Text

Enclosuflooding Hazard Reevaluation Report, Cover Through Page 2.2-46
	ML13079A807
Person / Time
Site:	South Texas
Issue date:	03/31/2013
From:	; South Texas
To:	; Office of Nuclear Reactor Regulation
References
	NOC-AE-13002975
	Download: ML13079A807 (118)
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NOC-AE-13002975

Enclosure:

South Texas Project Electric Generating Station Units 1 and 2 Flooding Hazard Reevaluation Report March 2013 (322 Pages Including Cover Page)

Enclosure NOC-AE-1 3002975 South Texas Project Electric Generating Station Units 1 and 2 Flooding Hazard Reevaluation Report March 2013

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 Fukushimna Response Project Table of Contents Intro d u ctio n .............................................................................................................................. xiii Introduction References ........................................................................................................... xiv 1 Site Information Related to Flooding Hazards ................................................................... 1-1 1.1 Detailed Site Information ........................................................................................ 1.1-1 1.1.1 Location and Size of the Site ...................................................................... 1.1-1 1.1.2 Description of Plant Environs ...................................................................... 1.1-1 1.1.3 Plan Layout and Topography ...................................................................... 1.1-2 1.1.4 Plant Description ........................................................................................ 1.1-2 1.1.5 Plant Safety-Related Structures .................................................................. 1.1-2 1.1.6 Major Hydrologic Features .......................................................................... 1.1-3 1.1.7 References ................................................................................................. 1.1-4 1.2 Current Design Basis Flood Elevations .................................................................. 1.2-1 1.2.1 Local Intense Precipitation .......................................................................... 1.2-2 1.2.2 Flooding in Streams and Rivers .................................................................. 1.2-2 1.2.3 Dam Breaches and Failures ....................................................................... 1.2-2 1.2.4 Storm Surge ............................................................................................... 1.2-4 1.2.5 Seiche ........................................................................................................ 1.2-4 1.2.6 Tsunam i...................................................................................................... 1.2-4 1.2.7 Ice Induced Flooding .................................................................................. 1.2-4 1.2.8 Channel Migration or Diversion ................................................................... 1.2-5 1.2.9 Com bined Effect Flood ............................................................................... 1.2-5 1.2.10 References ................................................................................................ 1.2-5 1.3 Licensing Basis Flood-Related and Flood Protection Changes .............................. 1.3-1 1.4 W atershed and Local Area Changes ...................................................................... 1.4-1 1.5 Current Licensing Basis Flood Protection and Mitigation Features ......................... 1.5-1 1.5.1 References ................................................................................................. 1.5-3 1.6 Additional Site Details ............................................................................................ 1.6-1 1.6.1 References ................................................................................................. 1.6-1 2 Flooding Hazard Reevaluation .......................................................................................... 2-1 2.1 Local Intense Precipitation ..................................................................................... 2.1-1 2.1.1 Probable Maxim um Precipitation Depths .................................................... 2.1-1 2.1.2 Drainage Areas and Local Drainage Parameters ........................................ 2.1-2 Introduction ii

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP I & 2 FukushiniaResponse Project 2.1.3 Peak Discharges ........................................................................................ 2.1-4 2.1.4 Hydraulic Model Setup ................................................................................ 2.1-4 2.1.5 Effect of local PM P ..................................................................................... 2.1-6 2.1.6 References ................................................................................................. 2.1-7 2.2 Flooding in Stream s and Rivers ............................................................................. 2.2-1 2.2.1 Probable Maxim um Precipitation (PM P) ..................................................... 2.2-3 2.2.2 Precipitation Losses .................................................................................... 2.2-4 2.2.3 Runoff and Stream Course Models ............................................................. 2.2-5 2.2.4 Probable Maxim um Flood Flow .................................................................. 2.2-5 2.2.5 Water Level Determ inations ...................................................................... 2.2-15 2.2.6 Coincident W ind W ave Activity ................................................................. 2.2-17 2.2.7 Other Impacts ........................................................................................... 2.2-18 2.2.8 References ............................................................................................... 2.2-18 2.3 Dam Breaches and Failures ................................................................................... 2.3-1 2.3.1 Upstream Dam Failures .............................................................................. 2.3-2 2.3.2 MCR Breach Evaluation ........................................................................... 2.3-12 2.3.3 Conclusions .............................................................................................. 2.3-28 2.3.4 References ............................................................................................... 2.3-28 2.4 Storm Surge ........................................................................................................... 2.4-1 2.4.1 Probable Maximum Winds and Associated Meteorological Parameters ...... 2.4-2 2.4.2 Storm Surge Water Levels .......................................................................... 2.4-3 2.4.3 Wave Actions ............................................................................................ 2.4-11 2.4.4 Protective Structures ................................................................................ 2.4-12 2.4.5 References ............................................................................................... 2.4-13 2.5 Seiche .................................................................................................................... 2.5-1 2.5.1 Seiches in the Essential Cooling Pond ....................................................... 2.5-1 2.5.2 Seism ic Seiche ........................................................................................... 2.5-4 2.5.3 Seiche in Main Cooling Reservoir ............................................................... 2.5-4 2.5.4 Seiche in Other W ater Bodies ..................................................................... 2.5-5 2.5.5 References ................................................................................................. 2.5-5 2.6 Tsunam i................................................................................................................. 2.6-1 2.6.1 Probable Maxim um Tsunam i ...................................................................... 2.6-1 2.6.2 Historical Tsunam i Record .......................................................................... 2.6-2 Introduction iii

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STPJ & 2 Fukushinma Response Project 2.6.3 Source Generator Characteristics ............................................................... 2.6-4 2.6.4 Tsunam i Analysis ..................................................................................... 2.6-11 2.6.5 Tsunam i W ater Levels .............................................................................. 2.6-13 2.6.6 Hydrography and Harbor or Breakwater Influences on Tsunami ............... 2.6-16 2.6.7 Hydrostatic Forces, Hydrodynamic Forces, Debris, and Waterborne Projectiles ................................................................................................. 2.6-16 2.6.8 Effects on Safety-Related Facilities .......................................................... 2.6-16 2.6.9 References ............................................................................................... 2.6-17 2.7 Ice Induced Flooding .............................................................................................. 2.7-1 2.7.1 Ice Conditions and Historical Ice Formation ................................................ 2.7-1 2.7.2 Ice Jam Events ........................................................................................... 2.7-1 2.7.3 Conclusion .................................................................................................. 2.7-2 2.7.4 References ................................................................................................. 2.7-2 2.8 Channel Migration or Diversion .............................................................................. 2.8-1 2.8.1 Historical Channel Migration or Diversions ................................................. 2.8-1 2.8.2 Stratigraphic Evidence ................................................................................ 2.8-2 2.8.3 Ice Causes ................................................................................................. 2.8-3 2.8.4 Flooding of Site Due to Channel Migration or Diversion .............................. 2.8-3 2.8.5 Human-Induced Changes of Channel Diversion ......................................... 2.8-4 2.8.6 References ................................................................................................. 2.8-7 2.9 Combined Effect Flood .......................................................................................... 2.9-1 2.9.1 References ................................................................................................. 2.9-1 3 Comparison of Current and Reevaluated Flood Causing Mechanisms ............................. 3-1 3.1 Local Intense Precipitation ..................................................................................... 3.1-1 3.1.1 References ................................................................................................. 3.1-2 3.2 Flooding in Streams and Rivers ............................................................................. 3.2-1 3.2.1 References ................................................................................................. 3.2-3 3.3 Dam Breaches and Failures ................................................................................... 3.3-1 3.3.1 Dam Failures .............................................................................................. 3.3-1 3.3.2 MCR Em bankment Breach ......................................................................... 3.3-3 3.3.3 References ................................................................................................. 3.3-4 3.4 Storm Surge ........................................................................................................... 3.4-1 3.4.1 References ................................................................................................. 3.4-3 Introduction iv

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP J & 2 Fukushima Response Project 3.5 Seiche .................................................................................................................... 3.5-1 3.6 Tsunam i................................................................................................................. 3.6-1 3.6.1 References ................................................................................................. 3.6-1 3.7 Ice Induced Flooding .............................................................................................. 3.7-1 3.8 Channel Migration or Diversion .............................................................................. 3.8-1 3.9 Com bined Effect Flood .......................................................................................... 3.9-1 4 Interim Evaluation and Actions Taken or Planned ............................................................. 4-1 5 Additional Actions ............................................................................................................. 5-1 Introduction V

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP J & 2 Fukushimna Response Project List of Tables Table 1.2-1 Current Maximum Flood Levels for Applicable Flooding Mechanisms per U F S A R ..................................................................................................................... 1.2 -1 Table 2.1 Short Duration PMP Depths ........................................................................... 2.1-9 T able 2.1 D rainage A reas ............................................................................................ 2.1-10 Table 2.1 Time of Concentration (Tc) Estimates .......................................................... 2.1-11 Table 2.1 Lag Time for Reach Routing ......................................................................... 2.1-12 Table 2.1 PM P Peak Discharges ................................................................................. 2.1-13 Table 2.1 Manning's Roughness Coefficient Values (Reference 2.1-15) Used in HEC-RAS Model C ross Sections ............................................................................ 2.1-14 Table 2.1 STP 1 & 2 Local PMP Maximum Water Levels ............................................. 2.1-15 Table 2.2-1 PMF and SPF Values at Mansfield Dam ......................................................... 2.2-21 Table 2.2-2 Drainage Areas, Unit Hydrograph Parameters, Rainfall Loss Rates, and PMP Depths for Subbasins from Mansfield Dam to Matagorda Bay ....................... 2.2-22 Table 2.2-3 10 Sq. Miles PMP Depth at Subbasin CC-06 .................................................. 2.2-25 Table 2.2-4 Dam and Spillway Outlet Data for Lake Travis Reservoir ................................ 2.2-26 Table 2.2-5 Elevation-Storage Data for Lake Travis Reservoir at Mansfield Dam .............. 2.2-27 Table 2.2-6 Estimated Peak PMF at Bay City for STP 1 & 2 .............................................. 2.2-28 Table 2.2-7 Location Description for Key Cross-Sections in the HEC-RAS Model .............. 2.2-29 Table 2.3-1 Summary of the 68 Dams in Colorado River Basin with 5,000 AF or More S torage C apacity .................................................................................................... 2 .3-3 1 Table 2.3-2 500-year and SPF Inflow Peak Discharges at Selected Locations along the C olorado R iver (in cfs) ............................................................................................ 2.3-35 Table 2.3-3 Breach Parameters for Buchanan and Mansfield Dams .................................. 2.3-35 Table 2.3-4 Initial Estimation of Manning's Roughness Coefficient ..................................... 2.3-36 Table 2.3-5 Estimated Water Levels due to Dam Break, Wind Setup, and Wave Run-up at STP 1 & 2 Power Block Structures ..................................................................... 2.3-37 Table 2.3-6 Estimated Water Levels due to Dam Break, Wave Transmission and Wave Run-up at ECW Intake Structure ............................................................................ 2.3-37 Table 2.3-7 Ground, Maximum Water and Buoyancy Elevations from the MCR Breach Analysis of the UFSAR for STP 1 & 2 (Reference 2.3-1) ........................................ 2.3-38 Table 2.3-8. Maximum Velocities, Depths, and Water Elevations for Selected Locations from the 2012 MCR Breach Analysis (Reference 2.3-19) ....................................... 2.3-38 Table 2.3-9 Debris Characterization Table from the 2012 MCR Breach Analysis (Reference 2.3-19) ................................................................................................. 2 .3-39 Table 2.3-10 Proportions of Debris Items Predicted to be Retained in the ECP from the 2012 MCR Breach Analysis (Reference 2.3-19) ..................................................... 2.3-40 Table 2.3-11 Extrapolated Inundation Period at Selected Locations from the 2012 MCR Breach Analysis (Reference 2.3-19) ....................................................................... 2.3-41 Introduction Vi

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project Table 2.4-1 Probable Maximum Hurricane Characteristics ................................................ 2.4-16 Table 2.4-2 Major Historic Hurricanes Impacting the Texas Coast 1900 to 2005 ............... 2.4-17 Table 2.4-3 Probable Maximum Hurricane Parameters and ADCIRC Model Scenarios ..... 2.4-18 Table 2.4-4 Summary of ADCIRC Results for the Selected Scenarios ............................... 2.4-19 Table 2.5-1 Short Duration PMP Depths at STP Site ........................................................... 2.5-7 Table 2.6-1 Areas of Potential Seismic Tsunamigenesis in the Caribbean (Reference 2.6-3, pp. 105 and 107) .......................................................................................... 2.6-22 Table 2.6-2 Source Parameters for Veracruz Scenario ...................................................... 2.6-22 Table 2.6-3 Initial Wave Deformation Characteristics and Maximum Runup for Simulations... 2.6-22 Table 2.7-1 Lowest Average Daily Temperature and Number of Days with Average Daily Tem perature below Freezing at STP Site ................................................................. 2.7-3 Table 2.7-2 Lowest Average Daily Temperature and Number of Days with Average Daily Temperature below Freezing at Bay City Climate Station ................................ 2.7-4 Table 3-1 Current Design Basis and Reevaluation Flood Elevations ....................................... 3-1 Table 3-2 Probable Maximum Hurricanes Considered in the UFSAR for STP 1 & 2 (taken from R eference 1.1-3) .................................................................................... 3.4-4 Introduction vii

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project List of Figures Figure 1.1-1 Site Region and Location ................................................................................. 1.1-5 Fig ure 1.1-2 S ite Layout ...................................................................................................... 1.1-6 Figure 1.1-3a STP 1 & 2 Safety-Related Structures ............................................................. 1.1-7 Figure 1.1-3b STP 1 & 2 Safety-Related ECP Embankments (Shaded Area on Plan V ie w) ........................................................................................................................ 1.1 -8 Figure 1.1-3c Aerial View of STP 1 & 2 and Environs ........................................................... 1.1-9 Figure 1.1-3d Topographic Details of STP 1 & 2 and Environs (elevations are in MSL) ..... 1.1-10 Figure 1.1-3e Topographic Details at STP 1 & 2 Area ........................................................ 1.1-11 Figure 1.4-1 C olorado R iver Basin ....................................................................................... 1.4-2 Figure 1.5-1 Typical Section Through Plant ......................................................................... 1.5-4 Figure 1.5-2 Maximum Flow Profiles Through the Plant ....................................................... 1.5-5 Figure 2.1-1 Fitting of PMP data to Obtain 2-hr and 3-hr PMP depths ................................ 2.1-16 Figure 2.1-2 6-Hour PMP Hyetograph for STP Units I & 2 Site .......................................... 2.1-17 Figure 2.1-3 Site Location, Flow Paths, and Drainage Areas ............................................. 2.1-18 Figure 2.1-4 STP I & 2 Site Layout ..................................... 2.1-19 Figure 2.1-5 HEC-HMS Model Hydrologic Diagram ........................................................... 2.1-20 Figure 2.1-6a HEC-HMS Model Junction US R2 Hydrograph ............................................. 2.1-21 Figure 2.1-6b HEC-HMS Model Junction US R3 Hydrograph ............................................. 2.1-22 Figure 2.1-7 Topography and Location of HEC-RAS Model Cross Sections ...................... 2.1-23 Figure 2.1-8 HEC-RAS Model Channel Network and Cross-Sections Schematic ............... 2.1-24 Figure 2.1-9a HEC-RAS Model Profiles (Main Channel Profile) ......................................... 2.1-25 Figure 2.1-9b HEC-RAS Model Profiles (North Branch Profile) .......................................... 2.1-26 Figure 2.1-9c HEC-RAS Model Profiles (South ECP Branch Profile) .................................. 2.1-27 Figure 2.1-10a HEC-RAS Model Cross Section 12809 ...................................................... 2.1-28 Figure 2.1-10b HEC-RAS Model Cross Section 13572 ...................................................... 2.1-29 Figure 2.1-10c HEC-RAS Model Cross Section 14135 ....................................................... 2.1-30 Figure 2.2-1a General Location of STP 1 & 2 Site in the Lower Colorado River Basin ....... 2.2-30 Figure 2.2-1b The Highland Lakes and Dams in the Lower Colorado River Basin (R efe rence 2 .2-1a) ................................................................................................. 2 .2-3 1 Figure 2.2-1c The Colorado River Streamflow Gauging Stations Downstream of Mansfie ld D a m ....................................................................................................... 2 .2-32 Figure 2.2-2a Drainage Delineation of Subbasins between Mansfield Dam and Matagoda Bay (Modified from Reference 2.2-8) ..................................................... 2.2-33 Figure 2.2-2b Drainage Delineation of Subbasins between Mansfield Dam and Matagoda Bay (Modified from Reference 2.2-8) .................................................... 2.2-34 Introduction viii

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project Figure 2.2-3 Lower Colorado River Basin from Lake O.H. Ivie to Matagorda Bay (Modified from Reference 2.2-8) ............................................................................. 2.2-35 Figure 2.2-4 Storm Orientation Pattern for Locations Upstream of Mansfield Dam (Modified from Reference 2.2-8) ............................................................................. 2.2-36 Figure 2.2-5 Storm Orientation Pattern for Locations Downstream of Mansfield Dam (Modified from Reference 2.2-8) ............................................................................. 2.2-37 Figure 2.2-6 96-hour PMP Hyetograph for Subbasin CC-06 .............................................. 2.2-38 Figure 2.2-7 PMF Hydrograph at Bay City for Scenario 1 ................................................... 2.2-39 Figure 2.2-8 Development of PMF Outflow Hydrograph at Lake Travis for Scenario 2 ....... 2.2-40 Figure 2.2-9 PMF Hydrograph at Bay City for Scenario 3 ................................................... 2.2-41 Figure 2.2-10 Extended Cross-Sections - Most Downstream Section to STP 1 & 2 Site .... 2.2-42 Figure 2.2-11 PMF Elevation at STP 1 & 2 Site for Normal Depth Boundary Condition (Manning's n values equal to 1.2 times those used in the Halff model) ................... 2.2-43 Figure 2.2-12 PMF Elevation at STP 1 & 2 Site for Normal Depth Boundary Condition (Manning's n values equal to those used in the Halff model) .................................. 2.2-44 Figure 2.2-13 PMF Water Levels at STP 1 & 2 Site (RS 891+46.0) and at Downstream Boundary (RS 383+64.5) (Manning's n values equal to 1.2 times those used in the Halff m odel) ...................................................................................................... 2.2-45 Figure 2.2-14 PMF Water Levels at STP 1 & 2 Site (RS 891+46.0) and at Downstream Boundary (RS 383+64.5) (Manning's n values equal to those used in the Halff m o d e l) .................................................................................................................... 2 .2 -4 6 Figure 2.3-1a Locations of Dams with Storage Capacity Over 10,000 AF in the Colorado River Basin Upstream of the STP Site .................................................................... 2.3-42 Figure 2.3-1b Locations of Dams with Storage Capacity of 5,000 AF to 10,000 AF in the Colorado River Basin Upstream of the STP Site ..................................................... 2.3-43 Figure 2.3-2 Model Cross Section at Buchanan Dam ......................................................... 2.3-44 Figure 2.3-3 Model Cross Section at Mansfield Dam .......................................................... 2.3-44 Figure 2.3-4 Locations of Model Cross-sections in the Dam Break Analysis ...................... 2.3-45 Figure 2.3-5 Model River Cross-section at About 365 River Miles Upstream of the GIWW 2.3-46 Figure 2.3-6 Model River Cross-section at About 163.5 River Miles Upstream of the G IWW ..................................................................................................................... 2 .3 -4 7 Figure 2.3-7 Model River Cross-section at About 10.5 River Miles Upstream of the G IWW............... ..................................................................................................... 2 .3-4 8 Figure 2.3-8 Model River Cross-section at Downstream Model Boundary at about 0.9 River Miles Upstream of the G IW W ........................................................................ 2.3-49 Figure 2.3-9 Based Case Flood and Stage Hydrographs at the STP Site ........................... 2.3-50 Figure 2.3-10 Sensitivity Case Flood and Stage Hydrographs at the STP Site ................... 2.3-51 Figure 2.3-11 Base Case Simulated Maximum Dam Break Surface Profiles from Buchanan Dam to 4,600 ft upstream of GIWW (Vertical Datum in NAVD 88) ........ 2.3-52 Figure 2.3-12 Sensitivity Case Simulated Maximum Dam Break Surface Profiles from Buchanan Dam to 4600 ft Upstream of GIWW ....................................................... 2.3-53 Introduction bV

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project Figure 2.3-13 Fetch Directions and Length ........................................................................ 2.3-54 Figure 2.3-14a ECP and ECW Intake Structure for STP 1 & 2 ........................................... 2.3-55 Figure 2.3-14b ECP Embankment Cross-section ............................................................... 2.3-56 Figure 2.3-15 Wave Transmission on Sloped Structure (Reference 2.3-12) ....................... 2.3-56 Figure 2.3-16 Model Configurations from the UFSAR for STP 1 & 2 (Reference 2.3-1) ...... 2.3-57 Figure 2.3-17 Coarse Grid and Plant Layout for Model No. 2 (Reference 2.3-1) ................ 2.3-58 Figure 2.3-18 Fine Grid and Plant Layout for Model No. 2 (Reference 2.3-1) ..................... 2.3-59 Figure 2.3-19 Fine Grid at STP Units 1 & 2 for Model No. 2 (Reference 2.3-1) .................. 2.3-60 Figure 2.3-20 Envelope of Maximum Water Level (Reference 2.3-1) ................................. 2.3-61 Figure 2.3-21 Location of STP 3 &4 in Relation to STP 1 &2 ............................................ 2.3-62 Figure 2.3-22 Layout of Modeling Domain for the Independent MCR Breach Analysis w ith Delft3 D............................................................................................................ 2 .3-6 3 Figure 2.3-23 Flood Hydrograph From FLDWAV and BREACH Simulations ...................... 2.3-64 Figure 2.3-24 Layout of Grid Model for STP 3 &4 COLA (with breach facing STP 3)

(Elevation datum is NG VD 29) ................................................................................ 2.3-65 Figure 2.3-25 Land Cover Types Assigned (STP 3 &4 COLA) .......................................... 2.3-66 Figure 2.3-26 Land Cover Types Assigned (Reference 2.3-19) ......................................... 2.3-67 Figure 2.3-27a Unit 1 Breaching Scenario Water Levels (Reference 2.3-19) ..................... 2.3-68 Figure 2.3-27b Unit 2 Breaching Scenario (Reference 2.3-19) ........................................... 2.3-69 Figure 2.3-27c NSC Breaching Scenario (Reference 2.3-19) ............................................. 2.3-70 Figure 2.3-28 Location of Monitoring Points (Reference 2.3-19) ........................................ 2.3-71 Figure 2.3-29a Unit 1 Breach Scenario Maximum Velocity Vectors (Reference 2.3-19) ..... 2.3-72 Figure 2.3-29b Unit 2 Breach Scenario Maximum Velocity Vectors (Reference 2.3-19) ..... 2.3-73 Figure 2.3-29c NSC Breach Scenario Maximum Velocity Vectors (Reference 2.3-19) ....... 2.3-74 Figure 2.3-30a Unit 1 Breach Scenario Velocity Time History (Reference 2.3-19) .............. 2.3-75 Figure 2.3-30b Unit 2 Breach Scenario Velocity Time History (Reference 2.3-19) .............. 2.3-76 Figure 2.3-30c NSC Breach Scenario Velocity Time History (Reference 2.3-19) ................ 2.3-77 Figure 2.4-1 Historic Hurricane Tracks of Major (i.e., Category 1 and Larger) Unnamned Hurricanes Impacting the Texas Coast Between 1852 and 1950 .............................. 2.4-0 Figure 2.4-2 Historic Hurricane Tracks of Major (i.e., Category 1 and Larger)

Unnamned Hurricanes Impacting the Texas Coast from 1950 to 2006 ..................... 2.4-1 Figure 2.4-3 Topographic Data Sources for the TX2008 Grid near the STP Site ................. 2.4-2 Figure 2.4-4 Bathymetric Data Sources for the TX2008 Grid near the STP Site ................... 2.4-3 Figure 2.4-5 Topographic Features of the TX2008 Grid Near the STP Site .......................... 2.4-4 Figure 2.4-6 Landward Extent of TX2008 Grid near the STP Site ........................................ 2.4-4 Figure 2.4-7 The TX 2008 Grid ............................................................................................. 2.4-5 Figure 2.4-8 PMH Track and STP COLA ADCIRC model Parameters for Scenario 2 .......... 2.4-6 Introduction X

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project Figure 2.4-9 STP COLA ADCIRC Model Results for the Maximum Water Level for S ce n a rio 2 ................................................................................................................ 2 .4-7 Figure 2.4-10 Elevation Contours (ft MSL) Surrounding the ECP ......................................... 2.4-8 Figure 2.5-1 ECP Embankment Cross-section ..................................................................... 2.5-8 Figure 2.5-2 STP 1 & 2 ECP Fetches and ECWIS .............................................................. 2.5-8 Figure 2.5-3a PMP Depth versus Time Period for STP Site ................................................ 2.5-9 Figure 2.5-3b Determination of 96-hr PMP Depth for STP Site ........................................... 2.5-9 Figure 2.6-1 Location Map of STP 1 & 2 from the Gulf Coast and Colorado River .................. 2.6-23 Figure 2.6-2 Regional Map of Plate Boundaries and Tsunami-Generating Earthquakes from 1530-1991 in the Caribbean Sea (modified from Reference 2.6-21) ............... 2.6-24 Figure 2.6-3 Landslide Source Regions in Gulf of Mexico .................................................. 2.6-25 Figure 2.6-4 Location of East Breaks Slump Relative to STP 1 & 2 (Source: Reference 2 .6-4 1) .................................................................................................................... 2 .6-2 6 Figure 2.6-5 Source Parameters for East Breaks Slump .................................................... 2.6-27 Figure 2.6-6 Grid Spacing for East Breaks Slump Modeling with MOST ............................ 2.6-28 Figure 2.6-7 Plan View of Palos Verdes (PV) Initial Deformation Condition at Location of the East Breaks Slump in the Gulf of Mexico .......................................................... 2.6-29 Figure 2.6-8 Side View of Palos Verdes (PV) Initial Deformation Condition ........................ 2.6-30 Figure 2.6-9 Plan View of Palos Verdes x20 (PVx20) Initial Deformation Condition at Location of the East Breaks Slump in the Gulf of Mexico ........................................ 2.6-31 Figure 2.6-10 Oblique View of Palos Verdes x20 (PVx20) Initial Deformation Condition .... 2.6-32 Figure 2.6-11 Plan View of Papua New Guinea (PNG) Initial Deformation Condition at Location of the East Breaks Slump in the Gulf of Mexico ........................................ 2.6-33 Figure 2.6-12 Oblique View of Papua New Guinea (PNG) Initial Deformation Condition .... 2.6-34 Figure 2.6-13 Plan View of Hypothetical "Monster" Initial Deformation Condition at Location of the East Breaks Slump in the Gulf of Mexico ........................................ 2.6-35 Figure 2.6-14 Oblique View of Hypothetical "Monster" Initial Deformation Condition .......... 2.6-36 Figure 2.6-15 Maximum Coastal Runup for the PV Simulation ........................................... 2.6-37 Figure 2.6-16 Time Series of Wave Amplitude for PV Simulation at 28.580 N and 95.980 W (i.e., Buoy Location Shown in Figure 2.6-4) ........................................................ 2.6-38 Figure 2.6-17 Maximum Coastal Runup for the PVx20 Simulation ..................................... 2.6-39 Figure 2.6-18 Time Series of Wave Amplitude for PVx20 Simulation at 28.580 N and 95.98°W (i.e., Buoy Location Shown in Figure 2.6-4) .............................................. 2.6-40 Figure 2.6-19 Maximum Coastal Runup for the PNG Simulation ........................................ 2.6-41 Figure 2.6-20 Time Series of Wave Amplitude for PNG Simulation at 28.580 N and 95.980 W (i.e., Buoy Location Shown in Figure 2.6-4) ............................................. 2.6-42 Figure 2.6-21 Maximum Coastal Runup for the Hypothetical "Monster" Simulation ............ 2.6-43 Figure 2.6-22 Time Series of Wave Amplitude for Hypothetical "Monster" Simulation at 28.580 N and 95.98 0 W (i.e., Buoy Location Shown in Figure 2.6-4) ....................... 2.6-44 Introduction xi

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STP I & 2 Fukushima Response Project Figure 2.7-1 Recorded Water Temperatures in the Lower Colorado River ........................... 2.7-7 Figure 2.8-1 STP Units 1 & 2 Relative to the Current Colorado River Channel (Dark Blue Line) and Relict Channels of the Colorado River Delta Plain (Red Lines) ................. 2.8-9 Figure 2.8-2 Quaternary and Tertiary Deposits of the Colorado River, from near Columbus, Texas, to the Gulf of Mexico ................................................................. 2.8-10 Figure 2.8-3 1838 Map of Texas Showing the Rio Colorado (i.e., the Lower Colorado River) between the Rio-La Vaca and Rio Navidad Rivers to the West and the Rio Brazos to the E ast .................................................................................................. 2 .8-11 Figure 2.8-4 Longitudinal Profiles for the Lower Colorado River Relative to Mean Sea Leve l (MS L) ............................................................................................................ 2 .8-12 Figure 2.8-5 Peak Discharge versus Water Year for the Colorado River at Austin, Texas, (USGS #08158000) before and after the Completion of Mansfield Dam and Lake T ra v is ..................................................................................................................... 2 .8 -1 3 Figure 2.8-6 Successive Growth Stages of the Modern Delta of the Colorado River, Te xa s ..................................................................................................................... 2 .8 -14 Figure 2.8-7 Graphic Representation of the Growth of the Colorado River Delta in Acres by Yea rs ................................................................................................................. 2 .8 -15 Figure 2.8-8 Historical Estuary Occupied by the Colorado River after Abandoning the Caney Creek Area (Solid Line) and the Estuary after Being Filled with Sediments in 1930 (Dashed Line) (Modified from Figure 3 of Reference 2.8-3 [p. 103]) ........... 2.8-16 Figure 2.8-9 Plan View of West Branch of the Lower Colorado River, Wild Cow Island, Baxter Island, and McNabb Island Near Matagorda, Texas .................................... 2.8-17 Introduction Xii

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 FukushiniaResponse Project Introduction Following the accident at the Fukushima Daiichi nuclear power plant resulting from the 2011 Great Tohoku earthquake and tsunami, the NRC established the Near Term Task Force (NTTF) and tasked it with conducting a systematic and methodical review of NRC processes and regulations to determine whether improvements were necessary.

The NTTF report concluded that continued U.S. nuclear plant operation did not pose an imminent risk to public health and safety, and provided a set of recommendations to the Commission. The Commission directed the Staff to determine those recommendations that should be implemented without unnecessary delay (Staff Requirements Memorandum (SRM) on SECY-11-0093).

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

Recommendation 2.1: Flooding (Reference 1, Enclosure 2)

" Recommendation 2.3: Flooding (Reference 1, Enclosure 4) to the NRC 50.54(f) letter addressed Recommendation 2.1 and requested a written response from licensees:

1 To gather information with respect to NTTF Recommendation 2.1, as amended by SRM on SECY-1 1-0124 and SECY-1 1-0137, and the Consolidated Appropriations Act, for 2012, Section 402, to reevaluate seismic and flooding hazards at operating reactor sites, 2 To collect information to facilitate NRC's determination ifthere is a need to update the design basis and systems, structures, and components (SSCs) important to safety to protect the updated hazards at operating reactor sites, 3 To collect information to address Generic Issue (GI) 204 regarding flooding of nuclear power plant sites following upstream dam failures This report is prepared in response to the March 12, 2012, 50.54(f) letter to provide information on the reevaluation of external flooding hazards at STP 1 & 2 using present day methodologies, data and guidance. Flooding hazards from external sources for the STP site and vicinity have been evaluated recently in support of the Combined License Application (COLA) for future units (Units 3 and 4) (Reference 2) to be located immediately to the northwest of Units 1 & 2 within the plant's property boundary. The approach and methods used for the STP 1 & 2 external flooding reevaluation are the same as the COLA analyses, which are consistent with the standards and requirements of present-day regulatory and industry guides, in particular, NUREG/CR-7046 (Reference 3), NUREG 0800, JLD-ISG-2012-06 (Reference 4), and ANSI/ANS-2.8-1992 (Reference 5).The results of the flooding hazard reevaluation, augmented by recent site specific information, are compared to the current design basis for the plant, which is documented in the Updated Final Safety Analysis Report (UFSAR) (References 6 through 12).

Introduction .°.i

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP I & 2 Fukushima Response Project Introduction References

1. U.S. Nuclear Regulatory Commission, Request for information pursuant to Title 10 of the Code of Federal Regulations 50.54(f) regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, ADAMS Accession No. ML12053A340, March 12, 2012.

2. South Texas Project Units 3 & 4 Combined License Application (COLA), Final Safety Analysis Report (FSAR), Rev. 7, Nuclear Innovation North America LLC (NINA),

February 1, 2012.

3. U.S. Nuclear Regulatory Commission, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, NUREG/CR-7046, Office of Nuclear Regulatory Research, November 2011.

4. U.S. Nuclear Regulatory Commission, Guidance for Performing a Tsunami, Surge, or Seiche Hazard Assessment, Interim Staff Guidance JLD-ISG-2012-06, Revision 0, Japan Lessons-Learned Project Directorate, January 2013.

5. "Determining Design Basis Flooding at Power Reactor Sites," ANSI/ANS-2.8-1992, American Nuclear Society, July 1992.

6. STPEGS (Units 1 & 2) Updated Final Safety Analysis Report (UFSAR), Section 1.2, "General Plant Description," Rev. 16.

7. STPEGS (Units 1 & 2) UFSAR, Section 2.1, "Geography and Demography," Rev. 16.

8. STPEGS (Units 1 & 2) UFSAR, Section 2.4, "Hydrologic Engineering," Rev. 15.

9. STPEGS (Units 1 & 2) UFSAR, Section 3.4, "Water Level (Flood) Design," Rev. 13.

10. STPEGS (Units 1 & 2) UFSAR, Section 3.2, "Classification of Structures, Components, and Systems," Rev. 16.

11. STPEGS (Units 1 & 2) UFSAR, Section 9.2, "Water Systems," Rev. 16.

12. STPEGS (Units 1 & 2) UFSAR, Subsection 2.5.6, "Embankments and Dams," Rev. 16.

Introduction xiv

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushimna Response Project 1 Site Information Related to Flooding Hazards Site information for STP 1 & 2 described in subsequent sections is obtained from References 1.1-1 through 1.1-7.

STP 1 & 2 adopts the Mean Sea Level (MSL) as the plant's reference vertical datum, which is also referred to as the National Geodetic Vertical Datum of 1929 (NGVD 29) in this report.

Site Information Related to Flooding Hazards !-!

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP I & 2 Fukushima Response Project 1.1 Detailed Site Information 1.1.1 Location and Size of the Site As indicated in Reference 1.1-1, the STP 1 & 2 site is located in south-central Matagorda County west of the Colorado River, 8 miles north-northwest of the town of Matagorda and about 89 miles southwest of Houston. It consists of approximately 12,220 acres of land and includes areas being used for a plant, a railroad, and a cooling reservoir. Centerline coordinates for the reactors of Units 1 and 2 are 28047' 41.772" latitude and 96002' 53.079" longitude, and 28047' 41.922" latitude and 96002' 59.820" longitude, respectively. The plant is located about 12 miles south-southwest of Bay City and about 13 miles east-northeast of Palacios between FM (farm-to-market road) 1095 and the Colorado River (Figure 1.1-1).

1.1.2 Description of Plant Environs As indicated in Reference 1.1-2, the local relief of the plant area is characterized by fairly flat land, approximately at 23 ft MSL. Through the site boundary flows the west branch of the Colorado River as well as several sloughs, one of which feeds 34.4-acre Kelly Lake, which is in the northeast corner of the site. The site and its immediate environs fall within the Coast Prairie, which extends as a broad band parallel to the Texas Gulf Coast.

As indicated in Reference 1.1-3, ground slopes are minimal, and both deciduous and coniferous trees are sparsely scattered throughout the site. Surface elevations range from about El. 30 ft MSL at the north end of the site to between El. 15 ft and 20 ft MSL at the south end. Plant grade is at 28 ft MSL. Figure 1.1-2 shows the site layout.

A major feature of the site is the Main Cooling Reservoir (MCR), which is formed by a 12.4-mile-long earthfill embankment constructed above the natural ground surface, Reference 1.1-3. The MCR has a surface area of 7000 acres with a normal maximum operating level of 49 ft MSL.

The MCR is not a safety-related facility. Makeup water to the MCR is supplied from the Colorado River and pumped into the MCR intermittently throughout the year via the Reservoir Makeup Pumping Facility (RMPF). Blowdown to control MCR water quality is discharged back to the Colorado River. A spillway is provided to release flood waters resulting from direct rainfall on the MCR surface level.

A smaller separate cooling pond, referred to as the Essential Cooling Pond (ECP), serves as the ultimate heat sink for STP 1 & 2 and is a safety-related facility, Reference 1.1-6a. The ECP, a man-made excavated pond 9 ft deep with an 8-foot-high embankment completely surrounding its perimeter as described in Section 9.2 of the UFSAR (Reference 1.1-6a) and is shown in Figure 1.1-3b. The ECP is sized to have a 30-day water supply for the Essential Cooling Water System (ECWS) to support the safe shutdown of both units. Water circulation within the pond flows clockwise and is controlled by a central dividing dike (Reference 1.1-6b). The crest elevations of the circumferential embankment and the dividing dike are at El. 34 ft MSL and El.

38 ft MSL, respectively. Both the dividing dike and the southern embankment are seismic Category I in addition to being designed to withstand the effects of MCR embankment breach.

The remaining portion of the northern embankment is designed to withstand the effects of the Colorado River dam failures. Failures of the ECP embankments are therefore not expected, as described in the UFSAR for STP 1 & 2 (Reference 1.1-3). The normal operating elevation of the ECP is between El. 25.6 ft MSL and El. 26.0 ft MSL (Reference 1.1-6a, Subsection 9.2.5), about 2 ft lower than the plant grade of El. 28 ft MSL. The ECP contains a surface area of 46.5 acres DetailedSite Information !.1-1

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP I & 2 FukushiniaResponse Project and a storage volume of approximately 112,000,000 gallons (344 acre-ft) at El. 25.5 ft MSL (Reference 1.1-6a, Subsection 9.2.5).

1.1.3 Plan Layout and Topography The plan layout of STP 1 & 2 is provided in Figures 1.1-3a and 1.1-3b, which identifies the safety-related structures of the plant. Figures 1.1-3c and 1.1-3d provide aerial view and topographic details of the STP 1 & 2 and environs, respectively. Figure 1.1-3e provides topographic details at STP 1 & 2. As part of the STP 1 & 2 flooding walkdown (Reference 1.1-7),

a review of site topography was undertaken to identify changes to topography from that assumed in the flooding evaluation for the current design basis as well as features added that might affect site drainage (e.g., direction or flow). The reviews and walkdowns of site topography concluded that no major changes to the topography of the STP 1 & 2 site have occurred that could significantly affect site drainage. The addition of features, such as vehicle barriers for security purposes, were reviewed and found not to significantly affect site drainage.

1.1.4 Plant Description The station is composed of two units, each having an identical pressurized water reactor (PWR)

Nuclear Steam Supply System (NSSS) and turbine generator (TG). The units are arranged using a "slide-along" concept which results in Unit 2 being similar to Unit 1, and 600 ft away.

The NSSS is a Westinghouse Electric Corporation four loop PWR. The rated core thermal power of each unit is 3,853 MWt. Each unit was originally designed for a net electrical power output of 1,250 MWe at 3.5 in. Hg abs. back pressure.

The limited work authorization was granted on August 12, 1975, and nonsafety-related construction was initiated in September 1975. A Class 103 construction permit was granted on December 22, 1975.

Fuel loading was completed in August 1987 and December 1988 for Units 1 and 2, respectively.

Commercial operation was declared in August 1988 and June 1989 for Units 1 and 2, respectively.

1.1.5 Plant Safety-Related Structures The safety-related structures of the plant are listed below (Reference 1.1-5):

Reactor Containment Building (RCB)

Fuel-Handling Building (FHB)

Mechanical-Electrical Auxiliaries Building (MEAB)

Isolation Valve Cubicle (IVC)

Diesel-Generator Building (DGB)

Auxiliary Feedwater Storage Tank (AFST)

Essential Cooling Water Intake Structure (ECWIS)

Essential Cooling Water Discharge Structure Essential Cooling Pond (ECP), except for the northern embankment Figures 1.1-3a and 1.1-3b show the locations of the safety-related structures.

Detailed Site Information 1.1-2

Enclosure NOC-AE-13002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project 1.1.6 Major Hydrologic Features Details provided in this subsection are obtained from STP 3 & 4 COLA, Reference 1.1-8 (Subsection 2.4S.4).

Colorado River Basin:

The Colorado River Basin extends across the middle of Texas, from the southeastern portion of New Mexico to Matagorda Bay at the Gulf of Mexico. The total drainage area of the Colorado River is 42,318 sq. miles, 11,403 sq. miles of which is considered non-contributory to the river water supply. The Lower Colorado River Basin is the part of the river system from Lake O.H.

Ivie to the Gulf Coast and comprises approximately 22,682 sq. miles of drainage area. The Upper Colorado River Basin has a drainage area of approximately 19,636 sq. miles. There are six major tributaries with drainage areas greater than 1000 sq. miles that contribute to the Colorado River: Beals Creek and Concho River in the upper Colorado River Basin and San Saba, Llano, Pedernales Rivers, and Pecan Bayou in the lower Colorado River Basin. All six major tributaries, and approximately 90% of the entire contributing drainage for the river, occur upstream of Mansfield Dam near Austin. Downstream of Austin, there are only two tributaries with drainage areas greater than 200 sq. miles: Barton Creek and Onion Creek in Travis County.

Buchanan Dam and Mansfield Dam are the two major dams on the Lower Colorado River that may influence conditions at STP 1 & 2. Mansfield Dam, forming Lake Travis, is located approximately 28 miles upstream from Austin. Mansfield Dam is the largest reservoir and the most downstream existing major control structure on the Colorado River. Buchanan Dam is another large dam on the main stream of the Colorado River. Its primary purpose is water supply and generation of hydroelectric power. The Lower Colorado River Authority (LCRA) operates six dams on the Lower Colorado River: Buchanan, Inks, Wirtz, Starcke, Mansfield, and Tom Miller. These dams form the six Highland Lakes: Buchanan, Inks, LBJ, Marble Falls, Travis, and Austin.

Even though there are several dams upstream of Mansfield Dam, Mansfield Dam provides most of the floodwater storage capacity. The other dams pass floodwaters downstream to Lake Travis, where the water is stored in a flood pool until it can be released safely downstream. Tom Miller Dam at Austin is downstream of Lake Travis. It impounds a portion of the Colorado River known as Lake Austin; however, because of the small storage capacity of its reservoir, it affords no major control of flood flows. Lake Travis and Lake Buchanan also serve as water supply reservoirs. Lake Travis has a water supply storage capacity of approximately 1,132,400 acre-feet and Lake Buchanan has a water supply storage capacity of approximately 875,000 acre-feet. With a combined capacity of about 2 million acre-feet, the two lakes store water for communities, industry and aquatic life along the river, as well as supply irrigation water for the agricultural industry near the Gulf Coast.

Little Robbins Slough:

Little Robbins Slough is a significant hydrologic feature near the STP site. It is an intermittent stream located nine miles northwest of Matagorda in southwestern Matagorda County and runs south for 6.5 miles to the point where it joins Robbins Slough, a brackish marsh, which meanders four more miles to the Gulf Intracoastal Waterway. During the construction of the MCR for STP 1 & 2, the water course of Little Robbins Slough within the STP site was relocated DetailedSite Information 1.1-3

Enclosure NOC-AE-13002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project to a channel on the west side of the west embankment of the reservoir and rejoined its natural course about one mile east of the southwest corner of the MCR.

Adiacent Drainage Basins To the west of the Colorado River in the coastal area is the Colorado-Lavaca River Basin. This basin includes the Tres Palacios Creek, which is not tributary to either of those rivers. The Colorado-Lavaca River Basin drains into Tres Palacios Bay, north of Matagorda Bay. In the event of interbasin spillage, flood waters from the Colorado River Basin flow into Caney Creek near Wharton, as in the case of the 1913 flood, or into the San Bernard River Basin on the east edge of the Colorado River Basin, or into the Colorado-Lavaca River Basin on the west.

Shore Regions The STP 1 & 2 site is located 10.5 miles inland from Matagorda Bay and 16.9 miles inland from the Gulf of Mexico. It is approximately 75 miles from the Continental Shelf. Matagorda Peninsula is a classic microtidal, wave-dominated coast with a mean diurnal tide range of approximately 2.1 ft. The shore region is also greatly affected by waves generated by tropical storms and hurricanes.

1.1.7 References 1.1-1 STPEGS (Units 1 & 2) Updated Final Safety Analysis Report (UFSAR), Section 1.2, "General Plant Description," Rev. 16.

1.1-2 STPEGS (Units 1 & 2) UFSAR, Section 2.1, "Geography and Demography," Rev. 16.

1.1-3. STPEGS (Units 1 & 2) UFSAR, Section 2.4, "Hydrologic Engineering," Rev. 15.

1.1-4. STPEGS (Units 1 & 2) UFSAR, Section 3.4, "Water Level (Flood) Design," Rev. 13.

1.1-5. STPEGS (Units 1 & 2) UFSAR, Section 3.2, "Classification of Structures, Components, and Systems," Rev. 16.

1 .1-6a. STPEGS (Units 1 & 2) UFSAR, Section 9.2, "Water Systems," Rev. 16.

1.1-6b. STPEGS (Units 1 & 2) UFSAR, Subsection 2.5.6, "Embankments and Dams," Rev.

16.

1.1-7. STP Nuclear Operating Company, "Flooding Walkdown Summary Report For South Texas Project Units 1 &2," Attachment 1, NOC-AE-12002932, November 26, 2012.

1.1-8. South Texas Project Units 3 & 4 Combined Licensing Application (COLA), Final Safety Analysis Report (FSAR), Rev. 7, Nuclear Innovation North America LLC, February 1,2012.

DetailedSite Information 1.1-4

Enclosure NOC-AE-13002975 Flooding Hazard Reevaluation Report STP1 & 2 Fukushima Response Project Elevation Scale, ft 0 1 2 4 6 8 0-5 M 30-40 5-10 M 40-50 10-20 M 50-60 20-30 l 60-70 Figure 1.1-1 Site Region and Location DetailedSite Information 1.1-5

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project Figure 1.1-2 Site Layout DetailedSite Infornmation 1.1-6

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project Figure 1.1-3a STP 1 & 2 Safety-Related Structures DetailedSite Information 1.1-7

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STPJ & 2 Fukushima Response Project SOUTH TEXAS PROJECT UNITS 1 & 2 Figure 1.1-3b STP I & 2 Safety-Related ECP Embankments (Shaded Area on Plan View)

DetailedSite Information 1.1-8

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STPJ & 2 Fukushima Response Project Figure 1.1-3c Aerial View of STP 1 & 2 and Environs DetailedSite Information 1.1-9

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STPJ1 & 2 Fukushima Response Project Figure 1.1-3d Topographic Details of STP I & 2 and Environs (elevations are in MSL)

DetailedSite Information 1.1-10

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP I & 2 Fukushima Response Project Figure 1.1-3e Topographic Details at STP I & 2 Area Detailed Site Information 1.1-11

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project 1.2 Current Design Basis Flood Elevations The current design basis flood (CDB) elevations of the safety-related SSCs at STP 1 & 2 are governed by the maximum flood levels resulting from the postulated MCR embankment breach event, as documented in Table 2.4.4-3 of the Updated Final Safety Analysis Report (UFSAR) for STP 1 & 2 (Reference 1.2-1). The CDB flood elevations in the power block vary from a minimum of 44.5 ft MSL at the Diesel Generator Building and the north face of the Mechanical Electrical Auxiliaries Building to a maximum of 50.8 ft MSL at the south face of the Fuel Handling Building.

In the ECP, the CDB flood elevation was established to be 40.8 ft MSL at the essential cooling water intake structure (ECWIS).

The maximum flood levels for the flooding mechanisms applicable to the site, in addition to the design basis flood, are documented in the UFSAR and summarized in Table 1.2-1. In the determination of the maximum flood levels, coincident wind action was considered for the upstream dam failures only, because the still water level from the other flooding mechanisms, except for the MCR embankment breach, are below the still water level for the upstream dam failure event.

Table 1.2-1 Current Maximum Flood Levels for Applicable Flooding Mechanisms per UFSAR Flood Still Water Maximum Critical Level Flood Level ft Sei Flooding Mechanism Plant ft MSL MSL Section Structures Local Intense Precipitation Site 32.0 32.0 2.4.2 Flooding in Streams and Rivers Site 29.0 Not Determined 2.4.3 Plant 34.1 43.7 ') 2.4.4 Upstream Dam Failures Structures ECWIS 34.1 39.3'2) 2.4.4 Plant Plnt 4.5 to 50.8 44.5 to 50.8 2.4.4 MCR Embankment Breach Structures ECWIS 40.8 40.8 2.4.4 Storm Surge Site 26.74 Not Determined 2.4.5 Seiche Site N/A N/A 2.4.5 Tsunami Site N/A N/A 2.4.6 Ice Induced Flooding Site N/A N/A 2.4.7 Channel Migration or Diversion Site N/A N/A 2.4.9 Notes: (1) Includes a wind-wave runup.

(2) Includes wind set-up and wind-wave runup (based on still water level of 32.0 ft MSL).

N/A indicates that no impact was identified.

Site includes the plant structures (power block) and the ECP areas.

Maximum flood levels were not determined for non-controlling flooding mechanisms.

Current Design Basis FloodElevations 1.2-1

Enclosure NOC-AE-13002975 Flooding Hazard Reevaluation Report STP I & 2 Fukushima Response Project Details of the flooding mechanisms that correspond to the flood elevations are given below.

1.2.1 Local Intense Precipitation Two local drainage areas adjacent to the plant structures were considered for the flooding analysis due to local intense precipitation. Considering a probable maximum precipitation (PMP) of a point rainfall magnitude, a probable maximum flood (PMF) on either of these two adjacent areas would result in water levels in the plant area that would be above plant grade. The larger of these two areas lies west and northwest of the plant structures and contains 4.5 mi2 of land surface. This area drains into relocated Little Robbins Slough. The PMF from this area is estimated conservatively to have a peak discharge of 8,000 ft3/s. It would cause a water level of about 32 ft MSL at the site. The inundation of plant grade is calculated to be greater for the flood conditions due to dam breaches and failure (Subsection 1.2.3 of this report), and therefore it was concluded that local PMP flooding is not critical to flood design.

1.2.2 Flooding in Streams and Rivers PMF determinations on the Lower Colorado River were made for multiple conditions which represent the most critical hydrometeorological events, or combinations thereof, which may be expected to affect the STP 1 & 2 site. The PMF flows were derived based on PMP depths that were calculated according to the procedures outlined in Hydrometeorological Reports 33 and 51 (HMRs 33 and 51) (References 1.2-2 and 1.2-3).

The PMF was determined to be the spillway design flood (SDF) from the proposed Columbus Bend Dam routed to the STP 1 & 2 site and occurring in coincidence with a standard project flood (SPF) on the 755-mi2 uncontrolled area between Columbus Bend Dam and the STP 1 & 2 site. Addition of a base flow of 50,000 ft3/s, results in the critical PMF of 958,000 ft3/s at the STP 1 &2 site and a corresponding still water elevation of 29 ft MSL. The PMF still water level is 1 ft above the plant grade of 28 ft MSL.

The flooding resulting from upstream dam failures was found to be more critical than that resulting from the PMF. Therefore, coincident wind-wave activity was considered for flooding resulting from dam failures only.

1.2.3 Dam Breaches and Failures Two dam breaches and failures were considered. The first one is in regards to the upstream dam failures and the second is in regards to the MCR embankment breach. Details of both conditions are given below.

Upstream Dam Failures For upstream dam failures, two dam failure sequences were analyzed that were found to be critical among multiple dam failure permutations considered. Details of the two cases are given below.

Buchanan-Mansfield Dam Failures Case: Mansfield Dam at Lake Travis was assumed to fail after having received floodwaters from the failure of Buchanan Dam, and the resulting flood peak was routed downstream to the plant site, considering existing conditions, i.e., with the proposed Columbus Bend Dam not in existence (the dam had been in a proposal stage in the Current Design Basis Flood Elevations 1.2-2

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STP I & 2 Fukushina Response Project past). The river was assumed to be at the stage corresponding to a flow equal to the standard project flood (SPF) for the uncontrolled area above the Bay City USGS gauge. The maximum still water level obtained was 32 ft MSL at STP 1 & 2 site.

The maximum water surface elevation realized at the plant structures is the sum of the still water elevation plus any wind setup and wave runup. The wind-wave phenomena produced by a 2-year, 50-mph wind speed were investigated assuming several different directions. The maximum wind setup and wind-wave runup at the plant structures were obtained to be 1.6 ft and 9.8 ft, respectively. This gave a maximum water elevation of 43.4 ft MSL except for the ECWIS. The ECWIS is protected against direct wave attack from the western quadrant by the wide berm in front of the structure, and from the other directions by the hardened ECP dikes.

The maximum runup on the ECWIS was obtained to be 39.3 ft MSL.

Cascaded Dam Failure Case: All of the dams above the proposed Columbus Bend Dam failed and released their top-of-dam contents into the river. The failure hydrographs were assumed to arrive at Mansfield Dam with such timing as to maximize the breach outflow hydrograph from Mansfield, which was assumed to fail when overtopped by five feet of water. The antecedent flow between Mansfield Dam and Bay City was derived from the Columbus Bend SDF and the SPF on the area between Columbus Bend Dam and the STP 1 & 2 site. Although this implies that Columbus Bend Dam would be in place, no credit was taken for any possible attenuation it might produce. The initial river stage assumed was that corresponding to standard project flood (SPF) flow for the uncontrolled drainage area above the Bay City USGS gauge. The maximum water level obtained at STP 1 & 2 site was 34.1 ft MSL.

Because STP 1 & 2 site would be inundated for the cascaded dam failure case, there would be no shoreline against which wind setup could develop. Therefore, only wind-wave runup from a 2-year, 50 mph, fastest-mile wind was considered. The maximum water level at the plant structures (power block) was obtained to be 43.7 ft MSL after including wind-wave runup. For the ECWIS, the maximum water level, including wind-wave runup, was obtained to be 38.9 ft MSL (at eastern and southern faces).

MCR Embankment Breach In addition to the above-described postulated failures of upstream dams, an analysis was made of the instantaneous removal of varying lengths of the embankment of the MCR near the plant site.

The MCR is enclosed by a rolled-earth embankment rising an average of 40 ft above the natural ground surface south of the plant site. The centerline of the north embankment is 800 ft south of the centerline of the power block. Plant grade is at 28.0 ft MSL and the top of the embankment is 65.75 ft MSL. Normal maximum operating level of the reservoir is 49.0 ft MSL, which is 21 ft higher than plant grade.

The breach was postulated to occur in coincidence with the peak elevation in the reservoir realized from an SPF, which was determined to be 50.5 ft MSL. The maximum water surface elevations at the power block structures vary from 44.5 ft MSL (at the Diesel-Generator Building and the north face of the Mechanical-Electrical Auxiliaries Building) to 50.8 ft MSL (at the south face of the Fuel Handling Building) resulting from breach lengths in excess of 2,000 ft. In view of the fact that a breach length of approximately 2.000 ft forming instantaneously is an extremely Current Design Basis Flood Elevations 1.2-3

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP I & 2 Fukushima Response Project unlikely event, the maximum water surface elevations that vary between 44.5 and 50.8 ft MSL are highly conservative. The corresponding maximum water level at the ECWIS is 40.8 ft MSL.

1.2.4 Storm Surge The storm surge was determined based on the Probable Maximum Hurricane (PMH). The coincidental conditions (combined events) included an initial rise of water surface of 2.5 ft and astronomical high tide of 2 ft above mean low water (MLW). The method and numerical values for PMH parameters (central pressure index, the radius of maximum wind, the hurricane translation speed, and the peripheral pressure) were based on NOAA's Technical Report NWS23 (Reference 1.2-4).

The maximum storm surge elevation at the mouth of the Colorado River was calculated to be El.

25.08 ft MSL. To determine water surface elevation at the STP 1 & 2 site resulting from the maximum PMH storm surge concurrent with a 100-year flood in the Colorado River, a backwater profile was run. The water surface elevation at the site was determined to be 26.74 ft MSL, which is below the plant grade elevation of 28.0 ft MSL. The PMH is not considered to be a design basis event for maximum water surface elevation at the plant structures or for hydraulic forces against them.

1.2.5 Seiche The large water bodies in the immediate vicinity of the site are the Gulf of Mexico and Matagorda Bay, and seiche has not been considered as the controlling influence for these bodies of water. Other than floods on the Colorado River, hurricane storm surge is the dominant factor responsible for coastal area flooding. Therefore, the flooding at the site due to seiche effect was considered to be insignificant.

1.2.6 Tsunami The geoseismic potential of the coastal study area was investigated as a possible generating source of a tsunami. Evaluation of potential tectonic faults in the coastal region near the site indicated that there is no known source that might generate such a fault. The coastal plain consists of flat, gently sloping coastal terraces, approximately 50 to 75 miles wide. There is no dominant physical relief to precipitate potential landslides of any significant magnitude. There is also no active volcanism in the coastal plain area. In view of the foregoing, a probable maximum tsunami water level for the site was not calculated.

1.2.7 Ice Induced Flooding The river water temperature record of the USGS gauge at Wharton (October 1944 through September 1975, with some gaps) indicated that the lowest river water temperature recorded was 35 0F, which occurred on December 23, 1963 and on January 14, 1964. A probability analysis of the annual minimum water temperature showed that the minimum river water temperature of 32°F has a probability of occurrence of one in every 10,000 years. Because of the low probability of a 32 0F water temperature, and because the plant site is adjacent to the reach of the Colorado River which is subject to tidal effects, it is concluded that ice flooding is not a potential hazard at the STP 1 & 2 site.

CurrentDesign Basis FloodElevations 1.2-4

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STPI & 2 Fukushima Response Project 1.2.8 Channel Migration or Diversion Due to flood regulation by upstream reservoirs and the responsibility for channel stabilization and improvement delegated by United States Congress to United States Army Corps of Engineers (USACE), channel migration or diversion is not considered to be a significant factor to the safety of the STP 1 & 2 site.

1.2.9 Combined Effect Flood Combined effect of different flood causing mechanisms is discussed in Subsection 1.2.1 through 1.2.7, where applicable.

1.2.10 References 1.2-1 STPEGS (Units 1 & 2) Updated Final Safety Analysis Report (UFSAR), Section 2.4, "Hydrologic Engineering," Rev. 15.

1.2-2 "Seasonal Variation of the Probable Maximum Precipitation, East of the 105th Meridian for Area from 10 to 100 Square Miles and Durations of 6, 12, 24, and 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months ," Hydrometeorological Report No. 33, United States Weather Bureau, 1956.

1.2-3 "Probable Maximum Precipitation Estimates, United States East of the 105th Meridian," Hydrometeorological Report No. 51, United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), June 1978.

1.2-4. Schwerdt, R. W., Meteorological Criteria for Standard Project Hurricane and Probable Maximum Hurricane Windfields, Gulf and East Coast of the United States, NOAA Technical Report NWS23, National Weather Service, Sept. 1979.

CurrentDesign Basis Flood Elevations 1.2-5

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STP1 & 2 Fukushimna Response Project 1.3 Licensing Basis Flood-Related and Flood Protection Changes There are no changes to the licensing basis flood-related and flood protection of STP 1 & 2.

Licensing Basis Flood Related and Flood Protection Changes 1.3-1

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project 1.4 Watershed and Local Area Changes The STP 1 & 2 site is located in the floodplain of the estuarial portion of the lower Colorado River Basin about 8 miles inland from Matagorda, Texas (Figure 1.1-1). As a result of the location, the site is subjected to hydrometeorological events relevant to inland sites as well as to coastal sites.

The Colorado River Basin, shown on Figure 1.4-1, contains approximately 41,800 mi 2 and is oriented generally along a northwest-to-southeast direction. The 600-mile length of the basin extends from the southeastern portion of New Mexico to Matagorda Bay in southeast Texas at the Gulf of Mexico. The width of the basin increases from 85 miles in the upper portion to about 170 miles in the area of Stacey, Texas, then narrows to about 30 miles near Austin, Texas.

From Austin the basin gradually continues to narrow to about 4 miles wide near Matagorda, Texas. The upper portion of the basin lies in a flat, semi-arid region containing numerous closed catchments. This area contains approximately 12,880 mi 2 which do not contribute runoff to the Colorado River.

Changes in watershed and local area could affect flooding conditions caused by different flood causing mechanisms. Changes in watershed properties particularly affect the estimation of the PMF and upstream dam failure flooding. It is expected that watershed characteristics such as land cover to change through the years with the expansion of urban areas and change in land use. However, all the changes that may have occurred in the past are captured in the current flooding hazard reevaluation. The reevaluation takes into account the existing watershed conditions, which were incorporated through the hydrologic and hydraulic model calibration efforts. It should be noted that since the construction and operation of STP 1 & 2, there has been no major upstream dam or impoundment on the Colorado River or tributaries constructed or proposed.

Any changes in local area that may have occurred in the past, including the vehicle barrier installed for a security measure, are incorporated in the local intense precipitation analysis of the current flooding hazard reevaluation. In addition, the potential impact of the proposed future STP 3 & 4 was also considered in the reevaluation.

Watershed and Local Area Changes 1.4-1

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project Elevation scale, ft close to 0 0 - 295 295 - 528 528 - 761 761 - 994 m

E:3 994- 1227 1227 - 1539 1539- 2001 2001 -2700 2700- 3629 3629- 4718 0 10 2030 40 50 Figure 1.4-1 Colorado River Basin Watershed and Local Area Changes 1.4-2

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP I & 2 Fukushima Response Project 1.5 Current Licensing Basis Flood Protection and Mitigation Features Floods that could result from all causes, or combinations thereof, were analyzed for the Colorado River, coastal area, local site drainage, upstream dams and the MCR. The critical flood levels at STP 1 & 2 determined from these analyses, result from a postulated breach of a portion of the north embankment of the MCR which determine controlling levels for the power block and the ECWIS as discussed in Section 1.2. Safety-related structures and components are designed to withstand the flood levels from these postulated events.

Plant-area drainage facilities are designed to pass a 50-year rainfall event without interference with traffic mobility, structures or process areas. More intense precipitation may cause flooding of certain roadways and yard areas. To determine the flooding caused by a local PMP, the plant area was analyzed as an impoundment with the peripheral roads and rail spurs as the impounding dike. Considering the discharge due to over-the-road weir flow alone, as would be the case with an incapacitated underground drainage system, flood levels thus caused would not exceed 32.0 ft MSL. However, because the MCR embankment breach described in the UFSAR for STP 1 & 2 (Reference 1.5-1, Subsection 2.4.4) produces more critical flood levels at the plant, further detailed analysis of local drainage was not undertaken.

External Flood Protection Measures for Seismic CateQory I Structures:

The flooding due to a postulated Main Cooling Reservoir (MCR) embankment breach produces the maximum water levels around the power block structures as well as the controlling water elevations for buoyancy calculations. This is also the controlling phenomena in determining the maximum water level at the Essential Cooling Water Intake Structure (ECWIS). Studies and analyses on the MCR embankment have demonstrated that an adequate margin of safety can be maintained for all credible failure mechanisms as described in UFSAR for STP 1 & 2 (Reference 1.5-2, Subsection 2.5.6). Accordingly, mechanistic effects (such as scour and erosion) associated with a postulated failure of the MCR embankment were not evaluated.

The maximum flood elevations at the safety-related SSCs at STP 1 & 2 are governed by the postulated instantaneous MCR embankment breach event, as documented in Table 2.4.4-3 of the UFSAR (Reference 1.5-1). The maximum flood elevations in the power block vary from a minimum of 44.5 ft MSL at the Diesel Generator Building and the north face of Mechanical Electrical Auxiliaries Building to a maximum of 50.8 ft MSL at the south face of the Fuel Handling Building. In the ECP, the maximum flood elevation was established to be 40.8 ft MSL at the essential cooling water intake structure (ECWIS).

Total inundation of the ECP occurs only under the condition of MCR embankment breach and does not affect the safe shutdown capability of the plant. The maximum water level calculated to occur at the ECWIS is 40.8 ft MSL.

The safety-related structures, systems and components (listed in UFSAR for STP 1 & 2, Table 3.2.A-1 (Reference 1.5-3) are protected against the effects of external flooding by:

1. Being designed to withstand the maximum flood level and associated effects and remain functional (such as seismic Category I structures and the Category I auxiliary feedwater storage tank) or CurrentLicensing Basis Flood Protectionand Mitigation Features 1.5-1

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP J & 2 Fukushiina Response Project

2. Being housed within seismic Category I structures which are designed as in item 1, above.

Flood protection of safety-related structures, systems, and components is provided for postulated flood levels and conditions described in the UFSAR for STP 1 & 2, Section 2.4 (Reference 1.5-1).

Seismic Category I structures are designed to withstand the maximum flood levels by:

1. Having external walls and slabs of structures designed to resist the hydrostatic and hydrodynamic forces associated with surge-wave runup and steady-state water level.

2. Ensuring the overall stability of the total structure against overturning and sliding due to the hydrostatic and hydrodynamic forces associated with surge-wave runup and steady state water level, and

3. Ensuring that the total structure will not float due to buoyancy forces.

Figure 1.5-1 shows a general section through the plant. Figure 1.5-2 shows the seismic Category I building maximum steady-state water surface profile, and the corresponding relationship of sill elevations for entrances to seismic Category I buildings.

All exterior seismic Category I building openings are located above the maximum steady-state flood level or are equipped with watertight doors when located below this profile, with the exceptions of the opening for the truck bay in the radwaste loading area of the MEAB and the opening for the rail car access in the spent fuel cask loading area of the FHB. These areas are not protected from flooding because they do not have any safety-related systems and components located near or below the maximum flood level which is required to perform any essential function. In addition, the two areas are separated from the remainder of the building by walls which do not contain openings below the maximum water surface elevation corresponding to their location. The Tendon Gallery Access Shaftcover (TGAS) is provided with a watertight cover to prevent flood waters from entering the MEAB.

The safety-related equipment in the ECWIS is protected from the effects of the design basis flood. The personnel access doors on the west wall are watertight; all other doors and openings are above the flood level. The dividing walls and doors between the ECWIS compartments minimize the potential for the propagation of flooding from one compartment to another.

The three maintenance knockout panels in the exterior walls of the DGB, which are located below the maximum water surface elevation of 45.0 ft MSL, are watertight and designed for the hydrostatic forces. Each knockout panel allows access to only one of the three separate compartments within the structure, and only one panel may be removed at one time. The dividing walls between the compartments preclude propagation of flooding from one compartment to another.

Current Licensing Basis Flood Protectionand Mitigation Features 1.5-2

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project 1.5.1 References 1.5-1. STPEGS (Units 1 & 2) UFSAR, Section 2.4, "Hydrologic Engineering," Rev. 15.

1.5-2. STPEGS (Units 1 & 2) UFSAR, Subsection 2.5.6, "Embankments and Dams," Rev.

16.

1.5-3. STPEGS (Units 1 & 2) UFSAR, Section 3.2, "Classification of Structures, Components, and Systems," Rev. 16.

Current Licensing Basis Flood Protectionand Mitigation Features 1.5-3

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STPJI & 2 Fukushima Response Project 2

STP 1 &

SOUTH NORTH SOUTH TEXAS PROJECT UNITS 1 & 2 TYPICAL SECTION THRUUGII PLANT I

Figure 1.5-1 Typical Section Through Plant CurrentLicensing Basis Flood Protectionand Mitigation Features 1.5-4

Enclosure NOC-AE-1 3002975 FloodingHazard ReevaluationReport STP1 & 2 Fukushi/na Response Project NOTES:

'ENVELOPESARE THE LOCI OF POINTS OF MAXIMUM WATERSURFACE ELEVATION ATTAINED ALONGTHE PROFILES SHOWNAT ANYTIME FOLLOWINGTHE EWBAMOCNT BREAC, ASSUM-ING AN INITIAl. RESERVOIR WATERSURFACE ELEVATION OF 505 (SI*) FOR WATERLEVELS USED IN BUOYANCY CN.CULATIONS SEE TABLE3.4-1 I'*afld*llUl* lld I*A*L*T*' * &T*'*I* LITt*I*'*pHDL'*

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1 DESCRIPTION IELV. OOIN DESCRIPTION ELEV.

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'U2 I DG8MUCUT PMS 0W I ENTRANCE TO MEAB 35.5*

SOUTH TEXAS PROJECT 0Q,) DOORTO STORAGEDOOR AREA~SK AREA T STORAE ,ASKi '

Ik*Bl Dwý TwOP nLQ 6 JI VEN BON TP sf3.o- L.EGEND, WATERTIGHT KNOOOUT PAMEILS(.OSED UNITS 1& 2 EQUIPMET REMOVALNIOCKI(T PAANELS 26.0 (*19 OPENINGTO HVAC EA IN FH 53.25' EXCEPTDURINGCOLDSHUTDOWN.

,FOR

IWl * *'oll l RCA AUXILIARYACCESS DOOR(6.-2IIA.) T9 ffg) (MOTSNE*OWN DOORS TO ESSqNTI*AL COOL.IN 34.0. U WATERTIGHT DOOR MAXIMUM FLOW PROFILES

~

THROUGHTHE PLANT ENTRNCTo 0 5.' BAY ** OFRC" XT MEA.1B 5ULIV=AN TO *DOORS ONWESTSIDE OF SMITIUlES ENTRA"MTO3 Figure 1.5-2 Maximum Flow Profiles Through the Plant CurrentLicensing Basis Flood Protectionand Mitigation Features 1.5-5

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project 1.6 Additional Site Details The results from the flooding walkdown of STP I & 2 are provided in Reference 1.1-7. The walkdown was conducted in response to NRC's request for information pursuant to 1 OCFR 50.54(f) on March 12, 2012, requesting information related to the Near-Term Task Force (NTTF)

Recommendation 2.3. STP Nuclear Operating Company (STPNOC) used the industry developed, NRC-endorsed, guidance in NE112-07 (References 1.6-1 and 1.6-2) in designing and executing the flooding walkdowns, as indicated in its 90-day response to the NRC's 50.54(f) letter (Reference 1.6-3).

In summary, the flooding walkdown report (Reference 1.1-7) concluded that the flood protection/mitigation measures at STP 1 & 2 were found to be available, functional, and maintained, consistent with the current licensing basis. A few deficiencies were identified during the walkdowns, and these either have been or are being addressed via the site corrective action program. Specifically, no features or changes to the plant identified in the flooding walkdown will affect the flooding reevaluation approach and results as described in Section 2 of this report.

1.6.1 References 1.6-1. NEI 12-07, Revision O-A, "Guidelines for Performing Verification Walkdowns of Plant Flood Protection Features", May 31, 2012 (ML12172A038).

1.6-2. NRC Letter, Endorsement Of Nuclear Energy Institute (NEI) 12-07, "Guidelines for Performing Verification Walkdowns Of Plant Flood Protection Features," May 31, 2012 (ML12144A412).

1.6-3. STPNOC Letter, D. W. Rencurrel to NRC Document Control Desk, "Response to NRC Request for Information Pursuant to 10 CFR 50.54(f) Regarding the Flooding Aspects of Recommendation 2.3 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," June 5, 2012, NOC-AE-12002864 (ML12163A127).

Additional Site Details 1.6-1

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STPJ & 2 Fukushima Response Project 2 Flooding Hazard Reevaluation The flooding hazard reevaluation for most of flood causing mechanism was adopted from the STP 3 & 4 COLA FSAR, Rev. 7, Section 2.4S (Hydrologic Engineering) (Reference 2.2-1c),

after conducting an applicability assessment for each mechanism. Because of the proximity of STP 1 & 2 and the proposed STP 3 & 4, most of the results and conclusions from STP 3 & 4 COLA FSAR are also applicable to STP 1 & 2. The exceptions are listed below:

" Reevaluation of the local intense precipitation flooding hazard was specifically performed for STP 1& 2.

" For the upstream dam failure reevaluation, wind setup and wind-wave runup were determined specifically for STP 1 & 2 safety-related structures, although the still water level from STP 3 & 4 FSAR COLA was adopted.

" For the MCR embankment breach analysis, in addition to the STP 3 & 4 COLA FSAR, a qualitative study conducted for the STP 1 & 2 was also used.

" For the storm surge reevaluation, wind-wave runup (due to PMH) were determined specifically for STP 1 & 2 safety-related structures, based on the still water level predicted for the site using the ADCIRC model as documented in the STP 3 & 4 COLA FSAR Subsection 2.4.5.

" Reevaluation of the seiche flooding hazard within the ECP was specifically performed for STP 1 & 2.

" For the reevaluation of the ice effects, the FSAR COLA STP 3 & 4 results and conclusions were supplemented with additional data collected since the completion of the FSAR.

Details of flooding hazard reevaluation from each potential flooding mechanism and the applicable combined effect on the flooding due to multiple mechanisms are described in Sections 2.1 to 2.8.

FloodingHazard Reevaluation 2-1

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project 2.1 Local Intense Precipitation The evaluation of the flooding hazards on the safety functions of South Texas Project (STP)

Units 1 & 2 due to local intense precipitation, also referred to as local probable maximum precipitation (PMP), is described in this section. Guidelines detailed in U.S. Nuclear Regulatory Commission (NRC) NUREG/CR-7046 (Reference 2.1-1), NRC Regulatory Guide 1.59 (Reference 2.1-2), and ANSI/ANS-2.8-1992 (Reference 2.1-3) are the basis for the approach and methodology used in this reevaluation. The analysis is performed assuming underground storm drains and culverts, as well as roof drains are clogged and not functioning during the local PMP storm event.

The local PMP flood hazards for the STP Units 1 & 2 are determined by performing site-specific hydrologic and hydraulic analyses. Natural Resources Conservation Service (NRCS) methods simulated in the US Army Corps of Engineers (USACE) computer program HEC-HMS (Reference 2.1-4) are used to determine runoff hydrographs and peak discharges along identified flow paths. Water surface elevations and flow velocities are determined using the USACE computer program HEC-RAS (Reference 2.1-5).

The evaluation considers both the present day (i.e., existing) condition and the effect of future conditions, in particular, when the future Units 3 & 4 (Reference 2.1-6) are in place.

The effect of local intense precipitation to STP Units 1 & 2 was evaluated previously as documented in Subsection 2.4.2.3 of the UFSAR (Reference 2.1-7), which reported a water level of 32 ft MSL (also referred to as NGVD 29) at the site. The reevaluated maximum water level at the plant structures, as described in the following subsections, is 1 foot higher at about 33 ft MSL. The increase in the water level is attributed to a more conservative delineation of the contributing drainage area used in this reevaluation, and changes in the drainage pattern due to the incorporation of vehicle barriers as part of the plant's security system. However, the higher water level, 33 ft MSL, estimated in this reevaluation for the local PMP event will not cause any adverse impact to the safety functions of the plant because there are substantial margins between 33 ft MSL and the design basis flood levels of the plant. As evaluated in Subsection 2.4.4 of the UFSAR and in Subsection 2.3.2 of this report, STP Units 1 & 2 is protected to the design basis flood levels between 44.5 and 50.8 ft MSL at the power block structures and 40.8 ft MSL at the Essential Cooling Water Intake Structure (ECWIS), both controlled by the postulated failure of the Main Cooling Reservoir (MCR) embankment. In addition to a lower flood level, the duration of the inundation at the plant is expected to be shorter for the intense precipitation event on local drainage areas which are characterized by short times of concentration (on the order of a few minutes to less than 5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months as shown below) and corresponding short critical storm durations.

A separate local intense precipitation evaluation was conducted in support of the Combined License Application (COLA) of the future units (Units 3 and 4). This reevaluation for STP Units 1 & 2 adopts similar modeling tools and basic PMP rainfall depths as used in the COLA analysis (Reference 2.1-6).

2.1.1 Probable Maximum Precipitation Depths The design basis for the local intense precipitation event is the all season one sq. mile or point PMP as obtained from the U.S. National Weather Service (NWS) Hydro-meteorological Reports 51 and 52 (HMR 51 and HMR 52) (References 2.1-8 and 2.1-9). The estimated depths from Local Intense Precipitation 2.1-1

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushitna Response Project HMR 51 are for precipitation durations ranging from 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months to 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months and for drainage areas from 10 to 20,000 sq. miles. These estimates are used with procedures outlined in HMR 52 to determine PMP depths of various durations for a specific watershed based on the watershed size, location, shape, and orientation. The estimates derived for HMR 52 are only applicable to watersheds greater than 10 mi 2 . The STP Units 1 & 2 site drainage area is below the applicable range at about 4.93 sq. miles. HMR 52 provides procedures for estimating short duration, point (1 sq. mile) PMP depths for up to 1 hour1.157407e-5 days 2.777778e-4 hours 1.653439e-6 weeks 3.805e-7 months of rainfall duration based on the HMR 52 values.

HMRs 51 and 52 do not provide a formulation to derive the PMP rainfall depths for 2-hour and 3-hour durations, which are used as part of the meteorological input in the HEC-HMS model.

These two rainfall depths are estimated by a logarithmic fit of the rainfall depths for storm durations available from HMR 51 and HMR 52 as shown on Figure 2.1-1. Table 2.1-1 presents the one sq. mile PMP depths for various durations at the STP Units 1 & 2 site. The 6-hour PMP hyetograph used for HEC-HMS runoff modeling is shown in Figure 2.1-2.

A recent search of the climate event database indicates that there has been no occurrence of extreme storm in the region that exceeds the PMP depth since the publication of HMRs 51 and

52. Specifically, according to the National Climatic Data Center, the national record 24-hr rainfall is 43 inches occurring on July 25-26, 1979 at Alvin, Texas (Reference 2.1-10), which is bounded by the PMP depth of 47.1 inches (10 square miles, 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months ) at the STP Units 1 & 2 site as obtained from HMR 51 (References 2.1-8).

2.1.2 Drainage Areas and Local Drainage Parameters STP Units I & 2 site is located in Matagorda County, Texas, 15 miles southwest of Bay City, Texas as shown on Figure 2.1-3. The contributing drainage area to the site includes an offsite area (defined by the natural drainage divide for Little Robbins Slough (LRS)) north of the road named Farm to Market 521 (FM 521) and the area between FM 521 and the Main Cooling Reservoir (MCR) where the plant is located. The site and the surrounding areas are characterized by flat terrain and extremely mild slopes.

The STP Units 1 & 2 power block area is protected within a security perimeter, which includes a concrete vehicular barrier. The concrete barrier is 3.5 ft high above grade level and the barrier width is 10 feet. Drainage across the vehicle barrier is permitted through narrow openings and grated culverts as well as the openings for normal pedestrian and vehicle access on the east side. For the flooding reevaluation, these openings are conservatively assumed to be clogged.

The power block area contains a storm drainage network of catch basins, pipes, and ditches that collect and drain the stormwater runoff toward the east into two main drainage ditches that discharge south to southeast towards Kelly Lake. One of the ditches borders the north side of the Essential Cooling Pond (ECP), and the other one is located between the ECP and the MCR.

During a local PMP storm event, all catch basins, pipes, openings and culverts are assumed inoperative, and consequently, surface runoff from the power block area would be collected in the drainage channels as overflows. The site areas west of the North Access Road drain in the westerly direction through two main ditches into LRS. One of the ditches located north of the developed area west of STP Units 1 & 2 and the other one between the developed area and the MCR.

In addition to the existing conditions, the local PMP flooding reevaluation also considers potential changes in the contributing drainage area that may impact the flooding condition at the plant. Future conditions considered include the construction of the proposed STP Units 3 and 4 Local Intense Precipitation 2.1I-2

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP I & 2 Fukushima Response Project northwest of STP Units 1 & 2 and the proposed interim spent fuel storage installation (ISFSI) for Units 1 & 2 . The effects of the local PMP on the proposed STP Units 3 & 4 have previously been studied in support of the Combined License Application (COLA) (Reference 2.1-6). The overall site arrangement including the proposed location of Units 3 &4 is shown in Figure 2.1-3.

Further details of the safety related facilities in the Units 1 & 2 power block and the two alternative plans of the proposed ISFSI are shown in Figure 2.1-4.

The numerical rainfall-runoff model HEC-HMS, developed by the US Army Corps of Engineers (USACE) (Reference 2.1-4) is used for the hydrologic modeling of the local PMP event. Two considerations are made in the delineation of the drainage areas: (1) during the PMP event, most part of the FM 521 as shown on Figure 2.1-3, would conservatively act as a floodplain boundary to confine the flood flow to the main flow path near the plant; (2) around the intersection of FM 521 and East Access Road, flood flow are allowed to overtop FM 521 to more realistically represent the natural course of the flood wave.

As describe in the above, the site has two main drainage systems: one that drains southeast into Kelly Lake and the other one that drains southwest through Little Robbins Slough (LRS),

which drains areas that are located north of FM 521. The local divide between these two systems is the North Access Road which runs from north to south. This local divide works as such only under more frequent, regular, rainfall events of low discharge in which the topography typically governs the direction of the flow. However, because the terrain is practically flat, the divide for the two drainage networks is less definable during the local PMP event, and the hydraulic slope will determine the distribution of the flood flow in the two flow paths. One (1) sub-basin, SW, drains to the southwest hydrologic flow path, and the other thirteen (13) sub-basins drain and contribute to the southeast hydrologic flow path as shown on Figure 2.1-3. The areas west of the North Access Road, as well as the areas north of FM 521, would naturally drain to LRS; however, due to the flat terrain, the local divide is not well defined, especially during high runoff conditions as in a local PMP event. For this reevaluation, these areas are conservatively assumed to flow eastward towards the power block and the ECP to make sure the flooding impact on STP Units 1 & 2 are properly reflected and conservatively bounded. To account for water draining southwest through LRS, a lateral weir is simulated along the West Access Road in the HEC-RAS model. The total area that discharges to the southeast is 4.930 sq. miles; and to the southwest 0.382 sq. mile. Figure 2.1-3 shows the drainage area delineations; Figure 2.1-5 shows a schematic of HEC-HMS model; and Table 2.1-2 shows the details of each drainage area.

From the NRCS methodology (References 2.1-11 and 2.1-12), curve numbers for specific soil groups are defined. However, ANSI/ANS 2.8-1992 standard (Reference 2.1-3) requires the assumption that prior to the PMP event an event equivalent to the 40% PMP has occurred, with 3 to 5 dry days between the events, leaving the ground saturated. To simulate saturated ground conditions, all areas are conservatively assumed impervious. The NRCS runoff curve number for impervious surfaces is 98, regardless of the soil type.

The methodologies given in the NRCS Technical Report (TR)-55 (Reference 2.1-11) are used for the computation of time of concentration (Tc). The time of concentration flow paths are divided into three segments: sheet flow, shallow concentrated flow, and ditch flow. The surface roughness coefficients for the sheet flows are determined as 0.15, 0.06, and 0.011 for the surface conditions of short grass, cultivated soil, and pavement, respectively (Table 3-1 of Reference 2.1-11). The shallow concentrated flow is used for the portions between the sheet Local Intense Precipitation 2. 1-3

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project flow and the ditch flow with the travel time estimated by the velocity equations of V=16.1345(slope)0 5 for an unpaved surface condition and V=20.3282(slope)0 5 for a paved surface condition (Reference 2.1-11). The ditch flow is used for the channelized portions along the flow paths with the Manning's equation with an average channel flow velocity of 3 feet per second. The Tc values are reduced by 25% for the PMP event to account for the non-linearity effect during extreme flood condition in accordance with the guidelines from USACE (Reference 2.1-13). The hydrologic HEC-HMS model uses lag time instead of Tc. Lag time was estimated as 60% of Tc (Reference 2.1-14). The calculated time of concentrations and lag times are shown in Table 2.1-3. The lag method of the HEC-HMS model is used for routing the flow through the reach. The lag time for reach routing was estimated based on the reach length and an average channel flow velocity of 3 feet per second, which is comparable to the results from the HEC-RAS model. The lag times for reach routing are shown in Table 2.1-4.

2.1.3 Peak Discharges The HEC-HMS model is used to simulate the PMP storm event; and yields flood flow hydrographs for each drainage area, as well as, composite hydrographs for the overall system.

The peak discharges for the various drainage areas will be used as input to the HEC-RAS hydraulic model analysis to estimate the water levels and flow velocities at the designated flow paths. The results indicate that the peak discharge from the overall southeast system to Kelly Lake is about 24,900 cfs, and the peak discharge for the sub-basin, SW, which contributes to the stream, LRS, is about 5,050 cfs. A summary of the results is included in Table 2.1-5.

Figures 2.1-6a and 2.1-6b are the PMP flood hydrographs at Junctions US R2 and US R3 and their contributing sub-basins and reaches from the HEC-HMS model.

2.1.4 Hydraulic Model Setup The computer program HEC-RAS, developed by the USACE (Reference 2.1-5), is used to estimate the peak water levels in the STP Units 1 & 2 site area based on the peak PMP discharges estimated using the HEC-HMS model. The location of cross sections and drainage channels configuration used in the model are shown on Figure 2.1-7. Cross-section coordinates and stationing data are obtained from the surface topographic contours prepared from recent aerial survey data collected in support of the STP Units 3 &4 COLA and the USGS topographic map (Blessings SE Quadrangle) (Reference 2.1-16). The elevations used follow the vertical datum NGVD 29, also referred to as MSL.

The drainage pattern identified for the hydraulic model follows the expected and the most conservative flood movement behavior across the site during a postulated PMP rainfall event.

The drainage pattern adopted is characterized by a main flow path in the northwest to southeast direction which starts at an upstream location where LRS meets FM 521; follows the drainage ditch located at the future site for STP Units 3 &4 (Figure 2.1-3); continues over the North Access Road; and follows the ditch that borders the north side of ECP until it discharges to Kelly Lake. Between cross sections 8359 and 12809, the ECP and the mound to north divide the flood flow in three (3) potential flood courses: STP Main Channel 3, North Branch and South ECP Branch just downstream of STP Units 1 & 2. The hydraulic model uses the split flow option through junctions to simulate the distribution of flow. The North Branch runs around the mound located north of the ECP, and the South ECP Branch borders the south side of the ECP. The total length of the STP Main Channel is divided in 4 reaches due to the split and the flow junctions (Figure 2.1-7). To account for the flow heading southwest through LRS, an overflow through a 3,922-feet-long lateral weir representing the West Access Road is simulated in the Local Intense Precipitation Z. 1-4

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushimna Response Project model. This weir is located between cross sections 16218 and 18127. A portion of the channel that converges with LRS downstream of the West Access Road is modeled to account for potential backwater effects at the lateral weir. During the PMP event, the FM 521 is expected to be overtopped with the flood flow draining in directions away from the STP Units 1 & 2 site.

However, the hydraulic analysis conservatively assumes that the flood flow would be confined inside the floodplain boundary created by FM 521. Downstream of cross section 8359 the flood waters are allowed to overtop FM 521 following the topography to represent a more realistic flow pattern.

The HEC-RAS model simulation is performed for a steady state sub-critical flow condition for which the downstream boundary conditions used at both Kelly Lake and LRS are normal depth based on hydraulic slopes of 0.0003 and 0.0018 approximated by the downstream slopes of the southeast and southwest principal flow paths, respectively. The junction was modeled using the energy equation to obtain a balance in energy through the junction in the HEC-RAS model.

Water surface elevation convergence at the junctions is analyzed through iterations of flow optimization.

All culverts and stream channels are assumed to be blocked under the local PMP event, and outflow would take place only by overtopping of the roads. The four roads across the Main Channel flow path are treated as inline weir structures: (1) the North Access Road, (2) the old rail tracks, (3) the East Access Road and (4) FM 521 at Kelly Lake. In addition, the concrete vehicle barrier shown in Figure 2.1-7 at the power block was modeled as an inline weir structures on the South ECP Branch. Figure 2.1-8 shows HEC-RAS channel network and cross section schematic.

The peak discharges in the HEC-RAS hydraulic model are obtained at the following HEC-HMS model basin and junctions (Table 2.1-5): sub-basin SW, and Junctions US R4, US R3, US R2, US R1 and Kelly Lake. The peak discharge values at the cross sections between the HEC-HMS junctions are determined by prorating in accordance with the corresponding contributing drainage areas. The flow distribution at the three reaches downstream of Junction J3, STP Main 3, North Branch and ECP South Branch, is determined to be 62%, 19% and 19%, respectively, through an iteration process based on energy balance at the split.

The Manning's roughness coefficients in the model vary laterally across each cross section, because of the wide floodplain delineated. Depending on the surface cover of the drainage channels and floodplains, various Manning's n values are selected as recommended in Reference 2.1-15. Manning's n values of 0.040 and 0.033 were used for natural and excavated channels, respectively. As for the floodplains, 0.160 was used for dense brush; 0.050 for high grass; 0.0 16 for concrete and asphalt surfaces; and 0.025 for gravel surfaces. Table 2.1-6 shows more details on the Manning's roughness coefficients used in the model.

Buildings and other structures are accounted for with the use of obstructions. Ineffective areas in the HEC-RAS model are used to simulate areas of negligible flow upstream and downstream of buildings, inside the ECP, and to account for ponding inside the power block concrete vehicle barrier, and the flow path is confined toward downstream beyond the East Access Road between cross sections 3653 and 8121 by FM 251 and the MCR embankments. There are currently two proposed locations and footprints for the future ISFSI for STP Units 1 & 2. Both plans are included in the HEC-RAS model as building obstructions and ineffective areas to maximize the flooding impact at the site.

Local Intense Precipitation 2.1-5

Enclosure NOC-AE-13002975 FloodingHazard Reevaluation Report STP I & 2 Fukushlina Response Project The proposed STP Units 3 & 4, to be located approximately 2,000 feet northwest of STP Units 1

& 2, will have no adverse effect on the current flood water elevation estimated for STP Units 1 &

2 because of the sub-critical regime conditions. Moreover, the obstruction of flow caused by the new structures may have a backwater effect upstream of the new construction; promoting more flood flow over the LRS lateral weir in the west to southwest direction and reducing the discharge towards STP Units I & 2. Even if the Main Drainage Channel (MDC) around the proposed STP Units 3 & 4 site has been relocated just north of the STP Units 3 & 4 site, it will not adversely affect the results of the hydraulic analysis because the channel has insignificant conveyance during the local PMP storm event when flood flow will overtop the channel into the much wider floodplain. The effect of the future development of STP Units 3 & 4 is captured with the use of a conservative curve number of 98 for all sub-basins, which represents impervious surface conditions, a bounding runoff generation characteristics for all surfaces including developed areas.

For sensitivity evaluation, the main HECRAS channel is modeled without flow splitting between cross sections 8359 and 12809. The result show that the split flow approach provided more conservative water surface elevations at the power block area; 0.5 feet higher on average.

2.1.5 Effect of local PMP A summary of the HEC-RAS results which includes discharge, minimum channel elevation, peak water levels, channel velocity, and channel Froude numbers are provided on Table 2.1-7.

The PMP peak flood elevation at the STP Units 1 & 2 power block is 33 ft (NGVD 29). In comparison, UFSAR Subsection 2.4.2.3 (Reference 2.1-7) concluded that the local PMP water level at STP Units 1 & 2 site was 32 feet, NGVD29.

With a plant grade at 28 ft MSL, the facilities at STP Units 1 & 2 would likely be inundated during a local PMP event. However, there would be no adverse impact to the safety function of the plant because the maximum water surface elevation of 33 ft MSL predicted is much lower than the design basis flood elevations between 44.5 and 50.8 ft MSL at the power block structures and 40.8 ft MSL at, predicted for a postulated failure of the Main Cooling Reservoir (MCR) embankment. Flood protection measures currently in place for the safety-related facilities against MCR embankment breach are sufficient to provide protection against flooding impacts from a local PMP storm event.

The channel velocities around the STP Units 1 & 2 site including the ECP vicinity predicted by the HEC-RAS model are relatively slow in a subcritical flow regime as shown in Table 2.1-7.

There is no risk of the plant's safety functions being affected by scouring and erosion during a local PMP event.

The resulting water surface profiles along the model reaches including the three branches downstream of the split are illustrated in Figures 2.1-9a, 2.1-9b, and 2.1-9c. The HEC-RAS results at selected cross sections are shown in Figures 2.1-10a, 2.1-1Ob, and 2.1-1Oc that cross the STP Units 1 & 2 site area.

The possible impact on the flooding hazards at the STP Units 1 & 2 site as a result of future changes is considered as part of the local PMP reevaluation. Potential changes include the construction of the proposed STP Units 3 & 4 and the proposed ISFSI. The future Units 3 & 4 would be located approximately 2,000 feet northwest of Units 1 & 2 (Figure 2.1-3) on sub-basins

'STP5a' and 'STP5b'. The sub-basin delineation will not be significantly affected as the STP Local Intense Precipitation 2.1-6

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushitna Response Project Units 3 & 4 facilities only occupy a fraction of the drainage area of both sub-basins and the proposed grading would not affect the overall existing drainage flow paths. In addition, the conservative runoff curve number of 98 used in the HEC-HMS model for all the sub-basins to represent saturated ground conditions will adequately capture the future developed condition of the future units. Further, the effect of the proposed ISFSI is also included by the conservative incorporation of both plans of the installation as building obstructions and ineffective areas in the HEC-RAS model, thereby reducing the conveyance area available for flood flow to maximize the flooding impact.

2.1.6 References 2.1-1 Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, NUREG/CR-7046, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, November 2011.

2.1-2 Regulatory Guide 1.59, Design Basis Floods for Nuclear Power Plants, Revision 2, Office of Standards and Development, U.S. Nuclear Regulatory Commission, August 1977.

2.1-3 American Nuclear Society, ANSI/ANS-2.8-1992, Determining Design Basis Flooding at Power Reactor Sites, July 1992.

2.1-4 U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-HMS, Hydrologic Modeling System, Version 3.5, August 2010.

2.1-5 U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-RAS, River Analysis System, Version 4.1, January 2010.

2.1-6 South Texas Project Units 3 & 4 Combined Licensing Application (COLA), Final Safety Analysis Report (FSAR), Rev. 7, Subsection 2.4S.2, Nuclear Innovation North America LLC, February 1, 2012.

2.1-7 STPEGS Updated Final Safety Analysis Report (UFSAR) for Units 1 & 2, Section 2.4, Revision 15.

2.1-8 U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Hydrometeorological Report No. 51, Probable Maximum Precipitation Estimates, United States East of the 105th Meridian, June 1978.

2.1-9 U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Hydrometeorological Report No. 52, Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian, August 1982.

2.1-10 National Climatic Data Center (National Climate Extremes Committee) official website. Available at http://www.ncdc.noaa.gov/extremes/ncec, accessed on October 16, 2012 2.1-11 U.S. Department of Agriculture, Soil Conservation Service (now known as Natural Resources Conservation Service), Technical Release 55, Urban Hydrology for Small Watersheds, June 1986.

Local hitense Precipitation 2.1-7

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushinia Response Project 2.1-12 U.S. Department of Agriculture, Natural Resources Conservation Service (In Cooperation with Texas Agricultural Experiment Station), Soil Survey of Matagorda County, Texas, 2001.

2.1-13 U.S. Army Corps of Engineers, EM 11 10-2-1417Flood-Runoff Analysis, August 1994.

2.1-14 U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-HMS, Hydrologic Modeling System, Technical Reference Manual, August 2010.

2.1-15 Chow, Ven Te, Open-Channel Hydraulics, 1959.

2.1-16 U.S. Department of Interior, U.S. Geological Survey, Blessing SE Quadrangle, TX, 2010 Local Intense Precipitation 2.1-8

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 FukushirnaResponse Project Table 2.1 Short Duration PMP Depths PMP Duration & 6-hr, 10 l-hr, Point Source PMP Area mi2 Ratio Location Depth (in)

Ratio 72 hr, 10 mi' HMR 51 - Fig. 22 55.7 2

48 hr., 10 mi HMR 51 - Fig. 21 51.8 24 hr, 10 mi2 HMR 51 - Fig. 20 47.1 12 hr, 10mi 2 HMR 51 - Fig. 19 38.7 6 hr, 10 mi 2 HMR 51 -Fig. 18 32.0 3 hr Fitted From Fig. 2.1-1 29.7 2 hr Fitted From Fig. 2.1-1 26.6 1 hr., point location 0.62 HMR 52 - Fig. 23 19.8 30 min, point 0.73 HMR 52 - Fig. 38 14.5 15 min, point 0.50 HMR 52 - Fig. 37 9.9 5 min, point 0.32 HMR 52 - Fig. 36 6.4 Local Intense Precipitation Z.1-9

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project Table 2.1 Drainage Areas Area Sub-Basin (ni2)

STPI 1.099 STP2a 0.619 STP2b 0.108 STP3a 0.100 STP3b 0.017 STP3c 0.124 STP4a 0.072 STP4b 0.054 STP5a 0.571 STP5b 0.211 North 1 1.466 North 2 0.298 North B 0.191 Total 4.930 SW 0.382 Local Intense Precipitation 2.1-10

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STPJ & 2 Fukushina Response Project Table 2.1 Time of Concentration (Tc) Estimates Sheet Flow Shallow Concentrated Flow Ditch Flow (Tell) (Tc2) [Tc3)

Cale. 25%

Sub- Est. Time of Reduced Time Ve]..Cal.

Travel Travel Travel Cone Time of ae- Tme vencth Time Length T (min)

Telfst LenithTime RhrI (fpst Tim (hrS (hr) Conc. ( lIfnl LIft) n' S IftUftI TI (hr) LIft)V (fps) T r T2 (hr)

STPI 50 0.15 0.18 0.03 380 008 4.6 0.02 7955 3 0.74 0.79 0.59 21 STP2a 46 0.06 0.04 0.03 1947 00007 0.4 1.27 5870 3 0.54 I 84 1.38 50 STP2b 47 0.011 0.12 0 004 91 0.024 2.5 0.01 2026 3 0.19 0 20 0.15 5 STP3a 49 0.06 00007 0.14 2059 0003 09 0 65 2036 3 0 19 t097 0.73 26 STP3b 24 0 15 00015 0.12 192 00015 06 0.09 810 3 00S 0.28 0 21 8 2

STP3c 48 0.15 0.2 003 2898 0004 t3 Ct63 61t 3 0.06 1.71 053 1)

STP4a 27 0.15 0.002 0 12 1937 0.002 07 075 840 3 008 0.94 0 71 25 2

STP4b 49 0.011 0.001 0.03 597 000006 02 1 05 1471 3 0 14 1.22 091 33 STP5a 47 0.15 0.04 0.06 1199 0.0006 04 084 4566 3 0.42 I 32 0.99 36 STP5hh 49 0.011 0.0007 0t.04 3147 00013 0.7 1.19 783 3 0.07 1 30 fOt8 35 SW 51 0.011 0.0008 004 1339 0.0008 0.5 0.82 4626 3 0.43 1 28 0.96 35 North B 51 0.06 0.001 0 12 1274 0 0006 04 090 3001 3 0.28 1.30 0.97 35 North 1 100 0.06 0.001 021 4760 0.000315 03 462 10200 6 047 5.30 3 98 143 North 2 100 006 0.001 0.21 5000 0.0004 03 430 4060 6 0.19 4.70 3.53 127 t Reference 2.1-11.

2 Slopes for sub-basins North I, North 2 and North B are estimated from 5 ftcontours developed from the USGS 7.5 minute series quad map tBlessing SE Quad) (Reference 2.1-16), for the rest of the sub-basin areas, contours from aerial survey data collected for STP 3 & 4 COLA are used.

Assumed and compared to hydraulic model's average velocities. In general, the velocity value of 3 fps is conservatively greater than the average hydraulic model values.

Paved surfaces velocity equation (Reference 2.1- Il1 is used to calculate shallow concentrated flow on sub-basins STP3c, STP4b and STP5b.

Local Intense Precipitation 2.1-11

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STP1 & 2 Fukushima Response Project Table 2.1 Lag Time for Reach Routing Velocity Lag Reach Length i Time (ft) (fps) jmin" R1 8226 3 46 R2 3857 3 21 R3 804 3 4 R3a 347 3 2 R4 1343 3 7 R5 4938 3 27 R6 4106 3 23 Local Intense Precipitation 2.1-12

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STPI & 2 Fukushima Response Project Table 2.1 PMP Peak Discharges Runoff oTime Drainage Peak Discharge of Peak Volume Hydrologic 2

Area (mi ) (cfs) (in)

Element Kelly Lake 4.9297 24878.8 07Sep2012, 04:45 31.68 North I 1.4660 7975.8 07Sep20 12, 05:30 31.68 North 2 0.2980 1773.1 07Sep2012, 05:15 31.68 North B 0.1905 2510.2 07Sep2012, 03:40 31.68 RI 3.8308 22657.8 07Sep2012, 04:50 31.68 R2 3.1036 15892.9 07Sep2012, 04:10 31.68 R3 2.8622 13743.5 07Sep2012, 03:50 31.68 R3a 0.1244 2177.5 07Sep2012, 03:20 31.68 R4 2.7362 12399.9 07Sep2012, 03:50 31.68 R5 1.9545 10214.4 07Sep2012, 05:40 31.68 R6 0.1905 2507.3 07Sep2012, 04:00 31.68 STPI 1.0989 18690.0 07Sep2012, 03:25 31.68 STP2a 0.6189 6712.8 07Sep2012, 03:55 31.68 STP2b 0.1083 3007.0 07Sep2012, 03:05 31.68 STP3a 0.0996 1528.7 07Sep2012, 03:30 31.68 STP3b 0.0174 453.9 07Sep2012, 03: 10 31.68 STP3c 0.1244 2215.4 07Sep2012, 03:20 31.68 STP4a 0.0724 1129.1 07Sep20I2, 03:3(0 31.68 STP4b 0.0536 729.9 07Sep2012, 03:35 31,68 STP5a 0.5706 7418.2 07Sep2012, 03:40 31.68 STP5b 0.2111 2781.7 07Sep2012, 03:40 31.68 US RI 3.8308 22657.8 07Sep2012, 04:05 31.68 US R2 3.1036 15948.6 07Sep20l2, 03:45 31.68 US R3 2.8622 13753.4 07Sep2012, 03:50 31.68 US R4 2.7362 12421.2 07Sep2012, 03:45 31.68 US R5 1.9545 10220.3 07Sep2012. 05:15 31.68 US R6 0.1905 2510.2 07Sep2012, 03:40 31.68 SW 0.3825 5040.2 07Sep2012, 03:40 31.68 Start time at 00:00 hrs on September 7, 2012 was arbitrarily chosen.

Local Intense Precipitation 2.1-13

Enclosure NOC-AE-13002975 FloodingHazard Reevaluation Report STP I & 2 Fukushimna Response Project Table 2.1 Manning's Roughness Coefficient Values (Reference 2.1-15)

Used in HEC-RAS Model Cross Sections Channel/Floodplain Type Description Natural channel clean, straight full, no rifts or deep pools, more 0.040 stones and weeds Excavated channel excavated channels, earth straight and uniform with 0.033 short grass and few weeds Dense brush Floodplain floodplain, brush, medium to dense brush in summer 0.160 High grass Floodplain Floodplain, Pasture no brush, high grass 0.050 Concrete and Asphalt Surfaces Rough asphalt: Concrete float finish 0.016 From visual inspection, and considering the average Gravel Surfaces size of the gravel compared with the empirical "n" 0.025 values Local Intense Precipitation 2.1-14

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project Table 2.1 STP I & 2 Local PMP Maximum Water Levels Average River River Channel Invert Water Surface Channel Section Channel Froude River Reach Station Station Total Discharge Eleation Elevation Velocity Velocity Number (cfs) Ift, NGVD 29) (ft, NGVD 29) (ft/s) (ft/s I STPDrain Main 4 18127.19 18127 10408 28.5 33.5 0.5 0.6 0.04 STPDrain Main 4 18125 18125 Lat Struct STPDrain Main4 17608.24 17608 8423 28.6 33.3 0.5 0.6 0.04 STPDrain Main 4 16899.61 16900 3635 22.0 33 0 1.8 0.5 0.13 STPfDrain Main4 16217.9 16218 2715 23.0 33.0 0.6 0.2 0.04 STPDrain Main 4 15722.88 15723 3035 23.5 33.0 0.4 0.2 0.03 STPDrain Main 4 15205.69 15206 3335 24.0 33.0 0.6 0.3 0.05 STPDrain Main 4 14693.5 14694 3634 23.5 33.0 0.3 0.2 0.02 STPfDrain Main 4 14302.09 14302 3936 24.0 33 0 0.4 0.3 0.03 STPDrain Main 4 14200 14200 Inl Struct STPDrain Main 4 14134.82 14135 4046 24.3 33.0 0.3 0.2 0.02 STPDrain Main 4 13572.4* 13572' 4592 25.0 33.0 0.5 0.3 0.03 STPDrain Main 4 12990.43* 12990* 5269 26.3 33.0 0.4 0.3 0.03 STPDrain Main 4 12900* 12900* Int Struct STPDrain Main4 12809.13" 12809* 6029 23.7 33.0 0.8 0.3 0.05 STPDrain Main 3 12627.04 12627 4376 23.5 32.9 1.8 1.1 0.11 STPDrain Main 3 11914.82 11915 4668 23 5 32.6 4.4 1.8 027 STPDrain Main 3 11069.19 11069 5113 20.0 32.1 4.3 1.6 026 STPDrain Main 3 10241.5 10242 5553 21.9 320 2.4 0.7 0.15 STPDrain Main 2 9927.655 9928 10197 20.0 31.9 2.7 0.9 0.16 STPDramn Main 2 8897.002 8897 11109 19.0 31.7 2.2 0.9 0.13 STPDrain Main I 8359.243 8359 14173 19.0 31.6 0.9 0 STPDrain Main I 8200 8200 Inl Stnict STPDrain Main I 8120.527 8121 14212 18.9 31.5 1.3 0 STPDrain Main I 7222.287 7_22 14352 18.0 31.3 1.4 1.2 0.15 STPDrain Main I 6541.212 6541 14479 18.0 31 0 2.5 2.1 0.35 STPDrain Main I 5552.233 5552 14673 18,0 29.5 1.7 1.2 0.29 STPDrain Main I 4596.777 4597 14903 18.0 27.6 2.3 2.2 0.31 STPDrain Main I 3652.693 3653 15171 17.5 26.5 3.8 1.7 0.28 STPDrain Main 1 2537.857 2538 15567 170 25.8 2.0 1.8 0 14 STPDrain Main I 2123.587 2124 15852 15.0 25.7 1.4 0.9 0.09 STPDrain Main I 2000 2000 Inl Struct STPDrain Main I 1839.389 1839 15960 13 0 21.5 3 6 2.6 0.27 STPDrain Main I 975.2599 975 16164 12.5 21.4 1.5 1.2 0.09 STPDrain Main I 0 0 16394 12.5 21 I 2.6 2.2 0.16 South SouthECP 2824.041* 2824* 1313 28.0 33.0 1.0 0.9 0.12 South SouthECP 2265.737* 2266* 1604 29.2 32.9 1.4 1.1 0.18 South SouthECP 2095 2095 Inl Struct South SouthECP 1516.087 1516 1923 25.0 31.6 . 0.7 0 South SouthECP 893.9085 894 2239 24.9 31.6 0.6 0.7 0.04 South SoushECP 248.0091 248 2412 21.0 31.6 - 0.9 0 North North 4465.82 4466 1443 280 33.0 0.7 0.3 0.06 North North 3363.364 3363 2389 28.0 329 1.0 0.6 0.09 North North 2276.247 2276 3268 27.0 32.6 1.8 0.9 0.15 North North 1055.887 1056 4170 260 32.2 20 0.6 0.14 LRS Latmeir 3891.809 3892 721 20 5 33.2 0.2 0. I 0.01 LRS Latweir 3381.533 3382 9648 20.0 33 I 2.5 0.9 0 16 LRS Lamweir 2535.23 2535 (0755 19.5 32.9 2.5 0.9 0.15 LRS Latweir 1718.763 1719 11807 (9 5 32.6 3.1 1.2 0.2 LRS Latweir 873.0794 873 (2767 (.6 32.2 3.6 1.4 0.24 LRS Latweir 0 0 (3525 (8.0 30.8 7.6 2.2 0.47

Cross sections that intersect STP I & 2 power block. River station numbers are rounded up.

2' Channel area was blocked with ineffective flow areas.

Local Intense Precipitation 2.1-15

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project 60

From HMR-51 I 4

From HMAR-52 50-

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Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STPJ & 2 Fukushima Response Project 2.2 Flooding in Streams and Rivers The STP 1 & 2 site is located near the west bank of Lower Colorado River in Matagorda County, Texas, as shown in Figure 2.2-1a. The site is 12 miles south-southwest of Bay City, Texas and 8 miles north-northwest of Matagorda, Texas. There are a total of 68 dams with storage capacity in excess of 5000 acre-ft upstream of the STP 1 &2 site on the Colorado River and its tributaries, as discussed in Section 2.3. The Lower Colorado River Authority (LCRA) operates six of these dams that together form the six Highland Lakes: Buchanan, Inks, LBJ (with Wirtz Dam), Marble Falls (with Starcke Dam), Travis (with Mansfield Dam), and Austin (Tom Miller Dam), Reference 2.2-1a. The Highland Lake System was designed for flood management, water supply management, and hydroelectricity generation purposes. These lakes on the Colorado River are shown in Figure 2.2-1b.

Subsection 2.4.3 of the STP 1 & 2 UFSAR (Reference 2.2-1 b) documented the Probable Maximum Flood (PMF) study for the Lower Colorado River with a predicted PMF still water level of 29 ft MSL (also referred as NGVD29 vertical datum). In this section, the PMF in the Lower Colorado River is reevaluated to assess the flooding hazard on the safety-related facilities at STP I &2.

The reevaluation adopts the approach and methodology of the probable maximum flood study prepared in support of the STP 3 & 4 Combined License Application (COLA) (Reference 2.2-1c). The STP 3 & 4 COLA PMF analysis provides a detailed assessment of streams and river flooding at the STP site using the most current data available and the industry standard numerical modeling tools, HEC-HMS and HEC-RAS of the U.S. Army Corps of Engineers (USACE) (Reference 2.2-17 and 2.2-18). A recent review in 2012 of the COLA analysis indicates that it still provides a bounding analysis and meets the objectives of using present-day methodology and data for the flooding reevaluation. In particular, the STP 3 & 4 COLA analysis follows the guidance of ANSI/ANS-2.8-1992 (Reference 2.2-13) on the assessment of PMF and the specification of the antecedent and combined event conditions, consistent with the recommendations of NUREG/CR-7046.

The probable maximum precipitation (PMP) initiating event for the PMF evaluated in the STP 3

&4 COLA was defined in accordance with National Weather Service (NWS)

Hydrometeorological Reports (HMRs) 51 and 52 (References 2.2-10 and 2.2-11). A recent search of the climate event database indicates that there has been no occurrence of extreme storm in the region since the publication of HMRs 51 and 52 that exceeds the PMP depth.

Specifically, according to the National Climatic Data Center, the national record 24-hr rainfall is 43 inches occurring on July 25-26, 1979 at Alvin, Texas (Reference 2.2-29), which is bounded by the PMP depth of 47.1 inches (10 square miles, 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months ) at the STP site as obtained from HMR 51 (Reference 2.2-10). In addition, since the publications of HMRs 51 and 52, no record storm and associated flood occurred near the STP site and the contributing watershed in the Lower Colorado River basin, according to the streamflow measurements at Bay City and Wharton (References 2.2-30a and 2.2-30b). The record Alvin storm event appears to be a local system as 1979 is not a record flood year, outside of the top 10 historical floods, for the Lower Colorado River basin. Further, the annual peak flow in 1979 at Bay City and Wharton occurred in June, earlier in the year than the Alvin storm.

Since the completion of the COLA PMF analysis in 2007, there has been no major upstream dam or impoundment on the Colorado River or tributaries constructed or proposed, and no change to the hydrologic and hydraulic properties of the watershed evaluated, as well as those Floodingin Streams and Rivers 2.2-1

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukiushima Response Project of the affected channel and floodplain, is identified or expected. There is also no new hydraulic control or modifications on the Lower Colorado River that could potentially affect the boundary conditions used in the HEC-RAS model simulation.

As part of the PMF evaluation for STP 3 & 4 COLA, several publicly available flood hydrologic studies performed on the Lower Colorado River basin from 1985 to 2002 (References 2.2-1b to 2.2-8) by STPNOC, Federal, State, and other local agencies were reviewed to establish the combination of events that constitute the probable maximum flood condition at the location of STP 1 & 2.

The following probable maximum flood studies were reviewed:

Possible PMF scenarios considered for STP 1 & 2 and reported in the Updated Final Safety Analysis Report (UFSAR) (Reference 2.2-1b).

PMF estimates and dam safety evaluation studies for Mansfield Dam by the United States Bureau of Reclamation (USBR) (References 2.2-2, 2.2-3, and 2.2-4) and others (References 2.2-5 and 2.2-6).

Dam safety evaluation project for the six Highland Lakes in the Lower Colorado River (from Lake O.H. Ivie to Mansfield Dam), Phase II (Preliminary Design and Final Design),

by Freese & Nichols Inc. (Reference 2.2-7).

" Flood damage evaluation project for the Lower Colorado River (from Lake O.H. Ivie to the Gulf of Mexico at Matagorda Bay) by Halff Associates Inc. (Reference 2.2-8).

A brief overview of each of these studies is given in Subsection 2.2.4.1.

The possible PMF scenarios considered in the STP 1 & 2 UFSAR analysis (Reference 2.2-1 b) were assessed for their applicability to the present and forecast future conditions of the Lower Colorado River based on information provided in the hydrologic studies reviewed (References 2.2-2, 2.2-7, and 2.2-8) and in the Region "K" Plan of the 2007 State Water Plan adopted by the Texas Water Development Board (TWDB) (see Section 8.4.2 in Reference 2.2-23). Based on this assessment, the following three possible PMF scenarios were selected to reevaluate the maximum flood elevation caused by river and stream flooding at the STP 1 & 2 site.

Scenario 1 The flow resulting from the PMF for the drainage area between Mansfield Dam and the Bay City United States Geological Survey (USGS) gauging station (see Figure 2.2-1c) combined with an antecedent storm equal to 40% of the PMP occurring over the same drainage area (3555 sq. mi), three days before the PMF. This combined flow is added to the flow release of 90,000 cfs from Mansfield Dam and to the base flow at Bay City to determine the peak PMF flow for this scenario.

The flow release from Mansfield Dam is added to this scenario to accommodate any rainfall contributions upstream of the Mansfield Dam during the PMF event. The "Water Management Plan for the Lower Colorado River Basin" (Reference 2.2-26) states that "if the reservoir level is forecast to exceed 714 ft MSL but not to exceed 722 ft MSL: release will be made at 90,000 cfs" from the Mansfield Dam. It also states that "if the reservoir level is forecast to exceed 722 ft MSL, the Bureau of Reclamation will schedule releases as required for the safety of the structure."

Floodingin Streams and Rivers 2.2-2

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 FukushiinaResponse Project Scenario 2 The flow resulting from the PMF inflow hydrograph to Mansfield Dam, generated by the PMP storm over the watershed upstream of the dam (from Lake O.H. Ivie to Mansfield Dam), routed through Lake Travis and combined with the flood hydrograph from a sequential storm equal to 40% of the PMP occurring over the drainage area between Mansfield Dam and Bay City (3555 sq. mi), three days after the PMP storm upstream of Mansfield Dam. This combined flow is added to the base flow at Bay City to determine the peak PMF flow for this scenario. The total contributing drainage area for this scenario is about 18,197 sq. mi.

Scenario 3 The flow resulting from the PMF for the entire Lower Colorado River basin area between Lake O.H. Ivie and Bay City (18,197 sq. mi) combined with the flood hydrograph from an antecedent storm equal to the Standard Project Storm (SPS)1 for the same drainage area occurring three days before the PMF. This combined flow is added to the base flow at Bay City to determine the peak PMF flow for this scenario. This scenario does not account for the storage effect of Lake Travis at Mansfield Dam. The total contributing drainage area for this case is about 18,197 sq. mi.

From these three possible PMF flow scenarios, the most critical flow scenario, which produces the highest PMF peak at the Bay City gauging station, is selected to evaluate flooding potential at the STP 1 & 2 site. The Bay City gauging station is located about 18 miles upstream from the STP reservoir makeup pumping facility located on the west bank of Lower Colorado River (see Figure 2.2-1a). The discussions on the PMF developments of these three scenarios are given in Subsection 2.2.4.2.

Failure of the upstream dams was not considered as part of these probable maximum flood scenarios. The reevaluation of potential hydrological dam failures is discussed in Section 2.3.

2.2.1 Probable Maximum Precipitation (PMP)

PMP depths for the drainage basins upstream of STP 1 & 2 were derived following the procedures described in the National Weather Service (NWS) Hydrometeorological Reports 51 and 52 (HMRs 51 and 52) (References 2.2-10 and 2.2-11). The PMP estimates obtained from the HMRs 51 and 52 procedures are location-specific and have accounted for orographic and seasonal effects.

The PMP storm spatial distribution, centering, and orientation pattern adopted for the drainage basin upstream of the STP 1 & 2 site were determined as follows:

The storm spatial distribution for the PMP was selected based on the procedures in HMRs 51 and 52 (References 2.2-10 and 11), as discussed in detail in Subsection 2.2.4.1.4.

1ANSI/ANS 2.8 (Reference 2.2-13) states that the antecedent storm should be equal to 40% of the PMP or the 500-yr storm, whichever is less. However, for this scenario, the SPS event is adopted conservatively as the antecedent storm, considering the fact the SPS event produces a higher flood peak compared to the 500-yr event (see Vol. Il-B, Chapter 4, Table VI-7, Reference 2.2-8).

Floodingin Streams and Rivers 2.2-3

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushiina Response Project

A critical storm centering approach that produces the largest peak flow rate at the STP 1

& 2 site for the PMP event was determined in a manner that maximizes the volume of precipitation within the basin, as discussed in detail in Subsection 2.2.4.1.4.

Two different storm orientation patterns were analyzed for the Lower Colorado River basin to derive the most critical PMF flood hydrographs at STP 1 & 2 site. A detailed description of the orientation patterns used for the analyses is provided in Subsection 2.2.4.1.4.

Halff Associates Inc. adopted a storm duration of 96 hours0.00111 days 0.0267 hours 1.587302e-4 weeks 3.6528e-5 months for the rainfall hyetographs used for the Lower Colorado River flood damage evaluation study (see Vol. II-B, Chapter 4, pg. 18 of Reference 2.2-8). The Halff study stated that "a 96-hour storm duration was selected because the upper basin peak could travel to Mansfield Dam during that period and storm events below Mansfield Dam could reach Wharton 2 by the end of the storm event." The UFSAR also adopted the 96-hour storm duration for the PMP hyetograph used for the STP 1 & 2 site (Reference 2.2-1b). Thus, a 96-hour storm duration was selected for the PMP hyetograph developed for the STP 3 & 4 COLA analysis and is considered appropriate for the STP 1 & 2 PMF reevaluation.

The time distribution of the 96-hour rainfall hyetographs adopted in the Halff study was used to derive PMP hyetographs for the drainage basins upstream of the STP site in Subsection 2.2.4.2.1.

Previous investigations of the Probable Maximum Flood (e.g., Reference 2.2-2, p. 5) have noted that frequent and intense rainfall events occurring simultaneously over several sub-basins of the Colorado River have produced the largest recorded floods in the watershed. The occurrence of flooding from snow melt or antecedent snowpack was not considered a factor in the PMF analysis of Mansfield Dam by the US Bureau of Reclamation (Reference 2.2-2), the Halff study (Reference 2.2-8), or 2006 Region K Water Plan (Reference 2.2-23). Therefore, snow melt and antecedent snow pack are not considered as factors in PMF flooding of the STP site.

The PMP estimates for the subbasins between Mansfield Dam and the STP 1 & 2 site are provided in Subsection 2.2.4.2.1.

2.2.2 Precipitation Losses The rainfall-runoff analysis requires estimation of initial rainfall loss and constant rainfall loss rate to determine the direct runoff hydrograph corresponding to the excess rainfall (i.e. total rainfall minus rainfall loss). The initial rainfall loss quantifies the amount of infiltrated or stored rainfall before surface runoff begins. The constant rainfall loss rate determines the rate of infiltration that will occur after the initial loss is satisfied. Conservative assumptions were made for initial and constant loss rates to account for absorption and wet watershed antecedent conditions that would maximize the PMF peak flow, as discussed in detail in Subsection 2.2.4.2.1.

2 It should be noted that the difference in drainage areas at Wharton (30,600 sq. mi) and Bay City (30,837 sq. mi) gauging stations is less than 1% of the contributing drainage area at Bay City (Vol. II-B, Chapter 4, Table IV-1 of Reference 2.2-8).

Floodingin Streams and Rivers 2.2-4

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project 2.2.3 Runoff and Stream Course Models The PMF hydrograph for the drainage area between Mansfield Dam and Bay City was estimated using the HEC-HMS model developed by Halff Associates Inc. (Reference 2.2-8).

This model included the calibrated rainfall losses (i.e. initial loss and constant loss rate) and the Snyder unit hydrograph parameters (i.e. basin lag-time and peaking coefficient) for each of the subbasins located between Mansfield Dam and Matagorda Bay (see Vol. II-B, Chapter 4, Attachment B-1 of Reference 2.2-8). A total of 80 subbasins are included in the lower part of the river basin from Mansfield Dam to Matagorda Bay as shown in Figures 2.2-2a and 2b. The subbasin drainage areas (between Mansfield Dam and Bay City) and the calibrated Snyder unit hydrograph parameters used for the analysis are presented in Subsection 2.2.4.2.1.

Because the Halff HEC-HMS model was calibrated only for floods up to the 100-year storm event, the calibrated unit hydrograph basin lag-time parameter for each subbasin was conservatively decreased by 25% to account for non-linearity effects in the runoff process under extreme flood conditions such as the PMF based on recommendations stated in the United States Army Corps of Engineers (USACE) EM 1110-2-1417 (Reference 2.2-24). The modified unit hydrograph basin lag-time parameters used for the PMF analysis are presented in Subsection 2.2.4.2.1, for the subbasins from Mansfield Dam to Matagorda Bay.

In the Halff HEC-HMS model, the flow routing from an upstream reach to a downstream reach was performed using the modified Puls method, which defined a storage-outflow rating curve for each of the channel reaches in the model (see Vol. II-B, Chapter 4, pg. 26 and pg. 29 of Reference 2.2-8). Because the Halff HEC-HMS model (developed for the lower part of river basin) was calibrated only for floods up to the 100-year storm event, it was also necessary to revise the storage-outflow channel rating curves for a few channel reaches between Mansfield Dam and Matagorda Bay to accommodate the PMF conditions. Only three out of a total of 58 channel rating curves needed to be revised. The rating curves for these three channel reaches were extended by linear extrapolation.

2.2.4 Probable Maximum Flood Flow 2.2.4.1 Previous Hydrologic Studies for Lower Colorado River The following publicly available hydrologic and hydraulic studies performed for the Lower Colorado River basin by Federal, State, and other local agencies were reviewed in detail to determine PMF conditions in Lower Colorado River and their potential to flood the facilities at STP I & 2. These studies were listed in the beginning of Section 2.2 and are discussed in detail in this subsection.

2.2.4.1.1 PMF Flow Scenarios at the STP 1 & 2 Site - UFSAR The UFSAR prepared for the existing STP 1 & 2 (Reference 2.2-1b) evaluated five hydro-meteorologically critical flow scenarios for the Lower Colorado River and selected among these the most critical PMF flow scenario to determine the maximum flood elevation at the STP 1 & 2 site. This study also included a proposed dam at Columbus Bend that was under consideration in the 1960s. These five PMF flow scenarios are summarized as follows (Reference 2.2-1 b):

The Spillway Design Flood (SDF) for the proposed Columbus Bend Dam, which would result from a Probable Maximum Precipitation (PMP) storm on the watershed above the Floodingin Streams and Rivers 2.2-5

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP J & 2 FakushimaResponse Project dam, was routed to the STP 1 & 2 site. It was assumed that this event would occur in coincidence with the peak of a Standard Project Flood (SPF) from the 755 sq. mi drainage area between the proposed Columbus Bend Reservoir and the STP 1 & 2 site.

It was assumed that the peaks of these two floods would be directly additive and that they would occur simultaneously with a base flow of 50,000 cfs. The peak flow at the STP 1 & 2 site for this scenario was estimated to be equal to 958,000 cfs (SDF: 648,000 cfs + SPF: 260,000 cfs + base flow: 50,000 cfs).

The PMF for the drainage area between Mansfield Dam and Bay City was assumed to occur three days after the occurrence of the SPF over the same area. A base flow of 50,000 cfs was adopted. The peak flow at Bay City for this scenario was estimated to be equal to 913,000 cfs, which includes a base flow of 50,000 cfs.

The SDF outflow hydrograph from Mansfield Dam, which results from the PMF inflow hydrograph into Lake Travis caused by a PMP storm on the watershed above the dam, was routed to the STP 1 & 2 site. This was combined with a SPS occurring over the drainage area between Mansfield Dam and the STP 1 & 2 site, three days after the PMP storm producing the Mansfield Dam SDF. It was also assumed that a base flow of 50,000 cfs occurs simultaneously with the resulting flood. The peak flow for this scenario was estimated to be equal to 698,000 cfs, which includes a base flow of 50,000 cfs.

The PMF for the drainage area between the proposed Columbus Bend Dam and the STP 1 & 2 site was assumed to occur in coincidence with an SPF peak discharge from the proposed dam. It was also assumed that a base flow of 50,000 cfs occurs simultaneously with the resulting flood. The peak PMF for this scenario was estimated to be equal to 894,000 cfs, (PMF: 520,000 cfs + SPF: 324,000 cfs + base flow: 50,000 cfs) at the site.

" A hypothetical PMF for the entire contributing drainage area of the Lower Colorado River basin above Bay City was assumed, with no credit taken for flood control in the numerous reservoirs upstream from Mansfield Dam, including Lake Travis. The peak PMF at Bay City for this scenario was estimated to be equal to 1,750,000 cfs.

The PMF flows in the UFSAR for STP 1 & 2 (Reference 2.2-1b) were derived based on PMP depths that were calculated according to the procedures outlined in Hydrometeorological Reports 33 and 51 (HMR 33 and 51) (References 2.2-9 and 2.2-10).

2.2.4.1.2 PMF at Mansfield Dam - USBR and Others The most recent publicly available PMF inflow hydrograph into Mansfield Dam was established in November 1985 by the USBR (Reference 2.2-2) and was developed using the procedures outlined in HMR 51 and 52 (References 2.2-10 and 2.2-11). According to the USBR study, the peak PMF inflow into Mansfield Dam was estimated to be equal to 931,600 cfs.

In July 1989, ATC Engineering Consultants Inc. (ECI) prepared a dam safety evaluation report (Reference 2.2-5) for Mansfield Dam, using the PMF inflow hydrograph established by the USBR in 1985. This report concluded that when all the bottom outlet gates are closed, the PMF outflow (or SDF) hydrograph has a peak discharge of 602,210 cfs with maximum reservoir water surface elevation at 750.28 ft NGVD29 (also referred to as MSL vertical datum) (Reference 2.2-5).

Flooding in Streams and Rivers 2.2-6

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 FukushimaResponse Project In March 2003 (Reference 2.2-3), the USBR reviewed the spillway of Mansfield Dam for its design, analysis, and construction features and confirmed the PMF hydrograph with a peak inflow of 931,600 cfs, which was established in 1985 (Reference 2.2-2).

The USBR official website states that the PMF inflow to Mansfield Dam is 931,600 cfs (Reference 2.2-4), i.e. the same as that published by USBR in November 1985.

2.2.4.1.3 Dam Safety Evaluation for Highland Lakes - Freese & Nichols In August 1992, Freese & Nichols Inc. (Reference 2.2-7) performed a dam safety evaluation study for the six Highland Lakes in the Lower Colorado River, including Lake Travis at Mansfield Dam, as part of the dam safety compliance study for the Lower Colorado River Authority (LCRA). The watershed area for this study extended from the Lake O.H. Ivie Reservoir to Mansfield Dam and was divided into 41 subbasins (Reference 2.2-7).

The computer models used for this study included HMR 52 for PMP estimates, HEC-1 for rainfall-runoff analysis, and NETWORK for runoff routing (Reference 2.2-7). The subbasin unit hydrographs and rainfall loss rates used in the HEC-1 model and channel roughness coefficients used in the NETWORK model were calibrated based on selected historical flood events in the Lower Colorado River.

In the Freese & Nichols study, PMF levels for the six Highland Lakes, including the PMF at Mansfield Dam, were calculated using the PMP depths derived from procedures outlined in HMR 51 and 52 (References 2.2-10 and 2.2-11). In computing the peak PMF water level at Mansfield Dam, an antecedent storm event was routed through Lake Travis, before the PMF hydrograph. For this routing, Freese & Nichols (Reference 2.2-7) assumed that the antecedent storm event was equal to 20% of the PMP, which was estimated using the HMR 51 and 52 (References 2.2-10 and 2.2-11).

This study concluded that the PMF outflow hydrograph at Mansfield Dam has a peak discharge of 837,094 cfs and that the maximum water surface elevation at the dam for this flood event was 752.02 ft NGVD29 (Reference 2.2-7).

2.2.4.1.4 Flood Damage Evaluation for Lower Colorado River - Halff Associates In July 2002, Halff Associates Inc. completed a comprehensive flood damage evaluation study for the Lower Colorado River basin (Reference 2.2-8). The study area extended from Lake O.H.

Ivie to the Gulf at Matagorda Bay (see Figure 2.2-3) with a total contributing drainage area of about 18,300 sq. mi. This overall study area was divided into 290 subbasins (see Vol. Il-B, Chapter 4, Figures 111-2 to 111-5 of Reference 2.2-8) to include major reservoirs, major tributary confluences, and the existing USGS stream gauging stations.

The USACE's HEC-HMS model, Version 2.2.2 (Reference 2.2-17) was used for this study as the hydrologic modeling framework to determine frequency flood hydrographs resulting from selected storm events with return periods of 2, 5, 10, 25, 50, 100, and 500-year and the Standard Project Storm. The HEC-HMS models developed for the Halff study were initially calibrated using three historic storm events selected based on availability of adequate rainfall gauge data. The three selected storm events occurred in June 1997, October 1998, and November 2000 (see Vol. I1-B, Chapter 4, pg. 12 of Reference 2.2-8). The calibrated HEC-HMS Floodingin Streams and Rivers Z.2-7

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 Fukushitna Response Project model parameters included: initial rainfall loss, constant rainfall loss rate, Snyder's basin lag-time, and Snyder's peaking coefficient (see Vol. II-B, Chapter 4, pg. 16 of Reference 2.2-8).

Also, the Halff study noted that "six special Points-of-Interest (POl's) were selected as target locations to compute/calibrate critical peak flow hydrographs (in addition to other, less critical gauge locations). These PO's were selected based on their location in the basin and because they were identified as key calibration points for this study. The six POr's are the Llano River at Llano, the San Saba River at San Saba, Lake Buchanan, Lake Travis, the Colorado River at Bastrop, and the Colorado River at Wharton" (Vol. II-B, Chapter 4, pg. 1 of Reference 2.2-8).

Additionally, "further adjustments to parameters, specifically loss rates, were necessary to match the peak discharges (historical frequencies) at the six PON's. Results compared closely to the historical frequency analysis results and the period-of-record analysis results." These initially calibrated model parameters were further adjusted to match the peaks of historic flood frequencies estimated at various stream gauging stations located within the study area (see Vol.

II-B, Chapter 4, pg. 23 of Reference 2.2-8).

The synthetic precipitation data used for this study were obtained from Hydro-35 (Reference 2.2-19), TP-40 (Reference 2.2-20), and TP-49 (Reference 2.2-21) for the storm events with return periods of 2, 5, 10, 25, 50, 100, and 500 years. For the SPS event, the SPF Index Rainfall was used. The storm spatial distribution, centering, and orientation pattern adopted for the Halff study were as follows:

" The storm spatial distribution was selected based on the procedures in HMR 51 and 52 (References 2.2-10 and 2.2-11) for storm events with return periods of 2, 5, 10, 25, 50, 100, and 500 years (see Vol. II-B, Chapter 4, pg. 18 of Reference 2.2-8).

" A critical storm centering approach was used for all the storm events (i.e. 2-, 5-, 10-, 25-,

50-, 100-, and 500-year storm and SPS). Using a trial and error approach, the storm center that produces the largest peak flow rate at a particular point-of-interest (POI) was determined as the critical storm center for that return period (see Vol. Il-B, Chapter 4, pg.

18 of Reference 2.2-8).

Two different storm orientation patterns were adopted for the Lower Colorado River basin; one for the upper part of the basin and the other for the lower part of the basin, to derive frequency flood hydrographs at different POls. For example, the storm orientation pattern shown in Figure 2.2-4 was used to estimate flood hydrographs at different PO's in the upper part of the basin, including Mansfield Dam (see Vol. II-B, Chapter 4, Figure VI-1 of Reference 2.2-8). The orientation pattern shown in Figure 2.2-5 was used to estimate flood hydrographs at different Po0's in the lower part of the basin, including Bay City (see Vol. II-B, Chapter 4, Figure VI-5 of Reference 2.2-8).

Based on the storm orientation pattern adopted for the upper part of the river basin, the peak SPF inflow to Mansfield Dam was estimated to be 801,996 cfs 3, with the critical storm center located at subbasin LR-24 (Vol. II-B, Chapter. 4, Table VI-5 of Reference 2.2-8) for unregulated flow conditions upstream of Mansfield Dam. Based on the same storm orientation pattern, with 3 The value of 801,996 cfs for the peak of the SPF peak inflow into Mansfield Dam was extracted from the computer files obtained from Halff Associates Inc. In the report documenting this work (Reference 2.2-8) this peak inflow was rounded to 800,000 cfs (see Vol. Il-B, Chapter 4, Table VI-5 of Reference 2.2-8).

Floodingin Streams and Rivers 2.2-8

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project the critical storm center located at subbasin SS18, and the unregulated flow conditions, the peak SPF at the Wharton USGS gauging station was estimated to be 423,321 cfs 4 , (Vol. Il-B, Chapter 4, Table VI-7 of Reference 2.2-8). The Wharton gauge is located at the Wharton POI shown on Figure 2.2-3. The unregulated flow conditions used to obtain these estimates were based on the assumption that there are no dams or reservoirs in the river basin.

2.2.4.1.5 Review Summary Table 2.2-1 summarizes the reported PMF and SPF values at Mansfield Dam in the hydrologic studies reviewed in Subsections 2.2.4.1.1 to 2.2.4.1.4.

2.2.4.2 PMF Flow Scenarios at STP I & 2 The five flood scenarios of possible PMF flows in the Lower Colorado River that were considered for the STP 1 & 2 in the UFSAR (see Subsection 2.2.4.1.1) were first evaluated for their applicability in determining the maximum flood elevation at the STP site for the present conditions. After careful consideration of the hydro-meteorological setting of the region, itwas determined that the five flood scenarios considered for the STP 1 & 2 in the UFSAR cover the permutation of the possible critical flood events that could occur in the region, thus acceptable for the current reevaluation of possible extreme flood conditions.

The first and fourth scenarios considered for STP 1 & 2 in the UFSAR (see Subsection 2.2.4.1.1) were eliminated because they include the Columbus Bend Dam that was proposed in the 1960s and which met with opposition by different groups at various times. This dam was also referred to later as the Shaw Bend Dam. Plans for the construction of this dam have been abandoned. This was confirmed by conducting an online search, a search of various sources, as well as inquiries to different engineers of the LCRA, none of which revealed any information regarding continuing plans for the construction of the Columbus Bend Dam. The Region "K" Plan for the Lower Colorado Region in the 2007 State Water Plan also states that "Large local opposition to this project was demonstrated at the various Lower Colorado River Water Planning Group (LCRWPG) public meetings and in correspondence during the 2001 LCRWPG plan preparation." The Planning Group's recommendation in the current water plan is to oppose the potential designation of the Shaw Bend site as a potential reservoir site (see Section 8.4.2 in Reference 2.2-23). Therefore, itwas concluded it is not likely that this dam will be constructed in the future.

The three remaining possible PMF flow scenarios in Lower Colorado River that are analyzed for their effects at STP 1 &2 are as follows:

(1) The PMF for the drainage area between Mansfield Dam and the Bay City USGS gauging station (3555 sq. mi) combined with an antecedent storm equal to 40% of the PMP occurring over the same drainage area, three days before the PMF. This combined flow is added to the flow release from Mansfield Dam and to the base flow at Bay City to determine the peak PMF flow for this scenario (see Section 2.2).

' The value of 423,321 cfs for the peak of the SPF peak at Wharton was extracted from the computer files obtained from Halff Associates Inc. Inthe report documenting this work (Reference 2.2-8) this peak inflow was rounded to 425,000 cfs (see Vol. Il-B, Chapter 4, Table VI-7 of Reference 2.2-8).

Floodingin Streams and Rivers 2.2-9

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STPJ & 2 FukushimaResponse Project (2) The PMF inflow hydrograph to Mansfield Dam, which results from a PMP storm on the watershed upstream of the dam (from Lake O.H. Ivie to Mansfield Dam), routed through Lake Travis and combined with the flood hydrograph from a sequential storm equal to 40% of the PMP occurring over the drainage area between Mansfield Dam and Bay City (3555 sq. mi),

three days after the PMP storm upstream of Mansfield Dam. This combined flow is added to the base flow at Bay City to determine the peak PMF flow for this scenario.

(3) The PMF for the Lower Colorado River basin area between Lake O.H. Ivie and Bay City (18,197 sq. mi) combined with the flood hydrograph from an antecedent storm equal to the SPS over the same area, occurring three days before the PMF. This combined flow is added to the base flow at Bay City to determine the peak PMF flow for this scenario. Conservatively, this scenario does not account for the storage effect of Lake Travis at Mansfield Dam nor any other dam in the Lower Colorado River basin.

From these three possible PMF flow scenarios, the most critical flow scenario, which would produce the highest PMF peak at the Bay City gauging station, is selected to evaluate flooding potential at the STP 1 & 2 site. The Bay City gauging station is located about 18 river miles upstream of the STP Reservoir Makeup Pumping Facility (RMPF) on the west bank of Lower Colorado River (see Figure 2.2-1a).

2.2.4.2.1 PMF between Mansfield Dam and Bay City for Scenario 1 For Scenario 1, the peak PMF for the drainage area between Mansfield Dam and Bay City (3555 sq. miles) was calculated by assuming an antecedent storm equal to 40% of the PMP occurs over the same area three days before the PMF event itself and combining those flows with the flow release from Mansfield Dam and the base flow at Bay City (Section 2.2). The analysis performed to determine the peak PMF for Scenario 1 is described below.

HEC-HMS Rainfall-Runoff Model The PMF hydrograph for the drainage area between Mansfield Dam and Bay City (3555 sq.

miles) was estimated using the HEC-HMS model developed by Halff Associates Inc. for the lower part of the river basin with the storm orientation pattern shown in Figure 2.2-5 (Subsection 2.2.4.1.4). This model consists of 80 subbasins between Mansfield Dam and Matagorda Bay.

In the Halff HEC-HMS model, the flow routing from an upstream reach to a downstream reach was performed using the modified Puls method, which defines a storage-outflow rating curve for each of the channel reaches in the model. As discussed in Subsection 2.2.3, three storage-outflow rating curves (out of 58) in the original Halff HEC-HMS model were extended to accommodate the PMF conditions. Note that there are nine dams/reservoirs with individual storage capacity in excess of 3000 acre-feet, but none of these reservoirs were included in the Halff HEC-HMS model. Only major reservoirs were included in the Halff study since "the effects of numerous other smaller reservoirs in the Colorado River Basin were considered to be insignificant to the overall accuracy of the study and impractical to model on a daily basis for a long period of record" (Vol II-A, Chapter 2, pg. 2 of Reference 2.2-8). Additionally, including these reservoirs in the model would produce a less conservative estimate of discharge due to attenuation of the flood peak by the reservoirs.

Flooding in Streams and Rivers 2.2-10

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 Fukushima Response Project Other input data to the HEC-HMS model included the unit hydrograph, the rainfall hyetograph, and rainfall losses for each of the subbasins in the lower part of the river from Mansfield Dam to Matagorda Bay as described below.

Unit Hydroqraph: The HEC-HMS model developed by Halff Associates Inc. included the calibrated Snyder unit hydrograph parameters (i.e. the basin lag-time and peaking coefficient) for each of the subbasins located between Mansfield Dam and Matagorda Bay (Subsection 2.2.4.1.4). As discussed in Subsection 2.2.3, the calibrated Snyder basin lag-time parameter for each of the subbasins was decreased by 25% to account for non-linearity effects in the runoff process under PMF conditions (Reference 2.2-24).

Table 2.2-2 presents the drainage areas and the unit hydrograph parameters for the subbasins from Mansfield Dam to Matagorda Bay extracted from the calibrated HEC-HMS model (see also Vol. II-B, Chapter 4, Attachment B-1 of Reference 2.2-8) and the Snyder lag times as modified to account for non-linearity effects.

Rainfall Hyetoqraph: PMP hyetographs were developed for each of the 80 subbasins located within the lower part of the river basin presented in Table 2.2-2, using the same storm spatial distribution and the critical storm centering location adopted by the Halff study (see Subsection 2.2.4.1.4) as follows:

In the Halff study, the critical storm centering location that produces the largest flow rate at Bay City for the 100-year storm event was found to be at subbasin CC-06, as shown in Figure 2.2-3 (see Vol. II-B, Chapter 4, Table VI-1 1 of Reference 2.2-8). Considering the unique elongated shape of the lower part of the Lower Colorado River basin (from Mansfield Dam to Matagorda Bay) and the storm orientation, it is reasonable to assume that the same critical storm centering location can be used for the PMP event.

The storm spatial distribution pattern adopted by the Halff study was based on the procedures in HMR 52 (Reference 2.2-11). The same procedures were used to spatially distribute the 96-hour PMP depth at subbasin CC-06 to the remaining subbasins.

2

" The 96-hour 10-mi PMP depth for subbasin CC-06 was estimated as 55.7 inches by extrapolating data obtained from Figures 18 to 22 in HMR 51 (see Table 2.2-3). The PMP hyetograph for subbasin CC-06 is shown in Figure 2.2-6.

The PMP hyetographs for the remaining 79 subbasins are the same, except that the rainfall intensity ordinates are multiplied by the ratio of the PMP depth for that subbasin (obtained from Table 2.2-2) to the PMP depth at subbasin CC-06 (55.7 inches, according to Table 2.2-3).

Rainfall Losses: The unit hydrograph approach requires estimation of initial rainfall loss and constant rainfall loss rate to determine the direct runoff hydrograph corresponding to the excess rainfall (i.e. total rainfall minus rainfall loss). The initial rainfall loss quantifies the amount of infiltrated or stored rainfall before surface runoff begins. The constant rainfall rate determines the rate of infiltration that is sustained during the rest of the storm after the initial loss is satisfied.

The PMF peak flow is often insensitive to the initial rainfall loss (Reference 2.2-12); therefore, this value was conservatively set equal to zero for each of the 80 subbasins in the HEC-HMS Floodingin Streams and Rivers 2.2-11

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukhshima Response Project model (see Table 2.2-2). Reference 2.2-12 also states that "for PMF runoff computations, the soil should be assumed to be saturated with infiltration occurring at the minimum rate applicable to the area-weighted average soil type covering each subbasin." Therefore, based on data provided in Table 8-8.1 of Reference 2.2-12, a minimum uniform rainfall loss rate of 0.05 in/hr was adopted in the model for the PMF analysis (see Table 2.2-2). The minimum uniform rainfall loss rate of 0.05 in/hr used in the model for the PMF analysis was based on a range of 0.05 to 0.15 in/hr provided in Table 8-8.1 of Reference 2.2-12. These conservative values were used in the model to account for absorption and wet watershed antecedent conditions that would maximize the peak PMF discharges for subbasins listed in Table 2.2-2. The use of minimum values for the rainfall loss rates increases the runoff volume of the PMF hydrograph and hence provides a conservatively higher peak PMF discharge.

Base Flow: The base flow rate at Bay City is estimated in accordance with the procedures in ANSI/ANS-2.8-1992 (Reference 2.2-13), which states that the mean monthly flow should be used as the base flow rate for the PMF analysis. The base flow rate at Bay City was conservatively set equal to the mean monthly average flow of 5200 cfs. This value was selected based on the published USGS mean monthly flow statistics at Austin (08158000), Columbus (08161000), and Bay City (08162500).

Peak PMF Discharge at Bay City for Scenario 1: The PMF hydrograph for the drainage area between Mansfield Dam and Bay City was estimated using the HEC-HMS model obtained from Halff Associates Inc. (Reference 2.2-8) with the input data described above. The peak discharge for this PMF hydrograph (without an antecedent storm event and a base flow) was estimated to be 1,096,807 cfs (see Figure 2.2-7). As shown in Figure 2.2-7, combining the PMF with an antecedent storm event equal to 40% of the PMP over the same drainage area occurring three days before the PMF event, the flow release of 90,000 cfs from Mansfield Dam (see Section 2.2), and the base flow of 5200 cfs gives a peak PMF discharge at Bay City of 1,397,432 cfs (see Figure 2.2-7).

2.2.4.2.2 PMF between Mansfield Dam and Bay City for Scenario 2 For Scenario 2, the PMF inflow hydrograph to Mansfield Dam is routed through Lake Travis and combined with the flood hydrograph from a sequential storm event equal to 40% of the PMP over the drainage area between Mansfield Dam and Bay City and the base flow at Bay City. The sequential storm event occurs three days after the PMP storm that produces the PMF inflow hydrograph into Lake Travis at Mansfield Dam.

The PMF inflow hydrograph into Lake Travis was estimated based on the SPF inflow hydrograph developed for the basin upstream of Mansfield Dam with unregulated flow conditions as reported in the Halff study (see Subsection 2.2.4.1.4). The PMF inflow was taken as equal to two times the SPF inflow into Lake Travis at Mansfield Dam. This assumption is based on guidelines given in the USACE EM 1110-2-1411 (Reference 2.2-14), which states that the SPF is usually equal 40 to 60% of the PMF for the same basin. A ratio of 50% is adopted in this PMF analysis.

The critical storm centering location for this SPF inflow hydrograph at Mansfield was found to be at subbasin LR-24, as shown in Figure 2.2-3 (Vol. II-B, Chapter 4, Table VI-5, Reference 2.2-8).

Routing of PMF Hydrograph through Lake Travis: The SPF inflow hydrograph at Mansfield Dam was routed through Lake Travis, using the USACE's HEC-1 model (Reference 2.2-15), in order Floodingin Streams and Rivers 2.2-12

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP I & 2 Fukushimna Response Project to establish the antecedent water level conditions in the reservoir. The SPF inflow hydrograph was assumed to occur three days prior to the routing of the PMF inflow hydrograph that was estimated as equal to two times the SPF inflow. The input data used in the HEC-1 model for the reservoir routing analysis are briefly described below:

" The initial reservoir water level prior to the routing of the SPF inflow hydrograph was set at elevation 681 ft NGVD29, i.e. the elevation of the reservoir conservation pool (see Table 2.2-4).

The reservoir elevation-storage data up to El. 740 ft NGVD29 were obtained from the Halff Reservoir Operation Model HEC-5 (see Vol. Il-B, Chapter 5, Reference 2.2-8) and are presented in Table 2.2-5. The storage values above El. 740 ft NGVD29 were estimated by logarithmic extrapolation of the Halff data.

The pertinent dam and spillway outlet data were obtained from various USBR publications (References 2.2-4, 2.2-5, and 2.2-16), the Freese & Nichols study (Reference 2.2-7) and from the Halff study (Reference 2.2-8). They are presented in Table 2.2-4. The main spillway at Mansfield Dam is an uncontrolled ogee crest spillway with a 700 ft clear length and crest at El. 714 ft NGVD29 (Reference 2.2-5). The low level outlets consist of twenty-four 102-in diameter conduits through the concrete section of the dam controlled by gates. The centerline elevation of the inlets to the conduits is at El. 540 ft NGVD29 (Reference 2.2-5).

" The main spillway rating curve for Mansfield Dam was computed from spillway capacity data given in References 2.2-4, 2.2-8, and 2.2-16.1.5 The discharge coefficient for the spillway is C = 4.0 in the expression Q = CLH , where Q is the spillway discharge, L is the spillway length, and H is the head over the spillway crest. This value is based on the model test result of C = 3.93 at the spillway design head and calculation of C value at other heads in accordance with data in Reference 2.2-22.

A rating curve for the 24 low level outlets was also developed from data given in References 2.2-4, 2.2-8, and 2.2-16. The low level outlets were treated as orifices in the HEC-1 model with a discharge coefficient of 0.87. The total discharge from all 24 outlet conduits with Lake Travis at El. 714 ft NGVD29 is 126, 000 cfs (Reference 2.2-5).

The combined inflow hydrograph (SPF + PMF) into the Lake Travis reservoir was estimated by adding the SPF hydrograph ordinates to the PMF hydrograph ordinates, after shifting the latter by three days, as presented in Figure 2.2-8. This figure also includes the routed outflow hydrograph for the combined SPF and PMF event.

As shown in Figure 2.2-8, the peak PMF inflow into Lake Travis at Mansfield Dam was estimated as 1,603,992 cfs (i.e. twice the peak SPF inflow of 801,996 cfs) (Reference 2.2-8).

The reservoir routing analysis showed that the peak PMF outflow at Mansfield Dam is about 944,138 cfs (see Figure 2.2-8).

The HEC-1 routing with this very conservative estimate of the PMF inflow shows that Mansfield Dam would be overtopped. For the purpose of the PMF analysis, it was assumed that Mansfield Dam would not fail. The dam break analysis for Mansfield Dam is addressed in Section 2.3. The peak outflow from Mansfield Dam obtained with the HEC-1 routing (944,138 cfs) exceeds all Floodingin Streams and Rivers 2.2-13

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 FukushimaResponse Project published values reviewed (see Table 2.2-1) and provides a conservative value for the peak outflow from Mansfield Dam.

Consequently, the PMF inflow and outflow hydrographs at Mansfield Dam developed as described above provide conservative, i.e. high, estimates of maximum water levels in the vicinity of STP 1 & 2 site.

Peak PMF Discharge at Bay City for Scenario 2: The peak PMF discharge at Bay City was calculated by adding the peak PMF outflow at Mansfield Dam (944,138 cfs) (see Figure 2.2-8) to the peak of the 40% PMP hydrograph for the drainage area between Mansfield Dam and Bay City (303,277 cfs) (see Figure 2.2-7) plus the base flow of 5200 cfs (see Subsection 2.2.4.2.1).

This approach provides a very conservative estimate for the peak PMF discharge at Bay City (1,252,615 cfs), because it does not account for the attenuation of the peak outflow from Mansfield Dam as the flood wave travels down the 290 mile long reach in the Lower Colorado River between Mansfield Dam and Bay City.

2.2.4.2.3 PMF between Lake O.H. Ivie and Bay City for Scenario 3 For Scenario 3, the peak PMF for the Lower Colorado River basin area from Lake O.H. Ivie and Bay City was estimated by assuming that the SPF occurs over the same basin area three days before the PMF event and combining those flows with the base flow at Bay City. This scenario does not account for the storage effect of Lake Travis at Mansfield Dam or any other dam in the Lower Colorado River basin.

The PMF hydrograph for this scenario was estimated based on the SPF hydrograph that was developed at Wharton by the Halff study (Reference 2.2-8) for unregulated flow conditions in Lower Colorado River Basin. The critical storm centering location for this SPF hydrograph at Wharton is found to be at subbasin SS-18 (see Vol. Il-B, Chapter 4, Table VI-7 of Reference 2.2-8), which is located in the upper portion of the Lower Colorado River basin, as shown in Figure 2.2-3. The SPF peak discharge at Wharton was estimated as 423,321 cfs 5 (Subsection 2.2.4.1.4).

Estimation of PMF Hydrograph at Bay City: The SPF hydrograph at Wharton (with a peak discharge of 423,321 cfs) was used to estimate the SPF hydrograph at Bay City. The ordinates of SPF hydrograph at Wharton were multiplied by the ratio of the drainage area at Bay City (30,837 sq. mi) over the drainage area at Wharton (30,600 sq. mi) to estimate the SPF hydrograph at Bay City. The SPF peak discharge at Bay City was estimated to be about 426,000 cfs. The drainage areas at Wharton and Bay City were obtained from the Halff study (Vol. Il-B, Chapter 4, Table IV-1 of Reference 2.2-8). The required PMF hydrograph at Bay City for Scenario 3 was estimated by assuming that the PMF is equal to twice the SPF at Bay City.

This assumption is based on guidelines in Engineering Manual EM 1110-2-1411 (Reference 2.2-14). The PMF peak discharge at Bay City was estimated to be 853,200 cfs (see Figure 2.2-9).

5The value of 423,321 cfs for the SPF peak at Wharton was extracted from the computer files obtained from Halff Associates Inc. In the report documenting this work (Reference 2.2-8) this peak inflow was rounded to 425,000 cfs (Reference 2.2-8, Vol. Il-B, Chapter 4, Table VI-7).

Flooding in Streams and Rivers 2.2-14

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP I & 2 Fukushima Response Project Peak PMF Discharge at Bay City for Scenario 3: The PMF hydrograph developed for the Lower Colorado River basin from Lake O.H. Ivie to Bay City has a peak discharge of 853,200 cfs at Bay City (see Figure 2.2-9) without the SPF event and base flow. As shown in Figure 2.2-9, adding the SPF event with a peak discharge of 426,600 cfs over the same area, occurring three days before the PMF event and the base flow of 5,200 cfs at Bay City (see Subsection 2.2.4.2.1), produces a peak PMF at Bay City equal to 994,060 cfs (see Figure 2.2-9).

2.2.4.3 Most Critical PMF Scenario at Bay City The analyses discussed in Subsections 2.2.4.2.1, 2.2.4.2.2, and 2.2.4.2.3 show that Scenario 1 produces the highest peak PMF at Bay City (see Table 2.2-6). The highest flood peak at Bay City is caused by the PMP for the drainage area between Mansfield Dam and the Bay City combined with an antecedent storm equal to 40% of the PMP occurring over the same drainage area, the flow release of 90,000 cfs from Mansfield Dam, and the base flow of 5200 cfs.

Therefore, the peak flow of 1,397,432 cfs for Scenario 1 is used as the most critical PMF scenario to evaluate potential flooding at the STP 1 & 2 site.

2.2.5 Water Level Determinations The maximum still water surface elevation at the STP 1 & 2 site for the peak PMF discharge of 1,397,432 cfs was calculated using the UACE's HEC-RAS hydraulic model, Version 3.1.3 (Reference 2.2-18). The HEC-RAS model for the STP I & 2 site was developed on the basis of topographic data and hydraulic characteristics such as Manning's roughness coefficients that were established for the Halffs flood damage evaluation study (Reference 2.2- 8).

2.2.5.1 Halff HEC-RAS Hydraulic Model - Bay City to Matagorda Bay The Halff HEC-RAS model (from Bay City to Matagorda Bay), that was developed for the Halff's flood damage evaluation study used the most recent channel and floodplain topographic information obtained from LCRA and the USACE. The required channel topographic data were field-surveyed and provided by USACE. The floodplain topographic data obtained from LCRA included aerial digital ortho-photographs, digital contour maps (2 foot intervals), and USGS 30-m National Elevation Dataset (NED) Digital Elevation Model (DEM) data. The 30-m DEM data were used only to fill a 0.5 mile buffer zone area outside the 500-year floodplain that was mapped using the aerial digital data (Vol.1, pg. 20 of Reference 2.2-8).

The Halff HEC-RAS model from Bay City to Matagorda Bay covers approximately a reach length of 24 miles and includes two bridge crossings, one at the Missouri Pacific Railroad (RS 1350+15.3) and another at the FM 521 roadway (RS 843+40.0). The upstream-most cross-section in the Halff model is located at the Bay City USGS gauging station (RS 1665+21.6). The downstream-most cross-section (RS 383+64.5) in the model is located about 4600 ft upstream of the intersection of Lower Colorado River and the Intra-Coastal Waterway (RS 337+90) (see Vol. Il-C, Chapter 6, Table I-1 of Reference 2.2-8). Table 2.2-7 lists the key cross-sections in the Halff HEC-RAS model, which include two bridge crossings and 68 channel cross-sections.

The initial Manning's roughness coefficients used in the Halff HEC-RAS model were estimated from the USGS National Land Cover Dataset coverage and then adjusted using aerial photographs (see Vol. Il-C, Chapter 6, Table 111-2 of Reference 2.2-8). During the model calibration by Halff Associates, the roughness coefficients were subsequently adjusted in the model to match historical flood levels using USGS gauge data. The cross-sections, gauges, and storms used for adjustment are available in Table IV-1 of Reference 2.2-8 (Vol. Il-A, Chapter 6, Flooding in Streams and Rivers 2.2-15

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project pg. 21). Calibration values for steady HEC-RAS runs were based on a six-stage "clean-up" procedure discussed on pg. 18-19 of Reference 2.2-8 (Vol. Il-C, Chapter 6). The calibration for the unsteady HEC-RAS runs is described on pg. 20 of Reference 2.2-8 (Vol. Il-C, Chapter 6).

The validation of the calibration results is shown in Attachment A of Vol. Il-C, Chapter 6 of Reference 2.2-8. The calibrated Manning's roughness coefficients used in model are 0.035 for the river channel, 0.045-0.05 for the overbank, and 0.085-0.095 for the floodplain.

2.2.5.2 Extension of Cross-sections for the PMF Event A review of the geometry data used in the Halff HEC-RAS model showed that the cross-sections need to be extended to more accurately reflect the potential increase in the width of the floodplain during the passage of a PMF event. The cross-section data used in Halff's HEC-RAS model was therefore extended, as shown in Figure 2.2-10, to cover a larger floodplain area between the most downstream cross-section (RS 383+64.5) and the STP 1 & 2 site (RS 964+99.7). Because the stretch of the Colorado River from the site to Matagorda Bay is in a sub-critical flow regime, it is not necessary to extend the cross-sections any further upstream from the STP 1 & 2 site because the flood elevation at the site depends only on conditions downstream from the STP 1 & 2 site.

A total of 32 cross-sections were extended (between RS 383+64.5 and RS 964+99.7) for a distance up to about 19 miles towards the east of the Lower Colorado River to near Caney Creek. The source maps used for the extension of these cross-sections were high-resolution digital raster graphic (DRG) scans of the USGS 7.5-minute quadrangles 6. To be conservative, these cross-sections were not extended to the west of the Colorado River.

2.2.5.3 HEC-RAS Hydraulic Model for STP I & 2 The HEC-RAS hydraulic model (Version 3.1.3) for the STP 1 & 2 site was developed using the above extended cross-sections (from RS 383+64.5 to RS 964+99.7) and Manning's roughness coefficients adjusted for PMF flow conditions. As the flow depth increases, the flow encounters larger size obstructions, e.g. shrubs, trees, etc., which effectively increase the roughness of the floodplain. For this purpose the calibrated Manning's roughness coefficients used in the Halff HEC-RAS model (see Subsection 2.2.5.1) were increased by 20% for the postulated PMF flow condition to provide a conservative estimate of the maximum stream flooding elevation at the site. The Manning's roughness coefficients that were increased by 20% for the PMF had values of 0.042 for the river channel, 0.054-0.06 for the overbank, and 0.102-0.114 for the floodplain.

Since the Manning's roughness coefficients cannot be determined a priori to a PMF event occurring in the Lower Colorado River, this increase in the roughness coefficient was based on experimental results of flooding in meandering streams (Reference 2.2-28), and from roughness coefficients for the river channel, overbank, and floodplain areas listed in Table 3-1 of Reference 2.2-18.

The HEC-RAS model developed for the STP 1 & 2 covers an approximate reach length of 11 miles and includes a bridge crossing at the FM 521 (RS 843+40.0). Incorporation of this bridge crossing in the model gives a conservative (i.e. higher) estimate for the maximum flood level.

The vertical datum for the USGS 7.5-minute quadrangles is referenced to NGVD29. This datum is adjusted to match NAVD88 that was used as the vertical datum for the Halff model cross-sections.

Flooding in Streams and Rivers 2.2-16

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushiina Response Project The upstream-most cross-section in this model is located at RS 964+99.7 and the downstream-most cross-section is at RS 383+64.5.

2.2.5.3.1 Model Boundary Conditions Under PMF flow conditions, the water level in the river at the downstream-most cross-section (RS 383+64.5) is not controlled by tidal effects. From 1961 to 2001, the highest water level recorded for National Oceanic and Atmospheric Administration (NOAA) Station #8772440 at Freeport is 4.95 feet above mean sea level (MSL) (Reference 2.2-27). Therefore, normal depth for an estimated channel slope of 0.0001 is the appropriate boundary condition to use at the downstream-most cross-section of the model that is located approximately 7.3 mile upstream from the shoreline of the Gulf of Mexico (see Table 2.2-7).

Using the HEC-RAS model developed for the STP 1 & 2 site, the normal depth at the downstream boundary (RS 383+64.5) was estimated to be equal to 17.5 ft NAVD88 for the peak PMF discharge of 1,397,432 cfs (see Figure 2.2-11) with a steady state model simulation. This calculation was made using Manning's n values equal to 1.2 times those used in the Halff HEC-RAS model (see Subsection 2.2.5.1) to provide a conservative upper bound flood level at the site as a result of a PMF event.

Using the same Manning's n values as those used in the Halff HEC-RAS model (see Subsection 2.2.5.1), the normal depth at the downstream boundary (RS 383+64.5) was estimated to be equal to 16.2 ft NAVD88 for the peak PMF discharge of 1,397,432 cfs (see Figure 2.2-12).

2.2.5.3.2 PMF Still Water Surface Elevation at STP I & 2 As shown in Figure 2.2-11, the maximum PMF still water surface elevation at the STP 1 & 2 site (RS 891+46.0) for the normal depth downstream boundary condition and using conservative Manning's n values equal to 1.2 times those used in the original Halff model was estimated to be equal to 26.1 ft NAVD88 (26.3 ft NGVD29). In comparison, the PMF still water level for STP 1 & 2 obtained in this reevaluation study is lower than the corresponding water level of 29 ft NGVD29 from UFSAR (Reference 2.2-1b).

The PMF still water surface profile obtained using the same Manning's n values as those used in the Halff model is shown in Figure 2.2-12. In this case, the maximum PMF still water surface elevation at the STP 1 & 2 site (RS 891+46.0) was estimated as 24.8 ft NAVD88 (25.0 ft NGVD29).

PMF water levels at two selected cross-sections: the downstream boundary (RS 383+64.5) and the STP 1 & 2 site (RS 891+46.0) are shown in Figure 2.2-13 (with Manning's n values equal to 1.2 times in the Halff model) and Figure 2.2-14 (with same Manning's n values used in the Halff model).

2.2.6 Coincident Wind Wave Activity The flooding resulting from dam failures upstream of the STP 1 & 2 site was found to be more critical than that resulting from the PMF. For example, the calculated maximum still water level at the STP site due to a domino-type failure of the upstream dams would be at 28.4 ft NAVD88 (28.6 ft NGVD29) according to the reevaluation described in Section 2.3. That is about 2.3 ft higher than the calculated maximum still water level of 26.1 ft NAVD88 (26.3 ft NGVD29)

Floodingin Streams and Rivers 2.2-17

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project resulting from the PMF. Coincident wind wave activity and associated hydrodynamic forces are therefore considered in the upstream dam failures evaluation as described in Section 2.3.

2.2.7 Other Impacts Because the maximum still water level of 26.3 ft NGVD29 at STP 1 & 2 from the PMF in the Lower Colorado River is below the plant grade of 28 ft NGVD29, no impact on the safety facilities of the plant due to sedimentation, erosion, debris, and water-borne missiles is expected.

2.2.8 References 2.2-1a "Hazard Mitigation Plan Update," 2011-2016, Texas Colorado River Floodplain Coalition, prepared with technical support from Lower Colorado River Authority.

2.2-1b STPEGS (Units 1 & 2) Updated Final Safety Analysis Report (UFSAR), Section 2.4, "Hydrologic Engineering," Rev. 15.

2.2-1c South Texas Project Units 3 & 4 Combined License Application (COLA), Final Safety Analysis Report (FSAR), Rev. 7, Nuclear Innovation North America LLC (NINA),

February 1, 2012.

2.2-2 "Probable Maximum Flood, Marshall Ford Dam, Lower Colorado River Project, Texas," United States Department of Interior, Bureau of Reclamation, November 1985.

2.2-3 "Mansfield Dam Comprehensive Facility Review, Highland Lakes Dams, Lower Colorado River Authority, " United States Department of Interior, Bureau of Reclamation, Technical Service Center, Denver, Colorado, March 2003.

2.2-4 USBR official website. Available at http://www.usbr.,qov/dataweb/dams/txO0l87.htm, accessed on February 20, 2007.

2.2-5 "SEED Analysis Report - Marshall Ford Dam," ATC Engineering Consultants Inc.

(ECI), prepared for the United States Department of Interior, Bureau of Reclamation, Colorado River Project, July 1989.

2.2-6 "Civil Engineering Report of Intermediate Examination of Marshall Ford Dam, Colorado River Authority, Texas," Goodson & Associates Inc., December 1990.

2.2-7 "Phase II - Dam Safety Evaluation Project, Task Order B, Volume I," prepared for the Lower Colorado River Authority, Freese & Nichols, Inc., August 1992.

2.2-8 Colorado River Flood Damage Evaluation Project - Phase I," Volume I and Volume II, prepared for the Lower Colorado River Authority and Fort Worth District Corps of Engineers, Halff Associates, Inc, July 2002.

2.2-9 "Seasonal Variation of the Probable Maximum Precipitation, East of the 105th Meridian for Area from 10 to 100 Square Miles and Durations of 6, 12, 24, and 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months ," Hydrometeorological Report No. 33, United States Weather Bureau, 1956.

Floodingin Streams and Rivers 2.2-18

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushiina Response Project 2.2-10 "Probable Maximum Precipitation Estimates, United States East of the 105th Meridian," Hydrometeorological Report No. 51, United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), June 1978.

2.2-11 "Application of Probable Maximum Precipitation Estimates, United States East of the 105th Meridian," Hydrometeorological Report No. 52, United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), August 1982.

2.2-12 "Engineering Guidelines for the Evaluation of Hydropower Projects, Determination of the Probable Maximum Flood," Federal Energy Regulation Commission (FERC),

September 2001, 2.2-13 "Determining Design Basis Flooding at Power Reactor Sites," ANSI/ANS-2.8-1992, American Nuclear Society, July 1992.

2.2-14 "Standard Project Flood Determination," Engineering Manual 1110-2-1411, United States Army Corps of Engineers, March 1965.

2.2-15 "HEC-1 Flood Hydrograph Package, User's Manual, Version 4.0," United States Army Corps of Engineers, September 1990.

2.2-16 "Water and Power Resources Service - Project Data," United States Department of Interior Bureau of Reclamation, A Water Resources Technical Publication, 1981.

2.2-17 "Hydrologic Engineering Center- Hydrologic Modeling System, HEC-HMS Model, Version 2.2.2," United States Army Corps of Engineers, May 2003.

2.2-18 "Hydrologic Engineering Center - River Analysis System, HEC-RAS Model, Version 3.1.3," United States Army Corps of Engineers, May 2005.

2.2-19 "Technical Memorandum NWS HYDRO-35," National Oceanic and Atmospheric Administration (NOAA), June 1977.

2.2-20 "Technical Paper No. 40, Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years," United States Department of Commerce, Weather Bureau, May 1964.

2.2-21 "Technical Paper No. 49, Two- to Ten-Day Precipitation for Return Periods of 2 to 100 Years in the Contiguous United States," United States Department of Commerce, Weather Bureau, May 1964.

2.2-22 "Discharge Coefficients for Irregular Overfall Spillways, Engineering Monograph No.

9," Bradley, J.N, United States Department of the Interior, Bureau of Reclamation, 1952.

2.2-23 "2006 Region K Water Plan for the Lower Colorado Regional Water Planning Group,"

Texas Water Development Board (TWDB), January 2006.

Floodingin Streams and Rivers 2.2-19

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STPJ & 2 Fukushima Response Project 2.2-24 "Flood-Runoff Analysis," Engineering Manual 1110-2-1417, United States Army Corps of Engineers, August 1994.

2.2-25 "Engineering Data on Dams and Reservoirs in Texas," Report 126, Part III, Texas Water Development Board (TWDB), February 1971.

2.2-26 "Water Management Plan for the Lower Colorado River Basin," Lower Colorado River Authority (LCRA), March 1999.

2.2-27 "NOAA Tides and Currents", Station #8772440, Available at http://www.co-ops.nos.noaa.gov/datamenu.shtml?stn=8772440%20Freeport,%20TX&t ype=Datums, accessed May 23, 2008.

2.2-28 Smith, C.D. 1992. Reliability of flood discharge estimates: Discussion. Canadian Journal of Civil Engineering 19: 1085-1087.

2.2-29 National Climatic Data Center (National Climate Extremes Committee) official website. Available at http://www.ncdc.noaa.gov/extremes/ncec, accessed on October 16, 2012.

2.2-30a National Water Information System: Web Interface, United States Geological Survey, Peak stream flow data at Bay City, Texas available at the following URL:

http://nwis.waterdata.usas.aov/tx/nwis/peak?site no=08162500&apqency cd=USGS&

format=html, accessed on 12/12/2012 (Bechtel SDN 25799-000-3BD-KO0G-00006).

2.2-30b National Water Information System: Web Interface, United States Geological Survey, Peak stream flow data at Wharton, Texas available at the following URL:

http://nwis.waterdata.usgs.gov/tx/nwis/peak?siteno=08162000&agencycd=USGS&

format=html, accessed on 12/12/2012 (Bechtel SDN 25799-000-3BD-KO1G-00006).

Floodingin Streams and Rivers 2.2-20

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STPI & 2 FukushirnaResponse Project Table 2.2-1 PMF and SPF Values at Mansfield Dam PMF SPF Hydrologic Study Reviewed Outflow Inflow (cfs) Outflow (cfs) Inflow (cfs) (cfs)

UFSAR for STP I & 2 957,0001'1 706,0001]

(Reference 2.2-1b)

USBR and Others (References 2.2-21, 21.23, "_1.2-4, 2.2-5, and 2.2- 931,6001'1 602,2101' -1 6)

Freese Nichols Inc. (Reference 837,0941'1 2.2-7)

Halff Associates Inc. (Reference - 801.996 [21J3]

2.2-8)

Estimated based on HMR 52 (Reference 2.2-1 1).

[21Estimated based on Engineering Manual 1110-2-1411 (Reference 2.2-14).

[3] The value of 801,996 cfs for the peak of the SPF peak inflow into Mansfield Dam was extracted from the computer files obtained from Halff Associates Inc. In the report documenting this work (Reference 2.2-8) this peak inflow was rounded to 800,000 cfs (see Vol. Il-B, Chapter 4, Table VI-5 of Reference 2.2-8).

Flooding in Streams and Rivers 2.2-21

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STPI & 2 Fukushijna Response Project Table 2.2-2 Drainage Areas, Unit Hydrograph Parameters, Rainfall Loss Rates, and PMP Depths for Subbasins from Mansfield Dam to Matagorda Bay Areainag(t Snyder Snyder 12 Initial Constant Subbasin Nae (sq. I Percent p riuil Areall) NaeIpriu~lLossp Peaking Time Lag12113, Rainfall 31 Rainfall os 3 Loss'pl Depth141 eph 4 mi.) Coefficienti (hours) (in) (in/hr) (in)

AL-16 25.2 21.12 0.7 1.22 0 0.05 39.8 AL-17 43.7 5.39 0.5 2.17 0 0.05 41.2 AL-i8 1.4 9.56 0.5 0.71 0 0.05 43.1 AL-19 22.7 2.88 0.5 1.73 0 0.05 41.2 AL-20 9.2 2.08 0.5 1.04 0 0.05 42.4 AL-21 14.1 7.95 0.5 1.01 0 0.05 43.8 AL-22 8.7 10.69 0.5 1.04 0 0.05 44.1 AL-23 89.6 0.44 0.8 5.07 0 0.05 37.9 AL-24 17.8 0.00 0.8 2.59 0 0.05 41.9 AL-25 9.4 7.39 0.8 1.97 0 0.05 42.3 AL-26 3.1 8.46 0.5 0.98 0 0.05 44.3 AL-27 28.6 23.06 0.5 1.61 0 0.05 44.7 AL-28 1.9 27.99 0.5 0.80 0 0.05 46.5 AL-29 20.6 19.68 0.6 3.89 0 0.05 46.9 AL-30 51.6 12.44 0.7 3.19 0 0.05 43.3 AL-31 4.8 3.09 0.6 3.56 0 0.05 47.4 AL-32 14.0 16.05 0.6 3.86 0 0.05 47.6 AL-33 6.6 12.85 0.6 4.15 0 0.05 49.9 AL-34 104.6 0.33 0.7 3.62 0 0.05 34.7 AL-35 19.0 0.00 0.7 2.26 0 0.05 36.4 AL-36 43.7 0.94 0.8 3.70 0 0.05 36.2 AL-37 66.7 0.23 0.8 4.00 0 0.05 36.7 AL-38 89.7 3.91 0.8 4.64 0 0.05 41.7 AL-39 21.4 7.59 0.6 4.67 0 0.05 47.6 CC-01 6.3 6.12 0.6 3.71 0 0.05 51.0 CC-02 41.5 0.62 0.6 3.52 0 0.05 42.4 CC-03 33.8 6.51 0.6 4.50 0 0.05 48.6 CC-04 25.6 4.07 0.6 4.66 0 0.05 53.1 CC-05 55.0 0.86 0.6 5.53 0 0.05 48.8 CC-06 22.6 3.86 0.6 4.99 0 0.05 55.7 CC-07 163.7 0.61 0.6 7.76 0 0.05 44.5 CC-08 17.4 0.78 0.6 4.22 0 0.05 51.4 Flooding in Streams and Rivers 2. 2-22

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STPI & 2 FukushimaResponse Project Table 2.2-2 Drainage Areas, Unit Hydrograph Parameters, Rainfall Loss Rates, and PMP Depths for Subbasins from Mansfield Dam to Matagorda Bay (Continued)

Initial Constant Drainage Snyder Snyder 1Time 21 PMP141 Subbasin Percent Rainfall Rainfall AreaI"! Peaking Lag 131 131 Depth Name Impervious"l Loss Loss (sq. mi.) Coefficientl 1 (hours) (in/hr) (in)

(in)

CC-09 3.0 9.05 0.6 3.56 0 0.05 55.2 CC-10 62.6 0.69 0.6 4.82 0 0.05 43.3 CC-I 1 47.0 0.63 0.6 5.15 0 0.05 48.1 CC-12 52.2 4.79 0.6 4.76 0 0.05 50.2 CC-13 28.7 4.16 0.45 5.42 0 0.05 51.9 CC-14 39.6 0.58 0.45 6.66 0 0.05 46.7 CC-15 56.0 0.68 0.45 5.75 0 0.05 44.3 CC-16 34.8 0.80 0.45 5.22 0 0.05 50.2 CC-17 12.1 0.31 0.45 5.02 0 0.05 50.5 CC-18 137.5 0.52 0.45 7.08 0 0.05 43.5 CC-19 65.4 0.62 0.45 5.83 0 0,05 45.0 CC-20 5.5 1.03 0.45 4.66 0 0.05 50.5 CC-21 102.4 1.55 0.45 6.20 0 0.05 50.5 CC-22 41.8 3.91 0.3 4.65 0 0.05 48.1 CC-23 42.3 1.52 0.3 5.12 0 0.05 45.9 CC-24 17.4 2.94 0.3 4.43 0 0.05 46.0 CC-25 118A1 1.25 0.3 5.90 0 0,05 47.4 CC-26 38.7 3.65 0.3 4.78 0 0.05 45.7 CC-27 125.1 1.34 0.3 6.24 0 0.05 43,6 CC-28 28.1 2.85 0.3 4.35 0 0.05 44.3 CC-29 3.0 9.08 0.3 1.16 0 0.05 43.9 CC-30 91.6 1.04 0.9 11.10 0 0.05 41.4 CC-31 94.2 1.15 0.3 7.06 0 0.05 43.1 CC-32 103.2 5.63 0.3 5.51 0 0.05 43.1 CC-33 82.1 2.49 0.3 6.08 0 0.05 41.4 CC-34 78.6 2.2 0.3 6.50 0 0.05 39.0 CC-35 80.7 1.63 0.4 3.98 0 0.05 40.9 CC-36 95.9 1.11 0.4 4.33 0 0.05 40.7 CC-37 75.3 0.68 0.4 4.37 0 0.05 40.2 CC-38 63.4 1.12 0.3 5.93 0 0.05 38.8 LC-O1 94.5 6.99 0.3 6.44 0 0.05 37.1 LC-02 110.8 3.75 0.3 6.57 0 0.05 36.7 Floodingin Streams and Rivers 2.2-23

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project Table 2.2-2 Drainage Areas, Unit Hydrograph Parameters, Rainfall Loss Rates, and PMP Depths for Subbasins from Mansfield Dam to Matagorda Bay (Continued)

Drainage Snyder Snyder Initial Constant PMP Subbasin III Percent 12e Rainfall Rainfall 141 Area1 imperviousll Peaking Time Lag'2 Loss131 Loss131 Depth141 Name (sq. mi.) Coefficienti'l (hours) (in) (in/hr) (in)

LC-03 62.8 13.38 0.3 8.95 0 0.05 35.5 LC-04 63.2 8.35 0.3 6.50 0 0.05 34.2 LC-05 32.1 7.73 0.3 7.54 0 0.05 33.5 LC-06 29.7 10.44 0.3 5.72 0 0.05 32.8 LC-07 35.6 5.43 0.3 5.52 0 0.05 32.3 LC-08 29.3 4.40 0.3 5.63 0 0.05 32.9 LC-09 33.4 3.38 0.3 4.68 0 0.05 32.8 LC-10 20.4 11.78 0.3 5.27 0 0.05 31.9 LC-I 1 21.6 7.36 0.3 3.35 0 0.05 31.4 LC-12 50.3 4.59 0.3 4.28 0 0.05 31.9 LC-13 30.2 10.21 0.3 3.42 0 0.05 31.2 LC-14 31.0 7.63 0.5 3.50 0 0.05 30.9 LC-15 27.3 2.72 0.7 2.33 0 0.05 30.7 LC-16 38.4 7.07 0.7 1.94 0 0.05 30.4 LC-17 34.8 33.89 0.7 2.35 0 0.05 30.2 LC-18 2.6 60.91 0.7 1.09 0 0.05 30.0 I' Drainage areas, percentage impervious values, and calibrated Snyder peaking coefficients are extracted from the Halff HEC-HMS model (Vol. Il-B, Chapter 4, Attachment B-I of Reference 2.2-8).

[2] Snyder lag time values given here account for the non-linearity effect in the runoff process during a PMF event.

The calibrated Snyder lag time parameters extracted from the HaIffHEC-HMS model are decreased by 25% to obtain these values.

1'] Initial rainfall loss and constant rainfall loss rate values are obtained from Reference 2.2-12 for the PMF conditions.

141Estimated PMP depths used for the PMF calculations at STP I & 2 site (see Subsection 2.2.4.2.1).

Floodingin Streams and Rivers 2.2- 24

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 Fukushinta Response Project Table 2.2-3 10 Sq. Miles PMP Depth at Subbasin CC-06 Duration (hour) PMP Depth (inches) Remarks 6 31.0 Figure 18, HMR 51 12 37.5 Figure 19, HMR 51 24 44.8t1 Figure 20, HMR 51 48 50.0 Figure 21, HMR 51 72 53.1 Figure 22, HMR 51 96 55.7 Extrapolated According to National Climatic Data Center, the national record 24-hr rainfall is 43 inches occurring on July 25-26. 1979 at Alvin, Texas (Reference 2.2-29).

Flooding in Streams and Rivers 2.2-25

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 FukushiniaResponse Project Table 2.2-4 Dam and Spillway Outlet Data for Lake Travis Reservoir Description Elevation (ft) Length/or Diameter13 1(ft) Reservoir Storage (acre-ft)

Low level outlet 540 24 102-in conduits 32,500*"

Conservation pool 681 n/a 1,132,1 72['

Uncontrolled ogee spillway crest 714 700 clear opening 1,879.794111'4 Darn crest (concrete section) 750 2710 3,125,683[2].[41 Floodwall crest 754.1 4393151 3,308,03012]

L" Elevation vs. Storage data (from El. 540 ft to El. 714 ft NGVD29) are obtained from the Halff Reservoir Operation Model HEC-5 (see Vol. lI-B, Chapter 5, Reference 2.2-8).

121Storage values are estimated by logarithmic extrapolation of elevation-storage data from El. 691 ft to El. 740 ft NGVD29 (see Table 2.2-5).

13JElevation, length, and diameter values are obtained from Halff(Reference 2.2- 8), USBR (Reference 2.2-16), and TWDB (Reference 2.2-25).

[41Reference 2.2-5 states that at El. 714 ft NGVD29, the reservoir storage capacity is equal to 1,953,000 acre-ft and at El. 750 ft NGVD29, the storage capacity is equal to 2,893,800 acre-ft.

151Floodwall length is set equal to the length of the concrete dam (i.e., 5093 ft - 700 ft), where 5093 ft is the total length of the dam section as per USBR (Reference 2.2-16).

Flooding in Streams and Rivers 2.2-26

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushimna Response Project Table 2.2-5 Elevation-Storage Data for Lake Travis Reservoir at Mansfield Dam Reservoir Water Surface Elevation (in feet, NGVD29 Datum )

131 Reservoir Storage Volume (acre-ft) 536 18,2701" 630 436.,502ý'

650 652,977"'

670 939,110"'

691 1,329,5931" 710 1,772,9131" 722 2,109,1761" 732 2,428,210" j 740 2,710,5981" 750 3,125,683-12]

760 3,587,326121

'JElevation vs. Storage data (from El. 536 ft to El. 740 ft NGVD29) are obtained from the Halff Reservoir Operation Model HEC-5 (see Vol. lI-B, Chapter 5, Reference 2.2-8).

12]Storage values are estimated by logarithmic extrapolation of elevation-storage data from El. 691 ft to El. 740 ft NGVD29.

[31At Lake Travis reservoir, NAVD88 ft = NGVD29 ft + 0.22 ft.

Floodingin Streams aind Rivers 2.2-27

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STPI & 2 Fukushima Response Project Table 2.2-6 Estimated Peak PMF at Bay City for STP 1 & 2 Description of the PMF Flow Scenario Peak PMIF at Bay City (cfs)

Scenario 1: PMF for the drainage area between Mansfield Dam and the Bay City, combined with the flood hydrograph from an antecedent storm equal to 40% of the PMP occurring over the same drainage area, three days before the 1,397,432 PMF, a flow release of 90,000 cfs from Mansfield Dam, and a base flow of 5200 cfs.

Scenario 2: PMF inflow hydrograph to Mansfield Dam routed through Lake Travis and combined with the flood hydrograph from a sequential storm equal to 40% of the PMP occurring over the drainage area (Mansfield Dam to Bay 1,252,615 City), three days after the PMP storm upstream of Mansfield Dam and a base flow of 5200 cfs Scenario 3: PMF for the entire Lower Colorado River basin area between Lake O.H. Ivie and Bay City combined with the flood hydrograph fromLIan 994,060 antecedent storm equal to the SPS over the same area, occurring three days before the PMF and a base flow of 5200 cfs.

Flooding in Streams and Rivers 2.2-28

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STPI & 2 Fukushimna Response Project Table 2.2-7 Location Description for Key Cross-Sections in the HEC-RAS Model HEC-RAS River Station C ectio No. River Station (feet) RiveS)

Location Description of Cross-section Cross-section (miles)

Bay City USGS Station 1 RS 1665+21.6 31.54 Bridge Missouri Pacific Railroad 16 RS 1350+15.3 25.57 STP I & 2 Site 43 RS 891+46.0 16.89 Bridge at FM 521 47 RS 843+40.0 15.97 4600 ft upstream from Intra-Coastal 70 RS 383+64.5 7.27 Waterway Flooding in Streams and Rivers 2.2-29

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 Fukushima Response Project Elevation Scale, ft 0 1 2 4 8 8 0-5 1 30-40 5-10 - 40-50 10-20 1 50-60 20-30 60-70 Figure 2.2-1a General Location of STP I & 2 Site in the Lower Colorado River Basin Floodingin Streams and Rivers 2.2-30

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STPJI & 2 Fukushima Response Project IN Elevation scale, ft 295 - 528 E 528 - 994 994 - 1227 1227 - 1539 1539 - 2001 5 10 Figure 2.2-1b The Highland Lakes and Dams in the Lower Colorado River Basin (Reference 2.2-1a)

Flooding in Streams and Rivers 2.2-31

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project COLORADO RIVER BASIN Elevation scale, ft close to 0 0- 295 0 r--J 295 - 528 528 - 761

-- 761 - 994 r- 994 1227 rJ 1227-1539 1539 - 2001 Gulf of Mexico 0 20 40 W0 80 I miles Figure 2.2-Ic The Colorado River Streamf low Gauging Stations Downstream of Mansfield Dam Flooding in Streams and Rivers 2.2-32

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project Figure 2.2-2a Drainage Delineation of Subbasins between Mansfield Dam and Matagoda Bay (Modified from Reference 2.2-8)

Floodingin Streams and Rivers 2.2-33

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STPJ & 2 Fukushimna Response Project Figure 2.2-2b Drainage Delineation of Subbasins between Mansfield Dam and Matagoda Bay (Modified from Reference 2.2-8)

Floodingin Streams and Rivers 2.2-34

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 Fukushima Response Project Figure 2.2-3 Lower Colorado River Basin from Lake O.H. Ivie to Matagorda Bay (Modified from Reference 2.2-8)

Floodingin Streams and Rivers 2.2-35

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project Figure 2.2-4 Storm Orientation Pattern for Locations Upstream of Mansfield Dam (Modified from Reference 2.2-8)

Flooding in Streams and Rivers 2.2-36

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STPJ & 2 Fukushima Response Project Figure 2.2-5 Storm Orientation Pattern for Locations Downstream of Mansfield Dam (Modified from Reference 2.2-8)

Flooding in Streams and Rivers 2.2-37

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STP1 & 2 Fukushima Response Project 96-hour PMP Hyetograph at Sub-basin CC-06 14 12 -----------------------------------------------------------------------------

S10 4Z 8 -----------------------------------------------------------------------------

1 4 7 10 13 16 19 22 25 28 31 34 37 4043 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 Time (hours)

Figure 2.2-6 96-hour PMP Hyetograph for Subbasin CC-06 Floodingin Streams and Rivers 2. 2-38

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP1 & 2 Fukushima Response Project 1,600,000

!2/30 1/'2 I/5 1/8 1/111 1/14 1/tl7 1/20 Date Figure 2.2-7 PMF Hydrograph at Bay City for Scenario 1 Floodingin Streams and Rivers 2.2-39

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project 2,000,000 =t 3 -1,1163 ,473 ,S

.aBgcfs "-A=pdcipFt Ft-3 = 1M9cs --- - ------

1,600,000 ----- ------- ---------------------- 3- ------ --------

- - -I Outflow - -

Peak of- SPF I PN9 i43 944,138 ,

1,. 0000 --------- 1"-- --- ,----------

I ltrossext water leyMIs so~ A E1 811 NGr'V029 1,2O0,00*- -. -------- -L ------ ------------- - - -- - ---

I t-----

j-.----- ------

1------ It",

800,0000 8OOOO0- - ---------------- SPr--lt) 600,000 -------------- - 3--------

-SPF(t)+ PWF(t.3) kInow to LakeTriMs 400,000 - -- SPF(t)+PW(t43) Outlow from Lau Tfavis 200,0- - --- ------- j Nowelassitioe 12/tOi abism* cl...

0 12/31 1/3 1/6 1/9 1/12 1115 1/18 1/21 Date Figure 2.2-8 Development of PMF Outflow Hydrograph at Lake Travis for Scenario 2 Floodingin Streams and Rivers 2.2-40

Enclosure NOC-AE-13002975 FloodingHazardReevaluation Report STP1 & 2 Fukushima Response Project 1,2W0,000 t~atya say forScauwio 3 WA jf, i1000,000------ --- --- --------- ---- 4 ______________ -- -------- ---------------

PaskPWU0.3)- ft7VOw7cb I- Peek) PFP?) - t"3) + BaseF1t) 600,000 - - - - - - - - - - - - - - - - - - - - - - - - - -

200,000 ----------- ------ ------ --- ---------

- - k2M I~ordM.I23Ow hdsm Mae Figure 2.2-9 PMF Hydrograph at Bay City for Scenario 3 Floodingin Streams and Rivers 2. 2-41

Enclosure NOC-AE-1 3002975 FloodingHazard Reevaluation Report STP I & 2 Fukushima Response Project Figure 2.2-10 Extended Cross-Sections - Most Downstream Section to STP 1 & 2 Site Floodingin Streams and Rivers 2.2- 42

Enclosure NOC-AE-1 3002975 Flooding HazardReevaluation Report STP1 & 2 Fukushima Response Project J1

-1 FIii w I- PF IGrowld 40 LeftLft ILt Lwev 0

.10

~t:w S

I r 0S r3gI- 1A ?t 0 1oooo 3D000 60000 Figure 2.2-11 PMF Elevation at STP I & 2 Site for Normal Depth Boundary Condition (Manning's n values equal to 1.2 times those used in the Halff model)

Flooding in Streams and Rivers 2.2-43 I

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STP1 & 2 Fukushima Response Project C00400 1 A

Limits of cross-section extension Lef Levee 30-0- 10000

! lBS 2DOOO3000040000 Main Char* Distance (1t)

Figure 2.2-1 2 PMF Elevation at STP I & 2 Site for Normal Depth Boundary Condition (Manning's n values equal to those used in the Halff model)

Floodingin Streams and Rivers 2.2-44

Enclosure NOC-AE-1 3002975 Flooding Hazard Reevaluation Report STPJI& 2 Fukushima Response Project At STP I & 2 site (RS 891+46.0 ft) 401 Legend WS PF I 30O Grcund

-E-20D Lev"e a Ba2Sta

~ij 10 010 I

PMF elevalion at SIP I & 2 site 7/

261 ft

. . I

-10000 4 4o000 -400.0 -20000 0 . 2000 StawEnM~

At Downstream Boundary (RS 383+64.5 ft) 20-15- WS PF I 10-Levee 5-Ban?Sta 0-

/

PMF ckwiaticm at downst~ream txundar-*

17.5 ft 1

-120000 -860M -40000 -2000 0 20000 Staton (ft)

Figure 2.2-13 PMF Water Levels at STP I & 2 Site (RS 891+46.0) and at Downstream Boundary (RS 383+64.5) (Manning's n values equal to 1.2 times those used in the Halff model)

Floodingin Streams and Rivers 2.2-45

Enclosure NOC-AE-1 3002975 FloodingHazardReevaluation Report STP I & 2 FukushimaResponse Project At STP I & 2 site (RS 891+46.0 ft) 40 Legend WS PF I 30~

Leveed 2 -6t-E I 10 0-I l I a PMF elevation at STP I & 2 site 24.8 f

-100000 -60000 -40000 -20000 a 20000 S*taon (f1)

At Downstream Boundary (RS 383+64.5 ft) 20-Legend 15- WS PF I to- GrouNd Levee C 5- 0it 0

/

0-

.5- PMF elevation at dossnstrearm boundary 16.2 tl 2 - -. . . ! . . . .

-120000 -100000 -8000o -4000 40DDO -200oo 0 20000 Staon (ft)

Figure 2.2-14 PMF Water Levels at STP I & 2 Site (RS 891+46.0) and at Downstream Boundary (RS 383+64.5) (Manning's n values equal to those used in the Halff model)

Flooding in Streams and Rivers 2.2-46

NOC-AE-13002975, Enclosuflooding Hazard Reevaluation Report, Cover Through Page 2.2-46

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