed every 30 min.

7') For the sce*narios, select cross sections in the model are examined and evaluated for the following:

1) Maximum WSEL

2) Maximum flow

~) Depth-averaged velocity.

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NTTF Recommendation 2.1 {Hazard Reevaluations): Flooding Exelon Generation Co.

3.3.6 Results - Hydrologlc (HEC-HMS) Model 3.3.5.1 Overtopping Dam Failure Results Revision O July 10, 2015 For the most critical precipitation-driven scenario, which is Scenario 1

• br ach-by-overtoppin.. g..

scenarios were simulated, by including the effect of breaching of

• • • • • • individual dams.... and -~b~~l;!~ls(~l ri~l~~(~)~(~)

• E:Jcomposite dams. Table 3.3.5.1.1 shows the peak flows at TMI for the dam breach (4) (b)(7)(F)

(4), (b)(7)(F) scenarios.

Table 3.3.5.1.1 - Peak Flows for the Overtopping Dam Breach Scenarios Scenario Dam Breach Triggering Criteria Peak Flow at TMI (cfs)

Dam Breach Composite dams: WSEL in the reservoir equal to top of dam Scenario A elevation 1,535,239 Individual dams: no breach Composite dams: WSEL in the reservoir equal to top of dam Dam Breach elevation 1,535,064 Scenario B Individual dams: breaching set at the time of maximum WSEL In the reservoir Based on the results reported in Table 3.3.5.1.1, the governing precipitation event is a 40 percent PMP antecedent storm combined with an all-season site-specific PMP, and upstream dams breaching by overtopping, with a peak flow of 1,535,239 cfs at TMI.

The TMI PMF hydrograph Is shown graphically in Figure 3.3.5.1.1. The inflow hydrographs downstream of TMI are run in the unsteady HEC-RAS model to evaluate the overtopping dam failure at PBAPS (Section 3.3.6).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 1,800,000 1,600,000 ~ ~-+-.,-;..-1-+-+-11e-+-........;_-1-1-1--

1,400,000 ~~

, 1'--',~

--+-a---....______-'----'-+

1,200,000 Area-Avenged PredpiUIIOn At Three M lie bland Nudur Generatlna Station 0

J 1,000,000 3

Dam ll<Hch Scenario A Dam Breach Scenario 8 0

i.:

200,000 3.3.5.2 3

3.5 4

Figure 3.3.5.1.1 - Hydrologlc Breach Timing Estimation for PBAPS Seismic Dam Failure Results In order to determine the peak flow based on a seismic dam failure, 19 different breach times are evaluated to determine the critical peak flow rate associated with the limiting dam failure event.

Multiple model dam breach scenario runs are performed, with all dams failing simultaneously at various time ste s be innin at O hr of the rainfall up to 125 hr after the start of the rainfall. (bX3) 16 J s c § 8240-' (di r,4 (b (ll(Fl Dams is to be analyzed using an unsteady flow hydraulic mode in a separa e ca cu a,on.

A Fb)(3) 16 u s c § 8240-t(ctl (bX4> !bX7XF>

jthe start of rainfall produces a critical flow rate of 1,224,954 cfs at TMI. Figure 3.3.5.2.1 presents the hydrographs corresponding to the critical seismic dam failure scenario for TMI. The inflow hydrographs downstream of TMI are run in the unsteady HEC-RAS model to evaluate the seismic dam failure at PBAPS (Section 3.3.6).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

1,500,000 1,400.000 1,300,000 1,200,000 1,100,000

\\,<l00,000 i 900,000 I!.' 800,000

~ 700,000

~ 600,000 500,000 400,000 300,000 200,000 100,000 0

Revision 0 July 10, 2015 0

0.1 0.2 0.3 0.4-:-

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CL 0.7 c!

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1.2 1.3 1.4 1.5 0

24 48 72 96 120 t<<

168 192 216 240 264 288 312 336 360 Time (hr)

Precipitation

• Tlme of Bre,ch (74 hrsl

• !,()% PMP with No &reach 5°" PMP with Seismic Breach F1llu11 of All Dams Figure 3.3.5.2.1 - Seismic Dam Breach Timlng Estimation for PBAPS 3.3.6 Results - Hydraulic (HEC-RAS) Model Precipitation-driven hydrologic-induced dam failures resulting from the critical PMF over the Susquehanna River Basin yield the following results at PBAPS, as shown In Table 3.3.6.1 and Figure 3.3.6.1. Composite dams are used with a starting WSEL In the reservoir equal to top of dam elevation. Individual dams are breached at the time of maximum water volume in the reservoir.

Table 3.3.6.1 - Results for Precipitation-Driven Hydrologlc-lnduced Dam Failures Max Left Max Right Average Flow at Time of Max Overbank Overbank Velocity MaxWSEL Max Flow MaxWSEL Channel Channel Channel Entire Event Velocity Velocity Velocity River Width (cfs)

(cfs)

(ft*

(ft/s)

(ft/s)

(ft/s)

(ft/s)

NAVD88l Precipitation-lb) 1t Driven 1,622,776 1,748,897 l,SC,§ 7 38 1.69 1.39 7.13 Hydrologlc 8240 1 (d) (b)

Dam Failure (4) (blUXF)

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

lDJ(J) lb U :S (.; § lll40-1{0J {0)(4) {0)(/)(f)

Revision 0 July 10, 2015 Figure 3.3.6.1 - Precipitation Driven Hydrologlc Dam Failure - Stage-Flow Hydrograph at PBAPS Seismically-Induced dam failures coincident with the 0.5 PMF over the Susquehanna River Basin yield the following results at PBAPS, as shown in Table 3.3.6.2 and Figure 3.3.6.2.

Composite dams are used with a starting WSEL in the reservoir equal to top of dam elevation.

All dams are breached at the same time.

Table 3.3.6.2 - Results for Selsmlcally-lnduced Dam Failures Mu Max Left Max Right Average Event Flow at Time of Max Flow MlxWSEL Channel Overblnk Overblnk Veloclty MIXWSEL Velocity Channel Channel Entire River Velocity Velocltv Width (cfs)

(cfs)

(ft-(ft/s)

(ft/a)

(ft/s)

(ft/s)

NAV088}

0.5 PMF +

Sel1mlcally-(tl){J) lb U :S (.; § tU40 l(dJ (l>)(4) {l>J(7J(~I I 9.68 2.14 0.85 9.56 Induced Dam C 124 hr Peach Bottom Atomic Power Station Page 110 of 165

(ll)(3) 16 USC

§ 824o-1(d}:-(b)" '"

(11)(3) 16 USC

§ 824o-1(d): (bj'

/.II H It '1,,- \\

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

(b)(3) 16 U SC § 8240 1 (d) (0)(4 ), (b)(r)(~)

Revision 0 July 10, 2015 Figure 3.3.6.2 - 1/2 PMF and Seismic Dam Fallure (124 hours0.00144 days <br />0.0344 hours <br />2.050265e-4 weeks <br />4.7182e-5 months <br />)- Stage-Flow Hydrograph atPBAPS 3.3.7 Conclusions Excluding the effects of wind-generated waves (Section 3.6), the maximum WSEL for the precipitation-driv~

rologic dam failure from NUREG/CR-7046, Appendix H.1 Alternative

• 1* (L::::._J ft-NAVD88) is below the PBAPS CLB protection level of EL 134.87 ft-NAVO88 (El 135 ft-C.D.).

Excluding the effects of wind-generated waves (Section 3.6), the maximum WSEL for the

~

Uy-induced dam fallure from NUREG/CR-7046, Appendix H.2 Alternative 2

...... (l:__J ft-NAVD88) Is below the PBAPS CLB protection level of EL 134.87 ft-NAVD88 (EL 135 ft-C.D.).

3.4 Probable Maximum Storm Surge and Selche The storm surge analysis is performed following the guidance outlined in NUREG/CR-7046 (Reference 128), ANSI/ANS-2.8-1992 (Reference 2), JLD-ISG-2012-06 (Reference 129), and NUREG/CR-6966 (Reference 126). NUREG/CR-7046, Appendix H.4 describes the combined events criteria for an enclosed body of water, which Is appropriate for analyzing surge and seiche flooding at the PBAPS cooling pond. NRC JLD-ISG-2012-06 requires: "all coastal nuclear power plant sites and nuclear power plant sites located adjacent to cooling ponds or reservoirs subject to potential hurricanes, windstorms, and squall Unes must consider the potential for inundation from storm surge and wind waves.* The PBAPS is not a coastal location; however, the Conowingo Pond could be subjected to storm surge (wind setup and wave setup) due to severe windstorms.

The analysis is performed in:

Calculation PEAS-FLOOD-10, "BDBEE -

Flood Re-Evaluation -

Seiche and Surge Analysis" (Reference 27).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

The bathymetry and topography used in the analysis are compiled in:

Revision O July 10, 2015 Calculation PEAS-FL0OO-01, "BOBEE -

Flood Re-Evaluation -

Topography and Bathymetry Data Processing" (Reference 20).

A storm surge is a local rise of the water surface usually associated with a low-pressure weather system. Surge is the combined effect of low pressure and persistent wind over a shallow water body.

In the case of the Conowlngo Reservoir, the effects of low pressure during a storm are Ignored due to the inland location of PBAPS and the reservoir's small size. Negating the effects of low pressure, storm surge elevations in the Conowingo Reservoir are calculated exclusively from wind-induced surge.

The maximum wind-generated surge is calculated from three parameters: wind setup, wave setup, and wave runup. Each parameter is added independently to the antecedent water elevation.

Seiches are generally defined as an oscillation of a fluid body in response to a disturbing force at the natural frequency of the fluid system (Reference 50). When external forces are applied, the system responds by oscillating at its Eigen periods until the energy is dissipated through friction or by exiting the system. If the applied force is aperiodic, a seiche can form that degrades over time. If the force is periodic, a significant seiche can form when the period of the force Is at or near an Eigen period.

The analysis is performed in Calculation PEAS-FLOOD-10, "BDBEE-Flood Re-Evaluation - Seiche and Surge Analysis" (Reference 27).

3.4.1 Inputs The inputs for the analysis are described below.

3.4.1.1 Antecedent Water Level Two antecedent water levels were considered for the Conowingo Reservoir: EL 108.5 ft-C. D.

(EL 108.4 ft-NAVO88) (Reference 56), corresponding to the normal pool elevation, and El 109.25 ft-C.O. (EL 109.1 ft-NAVO88) (Reference 78), corresponding to the maximum controlled water elevation for the reservoir.

3.4.1.2 Bathvmetrv The Conowingo Pond bottom topography (bathymetry), In the format of a DEM referenced to NAVO88, is from the 2012 survey performed by Gomez and Sullivan (Reference 56).

3.4.1.3 Atmospheric Forcjng <Wind}

A constant overwater wind speed equal to 100. mph is used to initiate a storm surge event in the Conowingo Pond. The wind speed is selected conservatively following the guidance outlined In ANSI/ANS-2.8-1992 (Reference 2).

3.4.1.4 Two-Year Wind Speed Based on ANSI/ANS-2.8-1992 Annual Extreme-Mile (Reference 2), the fastest mile wind speed for PBAPS is estimated to be 50 mph. This value is converted to an overwater wind speed with a 1-hr duration, and is calculated to be 36. 7 mph.

3.4.1.5 Hourly Wind Data The wind data are obtained from the closest weather station to the Conowingo Reservoir at buoy Station 44057 -

Susquehanna, MO, which is located at the outflow where the Peach Bottom Atomic Power Station Page 112 of 165

NTTF Recommendation 2.1 {Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Susquehanna River meets the Chesapeake Bay (Reference 71 ). The data are used to calculate natural oscillation periods of external forcing events {windstorms).

3.4.2 Methodology 3.4.2.1 Storm Surge The maximum storm surge elevation from wind generated surge was calculated from the cumulative effects of three different parameters: wave runup, wave setup, and wind setup.

Each one of these parameters was calculated separately, and then added to an antecedent water elevation to determine a final storm surge water elevation.

The fetch is defined as the distance over which wind speed and direction are reasonably constant. Restricted fetches, like PBAPS, llmit wave development due to confined geometry, such as is present on a lake, river, bay, or reservoir. Restricted fetches from the north, northeast, east, and southeast are considered at PBAPS to determine the critical scenario (Figure 3.4.2.1.1). Six fetch directions were considered as shown on Figure 3.4.2.1.1; the characteristics of each cross section are listed in Table 3.4.2.1.1. These six directions have the most unobstructed route approaching PBAPS during an extreme wind event. The PMSS on the Conowingo Pond is simulated by applying the PMWS caused by a 100 mph overland wind speed. Cross Section E-E' is determined to be a critical scenario fetch length and direction.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Figure 3.4.2.1.1 - Cross Sections and Directional Cases Examined for Wave Runup Wind setup is determined for each direction from the two formulas depending on the average water depth over the fetch (Reference 95). For an average water depth of over 16 ft, the Zeider Zee formula is applied. The Zeider Zee formula was developed empirically in fjord settings and is more appropriate for deeper water (Reference 95).

For an average water depth of under 16 ft, the Sibul Method is used (Reference 4).The Sibul Method was developed through laboratory experiments and tested with field data (Reference 4).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Table 3.4.2.1.1 - Fetch Lengths and Average Depths for Wind Directions Evaluated Normal POOi Max. Controlled Cross Fetch Length Fetch Length Average Water Elev.

Wind Direction Section (ft)

(ml.)

Fetch Depth Average Fetch Depth (ft) lftl North to South A-A' 11,202 2.121 12.1 12.8 Northeast to Southwest B-8' 6,797 1.287 14.2 14.9 East to West C-C' 7,669 1.452 12.6 13.3 Southeast to Northwest 0-0' 10,866 2.057 11.7 12.4 Southeast lo Northwest E-E' 13,868 2.626 15.8 16.5 Northwest to Southeast F-F' 12,690 2.402 13.8 14.4 The wave height and wave period were calculated using methods detailed in the USACE Coastal Engineering Manual (CEM) (Reference 96). The wave period and wave height were calculated as a function of fetch length and friction velocity. The wave period and wave height were compared to the depth-limited value, which provides the upper limit of physically realistic wave periods possible for the basin. If the wave height exceeds 0.78 times the depth of the water, it shall be limited to 0.78 times the depth (Reference 97).

A wave transforms as it propagates from deeper to shallower water. Shoaling and refraction of waves occur when the waves approach shallow water. Refraction Is the bending of waves due to varying water depth, and shoaling is the change in wave height due to changes in energy transport velocity with water depth. Shallow water wave height can be computed using the significant wave height from deep water and applying shoaling (Ka) and refraction (K,) coefficients. To maximize the potential wave height at PBAPS, the wave refraction is considered equal to one (i.e., no reduction for waves approaching at an angle to the site).

The ratio of wave setup to breaking wave height is estimated from Figure 3.4.2.1.2 (Reference 101) using the slope at the point of the breaking wave, the height of the breaking wave, depth of water at the breaking wave, and wave period.

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NTTF Recommendation 2.1 (Hazard Reevaluations}: Flooding Exelon Generation Co.

Figure 3.4.2.1.2-Wave Setup Revision 0 July 10, 2015 The wave runup on a vertical wall is calculated using dimensionless wave parameter ratios as shown on Figure 3.4.2.1.3 (Reference 101).

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

0 '

0 I 1

~

• 0

• D

, 0 t O, 0 I tO, If)

Revision 0 July 10, 2015 0

Figure 3.4.2.1.3 - Wave Runup on Smooth, lmpenneable Slopes When ds/Ho > 3.0 3.4.2.2 Selches The HHA approach described in NUREG/CR-7046 1s used to determine whether a seiche can result in significant flooding at PBAPS. This approach Involves:

1) Determination of the natural oscillation periods of external forces such as extratroplcal storms

2) Evaluation of the natural period of the Conowingo Pond

3) Comparison of Step 1 and Step 2 oscillation periods to determine if resonance is possible

4) If resonance is possible, computation of the potential seiche amplitude by applying the external forcing at the resonance period of the Conowingo Pond.

To determine If a significant seiche will occur on a body of water, the Elgen periods can be compared to all of the potential driving forces. If the natural period of oscillation of the body of water is at or near the period of the driving force, it may be possible for a selche to occur.

If a selche may occur, further analysis would be required. The period of potential driving forces considered include: diurnal atmospheric forcing, wave forcing, Ekman transport, seismic forcing, and pressure gradients. The frequency of wind forcing Is compared to the resonance frequency of the Conowingo Reservoir Basin to determine the potential for seiche.

When determining Eigen periods, the results are called modes. Modes are expressed In the form of amphidromic points or lines called nodes and points or lines of maximum wave Peach Bottom Atomic Power Station Page 117 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 amplitude called antlnodes. The mode with the longest Elgen period and corresponding greatest amplitude antinodes is called the fundamental mode.

For the Conowingo Reservoir, the Elgen periods are determined for the simplified one-dimensional case of a closed rectangular basin with vertical walls and uniform depth.

The natural free osclllating periods are calculated using Merian's formula (Reference 98) for the north to south and the east to west directions.

After determining the two-year wind speed, a wind setup height is calculated using equations contained in Reference 10 for wind setup of long waves. The energy loss due to friction as the seiche travels across the Conowingo Reservoir must then be calculated. Reference 84 provides an equation for the energy loss In a wave due to friction per unit length. After subtracting the energy loss due to friction for a given distance, the resultant wave height can be solved. In this evaluation, the new wave height corresponds to the wave height when the selche returns to the originating shore, at which time the cycle starts again where another wind setup builds upon the returning seiche (i.e., seiche amplification occurs).

After a seiche height is calculated,.wave setup and runup based on the two-year wind speed are calculated in the same manner. In the end, a maximum elevation is obtained by adding the wave setup and runup to the calculated seiche height.

The closest weather station to the Conowlngo Reservoir Is a buoy called "Station 44057 -

Susquehanna, MD" and is located at the outflow where the Susquehanna River meets the Chesapeake Bay. Five years of wind data from 2008 to 2012 were recorded at the weather station. A Fast Fourier Transformation (FFT) analysis was performed and the FFT magnitude versus FFT frequency were plotted to determine the largest magnitude points. The largest magnitude points were then selected and the frequency value was converted to a period.

If it is determined that a seiche in the Conowingo_ Reservoir is possible, a maximum seiche oscillation height due to the effects of wind-induced forcing will be calculated. In accordance with NUREG/CR-7046 (Reference 128) as defined by ANSI/ANS-2.8-1992, Section 9.1.4, "Wind Influence" (Reference 2), a two-year return period wind wave activity Is considered in association with all the flood events. Therefore, a two-year wind speed is used to induce a potential selche on the Conowingo Reservoir, making appropriate duration and overwater adjustments dependent on the local observation data.

3.4.3 Results 3.4.3.1 Storm Surge The maximum elevation reached during a wind-induced surge event of 100 mph Is EL 117.63 ft-C.O. (EL 117.5 ft-NAVD88) with normal pool water level antecedent conditions, and EL 118.63 ft-C.D. (EL 118.5 ft-NAVD88) with maximum controlled water elevation antecedent conditions. Both maximum water elevations occur along Cross Section E-E'.

3.4.3.2 Seiches Table 3.4.3.2.1 shows a comparison of all Eigen periods of the Conowingo Reservoir to the closest matching wind frequencies (i.e., wind forcing periods). By comparing the resultant Elgen periods calculated with the periods of the external driving forces, it is determined that an occurrence of a lengthwise Mode 2 seiche is "possible" in the Conowingo Reservoir for

• normal and maximum pool elevations. However, the data from Buoy 44057 are only recorded every 10 min; no data are recorded at smaller increments. Therefore, because two of the Peach Bottom Atomic Power Station Page 118 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 resonance perlods for Mode 2 in the width direction of the Conowlngo Reservoir are less than 1 O min, the FFT will not show that matching resonance periods are possible from wind. As such, the possibility Is examined where the oscillation periods could potentially be less than 10 min and match the period of the Conowlngo Reservoir In the width direction. These directions are evaluated for potential seiche magnitude.

Tabla 3.4.3.2.1 - Elgen Periods for Conowlngo Reservoir Closest to Wind Forcing Periods Antecedent Water Moda1 Closest Meda 2 Period, Closest Direction Period, T Matching Wind Matching Wind Condition (hr)

Period T (hr)

T (hr)

Period T (hrl

'Mdth Conowinao Nonnal Pool 0.18 1.00 0.09 1.00 Width Conowlngo Max.

0.17 1.00 0.09 1.00 Controlled Water Elev.

Length Conowlnao Nonnal Pool 1.66 2.00 0.83 1.00 Length Conowingo Max.

1.64 2.00 0.82 1.00 Controlled Water Elev.

For both normal pool and maximum controlled water elevations In the Conowingo Pond, selche calculations are evaluated for the north-south (lengthwise) and east-west (widthwise) directions of the Conowingo Reservoir. The fundamental Elgen periods for the Conowjngo Reservoir, as calculated using Marian's formula, are listed In Table 3.4.3.2.2 and Table 3.4.3.2.3.

Table 3.4.3.2.2 - Elgen Periods for Length of Conowlngo Reservoir Antecedent Avg.

Period, Period, frequency

Period, PerlOd, Frequency Water Depth Mode, n T

T Mode, n T

T Condition (ft)

(sec)

(hr)

(1/hr)

(sec)

(hr)

(1/hr)

Conowlngo Normal 22.3 1

5981 1.66 0.6 2

2990 0.83 1.2 Pool Conowlngo Max.

23.0 1

5889 1.64 0.6 2

2944 0.82 1.2 Controlled Water Elev.

Table 3.4.3.2.3 - Elgen Periods for Width of Conowlngo Reservoir Antecedent Avg.

Period, Period, Frequency

Period, Period, Frequency Water Depth* Mode, n T

T Mode, n T

T Condition (ft)

(sec)

(hr)

(1/hr)

(sec)

(hr)

(1/hr)

Conowlngo Normal 14.2 1

636 0.18 5.7 2

318 0.09 11.3 Pool Conowlngo Max.

14.9 1

621 0.17 5.8 2

311 0.09 11.6 Controlled Water Elev.

Table 3.4.3.2.4 provides the highest elevation that runup from these four seiche scenarios could potentially reach after 25 cycles. Elevation increases are only calculated out to 25 seiche cycles because the probability that a two-year wind reoccurring at the exact frequency required to initiate a seiche in the Conowingo Reservoir, whether that be widthwise or lengthwise, Is highly unlikely. For demonstration, 25 cycles is considered to show that, even with repetitive cycling, the increase in water level is minimal, as shown in Peach Bottom Atomic Power Station Page 119 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding

. Exelon Generation Co.

Revision O July 10, 2015 Table 3.4.3.2.4. The four seiche scenarios are evaluated because the Eigen periods calculated across the two directions of the Conowingo Reservoir were close to the wind forcing frequencies, or the Elgen period calculated Is small enough that adequate wind data resolution does not exist.

Table 3.4.3.2.4 - Selche Runup Totals Setche Antecedent total Elevation Antecedent

Period, Wave wave Scenario Direction Water

• Height at Water Level T

Setup Runup Elevation After Cycle 25 26 Cvcies (ft-NAV088)

(hr)

(ft)

(ft)

(ft)

(ft*

NAV088) 1 length Conowlngo 108.4 0.18 0.3 2.3 (1.2)1 112.2 Normal Pool Conowlngo 2

length Max.

109.1 0.17 0.3 2.3 (1.1)1 112.8 Controlled Waler Elev.

3 width Conowlngo 108.4 1.66 0.1 1.6 1.4 111.5 Nonnal PooJ Conowlngo 4

width Max.

109.1 1.64 0.1 1.6 1.5 112.3 Controlled Water Elev.

11n Scenarios 1 and 2, the selche wave height completely dissipates by the time it completes one cycle, disabling its ability to build on itself. Therefore, the highest the wave will ever be Is during the initial wind setup.

3.4.4 Conclusions The CLB does not consider flooding due to the probable maximum surge and seiche.

However, the maximum still water elevations caused by the PMSS (EL 118.563 ft-NAVD88) and by the probable maximum seiche (PMS) (EL 112.8 ft-NAVD88) are significantly lower than the PBAPS protection level of EL 134:87 ft-NAVD88.

3.5 Tsunami A tsunami ls a series of water waves generated by a rapid, large-scale disturbance of a water body due to seismic, landslide, or volcanic tsunamigenic sources. Therefore, only geophysical events that release a large amount of energy in a very short time Into* a water body generate tsunamis. The most frequent causes of tsunamis are earthquakes. Less frequently, tsunamis are generated by submarine and subaerial landslides.

3.5.1 Inputs The NOAA National Geophysical Data Center* (NGOC) website Is searched for historical tsunamis (Reference 72).

3.5.2 Methodology Review the NOAA NGOC Interactive Map for historical tsunamis near PBAPS and review the credibility of a tsunami wave to propagate up the Susquehanna River to PBAPS..

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.5.3 Results Revision 0 July 10, 2015 A query of the NOAA NGOC (Reference 72) produced no recorded tsunami events or observations in the Susquehanna River near PBAPS. Very small tsunami runups (less than one meter) have been recorded on the U.S. mid-Atlantic coast and in the Chesapeake Bay; however, such small tsunamis would be unable to propagate upstream on the Susquehanna River to PBAPS because of further frictional attenuation due to river bathymetry and elevation differences and the presence of the Conowingo Dam between the Bay and PBAPS.

• 3.5.4 Conclusions Flooding due to tsunamis is not a credible flood source for PBAPS.

3.6 Combined Events Flood NUREG/CR-7046, Appendix H (Reference 128) states that the following combinations of flood-causing events provide an adequate design basis for shore and streamside locations::

H.1 Floods Caused by Precipitation Events H.2 Floods Caused by Seismically-Induced Dam Fallures H.3 Floods Along the Shores of Open and Semi-Enclosed Bodies of Water H.4 Floods Along the Shores of Enclosed Bodies of Water H.5 Floods Caused by Tsunamis H.1.floods Caused by Precipitation Events NUREG/CR-7046, Combination H.1 specifies three combined effect flood alternatives.

Alternative 1 - Combination of:

Mean monthly base flow Median soil moisture Antecedent or subsequent rain: the lesser of (1) rainfall equal to 40 percent PMP and (2) a 500-year rainfall The PMP Waves Induced by two-year wind speed applied along the critical direction.

Alternative 2 - Combination of:

Mean monthly base flow Probable maximum snowpack A 100-year snow-season rainfall Waves induced by two-year wind speed applied along the critical direction.

Alternative 3 - Combination of:

Mean monthly base flow A 100-year snowpack Snow-season PMP Waves induced by two-year wind speed applied along the critical direction.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 The three PMF alternatives described above are addressed in Section 3.2, except the coincident wind wave activity caused by the two-year wind speed applied along the critical direction.

Alternative 1, the all-season PMF, is the critical scenario for PBAPS because it results In the highest WSEL at PBAPS. The combination of the flood with the wind waves is addressed In this section of the report. Wind waves are analyzed in:

Calculation PEAS-FLOOD-17, "BDBEE - Flood-Re-Evaluation - Co-Incident Wind Wave Run-Up Calculation" (Reference 30).

H.2 f loods caused by Seismically-Induced Dam Failures NUREG/CR-7046, Combination H.2 specifies two combined effect flood alternatives for seismically-induced dam failure:

Alternative 1 - Combination of:

A 25-year flood A flood caused by dam failure resulting from a Safe Shutdown Earthquake (SSE),

and coincident with the peak of the 25-year flood Waves induced by two-year wind speed applied along the critical direction.

Alternative 2 - Combination of:

The lesser of one-half of the PMF or the 500-year flood A flood caused by dam failure resulting from an Operating Basis Earthquake (OBE), and coincident with the peak of the flood In Item 1 above Waves induced by two-year wind speed applied along the critical direction.

Although not given as an alternative in NUREG/CR-7046, one other alternative Is the hydrologic dam failure of dams upstream of PBAPS as prescribed by JLD-ISG-2012-06. The precipitation-driven PMF peak discharge hydrographs to the Susquehanna River downstream of Marietta, Pennsylvania are determined from the evaluation of three combined effect flood scenarios, as defined by NUREG/CR-7046, Appendix H.1, Floods Caused by Precipitation Events. The critical scenario for PBAPS, Combination H.1, Alternative 1 Is examined in combination with all dams assuming to fail at the time of maximum water volume in the reservoir and the failure of Safe Harbor Dam and Holtwood Dam In a domino-type cascading failure at the peak of the flood wave. The hydrologic dam failure scenario coincident with the PMF Is the critical scenario for PBAPS because It results in the highest WSEL at PBAPS. This scenario is addressed in Section 3.3, except the coincident wind wave activity caused by the two-year wind speed. The combination of the flood with the wind waves Is addressed In this section of the report. Wind waves are analyzed in:

Calculation PEAS-FLOOD-17, "BDBEE - Flood-Re-Evaluation - Co-Incident Wind Wave Run-Up Calculation" (Reference 30).

H.3 floods Alona the Shores of Open and Semi-Enciosed Bodtes of Water The NUREG/CR-7046, Combination H.3 flooding scenarios are not applicable to PBAPS because PBAPS is not located on an open or semi-enclosed body of water subject to.tidal Influence.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

H.4 Floods Along the Shores of Enctosed Bodjes of Water {If Apoticable)

Revision O July 10, 2015 The NUREG/CR-7046 Combination H.4 specifies two combination effect flood alternatives for floods along shore of open and semi-enclosed bodies of water:

H.4.1 Shore Location Combination of:

• Probable maximum surge and seiche with wind wave activity 100-year or maximum controlled level In water body, whichever is less.

The NUREG/CR-7046, Combination H.4.1 flooding,events are analyzed for PBAPS in Calculation PEAS-FLOOD-10, "BDBEE - Flood Re-Evaluation - Seiche and Surge Analysis," and summarized in Section 3.4 of this report.

H.4.2 Streamside Location Alternative 1 - Combination of:

The lesser of one-half of the PMF or the* 500-year flood Surge and seiche from the worst regional hurricane or windstorm with wind wave activity The lesser of the 100-year or the maximum controlled water level in the endosed body of water.

Alternative 2 - Combination of:

PMF In the stream A 25-year surge and seiche with wind wave activity The lesser of the 100-year or the maximum controlled water level in the enclosed body of water.

Alternative 3 - Combination of:

A 25-year flood In the stream Probable maximum surge and selche with wind wave activity The lesser of the 100-year or the maximum controlled water level In the enclosed body of water.

The PMF alternatives described above are addressed In Section 3.4 of the report.

The NUREG/CR-7046, Combination H.4.2 altematlves are not applicable ~o PBAPS because the Susquehanna River (Conowlngo Reservoir) Is not considered to behave as an "enclosed body of water" during flooding events.

H.5 floods Caused by Tsunamis NUREG/CR-7046, Combination H.5 specifies two tsunami-induced combined effect flood scenarios.

However, PBAPS Is approximately 19 mi: upstream from the mouth of the Susquehanna River at the Chesapeake Bay. The downstream dam (Conowingo Dam), elevation difference, and topography of the Susquehanna River would not allow a tsunami wave generated in the Chesapeake Bay to reach PBAPS. Therefore, a tsunami is not an applicable flood-causing mechanism to be evaluated at PBAPS.

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3.6.1 Inputs The inputs for the analysis are described below.

3.6.1.1 Two-Year Wind Speed Revision 0 July 10, 2015

-Based on ANSI/ANS-2.8-1992 Annual Extreme-Mile (Reference 2), the fastest mile wind speed for PBAPS ls estimated to be 50 mph.

3.6.1.2 Bathymetry The Conowingo Pond bottom topography (bathymetry) in the format of a DEM, referenced to NAVD88, Is from the 2012 survey performed by Gomez and Sullivan (Reference 55).

3.6.1.3 Initial Condition water Surface Elevation The PMF WSEL from NUREG/CR-7046, Combination H.1, Alternative ~ with hydf olo. g. le.. d.a. m..

failure scenario coincident with the PMF is determined to be E

... tt~NAVDBB (b)(3) 16 u s c (Section 3.3.6).

§8246:l(d), (b)

IA\\ IL.\\17\\Jr\\

3.612 Methodology As part of the HHA process provided in NUREG/CR-7046 (Reference 128), the less critical scenarios of wind wave activity that generate runup were screened out and only the most critical case Is examined. The effects of the two-year. wind waves are computed only for this critical scenario.

Wave runup ls evaluated for the spectrum of waves that can potentially Impact PBAPS coincident with the PMF event. The analysis of the combined events Is performed based on the guidelines outlined in ANSI/ANS-2.8-1992 and NUREG/CR-7046. The wind wave parameters and effects are determined using the guidance and methodology outlined In USACE EM 1110-2-1100 (Reference 99}. The wind-generating wave mechanism considers the two-year period wind speed along the critical fetch length, ~ind setup, and wave setup and the potential for wave breaking, wave refraction, and wave shoaling Is examined from topography and bathymetry near PBAPS.

3.6.2.1 fetch Length The critical fetch Is determined from Cross Section E-E' as described in Section 3.4.2.1.

3.6.2.2 Wind Speed Adjustments The two-year wind speeds are adjusted following the guidance outlined in the USACE CEM (Reference 99) for level (measurement height}, duration, and wind speed over water versus wind speed over land. The 50 mph ANSI/ANS-2.8-1992 Annual Extreme-Mlle wind speed Is converted to an overwater wind speed with a duration of 1 hr, which is 36. 7 mph.

3.6.2.3

• Waye parameters. Wjnd Setup, Wave Setup. and Wave Runup Wave parameters, wind setup, wave setup, and wave runup were computed using the same method as outlined in Section 3.4.2.1, using the two-year wind speed as the forcing mechanism.

The maximum WSEL is determined by summing the maximum still water elevation, wind setup, wave setup, and wave runup.

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/.4\\ H.\\/"'/\\lr\\

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3.6.3 Results Revision O July 10, 2015 The critical combination of events is the NUREG/CR-7046, Appendix H.1, Alternative 1 with hydrologlc dam failure coincident with the PMF.

(b)(3) 16 USC

§ 8246:l(d), (b)

The maximum computed total water level elev~tlon Is at E ft (NAVD88), which is the '",w""*'

sum of:

Initial WSEL (PMF with hydrologic dam failure) - EG

• • • • • • • • • • • ** * -NAVD88 *

• • • ****........... *****........... ~b~~l~~(~)\\~)

Wind Setup - 0.12 ft

'",~ 1n 11n Wave Setup - 0.18 ft Maximum Wave Runup - 2.45 ft 3.6.4 3.7 Ice-Induced Flooding The HHA approach described In NUREG/CR-7046 (Reference 128) is used for the analysis. As per NUREG/CR-7046, Appendix G, ice-induced events may lead to flooding at a site due to two scenarios:

1. Ice jams or dams that form upstream of a site that collapse, causing a flood wave

2. Ice jams or dams that form downstream of a site that result in backwater flooding.

An evaluation of upstream and downstream structures and historical ice jam events near PBAPS is performed to demonstrate that ice jam flooding Is bounded by the PMF at PBAPS.

Ice jams are evaluated in:

Calculation PEAS-FLOOD-09, "BDBEE - Flood Re-Evaluation - Ice Effects" (Reference 26)

The ice jams are modeled In:

Calculation PEAS-FLOOD-16, "BDBEE -

Flood Re-Evaluation -

HEC-RAS Probable Maximum Flood (PMF) Water Level" (Reference 29):

The 25-year flood flow at Marietta, Pennsylvania is determined in:

TMI Calculation C-1101-122-E410-011, "Precipitation-Driven Discharge Calculation Package" (Reference 37).

3.7.1 Inputs The inputs for the analysis are described below.

3. 7.1.1 Historir&1I Ice Jam Data The USACE ice jam database (Reference 102) is queried to obtain the record of Ice jams that have occurred in the Lower Susquehanna River Basin.

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3.7.1.2 Hydraulic Model for Susquehanna River Revision 0 July 10, 2015 The one-dimensional, unsteady flow, HEC-RAS hydraulic model covering the lower portion of the Susquehanna River between Marietta, Pennsylvania and Conowingo, Maryland is used (refer to Section 3.2 for the model details). The same hydraulic model is used to determine the water level at PBAPS due to collapse of an upstream Ice jam, causing a flooding wave, or formation of a downstream ice jam, resulting in backwater flooding.

3.7.1.3 U.S. Geological Survey Gage Records USGS gage records (Reference 114) are used to establish elevation relationships (I.e., gage datum and minimum stage) for the historical ice jams.

3.7.1.4 25-Year Flood Flow The 25-year flood flow is from the results described In TMI Calculation C-1101-122-E410-011.

3.7.2 Methodology

3. 1.2.1 Historical Ice Jams The USACE Cold Regions Research and Engineering Laboratory ice jam database (Reference 102) is queried to obtain the record of ice jams that have occurred in the Lower Susquehanna River watershed. The hydrologic unit code (HUC) areas (Reference 111) considered (i.e., Lower Susquehanna watershed) are shown on Figure 3. 7.2.1.1. -

Figure 3.7.2.1.1 - HUC Units and Ice Jam Locations within Lower Susquehanna Watershed Peach Bottom Atomic Power Station Page 126 of 165

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3.7.2.2 Ice Jam Thickness of Historical Ice Jam Revision O July 10, 2015 The ice jam database presents the historical ice jam data as a combination of qualitative accounts and quantitative river stages and discharges resulting from ice-induced floods.

Data obtained from the ice jam database are used to determine the most severe historical ice jam or ice dam conditions, including the dam height and flood stages. The reported ice jam stage height Is used to determine the peak elevation by adding the ice jam height to the gage datum. The difference between the ice jam peak elevation and the minimum WSEL is used as an estimate of the ice jam height. The largest ice jams are listed in Table 3.7.2.2.1.

Table 3.7.2.2.1 - Largest Ice Jams Upstream of PBAPS Description Location Date Ice Jam Height (ft)

HUC-02050301, Lower Penn's Creek 2/26/1979 12.0 Susauehanna, Pennsylvania HUC-02050302,Upper Juniata, Frankstown Branch W/1965 8.5 Pennsylvania Juniata River HUC-02050303,Raystown, Raystown Branch 2/14/1966 12.3 Pennsylvania Juniata River HUC-02050304.Lower Juniata.

Juniata River 1/21/1959 12.4 Pennsylvania HUC-02050305,Lower Susquehanna River at 1/20/1996 25.0 Susquehanna-Swatara, Harrisburg Pennsvlvania HUC-02050306,Lower West Conewago Creek 3/19/1936 15.9 Susquehanna, Maryland.

Pennsvlvania HUC-02050306,Lower Susquehanna River at 1/3/1934 15.6 Susquehanna, Maryland, Marietta Pennsylvania

3. 7.2.3 Water Surface Elevation from Upstream Ice Jam This largest ice jam scenario is analyzed by creating an ice jam upstream from the PBAPS site. The ice jam location is determined by evaluating where possible constrictions or blockages could occur closest to PBAPS. The Holtwood Dam, located approximately 6 mi.

upstream of PBAPS, is determined to be the most likely location for an ice jam. Ice jam thickness is determined to be 25 ft using the maximum historical ice jam thickness from Table 3.7.2.2.1.

To calculate the potential effect to PBAPS from the ice jam, a 25-year flood flow is used in conjunction with the ice jam. An ice dam is created at the Holtwood Dam location and set to instantaneously collapse at the moment of overtopping. This causes a flood wave down the Susquehanna River to the PBAPS site.

To model the potential ice jam, 25 ft is added to the top of the Holtwood Dam spillway and then breached In the. HEC-RAS hydraulic model at the timing of the 25-year flood flow peak.

The height of the dam breach is conservatively expected to progress from the top of the Ice jam to a distance downward to the bottom of the ice jam 25 ft (the top elevation of the Holtwood Dam spillway). Because It is unlikely that the individual ice debris causing the ice jam will be cohesive once the failure starts, the entire length of the ice jam is conservatively expected to fail. The length is conservatively assumed to be approximately the entire length of the Holtwood Dam spillway.

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Revision O July 10, 2015 The value of 0.0 hr (instantaneous failure) is selected as being the most conservative ice dam breach progression time.

Breach progression is modeled as linear over the failure development time specified. Because the most conservative (i.e., fastest} failure development time is used, there is no significant difference between linear and other type(s) of failure progression.

A summary of the breach parameters is as follows:

Top Elevation - EL 189.92 ft-NAVD88 (top of the ice jam)

Bottom Elevation - EL 164.92 ft-NAVD88 (top of Holtwood Dam spillway)

Bottom Width - 2,900 ft (approximately the entire Holtwood Dam spillway section)

Left and Right Breach Slopes - 3:1 (horizontal to vertical side slopes)

Breach Progression - Linear via overtopplng Breach Weir Coefficient - 2.6 (generally accepted value for a broad crested weir)

Development Time- 0.0 hr (instantaneous)

Trigger Method - The trigger time Is adjusted by trial and error in order for the peaks to coincide with the peak of the 25-year flood.

3.7.3 Results Using the combination of events in the HEC-RAS hydraulic model:

25-year flood flow, 25-ft ice dam, and Ice dam at Holtwood Dam collapses at peak flow, this alternative/scenario results in a maximum WSEL of 111.48 ft-NAVD88 at PBAPS.

3.7.4 Conclusions In accordance with NUREG/CR-7046 (Reference 128), ice-induced flooding is analyzed to calculate the resulting WSEL at PBAPS. Ice-induced flooding is not specifically included as a mechanism to be combined with other extreme events (Appendix H of NUREG/CR-7046).

Ice-induced flooding is not considered in the PBAPS CLB; however, the calculated WSEL due to the breaching of an upstream ice dam is well below the PMF WSEL, which indicates that upstream ice-induced flooding Is bounded by the PMF on the Susquehanna River.

The Conowingo Dam is the only downstream structure on the Susquehanna River.

Historically, large ice jams have been recorded at dams upstream of PBAPS at Safe Harbor Dam and Holtwood Dam, as well as at the Conowingo Dam, such as the 1996 ice jam. The 1996 ice jam is the largest on record at the Conowingo Dam and did not flood PBAPS.

Therefore, based on historical evidence, large ice jams downstream are bounded by the PMF scenarios.

3.8 Channel Migration NUREG/CR-7046 (Reference 128) notes that natural channels may migrate or divert either away from or towards the site. There are no well-established predictive models for channel diversion.

Historical records and hydrogeomorphological data should be used to determine whether an adjacent channel, stream, or river has exhibited the tendency to meander towards the site.

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3.8.1 Inputs The inputs for the analysis are described below.

Revision 0 July 10, 2015 The 1912 to 2013 USGS topographic maps of PBAPS are examined to illustrate general continuity of the river shore for that period (References 118, 119, 120, 121 ).

3.8.2 Methodology The evaluation is conducted by reviewing USGS historical records and information to assess whether the Susquehanna River or Rock Run Creek exhibits the tendency to migrate towards the site.

3.8.3 Results A review of historical data and site information indicates that the Susquehanna River and Rock Run Creek have not exhibited tendencies to meander. Based on comparison of historical topographic maps and a present-day topographic map, the Susquehanna River and Rock Run Creek have been at the same approximate location for the past 100 years (Figure 3.8.3.1 ).

Construction of the Conowingo Dam (completed in 1928) did not significantly alter the channel configuration. Further, the channel edges near PBAPS are quite steep (with 1: 1 slopes, some steeper), which confine the flow and prevent significant migration.

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Revision 0 July 10, 2015 Figure 3.8.3.1 - Selected Hlstorlcal USGS Maps near PBAPS from 1912 to 2013 3.8.4 Conclusions The conditions of the Susquehanna River and Rock Run Creek have remained the same over the pest 100 years, so channel migration of the Susquehanna River and Rock Run Creek towards or away from the site is still not likely. Channel migration is not considered to be a potential contributor to flooding at PBAPS.

3.9 Error/Uncertainty The analysis evaluates the errors and uncertainties associated with the effects of LIP and reliability of the Conowingo Dam gates downstream of PBAPS.

Some degree of error/uncertainty is unavoidable in the development of the deterministic hydrologic and hydraulic models used In the PBAPS flood reevaluation.

The LIP flood mechanism ls selected for the error/uncertainty analysis because it has the greatest potential to adversely affect safety-related equipment under the current flood reevaluation process.

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Revision 0 July 10, 2015 The Conowingo Dam gates are selected for the error/uncertainty analysis because the Conowingo Dam controls the Susquehanna River WSEL adjacent to PBAPS. The error/uncertainty analysis considers a sensitivity analysis to the number of gates that fail to open during the PMF event.

Potential error/uncertainty is determined in:

Calculation PEAS-FLOOD-21, "BDBEE - Flood Re-Evaluation - Error and Uncertainty Calculation" (Reference 34).

The two-dimensional site runoff model used in the error and uncertainty analysis is from:

Calculation PEAS-FLOOD-03, "BDBEE - Flood Re-Evaluation - FLO-2D Local Intense Precipitation (LIP) Flooding" (Reference 22}.

The hydraulic model used in the error and uncertainty analysis Is from:

Calculation PEAS-FLOOC-20, "BCSEE -

Flood Re-Evaluation - Combination Flooding" (Reference 33).

The error/uncertainty analyses addressed parameters that were not assumed to be conservative or were not calibrated.

3.9.1 Inputs The inputs for the analysis are described below.

3.9.1.1 Local Intense Precipitation The two-dimensional FLO-2O model, developed as part of the LIP covering the PBAPS site, including the 1 hr, 1 sq. mi. LIP for PBAPS, is used (refer to Section 3.1 for the model details).

The maximum LIP WSELs at PBAPS computed in Section 3.1.3 are used in this analysis.

The maximum LIP WSELs are listed in Table 3.1.3.1. The locations of the doors are shown on Figure 2.2.3.

The range of Manning's n parameters is from the "FLO-2D Reference Manual" (Reference 48) and the Open-Channel Hydraulics textbook (Reference 6).

(b)(3) 16 USC § 8240 1(d) (b)(4), (b)(7)(F) 3.9.2 Methodology 3.9.2.1 Local Intense Precipitation A sensitivity analysis is performed by changing the Manning's n values in the LIP FLO-2D model to reflect the high end range values for the paved areas and the low end values of the recommended range for the paved areas. Note that only one value, 0.018, is available for the shot-concrete lining without smooth treatment (Reference 6). Therefore, this value is not Peach Bottom Atomic Power Station Page 131 of 165

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Revision 0 July 10, 2015 changed. The revised Manning's n for each surface type in the sensitivity analysis is as follows:

0.30 - Forested areas (shrubs and forest litter) (Reference 48) 0.20 - Areas with minimal grass cover over a rough surface or for open areas with debris (Reference 48) 0.04 - Open ground, no debris (Reference 48) 0.05 -Asphalt or concrete areas {Reference 48) 0.018 - Shot-concrete lining without smooth treatment, surface covered with fine algae and bottom with drifting sand dunes {Reference 6).

3.9.2.2 Site Topographic survey Evaluate the error and uncertainty involved in the PBAPS topography used to develop the model for the LIP flood mechanism. The error/uncertainty analysis was done by examining the National Geodetic Survey {NGS) Online Positioning User Service (OPUS) Solution Report produced in support of the site survey (Reference 9). The document includes a report of the root mean square vertical accuracy for the survey points.

The new maximum WSEL at PBAPS is computed as the PMF + Dam Failure + 2*year Wind Waves.

3.9.3 Results 3.9.3.1 Local Intense Precipitation The results of the LIP sensitivity analysis are presented in Table 3.9.3.1.1.

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Revision 0 July 10, 2015 Table 3.9.3.1.1 - Errors/Uncertainties for LIP Flooding Results Original Revised DIH111nce In Maximum Flow Depth Door Grid Cell Max Flow WSEL Max Flow WSEL Revised - Orlglnal Class 1 Structure Depth (rt*

Depth (rt*

Number Number lrtl NAVDBBI 1ft)

NAVO881 (ft)

T01 Emergency 13856 1.95 126.95 2.08 127.061

+0.13 Cooling Tower 246 Unit 3 Reactor 13611 0.60 135.17 0.73 135.30

+0.13 Building 244 Unit 3 Reactor 12327 0.82 135.43 1.02 135.63

+0.2 Building 239 Unit 3 Reclrc MG 13799 1.61 135.91 1.84 136.14

+0.23 s.. t Room 188 Unit 2 Reclrc MG 14882 1.55 135.85 1.69 135.99

+0.14 Set Room 183 Unit 2 Reactor 16853 0.83 135.53 0.99 135.69

+0.16 BuHdlng 198 Unit 2 Reactor

.19407 0.66 135.23 0.78 135.35

+0.12 Building Diesel Generator 22631 2.96 131.96 2.96 131.96 0

Bulldlng 23993 0.46 127.46 0.44 127.44

-0.02 Multiple 22646 0.12 117,62 0.15 117.65

+0.03 21450 3.33 120.83 3.26 120.76

-0.07 Multiple Pump Structure 25379 2.00 117.50 2.07 117.57

+0.07 1 Door T01 Into the Emergency Cooling Tower is elevated well above lhe ground surface. The ground surface is appro,dmalely,t EL 125 ft NAVD08. The bo1tDm of do<><T01

• lW<>,..,,,. belowE8 -.

Peach Bottom Atomic Power Station Page 133 of 165 (b)(3) 16 U SC

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NTTF Recommendation 2.1 (Hazard Reevaluations}: Flooding Exelon Generation Co.

Revision 0 July 10, 2015 3.9.3.2 Site Topographic Suryey The vertical RMS error for the spot elevation survey at PBAPS is +/-0.028 m or 0.092 feet.

Uncertainty regarding onsite flood elevations can be partially attributed to the level of accuracy of the site survey~ For this analysis, it is estimated that the two dimensional flow model potential inaccuracy in the elevation of any single grid element, maintains a one to one relationship with the LIP water level.

rb)(l) 16 0 S C § 824-0 1 (di, (b)i4). (b)(/)(f)

Table 3.9.3.3.1 -WSEL Results at PBAPS forl(b)(J) 10 u 5 c § 8L40*1(d) (b)(4) (b)(l)(F)

WSELln New Uncertainty Original Maximum HEC-RAS In HEC-WSEL Wind Maximum Original Range Model RAS Change Wave Total Maximum (New Scenario from Model In HEC-Runup WSELfrom WSEL WSEL-Sensitivity WSEL RAS from Sensitivity (ft*

Original Analysis

'(ft-Model Sensitivity Analysis NAVD88)

WSEL)

(ft*

(ft)

(ft*

(ft)

NAVO88)

NAVD88)

Analysis NAVD88)

(ft).

(b)(3) 16

O}(J) 10 U:::, l, ~ tlL'IO-128.17 (DJ(J} 10 0.88

• 2.75 130.92 USC §

+0.88 1 (d), (b)(4) (b)(7)(F)

USC§ 133.83 824o-1(d).

+3.59 131.06 18240-1 (d) 3.57 2.77 (b)(4) (b)(7)

(b)(4), (b) 3.9.4 Conclusions Fl (7)(F) 3.9.4.1 Local Intense Precipitation floodlnq Mechanism The sensitivity analysis Indicates that the peak elevation is sensitive to changes in the Manning's n. There is a potential error and uncertainty range of-0.07 ft +0.23 ft In the WSEL at the doors around PBAPS.

3.9.4.2 Site Topographic Survey The LIP water surface uncertainty of +/-0.026 m (0.092 feet) due to survey error of +/-0.028 m (0.092 feet} could occur at PBAPS.

3.9.4.3

!(b)(3) 16 USC § 824o-1(d) (b)(4) (b)(7)(F)

The sensltivi anal sis indicates that the eak WSEL is sensitive to the (b)(3) 16 U SC § 8240-1 (d), (b)(4) (b)(7)(F)

~:--:-:-:-':':"~:-:-:-= :-=---------J w EL 134.67 ft-NAVD88.

no 3.10 Associated Effects and Flood Duration Parameters The associated effects of the LIP are computed for the selected doors of safety-related buildings at PBAPS. The effects of LIP are computed in:

Calculation PEAS-FLOOD-19, "BDBEE -

Flood Re-Evaluation -

Hydrostatic &

Hydrodynamic Loading and Flooding Associated Effects" (Reference 32).

Hydrostatic loads, hydrodynamic loads, wave loads, groundwater ingress, and erosion and deposition for the critical combination flooding scenario are computed in:

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Revision 0 July 10, 2015 Calculation PEAS-FLOOD-19, "BDBEE -

Flood Re-Evaluation -

Hydrostatic &

Hydrodynamic Loading and Flooding Associated Effects>> (Reference 32).

Debris impacts for the critical combination flooding scenario are examined in:

Calculation PEAS-FLOOD-18, "BDBEE - Flood Re-Evaluation - Waterborne Projectiles" (Reference 31 ).

Warning time for the critical combination flooding scenario is computed In:

Calculation PEAS-FLOOD-20, "BDBEE - Flood Re-Evaluation - Combination Flooding" (Reference 33).

The "Guidance for Performing the Integrated Assessment for External Flooding* (Reference 131) defines "flood height and associated effects" as follows:

"The maximum stillwater surface elevation plus the following factors:

wind waves and run-up effects; hydrodynamic loading, including debris; effects caused by sediment deposition and erosion; concurrent site conditions, including adverse weather conditions; groundwater ingress; and other pertinent factors."

3.10.1 Inputs Two scenarios were examined in this calculation for the potential associated effects due to flooding: LIP flooding and riverine flooding. The inputs for the analysis are described below.

3.10.1.1 Local Intense Precipitation Floodin9 For the LIP floodi_ng event, the maximum flow depth and maximum flow velocity at the PBAPS doors from the FLO-2D model are used as input (Table 3.1.3.1).

3.10.1.2 Riverine Flooding For riverine flooding, the inputs are as follows:

G (b)(3) 16 u s c The PMF hydrograph with a maximum elevation of EL

• • • • • • • • • • • • • • • • • -NAVO88atPBAPS §824o:1(d), (b) as a result of combination flooding.

A maximum wave height of 2.45 ft resulting from the two-year wind speed at PBAPS.

A flow velocity of 1.39 ft/s from the HEC-RAS hydraulic model, as given in Section 3.3.6, is used as the river velocity to determine the hydrodynamic force from riverine flooding.

Per the American Society of Civil Engineers (ASCE) 7-10, in riverine floodplains, large woody debris (trees and logs) predominates, with weights typically ranging from 1,000 lb to 2,000 lb (Reference 3), with an average of debris weight of approximately 1,000lbs. Debris weights in riverine areas subject to floating ice typically range from 1,000 lb to 4,000 lb (Reference 3). However, the flood level from the ice effects scenario is far less than the PMF elevation (Section 3.7.4). Therefore, Impacts from floating ice are not be considered, and a debris weight of 1,000 lb Is used.

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3.10.2 Methodology 3.10.2.1 Local Intense Precipitation Hydrostatic Loading Revision 0 July 10, 2015 Hydrostatic loads are those caused by water above or below the ground surface, free or confined. These loads are equal to the product of the water pressure multiplied by the surface area on which the pressure acts. Hydrostatic pressure is equal in all directions and always acts perpendicular to the surface on which it is applied. Hydrostatic loads can be subdivided Into vertical downward loads, lateral loads, or vertical upward loads (uplift or buoyancy). The

. hydrostatic force per unit horizontal unit length is calculated using Federal Emergency Management Agency (FEMA) P-259, "Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures (Fourth Edition)" (Reference 47).

3.10.2.2 Local Intense Precipitation Hydrodynamic Loading Water flowing around a building (or structure) imposes loads on the building. Hydrodynamic loads, which are a function of flow velocity and structure geometry, include frontal impact on the upstream face, drag along the sides, and suction at the downstream side. Hydrodynamic loading with velocities less than 10 ft/s is analyzed using the guidelines described in FEMA P-259, "Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures (Fourth Edition)" (Reference 47).

3.10.2.3 Local Intense Precipitation Groundwater Ingress The potential for external flood mechanisms to affect groundwater levels is analyzed by examining the duration of flooding and surface permeability.

3.10.2.4 Local Intense Precipitation Sediment Deposition and Erosion High velocity flood flows may result in scour or erosion. Maximum velocities at the powerblock area were taken from the FLO-2D model as described in Section 3.1. The maximum velocity during the LIP event is compared to the maximum permissible mean velocity orlginally from Table 10-1, "Suggested Coefficients of Roughness and Maximum Permissible Mean Velocities for Open Channels," in USACE EM 1110-3-136, "Drainage and Erosion Control Mobilization Construction" (Reference 100), and replicated in Table 3.10.2.4.1 below:

Table 3.10.2.4.1 ~ Coefficents of Roughness and Maximum Permissible Mean Velocities Manning's Maximum Permissible Mean Material Roughness Velocity Coefficient, n (ftls)

Asphalt; smooth 0.012 15.0 Asphalt; rough 0.016 12.0 Natural earth, with 0.03-0.150 6.0 veaetation 3.10.2.5 Riverine Hydrostatic Loading The hydrostatic force per unit horizontal unit length Is calculated using FEMA P-259, "Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures (Fourth Edition)" (Reference 47).

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3.10.2.s Riverine Hydrodynamlc Loading Revision 0 July 10, 2015 Hydrodynamic loading with velocities less than 10 ft/s is analyzed using the guidelines described in FEMA P-259, "Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures (Fourth Edition}" (Reference 47).

3.10.2.7 Riverine Debris Impact Loading Debris impact is examined using the parameters for debris presented In ASCE 7-10 (Reference 3). The parameters for the debris weight, velocity, length, and diameter as prescribed by ASCE 7-10 are compared to those used in the PBAPS UFSAR (Reference 18).

3.10.2.8 Riverine Wind Wave Loading Wave loads are those loads that result from water waves propagating over the water surface and striking a building (or other structure). Loads due to nonbreaking waves are calculated using the Sainflou method as given in the Coastal Engineering Manual (Reference 99).

3.10.2.9 Riverine Groundwater Ingress The following steps are used to determine the potential effects of groundwater ingress:

The site Information, such as available drawings, is utilized to identify such SSCs.

The potential for external flood mechanisms to affect groundwater levels is examined.

The results of the riverine PMF calculation are also used to compare water elevations

• with those of safety-related structures to evaluate the potential for groundwater ingress.

3.10.2.10 Riverine Sediment Deposition and Erosion The U.S. Department of Agriculture's (USDA) NRCS Web Soil Survey (Reference 108) is utilized to determine the physical properties of the soils at PBAPS near the Susquehanna River to assess the potential for soil erosion.

The soi.I erodibility factor "K," according to "Revised Universal Soil Loss Equation Version 2 (RUSLE2),

• represents susceptibility of soil to erosion, transportability of the sediment, and the amount and rate of runoff given a particular rainfall Input, as measured under the standard unit plot condition (Reference 109). The soil erodibility factors are given in Table 3.10.2.10.1.

Table 3.10.2.10.1 - Soil Erodibility Factor Soil Class Soll Erodlblllty Factor Susceotibilltv to Erosion Fine textured soils high in clay 0.05 to 0.15 Low "K" values, because they are resistant to detachment Coarse textured soils, such as 0.05to0.2 Low "K" values, because of low runoff sandy soils even though these soils are easlly detached Medium textured soils, such as 0.25to0.45 Moderate "K" values, because they are the slit loam soils moderately susceptible to detachment and thev oroduce moderate runoff Soils having a high silt content 0.45to0.65 High "K" values, because they are easily detached and they tend to crust and produce large amounts and rates of runoff Peach Bottom Atomic Power Station Page 137 of 165

(b)(3) 16 U SC

§ 824o-1(d): (br NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.10.3 Results Revision O July 10, 2015 3.10.3.1 Local Intense Precipitation Hydrostatic/Hydrodynamic Loads The hydrostatic and hydrodynamic loads are determined as part of the FL0-2D computation as force per unit length of structure (lb/ft) for selected doors of safety-related buildings and are presented in Table 3.10.3.1.1.

Table 3.10.3.1.1 - Hydrodynamic and Hydrostatic Loads on Safety-Related Buildings Hydrostatic Hydrodynamic Height Above Door Total Force Ground at ID Bulldlng Force Force (lb/linear ft) which Total (lb/linear ft)

(lb/linear ft)

Force Acts (ft)

T01 Emergency N/A1 N/A1 N/A1 N/A1 Coolina Tower 246 Unit 3 Reactor 11

~

14 0.22 Bulldina 244 Unit 3 Reactor 21 5

26 0.30 Bulldlna 239 Unit 3 Reclrc MG 81 63 144 0.65 Set Room 188 Unit 2 Reclrc MG 75 60 135 0.63 Set Room 163 Unit 2 Reactor 21 6

28 0.31 Buildlna 198 Unll 2 Reactor 14 4

18 0.25 Bulldina All Diesel Generator 346 1274.

1620

• 1.55 Doors Building C01 Pump Structure 125 8

133 0.69 1 Door T01 Into the Emergency Cooling Tower Is elevated well above the ground surface. The

---~~uace is approximately at EL 125 ft NAVD68. The bottom of door T01 Is two inches below 3.10.3.2 Local Intense Precjpltatlon Debris Impact Loading Debris impact loading is not applicable at the main powerblock due to shallow depths and limited velocities.

3.10.3.3 Local lnten§ft Precjpitation Wind Waye and Rynup Effects Wind wave and runup effects are not applicable due to the presence of multiple obstructions and limited fetch which prevent any appreciable wave formation.

3.10.3.4 Local Intense Precipitation Groundwater Ingress The low permeabllity of much of the site ground cover {paved ground cover) and the short duration of the elevated water surface levels caused by the LIP event are unlikely to result in a significant increase in groundwater elevations at the site.

3.1 o. 3. 5 Local Intense Precipitation Sediment Deposition and Erosion The flow velocities around the PBAPS site during the LIP are examined using the results from FL0-2D.

Peach Bottom Atomic Power Station Page 138 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 For scour and erosion to occur, the typical permissible velocities for selected ground cover materials must be less than actual observed velocity. The highest velocities are located in paved sections of the main powerblock, which at minimum would have a permissible velocity of 12 ft/s when conservatively using rough asphalt (Reference 100). The areas where the velocity is above 12 ft/s are limited to a few isolated areas at the northwest side of the emergency cooling tower, the western portion of the diesel generator building In the southern portlon of PBAPS, and southeast of the administrative building ln the southern portion of PBAPS. Scour/erosion resulting from the LIP is therefore insignificant since the locations with high velocity are in small confined areas away from any doors of interest at PBAPS.

3.10.3.6 Local Intense precipitation Flood Duration The duration of the flooding at each door is presented on Figure 3.10.3.6.1 and In Table 3.10.3.6.1.

ua,---------------------------.

W.I uu U~.l ----------------------~

0 O.l 04 0.6 O.I u

u Tim* (Hounl 0-JO Doo,JU DoorUt 0-lU

-o-m Doorl'll Proto<llon 1-1 Figure 3.10.3.6.1 -Water Surface Elevation vs. Time for Various Door Locations Table 3.10.3.6.1 - Duration Exceeding Flood Protection Level for Door Locations Duration Above Location FLO-20 Grid Cell ID Building Location Description Protection Level (min)

Door 246 13611 Unit 3 Reactor Building Pressure Resistant Door 29.4 Door244 12327 Unit 3 Reactor Bulldlng Hollow Metal Door 10.8 Door 239 13799 Unit 3 Recirc MG Set Room Hollow Metal Door 58.8 Door188 14882 Unit 2 Recirc MG Set Room Hollow Metal Door 49.8 Door183 16853 Unit 2 Reactor Building Hollow Metal Door 57.0 Door 198 19407 Unit 2 Reactor Building Pressure Resistant Door 14.4 Peach Bottom Atomic Power Station Page 139 of 165

(b)(3) 16 USC

§ 824o-1(df(bj (6)(3) Y~'(f s C

§ 8240-1{cl}(b)

I" fl \\t-,1,r-\\

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.10.3.7 Local Intense Precioitatjon Warning Time Revision 0 July 10, 2015 Strategies for a beyond design basis LIP involving manual actions, developed as part of an Integrated assessment, should incorporate warning time in site procedures. The Nuclear Energy Institute (NEI) "Local Intense Precipitation Warning Time Vvhite Paper" (Guideline 15-05) {Reference 75) can be used for this purpose.

3.10.3.8 Riverine Hydrostatic and Hydrodynamic Loading r7 (b)(3) 16 U S.C The design still water flood depth at PSAPS i~ calculated to beL_jf[- Tf\\e-st1nwater flood §82401(d}, (b) depth was determined from PMF Ell Jrn8, minus the general grade at the,., "*"711r,

~

plant area of EL 115 ft-NAVD88 {EL ft-NAVO88 - EL 1.1.5 ft-NAVD88. =* ~~~~-;~~)s(~

L_Jft).

Note that the grade rises abrupty,n e area of the reactor building to,.,,,,,..,,,,.,

approximately El 135 ft-NAVD88.

The combined force of the hydrostatic and hydrodynamic loading at PBAPS from riverine flooding without any wave effects is calculated to be 4,914 lb per linear ~ (lb/linear ft).

Hydrostatic force is 4,867 lb/linear ft and hydrodynamic force from the water velocity is 47 lb/linear ft.

3.10.3.9 Riverine Debris Impact Loading The parameters for the debris weight, velocity, length, and diameter presented in ASCE 7-10

{Reference 3) and the PBAPS UFSAR {Reference 18) are shown in Table 3.10.3.9.1.

Table 3.10.3.9.1 - Qualitative Comparison of Impact Force Parameters Case Weight Velocity Length Diameter (11>9)

(ft/s)

(ft)

(ft)

ASCE 7~10 1,000 1.39 30 1.0 PBAPS UFSAR 10,000 5.00 50 2.5 3.10.3.1 o Riverine Wave Loading The combined force of the hydrostatic and wave loading at PBAPS from riverine flooding is calculated to be 7,329 lb/linear ft. Hydrostatic force is 4,867 lb/linear ft and wave force is 2,462 lb/linear ft. The hydrodynamic load from water velocity Is small (47 lb/linear ft), It can be safely neglected in comparison to the wave load and hydrostatic load.

3.10.3.11 Riverjne Groundwater (ogress All critical structures essential to a safe shutdown of the reactor are flood protected to EL 134.87 ft-NAVD88.

Therefore, PBAPS is not subject to effects associated with groundwater ingress, 3.10.3.12 Riverine Sediment Deposition and Erosion Given the low overbank velocity of 1.39 ft/s from the Susquehanna River at PBAPS, the potential for scour is minor due to the allowable velocity of a natural earth channel with vegetation being 6.0 ft/s {Reference 100). Furthermore, the majority of the soil covering the PBAPS site is classified as Mt. Airy and Manor soils {MRF), which have a low susceptibility to detachment. The areas which are classified as loamy udorthents {Ud) over the PBAPS Peach Bottom Atomic Power Station Page 140 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 site, which are moderately susceptible to erosion, are either paved or already underwater and well below plant grade in the plant cooling water discharge area. Any sedimentation and erosion in the immediate vicinity of PBAPS will be minimal.

3.10.3.13 Riverine Flood Duration and Warning Time The riverine flood durations for the critical flooding scenarios are summarized in Tables 3.10.3.13.1 and 3.10.3.13.2 and shown on Figures 3.10.3.13.1 and 3.10.3.13.2.

Table 3.10.3.13.1 - Duration of Flooding for ComblnatJon Flooding Scenarios at PBAPS Duration Duration Duration T\\metoft\\H T\\m*t.oFal\\

Abo.,. EL 111 Abo.,. EL 120 Above EL 121 from Normal from PHk Event ft*NAVDU ft-NAVDU ft-NAVDII Pool toPHk WSiLto WSEL Normal Pool (hr)

(hr)

(hr)

(hr) lhrl Preclp1talion*Driven Hydrologlc 61 32 6 13.25 48 72 Dam Faflure Vanat -

0.5 PMF + SeismicaUy-lnduced 1

72 Rafar to Dam FaUure@ 124 hr FjgU(8 3.10.3.13.2 (0/(3) 1li U::, t !i 8240 l(d) (b)(4) (tl)(f)(f)

Figure 3.10.3.13.1 - Flood Duration - Precipitation-Driven Hydrologlc-lnduced Dam Failure Peach Bottom Atomic Power Station Page 141 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

(b)(J) 16 U SC !i t!.140-1 (d) (b)(4) (D)(/)(F)

Revision 0 July 10, 2015 Figure 3.10.3.13.2 - Flood Duration - Seismically-Induced Dam Failure Scenario Table 3.10.3.13.2 Is a tabular form of the values In Figure 3.10.3.13.1 for the Incremental time It takes to reach the maximum wate su ace e e ation at Peach Bottom durin the PMF event startln t elevation 109 ft NAVD88.

(b)(J) 16 USC § 824o-1(d), (b)(4) (b)(7)(F)

The flood hydrograph resulting from the 500-year flood event Is passed by the Conowingo Dam prior to the PMP event over the basin. As such, this hydrograph assumes that the gates are not already open at the Conowlngo Dam prior to the PMF. If any number of crest gates at the Conowingo Dam already open, the capacity for the dam to pass additional flow will decrease and so will the warning time. The values In Table 3.10.3.13.2 also assume hydrologic dam failure of all upstream dams and the failure of Safe Harbor Dam and Holtwood Dam at the time of maximum water surface elevation at each one of the dams. If these dams fail prior to when the water surface elevation Is maximum at each of the dams, then the warning time may decrease, but the water surface elevation at Peach Bottom will not reach the PMF level.

Peach Bottom Atomic Power Station Page 142 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Table 3.10.3.13.2 - PMF Warning Time for an Incremental Rise In the Susquehanna River at PBAPS Elevation HEC-RAS Model Flow Cumulative (NAVDBB)

Time (Cfs)

Incremental Time to Increase 1 ft Time 109 4118114 3:30 205 020 0:00 0:00 110 411811416:30 628,781 13:00 13:00 111 4118114 20:30 800 554 4:00 17:00 112 4/18114 22:30 877 364 2:00 19:00 113 4/19/14 0:00 938 735 1:30 20:30 114 4119/14 2:00 1,020,780 2:00 22:30 115 4/19/14 4 :00 1088971 2:00 24:30 116 4/19/14 6:00 1154 994 2:00 26:30 117 4/19/14 8:30 1 235*842 2:30 29:00 118 4/19/1410:30 1 290,755 2:00 31:00 119 4/19/14 12:30 1337750 2:00 33:00 120 4/19/1414:30

• 1379 403 2:00 35:00 121 4/19/14 16:00 1409,096 1:30 36:30 12.2 4119/14 17:30 1 436,540 1:30 38:00 123 4119/14 19:30 1468,538 2:00 40:00 124 4/19/14 21:30 1495;013 2:00 42,00 125 4/20/14 0:30 1,526,023.

3:00 45:00 126 4/20/14 3:30 1546954 3:00 48:00 127 4/20/14 4:30 1654493 1:00 49:00

[D)(J) 1ti U S.G !j Ol'.'10-4/20/14 5:00 1,622 551 0:30 49:30 1 (d) (b)(4) (b)(7)(F)

Note: Time is rounded to the nearest half hour 3.10.4 Conclusions

3. t0.4.1 Local Intense Precipitation Hydrostatic/Hydrodynamic Loads LIP is not addressed in the PBAPS CLB and the maximum water surface elevation from the LIP is above the protection level of several doors at PBAPS. LIP loading will need to be addressed in the Integrated Assessment.

3.10.4.2 Local Intense Precipitation Debris Impact Loading The debris load for the LIP event is negligible due to the absence of heavy objects at the plant site and due to low flow velocity, the factors combination of which could lead to a hazard due to debris load. Debris Impact loading from the LIP event is not applicable to PBAPS.

3.10.4.3 Local Intense Precipitation Wind wave and Runup Effects Consideration of wind wave action for the LIP event is not explicitly required by NUREG/CR-7046 and is judged to be a negligible associated effect because of limited fetch lengths and flow depths. Wind wave and runup effects from the LIP event are not applicable to PBAPS.

Peach Bottom Atomic Power Station Page 143 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.10.4.4 Local Intense Precipitation Groundwater Ingress Revision 0 July 10, 20t5 Due to the relatively impervious cover Immediately around the powerblock *buildings, the infiltration of precipitation and groundwater seepage would likely be min lmal. Additionally, the event is a short duration (1-hr precipitation) which limits the amount of soil.infiltration.

Therefore, groundwater level changes are not expected to occur and PBAPS Is not subject to effects associated with LIP groundwater ingress.

3.10.4.5 Local Intense Precipitation Sediment Deposition and Erosion Because of generally low velocities around the powerblock area, sediment transport is not expected to be an effect of LIP flooding. The maximum velocity in the powerbiock area near the doors is well below permissible velocities for paved surfaces, so erosion and localized scour are also not expected to be an effect of LIP flooding. Deposition and erosion from the LIP event are not applicable to PBAPS.

3.10.4.6 Local Intense precipitation flood Duration LIP flood duration is not addressed in the PBAPS CLB and the maximum water surface elevation from the LIP is above the protection level of several doors at PBAPS. LIP flood duration will need to be addressed In the lntegratect Assessment.

3.10.4 *. 7 Local Intense Precipitation Warning Time.

Strategies for a beyond design basis LIP involving manual actions, developed as part of an integri:lted assessment, should incorporate warning time in site procedures. T~e Nuclear Energy Institute (NEI) "Local Intense Precipitation Warning Time White Paper" (Guideline 15-05) (Reference 75) can be used for this purpose and will need to be addressed in the integrated assessment.

3.10.4.8 Riverine Hydrostatic and Hydrodynamic Loading Hydrostatic and hydrodynamic loading is not quantified in the PBAPS UFSAR and will need to be addressed in the Integrated Assessment.

3.10.4.9 Riverine Debris Impact Loading The critical waterborne projectile parameters from ASCE 7-10 (Reference 3) and the velocity from the PMF with precipitation driven hydrologlc dam failure at PBAPS are less thari those stated in the PBAPS UFSAR (Reference 18). Therefore any debris impact loads given in the PBAPS UFSAR are bounding.

3.10.4.10 Riverine Wave Loading Wave loading is not quantified in the PBAPS UFSAR and will need to be addressed in the Integrated Assessment.

3.10.4.11 Riyerjne Groundwater Ingress Ali critical structures essential to a safe shutdown of the reactor are flood protected to EL 134.87 ft-NAVD88.

Therefore, PBAPS is not subject to effects associated with groundwater ingress.

Peach Bottom Atomic Power Station Page 144 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

3.10.4.12 Riverine Sediment Deposition and Erosion Revision 0 July 10, 2015 Velocities in the overbank from the Susquehanna River flooding are lower than the permissible (for erosion resistance).. Given the low overbank velocity of 1.39 ft/s from the Susquehanna River at PBAPS, the potential for scour ls minor due to the allowable velocity of a natural earth channel with vegetation being 6.0 ft/s (Reference 100). Any sedimentation and erosion In the Immediate vicinity of PBAPS will be minimal.

3.10.4.13 Riverine Flood Duration and Warning Time As part of the PBAPS Flooding Walkdown Report (Reference 138), a procedure walk-through, or 'Reasonable Simulation', was conducted for temporary and/or active features that require manual/operator actions to perform their intended flood protection function. The purpose of the reasonable simulations was to verify that the procedure or activity could be executed as specified/written. Six (8) reasonable simulations, were performed to demonstrate compliance with licensing basis requirements in regard to protection from external floods. The simulations are walk-throughs of specific steps in Flood Procedure SE-4 that align with licensing basis flood protection commitments and/or protect SSCs required for safe shutdown. The Procedure Instructions are as stated In SE-4 and SO 48.1.B for all simulations.

The reasonable simulations performed showed that credited procedures/actions could be accomplished before being Impeded by rising flood waters. A flood level increase of 1 foot/hour from the PBAPS UFSAR was used to assess the ability to perform the actions.

Per UFSAR Section 2.4.3.5.5 (Reference 18), It is estimated that 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> will elapse as the water rises from elevation 111 ft C.D. to 113 ft C.D., allowing the greatest portion of decay heat to be rejected to the river. Based on this statement, a flood level Increase rate of 1 foot/hour Is determined. The reasonable simulation initially was 156 minutes for the steps that were necessary to perform within the targeted available time of 120 minutes. Therefore, the procedures were entered into the PBAPS Corrective Action Program (CAP). PBAPS subsequently revised the procedures to optimize steps in the performance of the procedures such that the actual performance time would be 11 o* minutes and fall within the targeted available time of 120 minutes. The activities were timed, and in all cases the duration of the simulation showed that the activity could be completed prior to flood water rising to the grade level. The total time to complete was compared to this rate of rise to determine If the flood level would prevent the activity from being performed. The material and equipment necessary to complete the activities was avallable. Adequate personnel resources would be ava(lable at the site during a flooding event to perform the activities In parallel.

The reevaluated flood level rate of Increase is less than the 1 foot/hour used to assess these actions; therefore, the current design basis flood Is bounding.

For the flood duration and recession time, all critical structures essential to a safe shutdown of the reactor are flood protected to EL 134.87 ft-NAVD88. The flood warning arrives at PBAPS In one of two ways: (1) a high river surface water elevation alarm In the control room monitored by sensors at the intake canal; or (2) an offsite federal agency notification.

Warnings for high river level leading to an external flood can come from.either a high alarm on the Intake canal level or notification to the PBAPS Main Control Room from any offslte agency that high river flows are expected. Notification from an off-site agency would be made via the following chain of communications: Personnel from Muddy Run dam or Conowingo dam would notify the Power Team or PJM (grid operators), who in turn would notify the Exelon Nuclear Duty Officer, who would notify PBAPS. Initial actions include contacting the Conowingo Control Room to verify actions are being taken to control river level. If river level Peach Bottom Atomic Power Station Page 145 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 continues to rise above 109.5 ft C.D. and Is greater than or equal to 111 ft C.D. and river flow is greater than 600,000 cfs, and If not previously entered, the flood procedure, SE-4 is entered.

At a river elevation of +111.0 ft C.D. flood-related shut down actions are initiated. If the river elevation reaches +111.0 ft C.D. and river flows higher than 600,000 cfs are predicted, the reactors will be shut down at this time to their cold condition using normal operating procedures. Heat removal will then be transferred from the high water demanding main condenser system to the low water demanding service water systems, and the decay heat removed to the river. If the river level exceeds +112.0 ft C.O., the reactors will be manually scrammed and placed in the cold condition using the applicable special event procedure.

Should the river flow continue to increase causing additional rise in the water level, the double set of sluice gates, one on each side of the rotating screens which screen water to the service water pumps, will be closed when the river level reaches Elevation +113 ft C.D. and the operation of the service water systems is transferred from river supply to the on-site emergency reservoir. This isolation of the service water systems wlll be accomplished utlllzlng approved written procedures. It Is estimated that 2 hr time will elapse as the water rises from Elevation +111.0 ft C.D. to +113.0 ft C. D., allowing the greatest portion of decay heat to be rejected to the river. The emergency cooling water system and the emergency dlesel-generator systems are flood protected up to Elevation +137.5 ft C.D. and are capable of continuous operation, so the diesels will supply power to the essential shutdown equipment and engine~rlng safeguard equipment.

The NRC staff concluded that the licensee, through the implementation of the walkdown guidance activities and In accordance with plant processes and procedures, verified the plant configuration with the current flooding licensing basis; addressed degraded, nonconforming, or unanalyzed flooding conditions; and verified the adequacy of monitoring and maintenance programs for protective features (Reference 139). Given the procedures that are in place to shut down the plant if the river exceeds +112.0 ft (C. D.) and once the protective features are in place, flood duration and recession time are not applicable.

4 FLOOD PARAMETERS AND COMPARISON WITH CURRENT DESIGN BASIS Per the March 12, 2012, 50.54(f) letter (Reference 127), Enclosure 2, the following flood-causing mechanisms were considered in the flood hazard reevaluation for PBAPS:

1. LIP

2. Flooding in streams and rivers

3.

• Dam breaches and failures

4. Storm surge

5. Selche

6. Tsunami

7. Ice-Induced flooding

8. Channel migration or diversion.

Some of these individual mec::hanisms are Incorporated Into alternative "Combined Effect Flood" scenarios per Appendix Hof NUREG/CR-7046 (Reference 128)..

• Peach Bottom Atomic Power Station Page 146 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 The March 12, 2012, 50.54(f) letter, Enclosure 2, requests the licensee to perform an integrated assessment If the reevaluated flood hazard Is not bounded by the current design basis. This section provides comparisons with the current design basis flood hazard and applicable flood scenario parameters per Section 5.2 of JLD-ISG-2012-05 (Reference 131), Including:

1. Flood height and associated effects

a. Still water elevation;

b. Wind waves and runup effects;

c. Hydrodynamic loading, including debris;

d. Effects caused by sediment deposition and erosion (e.g., flow velocities, scour);

e. Concurrent site conditions, including adverse weather conditions; and

f. Groundwater Ingress.

2. Flood event duration parameters (Figure 4.0.1 below; per Figure 6 of JLD-ISG-2012-05)

a. Warning time [may include information from relevant forecasting methods (e.g.,

products from local, regional, or national weather forecasting centers) and ascension time of the flood hydrograph to a point (e.g., Intermediate WSELs) triggering entry Into flood procedures and actions by plant personnel];

b.

• Period of site preparation (after entry into flood procedures and before floodwaters reach site grade);

c. Period of inundation; and

d. Period of recession (when floodwaters completely recede from site and plant Is in safe and stable state that can be maintained).

3. Plant mode(s) of operation during the flood event duration.

4. Other relevant plant-specific factors (e.g., waterborne projectiles).

t......................_ __

"ood-*v_en_*d_u,._,oo _ _ _ **- _ -*- __ j,

1i,-p,.p111fon I portOdol j

IOCflS&fonol j

-,,wooc1ew1n1 1

/nundtlio"

, I 11111trl!offl 1th! i CondlUonu111 mel Amval ofnood W.ltr begins to W8Car completely for enlry In lo flood w11arii on ~ la 111ced1 fn,m 1N1 111c1dtd fn,m lllll PtOeOdUIH or ond p!anlln..,.

nouncawon ol and*tablu1a1a lmotndblg flood Iha l ean bl maln181nod lndonn111y Figure 4.0.1 - Illustration of Flood Event Duration (Ref~rence 131, Figure 6)

Per Section 5.2 of JLD-ISG-2012-05 (Reference 131), flood hazards do not need to be considered individually as part of the Integrated assessment. Instead, the integrated assessment should be

. performed for a set(s} of flood scenario parameters defined based on the results of the flood hazard reevaluations. In some cases, only one controlling flood hazard may exist for a site. In this case, licensees should define the flood scenario parameters based on this controlling flood hazard.

However, sites that have a diversity of flood hazards to which the site may be exposed should define multiple sets of flood scenario parameters to capture the different plant effects from the diverse flood parameters associated with applicable hazards. In addition, sites may use different flood protection systems to protect against or mitigate different flood. hazards. In such Instances, the Integrated assessment should define multiple sets of flood scenario parameters. If appropriate, It Is acceptable Peach Bottom Atomic Power Station Page 147 of 165

(b)(3) 16 U SC

§ 824o-1(d}:(b)

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 to develop an enveloping scenario (e.g., the maximum WSEL and Inundation duration with the minimum warning time generated from different hazard scenarios) Instead of considering multiple sets of flood scenario parameters as part of the integrated assessment. For simplicity, the licensee may combine these flood parameters to generate a single bounding set of flood scenario parameters for use In the integrated assessment.

For PBAPS, the followlng flood-causing mechanisms were either determined to be implausible or completely bounded by other mechanisms:

1. Rock Run Creek flooding

2. Seismic dam failure

3. Sunny day dam failure

4. Storm surge and selche

5. Tsunami

6. Ice-induced flooding

7. Channel migration or diversion.

PBAPS is considered potentlally exposed to the flood hazards (Individual flood-causing mechanisms and/or combined effects flood scenarios per Appendix H of NUREG/CR-7046) listed below. In some Instances, an indlvldual flood-causing mechanism (e.g., flooding In streams and rivers) Is addressed In one or more of the combined effect flood scenarios.

1. LIP (Table 4.0.2)

2. Combinations In Section H.1 of NUREG/CR-7046 (Floods Caused by Precipitation Events, includlng precipitation-driven hydrologlc dam failure) for the Susquehanna River (Table 4.0.3).

The Current License Basis (CLB) PMF water level was computed using steady state backwater computations made with a back-water program for the IBM 1130 by the firm of Tlppetts-Abbett-McCarthy-Stratton (TAMS) Engineers and Architects. In the flood studies performed by TAMS, It was assumed that the PMF hydrograph at Conowlngo Dam ls the same as that at Harrisburg, or a peak discharge of 1,750,000 cfs. Channel characteristics were computed from surveyed cross sections covering a distance of 60,000 ft upstream of the Conowingo Dam at intervals of 5,000 ft where the flood elevation at Peach Bottom is dependent upon the discharge capability of Conowingo Dam. The flow through the Conowlngo Dam utilized the computed Conowlngo Dam rating curve that was established by Alden Research Laboratories and the scale model discharge coefficients observed in 1927.

The CLB PMF flood elevation at PBAPS of 131.5 ft C.D. (131.37 ft NAVD88) Is based on a flow of 1,750,000 ffJ/s. In the CLB, coincident with the PMF, I

........ joam was... ass.umed to faUJn.a manner (b)(3) 16 u s c that would result in an instantaneous additional outflow Of!

. Jets, andaUhepreo!§C:!Jim~ (~~I ::~gg~M~wr~l will produce a maximum water elevation at the Peach Bottom site. I ne transient wave produced by (4l (~i[il(F)' (

.......... this failure Is estimated to bLJft at Peach Bottom. Superimposing the height of the ve, conservati,,.,ly estirriated ai[___lt, on the steady-stale backwater l'°fii at a PMF o

-- - - Is.-. "" 16 u 5 c produces a maximum water level at Peach Bottom.of Elevation+

ft C.D.

ft NAVO 88)

M~~~lwi~~

without wind-wave activity.

8240:::l(d), (b)

'---.....1 IA\\ /L\\n\\/r\\

The difference in the PMF elevations between the TAMS model and the HEC-RAS model (as used in the flood hazard reevaluation), with the same computed maximum flow, can largely be attributed Peach Bottom Atomic Power Station Page 148 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 to the type of model and the complexity of the model that was used to compute the maximum water surface elevation.

The TAMS model used a steady-flow model. A steady flow model refers to the condition where the fluid flow properties at any single point In the system do not change over time. Under steady flow, the user specifies: (1) discharge at the upstream boundary, and (2) stage at the downstream boundary. The steady flow model proceeds to calculate stages throughout the interior points, while keeping the discharge constant. Under steady flow, such as the TAMS model, the discharge-stage ratings are unique and the model will not account for flow attenuation through storage.

The HEC-RAS model (as used in the flood hazard reevaluation) computed the PMF water surface elevation at PBAPS*using an unsteady dynamic flow model with hundreds of cross sections and the gate rules set for the Conowingo Dam. An unsteady flow model refers to the condition where the fluid flow properties at any single point in the system do change over time. Under unsteady flow, the user specifies: (1) a discharge hydrograph at the upstream boundary, and (2) a discharge-stage rating at the downstream boundary. The model proceeds to calculate discharges and stages throughout the interior points. Under unsteady dynamic flow, the model calculates looped discharge-stage ratings according to the variability of the flow allowing for better accuracy. As such, the maximum flow rate may not correspond the maximum water surface elevation at a given cross section with the model system, leading to differences between a steady flow model and an unsteady flow model. Figure 4.0.2 from the Design Manual - Hydrometry Volume 4 (Reference 140) illustrates this relationship.

h FaRing stages Steady state SUge-dlscharae rallfllCUM!

Rising stages Q.. f(h,dh/dt)

Deviations A and 8 effect of unsteady flow, 11enerallv A> B Q.a steadv uniform flow Q=flow h = flow depth Q

Figure 4.0.2 - Effect of Unsteady Flow on Stage-Discharge Relationship Additionally, the HEC-RAS model demonstrates, through calibration, the accuracy of the model to reproduce the largest floods on record at Peach Bottom. The TAMS only calibrated to one historic event with limited data at PBAPS. The HEC-RAS model uses a calibration event and as well as several validation events with much more detailed flood related data at PBAPS.

Tables 4.0.1 through 4.0.3 summarize the parameters for each flood hazard and provide comparisons with the current design basis flood.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015 Table 4.0.1 - Summary of Licensing Basis and External Flooding Study Parameters Parameter CLB Reevaluation Study Value/Methodology Value/Methodology LIP Methodology HMRs 51 and 52 1-Hr LIP (in.)

18.0 5-Min Peak Intensity (In.)

5.5

.Hydrodynamic Modeling using FL0-2D Computer Software (flood Not Evaluated velocity and depths)

Effects of LIP FEMA P-259 Equatlon*s (hydrostatic and hydrodynamic loading)

USAGE EM 1110-3-136 (erosion and deoosition}

PMP (Rock Run Creek)

Methodology HMRs 51, 52, and 53 Storm Duration 72 hr Not Evaluated 40.42 in. (All Season)

Cumulative PMP 18.67 in. (Cool Season)

(not including antecedent storm) 16.56 (1 hr, 4 sq. mi.)

PMF (Rock Run Creek)

Hydrologic Model HEC-HMS Infiltration Method Initial and Constant Evapotranspiration None Rainfall to Runoff Method Snyder Unit Hydrograph Snowmelt Method USAGE 1110-2-1406 (Cool Season)

Routing Method No Routing (Instantaneous)

Gage Weighted (All Season)

Base Flow Not Evaluated None (Cool Season)

Antecedent Storm Yes (All Season)

Nonlinear Basin Response Yes Total Area 3.8 sq. mi.

Hydrologic Model None CalibrationNalidation Events Hydraulic Stream Model HEC-RAS Hydraulic Model CallbrationNalidation None Events PMP (Susquehanna River)

Methodology HMR40 Site-Specific PMP Storm Duration 72 hr 120 hr Cumulative PMP (All Season) 9.25 in.

10.8 in.

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NTTF Recommendation 2.1 (Hazard Reevaluations}: Flooding Exelon Generation Co.

Parameter CLB Value/Methodology PMF (Susquehanna River)

Hydrologic Model USACE HEC-1 Infiltration Method Initial and Constant Evapotranspiration None Rainfall to Runoff Method Basin Unit Hydrographs Snowmelt Method Not Evaluated Routing Method Muskingum Base Flow Not Specified Antecedent Storm Not Specified Nonlinear Basin Response None Total Area 24,100 sq. mi.

Hydrologic Model Not Specified CalibrationNalldatlon Events Hydraulic Stream Model TAMS Steady Flow Model Hydraulic Model CalibrationNalidation Events March 1936 Dam Failure Failure Mechanisms Examined Precipitation-Driven Dam Failure Onlv Hydrologic Model None Transient Wave from Hydraulic Model Holtwood Dam Failure Only Probable Maximum Surge and Seiche Methodology Not Evaluated Probable Maximum Tsunami Methodology Not Evaluated Revision 0 July 10, 2015 Reevaluation Study Value/Methodology HEC-HMS Initial and Constant None Clark Unit Hydrograph USACE 1110-2-1406 (Cool Season)

Muskingum Greatest Mean Monthly Base Flow -

April (All Season and Cool Season)

Yes None 27,466 sq. mi.

Agnes 1972, Ivan 2004, and Lee 2011 HEC-RAS Lee 2011 (Calibration)

Agnes 1972, Ivan 2004, and March 1936 (Validation)

Precipitation-Driven Dam Failure and Seismicallv-lnduced Dam Failure HEC-HMS (All dams breach upstream of PBAPS, except Safe Harbor and Holtwood)

HEC-RAS (Safe Harbor Dam and Holtwood Dam breaches)

Equations from USACE EM 1110 1100 and Literature Review Historical Literature Review Combined Event* Wind Wave Activity coincident with PMF and Dam Failure on Susquehanna River Equations from USACE EM 1110 Methodology Not Specified 1100 and Methods from ANSI/ANS-2.8-1992 Fetch Length 2mi.

3.24mi.

Wind Speed 45 mph 36.7 mph Wave Runup from Significant Wave 1.8 ft 1.47 ft Wave Runup from Maximum Wave 5.4 ft 2.45 ft Total Summation of Combined Events PMF with Holtwood Dam PMF with All Upstream Dam Failure +

Water Level Failure+ Max Wave Runup Wind Setup + Wave ~etup + Max Wave Runup Peach Bottom Atomic Power Station Page 151 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Parameter CLB Value/Methodology Effects of Combined Event Not Specified Ice-Induced Flooding Upstream Ice Jam Methodology Not Evaluated

. Downstream Ice Jam Methodology Hydraulic Model Channel Migration/Diversion Methodology Not Evaluated Peach Bottom Atomic Power Station Revision 0 July 10, 2015 Reevaluation Study Value/Methodology Hydraulic Modeling using HEC-RAS (flood velocity and depths)

ASCE 7-10 (debris loading)

. USACE EM 1110-2-1100 and FEMA P-259 (hydrostatic, hydrodynamic, and wave loading)

USACE EM 1110-3-136 and NRCS Soil Surveys (erosion and deoositlon)

_ 25 yr Flood + Maximum Historical Ice Dam with Ice Dam Break at Holtwood Dam Literature Review HEC-RAS Historical Literature Review Page 152 of 165

NTTF Recommendation 2.1 _(Hazard Reevaluations): Floodlng Exelon Generation Co.

Revision 0 July 10, 2015 Table 4.0.2 - Local Intense Precipitation Bounded(B)or Flood Scenario Parameter CLB

. Reevaluated Not Bounded CNB)

Max Stilt Water Elevation (ft-NAVD88)

Not Determined Table 3.1.3.1 NB

~i Max Wave Runuo Elevation (ft-NAVD88)

Not Determined Note 1)

NIA Max Hydrodynamic I Hydrostatic I Debris Table a; w Loading Not Determined 3.10.3.1.1 and NB

> -0

~ !!

Ob/linear ft)

Note2 l 8 Effects of Sediment Deoositlon/Erosion Not Determined Note 31 N/A u: i Concurrent Site Conditions Not Determined Note41 NIA Effects on Groundwater Not Determined Note 5)

N/A 5

Warnino Time (hr)

Not Determined Note 6)

NB Period of Site Preparation (hr)

Not Determined Note 6)

NB

]

Figure i:

Period of Inundation (hr)

Not Determined 3:10.3.6.1 ~nd NB

~

tNote 7) w Figure "8

0 Period of Recession (hr)

Not Determined 3.10.3.6.1 and NB Ii:

(Note 8)

Other Plant Mode of Ooerations Not Determined Note 9l NIA Other Factors Not Determined Note 1m N/A Additional notes: 'NIA' justifications (why a particular parameter is judged not to affect the site) and explanations regarding the bounded/nonbounded determination.

1. Consideration of wind wave action for the LIP event is not explicitly required by NUREG/CR-7046 and is judged to be a negligible associated effect because of limited fetch lengths and flow depths.

2. These loads need to be evaluated against the permissible loads for the doors that provide flood protection for safety-related equipment. The CLB does not state the acceptable loads; therefore, the result is not bounded. The debris load for the LIP event is negligible due to the absence of heavy objects at the plant site and due to low flow velocity. The combination of these factors could lead to a hazard due to debris load.

3. Because of generally low velocities around the powerblock area, sediment transport is not expected to be an effect of LIP flooding. The maximum velocity in the powerblock area near the doors is well below permissible velocities for paved surfaces, so erosion and localized scour are also not expected to be an effect of LIP flooding.

4. High winds could be generated concurrent to a LIP event.

5. Due to the relatively impervious cover immediately around the powerbiock buildings, the infiltration of precipitation and groundwater seepage would likely be minimal. Additionally, the event is a short duration (1-hr precipitation) which limits the amount of soll lnfiltratlon. Therefore, groundwater level changes are not expected to occur.

6. NEI Guideline 15-05 (Reference 75) can be used to evaluate warning time and will need to be addressed in an integrated assessment.

7. Period of inundation is determined as time for which the maximum WSEL exceeds current protection levels.

8. Period of inundation includes the period of recession.

9. Any plant mode of operation can be expected to be in place when the LIP event occurs.

10. There are no olant-soecific factors annlicable to the LIP flood.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision O July 10, 2015 Table 4.0.3 - Combinations In Section H.1 of NUREG/CR-7046 for the Susquehanna River with Precipitation-Driven Hydrologlc Dam Failure Flood Scenario Parameter CLB RHvaluated Bounded (B) or Not Bounded (NB)

Max Still Water Elevation

\\0)\\.)/ 1ti U ::; (.;

I "O

lft-NAVD88l

§ 8240-1 (d) (b)

,... ft ** 1\\

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 8...... ********************** ~*

b)(3)16USC 824C0 1(d), (b)

~)(l~)(ll)lfS C

• 824o*1 (d) (b)

4) (b)(7)(F)

.l!!

Max Wave Runup Elevation (4) (b)(7)(F)

I

• • • • • • • • • • • • • • • I...,.......

8 j

(ft-NA V088l

.................... ~

(NOie -'1 Ma.x Hydrostali.c I Hydrodynamic/ Wave I 7,329 Debris Loading Not Determined NB ii (lb/linear ft)

(Note 3) 10,000 lb object, 50 ft 1,000 lb object, 30 ft long and 4iW long and 2 ft In 1 ft in diameter traveling at s

Debris Loading diameter traveling at 1.39 ft/s B

"O 5 ft/s (Note4

.a Effects of Sediment Deoosition/Eroslon Not Detenmlned 1Note5 NIA

u.

Concurrent Site Conditions Not Detenmlned (Note 6 NIA Effects on Groundwater Not Oetenmfned 1Note7 NIA Figures 3.10.3.13.1 and Warning Time (hr)

Not Detenmlned 3.10.3.13.2 and Tables B

3.10.3.13.1 and 3.10.3.13.2

§

• INole 8)

Ffgures 3.10.3.13.1 and

'i!

Period of Site Preparation (hr)

Not Detenmlned 3.10.3.13.2 and Tables B

i 3.10.3.13.1 and 3.10.3.13.2 C

i (Note 8)

Figures 3.10.3.13:1 and w

. Period of Inundation (hr)

Not Determined 3.10.3.13.2 and Tables NIA 1

3.10.3.13.1 and 3.10.3.13.2

!Note 91

u.

Figures 3.10.3.13.1 and Period of Recession (hr)

Not Determined 3.10.3.13.2 and Tables NIA 3.10.. 3.13.1 and 3.10.3.13.2

{Note 9)

Other Plant Mode of uoeralions Not Detennlned All Modes (Note* 10)

NIA Other Factors Not Oetennlned NIA !Note 111 NIA Additional notes: 'NIA' justifications (why a particular parameter Is Judged not to affect the site) and e,cplanatlons regarding the boundedlnonbounded detenmlnalion.

1.

The reevaluated still water elevation is bounded by the current design basts flood.

2.

The reevaluated PMF with wind wave runup elevatlon Is bounded by the current design basis ftood.

3.

Loading due to hydrostatlc/hydrostalic'wave loading Is not given In the current design basis ftood; therefore, the result is not bounded and will need to be addressed-In the Integrated assessment.

4.

The waterborne projectile debris parameters from ASCE 7-1 O (Reference 3) and the velocity from the PMF with precipitation driven hydrologlc dam failure at PBAPS are less than those stated In the current design basis ftood. Therefore any debris Impact loads from the current design basis flo~ are b_oundlng.

5.

Any sedlmentaUon and erosion in the Immediate vicinity of PBAPS will be minima!. Velodtles In the overbank from the Susquehanna River ftoodlng are lower than the permissible (for erosion resistance).

6.

Adverse weather would Increase the time required for activity completion because II would take longer for operators lo walk outdoors, from building !o building. But walking time Is a small proportion of activity time, and adverse weather would not prevent an operator from completlng any action (Reference 138).

7.

AH crltlcal structures essential to a safe shutdown of the reactor are flood protected to EL 134.87 ft-NAVD88. The.refore, PBAPS Is not subject to effects assodated with groundwater Ingress.

8.

Sile preparation procedures are in place and have been verified to demonstrate that. procedures/actions could be accomplished before being impeded by rising flood water (Reference 136). The reevaluated PMF level rate of increase Is less than the1 ft/hOur used to assess these actions; therefore, the current design basis ftood Is bounding.

9.

Once the plant Is shut down, protective features are In place for the duration of the flood event. Therefore, the period of Inundation and the period of recession are not appMcable.

10. Any plant mode of operation can be expected prior to the PMF; however, the plant will be shut down prior to the peak of the PMF ftood wave arrival.

11. There are no olant-!IOP.dfic factors related to riverine ftoodlng.

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

5. REFERENCES Revision 0 July 10, 2015

1.

AMEC, Email from Matthew Lehrer (AMEC) to ENERCON, RE: Data for Conowingo Dam (Attachment Susquehanna River Historic Flood Events.xis), May 20, 2014.

2.

American National Standards Institute (ANSl)/American Nuclear Society (ANS), "American National Standard for Determining Design Basis Flooding at Power Reactor Sites,"

ANSI/ANS-2.8-1992, prepared by the American Nuclear Society Standards Committee Working Group ANS-2.8, La Grange Park, Illinois, 1992.

3.

American Society of Civil Engineers (ASCE), Standard 7-10, "Minimum Design Loads for Buildings and Other Structures," Chapter CS, 2010.

4.

Brater, Ernest, Horace King, James Lindell, and C. Wei, "Handbook of Hydraulics, 7th Edition,"

McGraw-Hill Company, 1996.

5.

Buckler, S.J., Probable Maximum Snowpack Spring Melt and Rainstorm Leading to the Probable Maximum Flood, Elbow River, Alberta, Canada Department of Transport, Meteorological Service, Prairie Hydrometeorological Centre, 37 pages, 1968.

6.

Chow, Ven Te, Open-Channel Hydraulics, McGraw-HIii Book Company, New York, 1959.

7.

Commonwealth of Pennsylvania Department of Highways Bridge Division, SR-372/Norman Wood Bridge S7021, Sheet 1 of 43 through Sheet 31 of 43, September 15, 1965.

8.

Commonwealth of Pennsylvania Department of Highways Bridge Division, US-30/Wrights Ferry Bridge S8926, Sheet 1 of 15 through Sheet 15 of 15, October 17, 1969.

9.

C,S. Davidson, Inc., Peach Bottom Atomic Power Station (PBAPS) Site Survey, September 2013.

10. Dean, R and R. Dalrymple, Water Wave Mechanics for Engineers and Scientists, Advanced Series on Ocean Engineering-Volume 2, pp. 194-199, 1991.

11. Donigian, A.S., Jr., J.C. Imhoff, B.R. Bicknell, and J.L. Kittle, Jr., Application Guide for Hydrological Simulation Program FORTRAN (HSPF),

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NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015

15. Exelon Generation Company, LLC (Exelon), Conowingo Dam Discharge Rating Curve Development, prepared by Gomez and Sullivan Engineers, Revised December 2012.

16.

Exelon Generation Company, LLC (Exelon), Final Study Report Effect of Project Operations on Downstream Flooding - RSP 3.29, Conowingo Hydroelectric Project FERC Project No. 405, August 2012.

17.

Exelon Generation Company, LLC (Exelon), Peach Bottom Atomic Power Station (PBAPS)

Units 2 and 3, Individual Plant Examination of External Events (IPEEE), May 1996.

18.

Exelon Generation Company, LLC (Exelon), Peach Bottom Atomic Power Station (PBAPS)

UFSAR Section 2.4, Revision 24, April 2012.

19.

Exelon Generation Company, LLC (Exelon), Peach Bottom Atomic Power Station (PBAPS)

UFSAR Appendix C, Revision 22, April 2012.

20.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-01, "BDBEE-Flood Re-Evaluation -Topography and Bathymetry Data Processing," Revision 0.

21.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-02, "BDBEE - Flood Re-Evaluation - Rock Run Creek Probable Maximum.Precipitation (PMP) and Local Intense Precipitation (LIP}," Revision 0.

22.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-03, "BDBEE -

Flood Re-Evaluation -

FLO-2D Local Intense Precipitation (LIP) Flooding,"

Revision 0.

23.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-06, "BDBEE - Flood Re-Evaluation - HEC-RAS Model of Susquehanna River Development and Calibration," Revision 0.

24.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-07, "BDBEE -

Flood Re-Evaluation - HEC-HMS Rock Run Creek Hydrologic Calculation,"

Revision 0.

25. Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-08, "BDBEE - Flood Re-Evaluation - HEC-RAS Rock Run Creek Probable Maximum Flood (PMF)

Hydraulic Calculation," Revision 0.

26. Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-09, "BDBEE-Flood Re-Evaluation - Ice Effects," Revision 0.

27.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-10, "BDBEE-Flood Re-Evaluation - Seiche and Surge Analysis," Revision 0.

28. Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-11, "BDBEE - Flood Re-Evaluation - Site-Specific Probable Maximum Precipitation (PMP) and Climatology Calculation," Revision 1.

Peach Bottom Atomic Power Station Page 156 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015

29.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-16, "BDBEE - Flood Re-Evaluation - HEC-RAS Probable Maximum Flood (PMF) Water Level,"

Revision 0.

30.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-17, "BDBEE - Flood Re-Evaluation - Co-Incident Wind Wave Run-Up Calculation,* Revision 0.

31.

Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-18, "BOBEE - Flood Re-Evaluation -Waterborne Projectiles,* Revision 0.

32. Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-19, "BDBEE -

Flood Re-Evaluation -

Hydrostatic & Hydrodynamic Loading and Flooding Associated Effects," Revision 0.

33. Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-20, "BDBEE-Flood Re-Evaluation - Combination Flooding," Revision 0.

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35. Exelon Generation Company, LLC (Exelon), Peach Bottom Calculation PEAS-FLOOD-23, "BDBEE - Precipitation Data Processing," Revision 0.

36. Exelon Generation Company, LLC (Exelon), Three Mlle Island Calculation C-1101-122-E410-010, "HEC*HMS Model Calculatlon Package - Three Mile Island and Peach Bottom Riverine Hydrology Calibration," Revision 3.

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38.

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39.

Exelon Generation Company, LLC (Exelon), Final Study Report Effect of Project Operations on Downstream Flooding - RSP 3.29, Conowlngo Hydroelectric Project FERC Project No. 405, August 2012.

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Flooding for the Peach Bottom Atomic Power Station," November 5, 2012, ADAMS Accession No. ML123250714.

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43. Exelon Generation Company, LLC (Exelon), Transmittal of Design Information (TOOi}

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65. National* Oceanic and Atmospheric Administration (NOAA)/National Weather Service (NWS),

Office of Hydrologic Development, Hydrometeorologlcal Design Studies Center (HDSC),

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68.

National Oceanic and Atmospheric Administration (NOAA), "Hydrometeorological Report No. 53, Seasonal Variation of 10-Square-Mile Probable Maximum Precipitation Estimates, United States East of the 105th Meridian,* Silver Spring, Maryland, April 1980.

69.

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Susquehanna Rlver at

Marietta, http://www.erh.noaa.gov/marfc/Rivers/FloodClimo/

Top_Flood_Crests/Msl/Marletta-MSL-Top10-Table.pdf>, Accessed April 7, 2014.

Peach Bottom Atomic Power Station Page 159 of 165

NTTF Recommendation 2.1 (Hazard Reevaluations): Flooding Exelon Generation Co.

Revision 0 July 10, 2015

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