ML25324A309

Enclosure 2: PSAR Subsection 2.4.3, Probable Maximum Flood on Streams and Rivers

ML25324A309
Person / Time
Site:	05000614, 99902117
Issue date:	11/20/2025
From:	Long Mott Energy
To:	Office of Nuclear Reactor Regulation
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ML25324A306	List: ML25324A307 ML25324A308 ML25324A309 ML25324A310 ML25324A311 ML25324A312
References
2025-PLM-NRC-013
Download: ML25324A309 (1)

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Enclosure 2 to Long Mott Energy, LLC, Letter No. 2025-PLM-NRC-013 Long Mott Energy, LLC PSAR Subsection 2.4.3, Probable Maximum Flood on Streams and Rivers

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-i November 2025 CHAPTER 2 SUBSECTION 2.4.3 PROBABLE MAXIMUM FLOOD ON STREAMS AND RIVERS LIST OF TABLES Number Title 2.4.3-1 All-Season PMP Precipitation Depths in Inches for the Guadalupe River Basin 2.4.3-2 PMF Basin Runoff Model Parameters 2.4.3-3 Largest Five Recorded Peak Discharges for USGS Gage No. 08167000, Guadalupe River at Comfort, Texas 2.4.3-4 Largest Five Recorded Peak Discharges for USGS Gage No. 08167500, Guadalupe River near Spring Branch, Texas 2.4.3-5 Largest Five Recorded Peak Discharges for USGS Gage No. 08176500, Guadalupe River at Victoria, Texas 2.4.3-6 Subbasin Drainage Areas and NWS Rainfall Used in Basin Runoff Model Calibration 2.4.3-7 Subbasin Runoff Parameters 1998 Calibration Results 2.4.3-8 Channel Elements Muskingum K and X 1998 Calibration Results 2.4.3-9 Subbasin Parameters 2004 Calibration Results 2.4.3-10 Channel Elements Muskingum K and X Values 2004 Calibration Results 2.4.3-11 PMF Basin Runoff Model Muskingum Channel Routing Coefficients K and X

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-ii November 2025 2.4.3-12 Canyon Dam Watershed Subbasins 2.4.3-13 Canyon Dam Watershed 1-Hour Unit Hydrographs for Subbasins 2.4.3-14 Canyon Dam Watershed Channel Elements Guadalupe River Storage-Discharge Relationships 2.4.3-15 Canyon Dam Elevation-Storage-Discharge Relationship 2.4.3-16 X and Y Coordinates of Storm Center and Optimized Orientation for PMP Estimates 2.4.3-17 PMF Development Starting Reservoir Water Levels 2.4.3-18 PMF Water Surface Profile along Guadalupe River 2.4.3-19 All-Season PMP Precipitation Depths in Inches for the Basin with Storm Center at the Long Mott Generating Station Site 2.4.3-20 Depth Duration Precipitation Values for Envelope Basins in Inches with Storm Center at the Long Mott Generating Station Site 2.4.3-21 Precipitation Depths in Inches for Each Realigned 6-Hour Time Period According to the Standardized Temporal Distribution from HMR 52 2.4.3-22 Curve Number Combinations 2.4.3-23 Calculation of Wind-Driven Waves and Wind Setup Approaching the Long Mott Generating Station Site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-iii November 2025 LIST OF FIGURES Number Title 2.4.3-1 Guadalupe and San Antonio River Basin Stream Gages 2.4.3-2 West Coloma Creek Watershed 2.4.3-3 Subbasin Delineation U.S. Army Corps of Engineers 2.4.3-4 Subbasin Delineation U.S. National Weather Service 2.4.3-5 1998 Flood Observed and Computed Hydrographs, Guadalupe River above Comal River (USGS No. 8168500) 2.4.3-6 1998 Flood Observed and Computed Hydrographs, Blanco River at Wimberley (USGS No. 8171000) 2.4.3-7 1998 Flood Observed and Computed Hydrographs, Plum Creek at Lockhart (USGS No. 8172400) 2.4.3-8 1998 Flood Observed and Computed Hydrographs, San Marcos River at Luling (USGS No. 8172000) 2.4.3-9 1998 Flood Observed and Computed Hydrographs, Sandies Creek near Westhoff (USGS No. 8175000) 2.4.3-10 1998 Flood Observed and Computed Hydrographs, Guadalupe River at Cureo (USGS No. 8175800)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-iv November 2025 2.4.3-11 1998 Flood Observed and Computed Hydrographs, Guadalupe River at Victoria (USGS no. 8176500) 2.4.3-12 1998 Flood Observed and Computed Hydrographs, Coleto Creek at Road Crossing near Schroeder (USGS No.

8176900) 2.4.3-13 1998 Flood Observed and Computed Hydrographs, Coleto Creek near Victoria (USGS No. 8177500) 2.4.3-14 2004 Flood Observed and Computed Hydrographs, Guadalupe River above Comal River (USGS No. 8168500) 2.4.3-15 2004 Flood Observed and Computed Hydrographs, Blanco River at Wimberley (USGS No. 8171000) 2.4.3-16 2004 Flood Observed and Computed Hydrographs, Plum Creek at Lockhart (USGS No. 8172400) 2.4.3-17 2004 Flood Observed and Computed Hydrographs, Plum Creek near Luling (USGS No. 8173000) 2.4.3-18 2004 Flood Observed and Computed Hydrographs, San Marcos River at Luling (USGS No. 8172000) 2.4.3-19 2004 Flood Observed and Computed Hydrographs, Guadalupe River at Gonzales (USGS No. 8173900) 2.4.3-20 2004 Flood Observed and Computed Hydrographs, Peach Creek at Dilworth (USGS No. 8174600)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-v November 2025 2.4.3-21 2004 Flood Observed and Computed Hydrographs, Sandies Creek near Westhoff (USGS No. 8175000) 2.4.3-22 2004 Flood Observed and Computed Hydrographs, Guadalupe River at Cuero (USGS No. 8175800) 2.4.3-23 2004 Flood Observed and Computed Hydrographs, Guadalupe River at Victoria (USGS No. 8176500) 2.4.3-24 2004 Flood Observed and Computed Hydrographs, Coleto Creek at Road Crossing near Schroeder (USGS No.

8176900) 2.4.3-25 2004 Flood Observed and Computed Hydrographs, Coleto Creek near Victoria (USGS No. 8176500) 2.4.3-26 HEC-RAS Cross Section Locations 2.4.3-27 HUC-12 Subwatershed Boundaries Adjacent to the Long Mott Generating Station Site 2.4.3-28 Tributary Areas to the Long Mott Generating Station Site 2.4.3-29 Direct Watershed Upstream of the Long Mott Generating Station Site 2.4.3-30 Sparks Watersheds, Hydraulic Structures, and Flow Direction 2.4.3-31 Drainage Ditches North of Site Tributary Area 2.4.3-32 Drainage Ditches North of Site Tributary Area - Blowup

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-vi November 2025 2.4.3-33 Topography of West Coloma Creek Watershed below Sparks Road 2.4.3-34 PMP Hyetograph for PMF over West Coloma Creek 2.4.3-35 Watershed Land Use 2.4.3-36 Watershed Hydrologic Soil Group 2.4.3-37 HEC-RAS 2D Model Outline and Boundary Conditions 2.4.3-38 HEC-HMS Model Input/Output for Sparks Watershed 2.4.3-39 Computational Mesh Overview 2.4.3-40 HEC-RAS Unsteady Computation Options and Tolerances 2.4.3-41 HEC-RAS 2D Model Results - Maximum Flow Depth 2.4.3-42 HEC-RAS 2D Model Results - Maximum Flow Depth at Long Mott Generating Station Site 2.4.3-43 HEC-RAS 2D Model Results - WSEL Upstream of Long Mott Generating Station Site 2.4.3-44 HEC-RAS 2D Model Results - Maximum Flow Velocity 2.4.3-45 HEC-RAS 2D Model Results - Maximum Flow Velocity at Long Mott Generating Station Site 2.4.3-46 HEC-RAS 2D Model Results - Maximum Fetch

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-vii November 2025 2.4.3-47 Guadalupe River Basin 2.4.3-48 San Antonio River Basin

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-viii November 2025 ACRONYMS AND ABBREVIATIONS Acronym/Abbreviation Definition 1D one-dimensional 2D two-dimensional ac.

acre(s)

ACES Automated Coastal Engineering System ac-ft acre-feet ANSI American National Standards Institute ANS American Nuclear Society CEDAS Coastal Engineering Design & Analysis System CEM Coastal Engineering Manual cfs cubic feet per second CI Conventional Island cm centimeter(s)

DWE Diffusion-Wave Equation FEMA Federal Emergency Management Agency FIS Flood Insurance Study fps feet per second ft.

feet ft2 square feet ha hectare(s)

HEC Hydrologic Engineering Center

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-ix November 2025 HEC-HMS Hydrologic Engineering Centers Hydrologic Modeling System HEC-RAS Hydrologic Engineering Centers River Analysis System HMR NWS Hydrometeorological Report hr.

hour(s)

HUC-12 12-Digit Hydrologic Unit Code in.

inch(es) km kilometer(s) km2 square kilometer kph kilometers per hour LMGS Long Mott Generating Station m

meter(s) m3 cubic meter mi.

mile(s) mi2 square mile mm millimeter mph miles per hour NAVD 88 North American Vertical Datum of 1988 NI Nuclear Island NLCD National Land Cover Database NOAA National Oceanic and Atmospheric Administration NRC U.S. Nuclear Regulatory Commission NWS U.S. National Weather Service

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-x November 2025 PMF probable maximum flood PMP probable maximum precipitation RFS River Forecast System s

second(s)

SCS Soil Conservation Service STA Station SWE Shallow-Water Equations TCEQ Texas Commission on Environmental Quality U.S.

United States USACE U.S. Army Corps of Engineers USGS U.S. Geological Survey VCS Victoria County Station WGRFC West Gulf Region Forecast Center WRCM Watershed Runoff Computer Model WSEL water surface elevation yr.

year(s)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-1 November 2025 Chapter 2 Site Characteristics 2.4 HYDROLOGY 2.4.3 PROBABLE MAXIMUM FLOOD ON STREAMS AND RIVERS The Long Mott Generating Station (LMGS) site is located at the southern end of the lower Guadalupe River on the east bank of the river downstream of its confluence with the San Antonio River and just upstream of the United States (U.S.) Geological Survey (USGS) gage near Tivoli, Texas, as shown on Figure 2.4.3-1. The natural ground at the site varies in elevation from approximately 27 ft (8.2 m) to above 29 ft (8.8 m) in North American Vertical Datum of 1988 (NAVD 88). The finished floor grade of all safety-related structures is at elevation 31.5 ft (9.6 m) NAVD 88. Although the LMGS site is not located inside the Guadalupe River basin, flooding from this river is analyzed as there is a possibility of overtopping the bluff areas on the east side of the Guadalupe River and impacting the site.

Near the LMGS site, the Guadalupe River drains an area of about 5953 mi2 (15,418 km2). There are 29 dams upstream of the LMGS site on the Guadalupe River and its tributaries with storage capacity in excess of 3000 ac-ft (3.7 million m3). The data pertinent to these dams, such as the type, dam height, top-of-dam elevation, storage volume, ownership, and location, are described in Section 2.4.1 and Section 2.4.4.

The most significant dams, in terms of flood storage capacity, are the Canyon Dam at river mile 303 on the Guadalupe River and the Coleto Creek Dam on Coleto Creek, a tributary of the Guadalupe River. The Canyon Dam has a top-of-dam elevation at 974.34 ft (296.98 m) NAVD 88 and a storage capacity of about 1.21 million ac-ft (1492.5 million m3) at that level, as presented in the 2005 Probable Maximum Flood (PMF) Study Report for Canyon Dam by the U.S. Army Corps of Engineers (USACE) (USACE, 2005a). The Coleto Creek Dam has a top-of-dam elevation at 119.71 ft (36.49 m) NAVD 88 and a storage capacity of 149,800 ac-ft (184.8 million m3), as given by U.S. National Weather Service (NWS) River Forecast System (RFS) for the Guadalupe River Basin down to Bloomington, Texas (NWS, 2007). See Section 2.4.3.1.

West Coloma Creek passes east of the LMGS site (see Figure 2.4.3-2). Because the safety-related facilities are located inside of its watershed, flooding of this creek due to probable maximum precipitation (PMP) is considered and analyzed separate from Guadalupe River.

Flooding from analysis for the San Antonio River is presented in Subsection 2.4.3.3. not considered as it has a smaller watershed area (i.e., 4194 mi2 [10,862 km2]) than the Guadalupe River and aA simultaneous PMP event on both watersheds is not reasonable. Instead, according to American National Standards Institute/American Nuclear Society (ANSI/ANS) document ANSI/ANS 2.8-1992, proper river discharge is considered simultaneous with PMP over Guadalupe River watershed as described in following subsections (ANSI/ANS, 1992).

Based on air temperature data given in Subsection 2.4.7, the low probability occurrence of snow within the Lower Guadalupe River Basin and its effect on flood-producing phenomena indicated that snow-melt and antecedent snow-pack are not critical factors in the production of floods at the LMGS site. In addition, because the drainage area of the Guadalupe River at the LMGS site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-2 November 2025 is about 5953 mi2 (15,418 km2), it is not expected that any urban developments in the basin would significantly alter the flood runoff characteristics of the watershed or the flood level at the LMGS site. Regarding the West Coloma Creek watershed, conservative assumptions are made to address such uncertainty.

In this subsection, the effects of the PMF in the Lower Guadalupe River and PMF in the West Coloma Creek on the safety-related facilities of LMGS site are evaluated. The effects of flooding resulting from local intense precipitation and from potential dam failures are addressed, respectively, in Section 2.4.2 and Section 2.4.4.

The following major hydrologic and hydraulic studies on the Guadalupe River Basin were performed by federal, state, and other local agencies. These studies/analyses include:

USACE, Reconnaissance Report, Canyon Lake Modification of Embankment, Guadalupe River, Texas, Fort Worth District, October 1979 (USACE, 1979).

USACE, Dam Assurance Study on Canyon Lake, Guadalupe Basin, Texas, Fort Worth District, June 2005 (USACE, 2005a).

USACE, Flood Forecast Model for Guadalupe River Basin, HEC-1 input data file, updated on August 12, 2004 (USACE,2004) with data on the Soil Conservation Service (SCS) reservoirs updated on February 4, 2008.

Albert H. Halff Associates, Inc., Dam Break Analysis for Coleto Creek Dam, for Guadalupe-Blanco River Authority, March 1989 (Halff, 1989).

Albert H. Halff Associates, Inc., Phase 1 Hydrologic Study, Coleto Creek Dam, prepared for the Guadalupe-Blanco River Authority, December 1992 (Halff, 1992).

URS Corporation, Bi-Annual Dam Inspection of Coleto Creek Dam, submitted to Guadalupe-Blanco River Authority, August 21, 2003 (URS, 2003).

Federal Emergency Management Agency (FEMA), Flood Insurance Study (FIS), Victoria County, Texas, Unincorporated Area, November 20, 1998 (FEMA, 1998).

FEMA, FIS, City of Victoria, Texas, Victoria County, July 21, 1999 (FEMA, 1999).

InFRM, Watershed Hydrology Assessment for the Guadalupe River Basin, September 2019 (InFRM, 2019).

Victoria County Station (VCS) nuclear plant Early Site Permit Application (VCS, 2007).

Among the existing flood study reports and data files listed above, there are four PMF studies performed for the Guadalupe River Basin. The first study was conducted by the USACE, who studied the Canyon Dam in the Upper Guadalupe River for a drainage area of about 1425 mi2 (3691 km2) (USACE, 2005a and USACE, 1979). The second study was performed by Albert H.

Halff Associates, Inc., for the Coleto Creek Dam on Coleto Creek, with a drainage area of about 491 mi2 (1272 km2), for the Guadalupe-Blanco River Authority (Halff, 1992). The third study is the InFRM analysis performed in 2019 that covers a large portion of the Guadalupe River Basin, but not the entire area. The main goal of the InFRM analysis was to establish the 100-yr. flood

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-3 November 2025 event, but it also provides 500-yr. flood event results. The fourth analysis is the one performed as part of the Early Site Permit application to the U.S. Nuclear Regulatory Commission (NRC) for the Victoria County StationVCS in 2007.

Because these studies are not directly applicable to the LMGS site, an independent PMF model study is being performed to evaluate the flooding impact on the safety-related facilities of LMGS.

The results of the VCS, 2007 analysis are provided in this submittal. Additional site-specific analyses and associated information that includes the postulated coincidental wind setup and wave setup will be provided by the end of 2025.

The PMF study performed for VCS adopted the basin characteristics from the USACE Flood Forecast Model (USACE, 2004), including the physical layout of the watershed elements such as sub-basin boundary definition, channel reach locations and physical characteristics, and dam/reservoir physical attributes.

In addition to the VCS analysis, an independent PMF analysis on the Guadalupe and San Antonio watersheds was performed as documented in Subsection 2.4.3.3 No study was found for the flooding in West Coloma Creek and therefore an independent analysis is performed as documented in Subsection 2.4.3.2.

2.4.3.1 Summary of VCS PMF Analysis The PMF hydrographs due to PMP over Guadalupe River Watershed were developed using the computer application HEC-HMS (Hydrologic Engineering Centers Hydrologic Modeling System)

(USACE, 2006b). The USACE Flood Forecast Model, which was developed in HEC-1 (USACE, 1990), was first converted to HEC-HMS format. The HEC-HMS model was then expanded to include the Coleto Creek Watershed using model data from the NWS RFS for the Guadalupe River Basin (NWS, 2007) and Halff Associates' model study for the Coleto Creek Watershed (Halff, 1992), as well as records from the USGS stream gaging stations.

The VCS analysis was based on USACE, 2005a and the calibrated basin runoff model used for the PMF development of the Canyon Dam Watershed was, therefore, applied directly in the VCS PMF model. The revised expanded model was further calibrated for the portion of the Guadalupe River Basin downstream of the Canyon Dam only.

The HEC-HMS model for the portion of the Guadalupe River Basin downstream of the Canyon Dam was calibrated during the VCS study using the observed rainfall and flood hydrograph data from two storms, which occurred in October 1998 and November 2004. According to the flood peak discharge data observed at the USGS gage on the Guadalupe River at Victoria, Texas, these are still the largest and fourth largest floods on record from 1935 to 2023. The fourth largest flood on record, the 2004 flood, was selected because the average basin rainfall data for the Guadalupe River Basin for the second largest flood (July 1935) and third largest flood (September 1981) on record are not available.

The calibrated HEC-HMS model for the Guadalupe River Basin downstream of the Canyon Dam to the LMGS site was expanded to include the Canyon Dam Watershed using the basin model parameters of the 2005 USACE PMF Study for that watershed (USACE, 2005a). A verification study was performed to ensure that the calibrated Watershed Runoff Computer

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-4 November 2025 Model (WRCM) HEC-1 model for the Canyon Dam Watershed was properly converted to the HEC-HMS model.

The 72-hr PMP estimates for the Guadalupe River near the LMGS site are derived following procedures described in NWS Hydrometeorological Reports (HMRs) Nos. 51 and 52 (NWS, 1978 and NWS, 1992 respectively), and using HEC-HMR 52, developed by Hydrologic Engineering Center (HEC) of the USACE (USACE, 1987).

For the watershed downstream of the Canyon Dam, the precipitation losses for the PMF model were conservatively established from the model calibrations of the 1998 and 2004 floods. For the Canyon Dam watershed, the precipitation losses used by the USACE in the 2005 PMF Study (USACE, 2005a) were adopted as given.

The Guadalupe River PMF hydrograph at the LMGS site was found to have a peak discharge of about 1,300,000 cfs (36,812 m3/s). The maximum PMF still water level at the LMGS site is estimated to be at elevation 30.7 ft (9.37 m) NAVD 88. This elevation is the average of Station (STA) 8.7744 and STA 11.1811 from HEC-RAS (Hydrologic Engineering Centers River Analysis System). The left bank elevation at this location is 33.9141.72 ft (12.710.34 m) NAVD 88, which indicates that the left bank of the river is not overtopped at the site location.

A 500-yr. flood event in the San Antonio River is postulated to be occurring coincidentally with the PMF event in the Guadalupe River. The 500-yr. flood flow from the San Antonio River is estimated to be about 180,000 cfs (5097 m3/s).

2.4.3.1.1 Probable Maximum Precipitation (PMP)

PMP depths for the Guadalupe River Basin are derived following the procedures described in NWS HMR Nos. 51 and 52 (NWS, 1978 and NWS, 1992 respectively) and using the computer program HEC-HMR 52 (USACE, 1987).

In using HEC-HMR 52, the PMP estimates and the storm orientation for the basin of interest for the various area sizes and durations are required as inputs to the program. They are derived from NWS HMR Nos. 51 and 52 and are presented in Table 2.4.3-1.

HEC-HMR 52 also requires the X and Y coordinates of the boundaries of the river basin and of each of the sub-basins, as well as the preferred storm orientation, which is 195 degrees°, as suggested in NWS HMR No. 52 for this area. The boundaries of the Guadalupe River Basin and its sub-basins are shown on Figure 2.4.3-3. The program estimates the hourly PMP values for each of the sub-basins for a particular storm center in the basin and the hourly PMP values are stored in the data storage system (USACE, 2006a) to be recalled for use in flood hydrograph developments.

In accordance with guidelines suggested by ANSI/ANS 2.8-1992, Subsection 9.2.1.1 (ANSI/ANS, 1992), an antecedent storm, equal to 40 percent of the 72-hr PMP, is assumed to end three days before the start of the 72-hr PMP.

2.4.3.1.2 Precipitation Losses For the watershed downstream of Canyon Dam, the adopted initial losses for the sub-basins vary from 0.05 in. (1.27 mm) to 1.0 in. (25.4 mm), while the constant loss rates of 0.05 in/hr

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-5 November 2025 (1.27 mm/hr) and 0.1 in/hr (2.54 mm/hr) are used. These losses are derived primarily from the results of the model calibrations of the 1998 and 2004 floods and adjusted conservatively for use in the PMF developments. For the Canyon Dam Watershed, the values used by the USACE in its 2005 PMF study, an initial loss of 1.0 in. (25.4 mm) and a constant loss of 0.15 in/hr (3.81 mm/hr), with the losses for the surface of Canyon Lake being zero, are adopted. The loss values for each of the sub-basins used in the PMF development are presented in Table 2.4.3-2.

2.4.3.1.3 Runoff and Stream Course Models The PMF model adopts the basin characteristics from the Flood Forecast Model developed by the USACE, Fort Worth District, Texas, for the Guadalupe River Basin down to the USGS gage at Victoria, Texas (USACE, 2004). The basin characteristics include the physical layout of the watershed elements, such as sub-basin boundary definitions, channel reach locations and physical characteristics, and dam/reservoir physical attributes for basin runoff calibration. The USACE model includes the Canyon Dam because of its large flood storage capacity. In addition, two small agriculture-related reservoirs located in the San Marcos River Basin were modeled. They are the SCS No. 3 and SCS No. 5 reservoirs with the top of dam elevations at 648.5 ft (197.66 m) NAVD 88 and 667.2 ft (203.36 m) NAVD 88, and maximum storage capacities of about 4000 ac-ft (4.93 million m3) and 7000 ac-ft (8.63 million m3), respectively (USACE, 2004).

The USACE model only covers the portion of the basin upstream of the USGS gage at Victoria, Texas. It does not include the drainage area from Coleto Creek, a tributary of Guadalupe River, which joins the main river downstream of the gage at Victoria. The Coleto Creek Watershed, together with the Coleto Creek Dam/Reservoir, is modeled by including the drainage areas given by the USGS at gaging station No. 08176900 (USGS, 2008) and by Halff Associates for the Coleto Creek Dam/Reservoir (Halff, 1992). The drainage area downstream of the Coleto Dam to its confluence with Guadalupe River, the sub-basin boundaries, and the elevation-storage-discharge relationships for the Coleto Creek Dam/reservoir are those given in the NWS RFS for the Guadalupe River Basin near Bloomington, Texas (Station DUPT2) (NWS, 2007).

In the 1979 PMF study for the Canyon Dam (USACE, 1979), USACE calibrated the runoff response characteristics of the watershed with the August 1978 flood flows observed at the Johnson Creek gage near Ingram, the North Fork gage near Hunt, and the Guadalupe River gages near Hunt, Comfort, and Spring Branch. USACE updated the model in 2005 (USACE, 2005a) using the WRCM (USACE, 1985) with the same basin runoff response characteristics, but including additional PMP data from NWS HMR 52 (NWS, 1992). The 1978 flood is still the flood of record to date for the part of the Guadalupe Watershed upstream of Canyon Dam, which has a drainage area of about 1432 mi2 (3709 km2) (Table 2.4.3-3 and Table 2.4.3-4). It is, therefore, reasonable to postulate that the runoff parameters established in the USACE PMF model adequately represent the basin response during extreme floods and no new calibration of the Canyon Dam Watershed is necessary. The calibration efforts, therefore, concentrate only on the portion of the Guadalupe Watershed downstream of the Canyon Dam.

The resulting composite watershed and the sub-basins, including those for the Canyon Dam Watershed used in the 2005 USACE PMF model, are shown on Figure 2.4.3-3.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-6 November 2025 2.4.3.1.4 Runoff Model Calibrations The HEC-HMS model developed for the watershed downstream of Canyon Dam is calibrated using the flood records of the October 1998 and November 2004 storms. The storms of October 1998 and November 2004 produced the largest and the fourth largest floods on record from 1935 to 2006 at the USGS gaging station at Victoria, Texas, as indicated in Table 2.4.3-5. The observed hourly rainfall depths for each of the sub-basins, from October 13, 1998, to October 30, 1998, and from November 15, 2004, to December 3, 2004, are obtained from the NWS West Gulf Region Forecast Center (WGRFC). There are a total of 13 USGS stream gaging stations on the Guadalupe River downstream of the Canyon Dam to the LMGS site for which flood hydrograph data are available for the 1998 and 2004 floods. The observed 15-minute flood flow hydrographs from these gages are used for the calibration (USGS, 2008). The gaging stations are:

USGS No. 08168500 - Guadalupe River above Comal River with a drainage area of 1518 mi2 (3932 km2)

USGS No. 08173900 - Guadalupe River at Gonzales with a drainage area of 3490 mi2 (9039 km2)

USGS No. 08175800 - Guadalupe River at Cuero with a drainage area of 4934 mi2 (12,779 km2)

USGS No. 08176500 - Guadalupe River at Victoria with a drainage area of 5198 mi2 (13,463 km2)

USGS No. 08171000 - Blanco River at Wimberley with a drainage area of 355 mi2 (919 km2)

USGS No. 08171300 - Blanco River near Kyle with a drainage area of 412 mi2 (1067 km2)

USGS No. 08172000 - San Marcos River at Luling with a drainage area of 838 mi2 (2170 km2)

USGS No. 08172400 - Plum Creek at Lockhart with a drainage area of 112 mi2 (290 km2)

USGS No. 08173000 - Plum Creek near Luling with a drainage area of 309 mi2 (800 km2)

USGS No. 08174600 - Peach Creek below Dilworth with a drainage area of 460 mi2 (1191 km2) (2004 Storm Only)

USGS No. 08175000 - Sandies Creek near Westhoff with a drainage area of 549 mi2 (1422 km2)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-7 November 2025 USGS No. 08176900 - Coleto Creek at Arnold Road Crossing near Schroeder with a drainage area of 357 mi2 (925 km2)

USGS No. 08177500 - Coleto Creek near Victoria with a drainage area of 514 mi2 (1331 km2)

In the calibration of the HEC-HMS model, the 1998 and 2004 observed flood hydrographs of Guadalupe River at the Sattler, Texas, gage (USGS Gage No. 08167800) are used as inflows to the model basin. The Sattler gage is located immediately downstream of Canyon Dam with a drainage area of 1436 mi2 (3719 km2). The locations of the USGS gauging stations are shown in Figure 2.4.3-1.

The model basin of the USACE Flood Forecast Model is subdivided into 22 sub-basins, linked respectively by 16 channel reaches. The model uses the Muskingum channel routing method for 13 of these 16 channels reaches and the Modified Puls method with prescribed storage-discharge relationship for the remaining three channel reaches. In the calibration process, only the K and X values are adjusted and the storage-discharge relationships remain unchanged as defined by the USACE.

As noted in Section 2.4.3.1, the USACE Flood Forecast Model does not include the Coleto Creek Watershed. Thus, the Coleto Creek Watershed and Reservoir are added to the model.

The stage-storage and storage-discharge relationships for Coleto Creek Dam/Reservoir from NWS RFS for the Guadalupe River Basin are adopted instead of those from URS, 2003 because they are more current (dated March 2007) and are used by NWS WGRFC in its current flood forecast model for the Guadalupe River Basin. The runoff model of the Coleto Creek Watershed at its confluence with the Guadalupe River is represented by three sub-basins, Sub-basins 29, 30, and 31 (see Figure 2.4.3-3), and three channel reaches.

The historical observed rainfall data for the 1998 and 2004 floods are obtained from NWS WGRFC for the sub-basins shown on Figure 2.4.3-4. Comparisons of the individual drainage boundaries of the respective sub-basins, as shown on Figure 2.4.3-3 and Figure 2.4.3-4, indicate that there are some minor differences in the sub-basin definitions. In some parts of the basin, the USACE definitions are more refined, resulting in more sub-basins, and the reverse is true for other areas. In areas where a USACE sub-basin consists of more than one NWS sub-basin, the area-weighted average of the NWS sub-basin rainfall depths is used to approximate the rainfall depth of the corresponding USACE sub-basin. In the case where the NWS sub-basin encompasses a number of USACE sub-basins, the average rainfall depth of that NWS sub-basin is assumed to be applicable to all the corresponding USACE sub-basins. Table 2.4.3-6 provides the names of the NWS sub-basin rainfall data files used as inputs to the HEC-HMS for the sub-basins downstream of Canyon Dam. With a given drainage area and rainfall input sequence, the runoff characteristics of a basin are defined by four groups of parameters in the HEC-HMS rainfall-runoff model, namely:

Basin losses Runoff characteristic of rainfall excess to the conveyance channels Base flow characteristics Channel and reservoir flood routing characteristics

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-8 November 2025 For the VCS PMF study, the basin losses are represented by an initial loss and a constant loss rate. The Snyder's lag and peaking are used to define the runoff characteristics. Further calibration is provided in Subsection 2.4.3 of VCS, 2007.

The plots of the observed and computed flood hydrographs at each of the USGS gaging stations used in the calibration process for the 1998 and 2004 floods are depicted in Figure 2.4.3-5 through Figure 2.4.3-25. The calibrated basin runoff parameters, namely, the basin loss values, the Snyder's lag and peaking coefficients, the base flow recession coefficients, and Muskingum K and X values for the 1998 and 2004 floods, are also presented in Table 2.4.3-7 through Table 2.4.3-10.

2.4.3.1.5 Probable Maximum Flood Flow The adopted basin runoff parameters for the PMF development for each of the basin elements for the Guadalupe River Basin downstream of the Canyon Dam are shown in Table 2.4.3-2 and Table 2.4.3-11. They are developed from the calibrated basin runoff parameters given in Table 2.4.3-7 through Table 2.4.3-10 by selecting the more conservative values of the two. To account for nonlinearity effects of extreme flood conditions, the calibrated Snyder's lags for the sub-basins are reduced by 15 to 20 percent. A lag reduction is typically suggested for PMF development (USACE, 1994) even though the model is calibrated with the extreme flood of October 1998.

In the 2005 PMF study for the Canyon Dam (USACE, 2005a), the USACE subdivided the watershed above the dam into nine sub-basins instead of the 24 in the Flood Forecast Model (USACE, 1979). The definitions of the sub-basins and respective drainage areas in the 2005 study are presented in Table 2.4.3-12. Note that the total drainage area for the Canyon Dam is shown as 1417.85 mi2 (3672 km2), which is slightly less than the drainage area of 1436 mi2 (3719 km2) given by USGS. The sub-basin delineation for the Flood Forecast Model is modified for the Canyon Dam Watershed to match those given in the 2005 PMF study for the same area, as shown in Figure 2.4.3-3. These revised sub-basin boundaries, including the watershed for the Coleto Creek Basin, are used in the PMF model development.

In the same 2005 study, the unit hydrographs for each of the sub-basins were specified by the USACE and are presented in Table 2.4.3-13. A storage-discharge relationship was defined by the USACE for each of the five channel elements for flood routing through the channel reaches in the watershed, as shown in Table 2.4.3-14. The elevation-storage-discharge relationship for the Canyon Dam and reservoir is presented in Table 2.4.3-15. The basin runoff routing parameters for each of these sub-basins are obtained from the WRCM.

The hourly PMP estimates for each of the sub-basins, with storm centers as given in Table 2.4.3-16, are used as input to the calibrated PMF HEC-HMS model with the loss and base flow parameters as given in Table 2.4.3-2.

In accordance with the combined-event criterion stated in Subsection 9.2.1.1 of ANSI/ANS 2.8-1992 (ANSI/ANS, 1992), an antecedent rainfall equal to 40 percent of the PMP should be simulated as part of the PMF flood level determination. As suggested in Subsection 5.2.7.1 of ANSI/ANS 2.8-1992 (ANSI/ANS, 1992), an antecedent storm preceding the PMP by three days is selected. Using a 72-hr PMP, this combined-event criterion requires generating a 40 percent PMP sequence and placing it six days ahead of the PMP estimates. To simulate this, the HEC-HMS is first run using the 40 percent PMP event. The simulation is re-started for the full PMP

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-9 November 2025 event with the starting water levels in the four reservoirs, Canyon Lake, SCS Nos. 3 and 5, and the Coleto Creek Reservoir, equal to the predicted reservoir levels at three days after the cessation of the 40 percent PMP event, as shown in Table 2.4.3-17.

2.4.3.1.6 Water Level Determinations The water surface profile in the Guadalupe River for the postulated PMF condition is estimated using the steady state routing option of the computer program HEC-RAS, version 3.1.3 (USACE, 2005b). The river channel and cross section geometry of the Guadalupe River from San Antonio Bay are established from the digital terrain map of the area (USGS, 2007). The Manning's n values are conservatively assumed to be 0.1 for both the channel and over-bank areas. Section 2.4.4 discusses the selection of this Manning's n value. The locations of these cross sections are presented in Figure 2.4.3-26. The downstream boundary condition is assumed to be at normal depth with a slope equal to 0.00016, which is the average of the bed slopes between the 10-ft to 20-ft (3-to 6-m) contour, equal to 0.00032, and that near the confluence between Guadalupe and San Antonio Rivers, which is basically flat.

The San Antonio River joins the Guadalupe River upstream of Tivoli, Texas. For the PMF prediction, a 500-yr. flood in the San Antonio River Basin is assumed to occur coincidentally with a PMF event in the Guadalupe River Basin.

The USGS gaging station on the San Antonio River closest to its confluence with the Guadalupe River and with a long stream flow record for flood frequency analysis is at Goliad, Texas (USGS Gage No. 0818850). At this gage, the San Antonio River drains an area of about 3921 mi2 (10,155 km2). A flood frequency analysis is performed using 75 yr. of data, assuming the Log-Pearson Type III distribution and following the formulations suggested by Hamead and Rao (Hamad and Rao, 2000) and USGS Bulletin 17 B (USGS, 1982). The 500-yr. flood peak discharge at Goliad is found to be about 164,000 cfs (4644 m3/s).

The San Antonio River drainage area at its confluence with the Guadalupe River is estimated to be about 4180 mi2 (10,826 km2). By prorating the peak discharge using a drainage area ratio, the San Antonio River 500-yr. flood peak discharge at its confluence with the Guadalupe River is determined to be 180,000 cfs (5097 m3/s). This flow rate is added to the Guadalupe River PMF peak discharge of 1,123,300 cfs (31,808.3 m3/s), yielding a total flood discharge of about 1,303,300 cfs (36,905 m3/s).

The PMF peak discharge value of 1,300,000 cfs (36,812 m3/s) is used for the cross sections downstream of the confluence of the San Antonio River in the HEC-RAS model in determining the PMF water level. Upstream of that confluence, the PMF peak discharge used in the model is 1,120,000 cfs (31,715 m3/s). The PMF inflow discharge from Coleto Creek to the Guadalupe River at the time of the PMF peak discharge in the Guadalupe River is estimated to be about 20,000 cfs (566 m3/s) from the HEC-HMS run. Therefore, for the reach of the Guadalupe River upstream of its confluence with Coleto Creek, the PMF peak discharge used in the simulation is about 1,100,000 cfs (31,149 m3/s). The PMF water surface profile along the Guadalupe River from STA 7.247 to STA 66.256 is shown in Table 2.4.3-18. The PMF flooding water level of the Guadalupe River near the LMGS site (average of river mile 11.1811 and 8.7744) is found to be about 30.74 ft (9.37 m) NAVD 88.

The HEC-RAS model does not include the inflatable Lower Guadalupe Salt Water Barrier and Diversion Dam (Fabridam) located near river mile 11. Because this inflatable dam would rupture

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-10 November 2025 when a hydraulic head against the dam exceeds about 4.8 ft (1.46 m), when inflated (GBRA, 1994), it would not have any effect on the PMF level.

2.4.3.1.7 Coincident Wind-Wave Activity The 2-yr. mean recurrence interval annual extreme-mile wind speed was obtained from ANSI/ANS 2.8-1992 (ANSI/ANS, 1992). Wave runup is estimated to be 6.4 ft (1.95 m) for a 50 mph (80.46 kph) design wind speed.

Wind setup is estimated to be 1.8 ft (0.55 m).

2.4.3.1.82.4.3.1.7 Probable Maximum Flood Water Level over Guadalupe River The maximum PMF still water level of the Guadalupe River at the LMGS site, before wind-wave induced setup and run-up, is predicted to be at elevation 30.74 ft (9.37 m) NAVD 88. Adding the conservative combined wind setup and maximum wave run-up prediction of about 8.2 ft (2.5 m),

the maximum PMF flooding water level at the LMGS site is postulated to be at elevation 38.94 ft (11.87 m) NAVD 88. This is higher than the 33.91 ft (10.34 m) NAVD 88 by 5.03 ft (1.53 m).

Therefore, the left bank of the river will be overtopped by wind-wave runup effect. The site will not be flooded during Guadalupe River PMF event as the overtopped spillage will be distributed over the low-lying area behind the riverbank toward the site. Additionally, the site is raised by a minimum of 4 ft (1.2 m) from the low-lying areas surrounding the site.

2.4.3.2 West Coloma Creek Probable Maximum Flood 2.4.3.2.1 Watershed Description West Coloma Creek watershed is in Calhoun County, Texas. To identify West Coloma Creek watershed, the 12-Digit Hydrologic Unit Code (HUC-12) watershed boundaries were used. The adjacent drainage boundaries with their respective names, along with the general topography of the area, are shown in Figure 2.4.3-27. The HUC-12 boundaries near the site were refined and a direct tributary area was delineated and presented in Figure 2.4.3-28. The entire watershed runs from downstream to upstream from Powderhorn Lake in Matagorda Bay to approximately 4.5 mi (7.2 km) north of Green Lake (see Figure 2.4.3-2). West Coloma Creek watershed has a size of 102.32 mi2 (265.01 km2). The drainage area north of Farik Road is 9.1 mi2 (1.7 mi2 + 5.5 mi2 + 1.9 mi2) (4.4 km2+ 14.2 km2 + 4.9 km2) (23.5 km2). The drainage area south of Farik Rd.

upstream of Jesse Rigby Road (i.e., the LMGS site) is 4.6 mi2 (11.9 km2). Therefore, the total watershed area upstream of the LMGS site is 23 mi2 (60 km2) (Figure 2.4.3-29). Of this area, 15.5 mi2 (40.1 km2) flows toward the site and the rest is part of East Coloma Creek. The portion of the watershed that is upstream of Sparks Road is 7.2 mi2 (18.6 km2) and is called Sparks Watershed in this analysis (Figure 2.4.3-30).

The 2018 USGS LiDAR data for South Texas (USGS, 2018) with nominal pulse spacing of 2.3 ft (0.8 m) and vertical accuracy of 8 in. (5.8 cm) at a 95 percent confidence level is used in this analysis. The data were developed based on a horizontal projection/datum of North American Datum of 1983 (2011), Universal Transverse Mercator Zone 14, meters and vertical datum of NAVD 88 (GEOID12B) meters. Elevations on the north and south side of Farik Road are presented in Figure 2.4.3-31 and Figure 2.4.3-32. They depict the approximate overtopping

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-11 November 2025 elevations that would need to be achieved for water to flow from the north tributaries into the site's direct tributary. Topography of the watershed downstream of the Sparks Road is presented in Figure 2.4.3-33 and shows that the watershed elevations vary from 3 ft (0.9 m)

NAVD 88 to 39 ft (12 m) NAVD 88 from downstream to upstream with the existing grade varying from at 26 ft (7.9 m) NAVD 88 to 28 ft (8.5 m) NAVD 88 around the LMGS site.

2.4.3.2.2 Probable Maximum Precipitation (PMP)

PMP depths for the West Coloma Creek basin are derived following the procedures described in NWS HMR Nos. 51 and 52 (NWS, 1978 and NWS, 1992 respectively).

In using HEC-HMR 52, the PMP estimates and the storm orientation for the basin of interest for the various area sizes and durations are required as inputs to the program. They are derived from NWS HMR Nos. 51 and 52 and are presented in Table 2.4.3-19. Instead of considering multiple storm centers and developing multiple PMPs, conservatively, the 10-mi2 (26-km2) PMP developed for the site is applied to the entire watershed. This is reasonable because the direct contributing watershed upstream of the LMGS site is only 15.5 mi2 (40.1 km2). 6-hourly precipitation depths were developed by interpolating between the 12-and 24-hr, 24-and 48-hr, and 48-and 72-hr depths (see Table 2.4.3-20).

In accordance with guidelines suggested by ANSI/ANS 2.8-1992, Subsection 9.2.1.1 (ANSI/ANS, 1992), an antecedent storm, equal to 40 percent of the 72-hr PMP, is assumed to end 3 days before the start of the 72-hr PMP.

The standardized distribution for a 72-hr PMP storm event from HMR 52, Section 3.1, page 16, is used to establish the PMP hyetograph. Table 2.4.3-21 presents the order of incremental PMP rainfall. The entire applied PMP hyetograph including the antecedent rainfall is presented in Figure 2.4.3-34.

2.4.3.2.3 Precipitation Losses The SCS curve number method is a standard, widely used, and efficient method for determining the amount of runoff from rainfall events. A combination of land use and hydrologic soil group are used to determine curve number. The Conterminous U.S. land cover (i.e., the National Land Cover Database [NLCD]) at a 30-m (98-ft) spatial resolution with a 16-class legend based on a modified Anderson Level II classification system (USGS, 2021) is used and presented in Figure 2.4.3-35. The majority of the basin is covered with cultivated crops or hay/pasture.

Soils are classified by the Natural Resource Conservation Service into four Hydrologic Soil Groups, A, B, C, and D, based on the soil's runoff potential, where soils classified as A generally have the smallest runoff potential and soils classified as D the greatest. The Soil Survey Geographic Database is used in this analysis (NRCS, 2024) and presented in Figure 2.4.3-36.

Most of the basin is covered with soil type D.

Table 2.4.3-22 shows the curve number for each soil group for each land use category. As described above, for the combination of hydrologic soil type D and agricultural land use in the watershed, the corresponding curve number is 87. A curve number of 90 is conservatively applied to the entire watershed.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-12 November 2025 2.4.3.2.4 Runoff Hydrograph Determination The Sparks watershed presented in Figure 2.4.3-30 combined with the Farik watershed contribute flow to the site. Sensitivity analysis showed that Farik Road is overtopped during a PMP event. The majority of the runoff hydrograph is captured by rain on grid in HEC-RAS 2D model Version 6.4.1 (See Section 2.4.3.2.5). However, the HEC-HMS Version 4.11 hydrology model is used to estimate the runoff hydrograph of the Sparks watershed as the upstream boundary condition, which is outside the footprint of the HEC-RAS 2D model, presented in Figure 2.4.3-37.

The following conservative inputs were applied to the HEC-HMS model:

Curve number of 90 (see Section 2.4.3.2.3)

Lag time is calculated as 5 hr, but, conservatively, a lag time of 10 minutes. was applied PMP hyetograph from Section 2.4.3.2.2 (Figure 2.4.3-34)

Although impervious area is less than 5 percent, conservatively, 20 percent impervious area was used in the model The runoff hydrograph generated for the Sparks watershed as the upstream boundary condition is presented in Figure 2.4.3-38 with maximum flow associated with 40 percent PMP and full PMP estimated as 10,000 cfs (283 m3/s) and 25,000 cfs (708 m3/s), respectively.

2.4.3.2.5 Hydraulic Model Setup and Flood Routing A detailed, two-dimensional (2D) flood routing model developed for the LMGS site and its direct watershed (see Figure 2.4.3-37) is used to establish depth of flooding and maximum velocities.

This model, HEC-RAS 2D (USACE, 2020), represents all the topographical and man-made features (i.e., buildings, tanks, and hydraulic structures) that significantly affect runoff at the LMGS site and its local watershed.

By using a 2D model, floodwater is routed in a natural manner without being forced to flow in predefined directions. This allows for a more accurate flood analysis than is possible with one-dimensional (1D) models. The HEC-RAS 2D Reference Manual (USACE, 2020) describes the HEC-RAS model as follows: HEC-RAS is designed to perform one-dimensional (1D), two-dimensional (2D), or combined 1D and 2D hydraulic calculations for a full network of natural and constructed channels. HEC-RAS solves the Diffusion-Wave Equation (DWE) and Shallow-Water Equations (SWE). Additionally, the HEC-RAS model is approved by FEMA FIS (FEMA, 2020). HEC-RAS is also approved by the NRC to perform local intense precipitation analysis, as described in NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America.

Topographic LiDAR (USGS, 2018) was used for construction of a terrain model used for the topography and bathymetry of the model domain. The extent of the HEC-RAS 2D model is illustrated in Figure 2.4.3-37.

The model boundaries are established away from the Nuclear Island/Conventional Island (NI/CI) area and safety-related structures, systems, and components to prevent boundary conditions

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-13 November 2025 from affecting flood levels evaluated within the NI/CI area and to ensure the stability of the model. The two ditches upstream of Farik Road redirect part of the upstream runoff toward Green Lake (see Figure 2.4.3-30). Farik Road is overtopped during a PMP event and HEC-RAS 2D determines the flow that passes over the road and flows toward the site. Model boundary conditions are comprised of the following:

Upstream: Sparks basin flow hydrograph (Figure 2.4.3-38)

Downstream: Normal flow boundary condition with conservative slope of 0.001 Western Boundary: selected based on sensitivity analysis and set to normal flow with slope of 0.001 Entire model domain: Rain on grid with hyetograph presented in Figure 2.4.3-34 The HEC-RAS 2D model covers an area of approximately 47,836 ac. (19,359 ha); about 5.8 mi (9.3 km) in the east-west direction and about 20.7 mi (33.3 km) in the northwest-southeast direction (Figure 2.4.3-37). The numerical grid was generated with the RAS Mapper module.

The horizontal grid size was 200 by 200 ft (61 by 61 m) within the entire model domain. The model grid is refined to 10 by 10 ft (3 by 3 m) at West Coloma Creek, the edges of the embankments, and ditches around the site. The model has 275,235 cells (Figure 2.4.3-39).

Sensitivity analysis on the size of the mesh was performed and confirmed that the selected cell size results in similar output as smaller cells (i.e., 50 ft x 50 ft [15 x 15 m]). This is mainly due to the underlying features of HEC-RAS 2D, which consider:

Volume rating curves of each cell based on detailed LiDAR data within footprint of each cell Profile along all faces of the cell based on detailed LiDAR data Each cell in the model is assigned a Manning's roughness coefficient. Roughness coefficients are assigned using land cover categories. Several sources report Manning's roughness coefficients for flow over various surfaces. The following three sources are referenced here:

The U.S. Department of Agriculture Natural Resources Conservation Service's Technical Release 55 (TR-55) (USDA-SCS, 1986)

HEC-RAS Reference Manual (USACE, 2020)

Open-Channel Hydraulics (Chow, 1988)

The references listed above provide a range of acceptable values for Manning's roughness coefficients for overland floodplain flow. Because flow is generally expected to be directed away from the NI/CI area, a higher Manning's roughness coefficient generally results in higher water levels. This conclusion is based on Manning's equation, which indicates that an increased roughness (n) will result in slowed velocities and an increased hydraulic radius (R), leading to deeper flow. Roughness associated with each land use type is presented in Figure 2.4.3-35.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-14 November 2025 1.49

/ /

(Equation 2.4-1) where:

V = velocity (ft/s) n = the Manning's roughness coefficient (dimensionless)

S = the topographic slope in the direction of flow (ft/ft)

R = the hydraulic radius (ft)

R = A/P (flow area [ft2]/wetted perimeter [ft])

The main model control parameters are provided in Figure 2.4.3-40. The computation interval is used in the unsteady flow calculations. This is one of the most important parameters entered into the model. Choosing this value should be done with care and consideration as to how it will affect the simulation. The computation interval should be based on several factors. First, the interval should be small enough to accurately describe the rise and fall of the hydrographs being routed. A general rule of thumb is to use a computation interval that is equal to or less than the time of rise of the hydrograph divided by 20 (USACE, 2020). A second way of computing the appropriate time step is by applying a numerical accuracy criterion called the Courant condition.

The Courant condition criteria looks at cross section spacing and flood wave velocity. The basic premise is that the computational interval should be equal to or less than the time it takes water to travel from one cross section to the next. A detailed description of the Courant condition can be found in the HEC-RAS2D user manual (HEC-RAS2D, 2020). Use of a time step based on the Courant condition will give the best numerical solution, but it may cause the model to take a lot longer to run. In this analysis, sensitivity analysis showed that a 20-s computation interval results in a stable model and captures the hydraulics of the PMP hyetograph.

The following main hydraulic structures were identified along West Coloma Creek during site survey and considered in the model as presented in Figure 2.4.3-30 and Figure 2.4.3-37:

Whitley Road: three reinforced concrete pipe culverts, each 4 ft (1.2 m) in diameter Highway 35: Five reinforced concrete box culverts, each 5 ft by 5 ft (1.5 m by 1.5 m)

Jesse Rigby Road: Four reinforced concrete box culverts, each 8 ft by 8 ft (2.4 m by 2.4 m)

Farmland FM 2235: A road crossing exists but is considered as blocked. This is conservative as the road crossing is downstream of the site.

Due to the intensity of PMP event and small size of West Coloma Creek compared with the flat floodplain, sensitivity analysis showed that hydraulic structures do not impact the results when 2D modeling is being used. Brake lines were added to the top of roads and basin embankments

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-15 November 2025 to capture their topography in the model. Basins 31 and 5 (see Figure 2.4.3-39) are considered to be at their normal operating level during PMP and they are not overtopped.

Water surface elevations, depths, and velocities around the LMGS site were investigated and velocities near edges of the site were compared to permissible velocities published in a comprehensive literature review performed by the U.S. Army Engineer Research and Development Center as part of the Ecosystem Management and Restoration Research Program (USACE, 2001).

Figure 2.4.3-41 and Figure 2.4.3-42 present the maximum water depth and water surface elevation for the entire 2D model domain and for the area focused on the site, respectively.

Model results show that the maximum water elevation around the site is approximately 31 ft (9.4 m) NAVD 88. This elevation is on the north-western and eastern boundaries of the site. Figure 2.4.3-43 presents the water surface elevation time series for a location at the northern edge of the LMGS site. This figure shows that the water surface elevation is less than 31 ft (9.4 m).

Minor inundation shown on Figure 2.4.3-42 at the edges of the site is due to direct rainfall on the site itself that will be drained with proper site grading.

Figure 2.4.3-44 and Figure 2.4.3-45 present the maximum velocity for the entire 2D model domain and for the area focused on the site, respectively. Maximum velocity occurs on the north-eastern corner of the site with an approximate velocity of 2 fps (0.6 m/s). According to Table 2 of USACE, 2001, the critical velocity for a 6-in. (15-cm) gravel is 4 fps (1.2 m/s) and for 2-in. (5-cm) gravel is 3 fps (0.9 m/s). Therefore, it is necessary to protect the berm around the site with boulders of medium to large size with proper geotextile and grading underneath to protect against scouring. Outside the footprint of the site velocity is around 2 fps (0.6 m/s) and because the area is covered with vegetation, erosion is not expected as the permissible velocity for short native and bunch grass is 3 fps (0.9 m/s) per Table 2 of USACE, 2001.V 2.4.3.2.6 Coincident Wind Activity Wind-driven waves were calculated using the Wind Speed Adjustment and Wave Growth module of the Automated Coastal Engineering System (ACES) in the Coastal Engineering Design & Analysis System (CEDAS) Version 4.03 (CEDAS-ACES).

Inputs to the ACES program included wind fetch option shallow restricted, elevation of observed wind, observed wind speed, duration of observed wind, duration of final wind, latitude of observation, restricted fetch geometry, and average fetch depth. ACES allows for the input of duration for both observed and final wind speed because it can convert between wind speed durations internally. For this calculation, the ACES inputs for both were the same because the wind speed durations were converted previously (prior to input).

The shallow restricted option was chosen because the waves are not expected to propagate under a deep-water condition for significant duration, and the fetch is not unlimited for wave formation; selecting the shallow restricted option therefore yields more accurate results. Outputs from the ACES program were wave height (Hmo) and wave period (Tp). The fetch geometry and average depth along the fetch were determined using the outputs of HEC-RAS 2D as presented in Figure 2.4.3-46. Average water depth along the fetches was calculated by obtaining the raster water depth information along the fetch line and then averaging these values. Various fetches with separation angles of 25 degrees were evaluated around the site. Fetch 12 is selected as the controlling fetch as it has the second longest fetch length and water depth. Fetch 11 is

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-16 November 2025 approximately 140 ft (42.7 m) longer than Fetch 12, but its water depth is about 0.8 ft (0.2 m) lower than Fetch 12.

Wind approaching along Fetch F12 (see Figure 2.4.3-46) is modeled in ACES to determine the controlling (maximum) significant wave height. The equations and calculation flow logic for the ACES formulations are included in the reference manual (USACE, 1992).

In accordance with Equation 2-2 of EM 1110-2-1614, (USACE, 1995), reproduced below, and ANSI/ANS-2.8-1992 (ANSI/ANS, 1992), the design wave height, also referred to herein as H1%

(the average wave height of the highest 1 percent of waves), used for wave runup and the calculation of wave effects was calculated using the following approximation.

(Equation 2.4-2)

Where:

Hs = significant wave height at embankment toe Note that the significant wave height represents the average wave height of the highest 1/3 of the waves.

Wind information was obtained from ANSI/ANS-2.8-1992, Figure 1 (ANSI/ANS, 1992), as an annual extreme-mile wind speed of 50 mph (80 kph) at the LMGS site. This value represents a 2-yr. mean recurrence interval at 30 ft (9 m) above ground. The annual extreme-mile wind speed was converted to a 1-hr duration wind speed using the Coastal Engineering Manual (CEM), Figures II-2-1 and II-2-2 (USACE, 1995). The wind speeds were then converted to 10-,

15-, and 20-minute wind speed durations (as appropriate for wave generation for the applied fetches) using CEM Figure II-2-1.

Wind setup was calculated using U.S. Bureau of Reclamation ACER Technical Memorandum No. 2 (USBR, 1981), Equation 4, reproduced below:

(Equation 2.4-3)

Where:

S = wind setup in ft U = design wind velocity in mph F = wind fetch in mi D = average water depth in ft The calculated value was added to the PMF water surface elevation (WSEL) for wave runup calculation. Results of the calculation of wind-driven waves, wind setup, and conversion to H1%

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-17 November 2025 are listed in Table 2.4.3-23. The wave height was calculated with an H1% of 1.196.31 ft (1.920.36 m).

West Coloma Creek is not a gaged stream and there is no USGS gage along the creek. The creek is dry most of the time and no constant flow runs through the Creek. It has been manually developed to drain excess water out of the croplands to Matagorda Bay. Therefore, no base-flow is considered in this analysis.

2.4.3.2.7 PMF Water Level over West Coloma Creek Model results show that the maximum water depth around the site is approximately 31 ft (9.4 m)

NAVD 88. The vertical extent of the runup on the embankment was calculated to be 33.56 ft (10.23 m) NAVD 88. This indicates that the total water level will conservatively be 33.56 ft (10.23 m) NAVD 88, which is 2 ft (0.6 m) above finish grade elevation of 31.5 ft (9.6 m).

2.4.3.3 Additional PMF Analysis on Guadalupe and San Antonio Watersheds Two independent analyses were performed. The first analysis is the PMF over Guadalupe River Basin combined with 500-year flow from San Antonio River Basin. The second analysis is the PMF over San Antonio River Basin combined with 500-year flow from Guadalupe River Basin.

The USACE provided HEC-HMS and HEC-RAS models for the Guadalupe River Basin. A comprehensive assessment verified, validated, and updated these models to serve as the base framework. The final calibrated HEC-HMS and HEC-RAS models were utilized for this study.

The extent of the models is from the most upstream point of the basin in Kerr County, Texas, at approximate river mile 230 to Mission Lake in Calhaun County, Texas, at approximate river mile 0 for the Guadalupe River Basin shown in Figure 2.4.3-47.

An independent HEC-HMS model is developed for San Antonio River Basin. The model extends from the most upstream point in Baxter County, Texas, at approximate river mile 2342 to the Guadalupe River in Calhaun County, Texas, at approximate river mile 7 of the Guadalupe River.

The extent of the modeled reach is shown in Figure 2.4.3-48. A full calibration of the San Antonio HEC-HMS model was performed. San Antonio River Basin HEC-HMS model generated PMF flow that was used as lateral flow on the same HEC-RAS model developed for the Guadalupe River Basin.

National Oceanic and Atmospheric Administration (NOAA/NWS) HMR 51 (NWS, 1978) and HMR 52 (NWS, 1992) are used to determine the probable maximum precipitation (PMP) over both Guadalupe and San Antonio Basins. Basin-wide average rainfall values derived from the HMRs were compared with those developed in a Texas Commission on Environmental Quality (TCEQ) study in 2016 (TCEQ, 2016). The HMR basin average values were higher than those from TCEQ. Therefore, it is expected that the PMF values derived from HMR PMPs are bounding. The PMP values from HMR 51 and 52 were used to perform the PMF analysis described in this subsection.

Co-incident wind-wave activity was analyzed considering the 2-yr. mean recurrence interval annual extreme-mile wind speed of 50 mph (80 kph) obtained from ANSI/ANS 2.8-1992 (ANSI/ANS, 1992). Maximum wave runup is estimated to be 10.83 ft (3.30 m) for Guadalupe River PMF and 9.52 ft (2.9 m) for San Antonio River PMF.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-18 November 2025 Maximum wind setup is estimated to be 0.79 ft (0.24 m) for Guadalupe River PMF and 0.68 ft (0.21 m) for San Antonio River PMF. Sea level rise at the end of the project life is estimated to be 1.18 ft (0.35 m).

The maximum PMF still water level of the Guadalupe River at the LMGS site, before wind-wave induced setup and run-up, is predicted to be at elevation 28.0 ft (8.53 m) NAVD 88. Adding the combined maximum wind setup, wave run-up predictions and sea level rise, the maximum Gudalupe River PMF flooding water level at the LMGS site is postulated to be at elevation 40.80 ft (12.44 m) NAVD 88.

The maximum PMF still water level of the Guadalupe River with PMP over San Antonio River Basin at the LMGS site, before wind-wave induced setup and run-up, is predicted to be at elevation 26.0 ft (7.92 m) NAVD 88. Adding the combined maximum wind setup, wave run-up predictions and sea level rise, the maximum San Antonio River PMF flooding water level at the LMGS site is postulated to be at elevation 37.38 ft (11.39 m) NAVD 88.

A barge canal is located between the Guadalupe River and LMGS site with embankment elevation at 41.72 ft. (12.71 m) NAVD88. Therefore, this embankment is not inundated with a margin of 11 in. (28 cm).

This is lower than the 41.72 ft (12.7 m) NAVD 88 by 0.92 ft (0.28 m). Therefore, the left bank of the river will not be overtopped by wind-wave runup effect. The site will not be flooded during Guadalupe River PMF event.

References 2.4.3-1 ANSI/ANS, 1992. Determining Design Basis Flooding at Nuclear Power Reactor Sites, ANSI/ANS 2.8-1992, American National Standards Institute/American Nuclear Society, July 1992.

2.4.3-2 CEDAS-ACES Version 4.03. Veri-Tech, Vicksburg, MS.

2.4.3-3 Chow, V. T., 1988. Open-Channel Hydraulics, McGraw-Hill, New York, 1959 (Reissued in 1988).

2.4.3-4 FEMA,1998. Flood Insurance Study, City of Victoria, Victoria County, Texas (Unincorporated Areas), Washington, D.C., Federal Emergency Management Agency, revised November 20, 1998.

2.4.3-5 FEMA,1999. City of Victoria, Texas, Victoria County, Federal Emergency Management Agency, July 21, 1999.

2.4.3-6 FEMA, 2020, Guidance for Flood Risk Analysis and Mapping, Accepting Numerical Models for Use in the NFIP, Guidance Document 98, December 2020.

2.4.3-7 GBRA, 1994. Operating Manual for Diversion System Operator, Calhoun Canal Division, Guadalupe-Blanco River Authority, September 1981, revised October 1994.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-19 November 2025 2.4.3-8 Halff, 1989. Dam Break Analysis for Coleto Creek Dam, for Guadalupe-Blanco River Authority, Albert Halff Associates, Inc., March 1989.

2.4.3-9 Halff, 1992. Coleto Creek Dam, Phase 1 Hydrologic Study, including HEC-HMR 52 and HEC-1 input and output data files, prepared for the Guadalupe Blanco River Authority, Albert H. Halff Associates, Inc., December 1992.

2.4.3-10 Hamed, K., and A. R. Rao (Eds.), 2000. Flood Frequency Analysis (1st ed.), CRC Press, 2000.

2.4.3-11 InFRM, 2019. Watershed Hydrology Assessment for the Guadalupe River Basin, September 2019.

2.4.3-12 NRCS, 2024. Web Soil Survey, Natural Resources Conservation Service, United States Department of Agriculture, Website: https://websoilsurvey.nrcs.usda.gov/,

Date accessed: June 2024.

2.4.3-13 NWS, 1978. Probable Maximum Precipitation Estimates, United States East of the 105th Meridian, Hydrometeorological Report No. 51, U.S. National Weather Service, June 1978.

2.4.3-14 NWS,1992. Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian, Hydrometeorological Report No. 52, U.S. National Weather Service, August 1992.

2.4.3-15 NWS, 2007. River Forecast System (RFS), Guadalupe River Basin, Input Data File, Version ob8.1, West Gulf Region Forecast Center (WGRFC), March 20, 2007.

https://water.noaa.gov 2.4.3-16 USACE, 1979. Reconnaissance Report, Guadalupe River, Texas, Canyon Lake, Modification of Embankment, U.S. Army Corps of Engineers, Fort Worth District, October 1979.

2.4.3-17 USACE, 1985. Watershed Run-off Computer Model (WRCM) for Historical and Hypothetical Storm Events, User Instructions, U.S. Army Corps of Engineers, Southwestern Division, Dallas, Texas, February 1985.

2.4.3-18 USACE, 1987. Probable Maximum Storm (Eastern United States), HEC-HMR 52, U.S. Army Corps of Engineers, March 1984, revised April 1987.

2.4.3-19 USACE, 1990. Flood Hydrograph Package (HEC-1), U.S. Army Corps of Engineers, 1990.

2.4.3-20 USACE, 1992. Automated Coastal Engineering System, Technical Reference, Version 1.07, Coastal Engineering Research Center, U.S. Army Corps of Engineers, 1992.

2.4.3-21 USACE, 1994. Flood Run-off Analysis, EM 1110-2-1417, U.S. Army Corps of Engineers, August 1994.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-20 November 2025 2.4.3-22 USACE, 1995. Engineer Manual 1110-2-1614, Design of Coastal Revetments, Seawalls, and Bulkheads, U.S. Army Corps of Engineers, 1995.

2.4.3-23 USACE, 2001. Fischenich, C., Stability Thresholds for Stream Restoration Materials, Ecosystem Management and Restoration Research Program, U.S. Army Engineer Research and Development Center, Vicksburg, MS, U.S. Army Corps of Engineers, May 2001.

2.4.3-24 USACE, 2004. Guadalupe River Basin Flood Forecast Model in HEC-1, U.S.

Army Corps of Engineers, Fort Worth District, updated August 12, 2004.

2.4.3-25 USACE, 2005a. Dam Assurance Study on Canyon Lake, Guadalupe Basin, Fort Worth District, with input and output data files for Watershed Run-off Computer Model (WRCM), U.S. Army Corps of Engineers, 2005.

2.4.3-26 USACE, 2005b. River Analysis System (RAS), Version 3.1.3, U.S. Army Corps of Engineers, May 2005.

2.4.3-27 USACE, 2006a. Hydrologic Engineering Center, HEC-DSS Vue, Data Storage System Visual Utility Engine, User's Manual, U.S. Army Corps of Engineers, May 2005,revised January 2006.

2.4.3-28 USACE, 2006b. Hydrologic Modeling System (HMS), Version 3.1.0, U.S. Army Corps of Engineers, November 2006.

2.4.3-29 USACE, 2020. HEC-RAS Hydraulic Reference Manual Reference Manual, U.S.

Army Corps of Engineers, December 2020.

2.4.3-30 USBR, 1981. Freeboard Criteria and Guidelines for Computing Freeboard Allowances for Storage Dams, ACER Technical Memorandum No. 2, United States Bureau of Reclamation, 1981.

2.4.3-31 USDA-SCS, 1986. Urban Hydrology for Small Watersheds. Technical Release No.

55 (TR-55), Washington DC, U.S. Department of Agriculture Soil Conservation Service, 1986.

2.4.3-32 USGS, 1982. Guidelines for Determining Flood Flow Frequency, Bulletin 17B, Hydrology Subcommittee, Inter-agency Advisory Committee on Water Data, U.S.Geological Survey,1982.

2.4.3-33 USGS, 2007. 30m National Elevation Dataset (NED) in ESRI RASTER GRID format, purchased on a Portable Hard-drive from Digital Data Services, Inc. on May 22, 2007. Website: https://www.usgs.gov/tools/national-map-viewer.

2.4.3-34 USGS, 2008. 15-minute Flood Hydrograph Data, U.S. Geological Survey, Website:

ida.water.usgs.gov/ida, Date accessed: January 15, 2008.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-21 November 2025 2.4.3-35 USGS, 2018. OCM Partners, 2024: 2018 USGS Lidar: South Texas from 2010-06-15 to 2010-08-15. NOAA National Centers for Environmental Information, U.S.

Geological Survey, Website: https://www.fisheries.noaa.gov/inport/item/57941, Date accessed:June 13, 2024.

2.4.3-36 USGS, 2021. National Land Cover Database (NLCD), Revision 2021, U.S.

Geological Survey, Website: https://www.usgs.gov/centers/eros/science/national-land-cover-database#overview, Date Accessed: June 2024.

2.4.3-37 URS Corporation, 2003. Bi-Annual Dam Inspection of Coleto Creek Dam, submitted to Guadalupe-Blanco River Authority, August 21, 2003.

2.4.3-38 VCS, 2007. Victoria County Station Early Site Permit Application, Site Safety Analysis Report Subsection 2.4.3, ADAMS Accession No. ML101030909, 2007.

2.4.3-39 TCEQ, 2017. Probable Maximum Precipitation Study for Texas, Applied Weather Associates for Texas Commission on Environmental Quality, September 2016, Website: https://www.tceq.texas.gov/downloads/compliance/enforcement/dam-safety/texas-pmp-final-report.zip, Accessed on November 14, 2025.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-22 November 2025 Table 2.4.3-1 All-Season PMP Precipitation Depths in Inches for the Guadalupe River Basin Area (mi2)

PMP (in.)

6-hr.

12-hr.

24-hr.

48-hr.

72-hr.

10 32.0 38.7 47.1 51.8 55.7 200 24.6 31.2 39.5 44.3 48.8 1000 18.2 24.9 33.2 37.7 41.3 5000 10.1 15.0 21.9 26.6 30.7 10,000 7.6 11.8 17.6 22.5 26.5 20,000 5.6 9.2 13.6 18.0 22.0

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-23 November 2025 Table 2.4.3-2 PMF Basin Runoff Model Parameters Model Sub-Basin(a)

Drainage Area (mi2)

Initial Loss (in.)

Constant Loss Rate (in/hr)

Synders Lag(b) (hr.)

Snyders Cp (b)

Base Flow STRTQ(b)

(cfs/mi2)

QRCSN(b)

Recession Coefficient(b) 1 179.14 1.0 0.15 (c)

(c)

(d)

(d)

(d) 2 102.00 1.0 0.15 (c)

(c)

(d)

(d)

(d) 3 31.69 1.0 0.15 (c)

(c)

(d)

(d)

(d) 4 131.82 1.0 0.15 (c)

(c)

(d)

(d)

(d) 5 382.53 1.0 0.15 (c)

(c)

(d)

(d)

(d) 6 473.43 1.0 0.15 (c)

(c)

(d)

(d)

(d) 8 64.07 1.0 0.15 (c)

(c)

(d)

(d)

(d) 9 32.04 1.0 0.15 (c)

(c)

(d)

(d)

(d) 10 20.14 0.0 0.0 (c)

(c)

(d)

(d)

(d) 11 86 1.0 0.1 2.5 0.6 2

0.05 0.887 12 130 0.5 0.1 4.0 0.625 0.4 0.08 0.887 13 456 0.5 0.1 8.0 0.55 0.3 0.05 0.887 14 355 0.5 0.05 2.0 0.625 6

0.1 0.92 15A 57 1.0 0.05 2.0 0.6 0.3 0.1 0.887 15B 24 1.0 0.05 2.0 0.625 0.3 0.1 0.887 16 48.5 1.0 0.05 2.5 0.6 0.3 0.05 0.788 17 46.5 1.0 0.05 2.5 0.6 0.3 0.05 0.788 18 82 1.0 0.05 3.5 0.6 0.3 0.05 0.788 19 143 1.0 0.05 3.5 0.6 0.3 0.05 0.788 20 82 1.0 0.05 5.5 0.6 0.3 0.05 0.788 21 23 1.0 0.05 2.3 0.6 0.3 0.1 0.887 22A 112 0.5 0.05 4.2 0.6 0.1 0.24 0.788 22B 277 0.5 0.05 5.0 0.6 0.1 0.1 0.788 23 108 0.5 0.05 5.0 0.6 0.1 0.05 0.887 24 69 0.5 0.05 3.5 0.6 0.3 0.05 0.887 25 483 0.5 0.05 22 0.55 0.1 0.6 0.887 26 209 0.5 0.05 12.5 0.55 0.3 0.05 0.887 27A 390 1.0 0.05 37.0 0.75 0.1 0.05 0.75 27B 159 1.0 0.05 37.0 0.75 0.1 0.05 0.75 27C 162 1.0 0.05 34.0 0.75 0.3 0.05 0.75 28A 132.5 0.5 0.05 12.5 0.55 0.1 0.05 0.887 28 132 0.5 0.05 12.5 0.55 0.1 0.05 0.887 29 357 1.0 0.05 4.2 0.52 0.1 0.1 0.75 30 133 1.0 0.05 5.0 0.58 0.1 0.01 0.75 31 148 1.0 0.05 5.0 0.55 0.1 0.01 0.75 a) Subbasin 7 is intentionally omitted.

b) No base flow is assumed by USACE (USACE, 2005a).

c) Use unit hydrographs (Table 2.4.3-13).

d) Definitions are given in HEC-HMS Users Manual (USACE, 2006a).

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-24 November 2025 Table 2.4.3-3 Largest Five Recorded Peak Discharges for USGS Gage No. 08167000, Guadalupe River at Comfort, Texas Water Year Date Peak Discharge (cfs) 1978 August 2, 1978 240,000 1900 July 16, 1900 182,000(a) 1935 June 14, 1935 148,000 1987 July 17, 1987 130,000(b) 2002 July 4, 2002 128,000(b) a) Discharge is a Historic Peak.

b) Discharge affected to unknown degree by Regulation or Diversion.

Source: (USGS, 2023a), the drainage area is 839 mi2.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-25 November 2025 Table 2.4.3-4 Largest Five Recorded Peak Discharges for USGS Gage No. 08167500, Guadalupe River near Spring Branch, Texas Water Year Date Peak Discharge (cfs) 1978 August 3, 1978 160,000 1932 July 3, 1932 121,000 1997 June 2, 1997 116,000 1935 June 15, 1935 114,000 2002 July 5, 2002 94,400 Source: (USGS, 2023b) and the drainage area is 1315 mi2.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-26 November 2025 Table 2.4.3-5 Largest Five Recorded Peak Discharges for USGS Gage No. 08176500, Guadalupe River at Victoria, Texas Water Year Date Peak Discharge (cfs) 1999 October 20, 1998 466,000(a) 1936 July 3, 1935 179,000(a) 1981 September 2, 1981 105,000(a) 2005 November 26, 2004 102,000(a) 2017 August 20, 2017 86,500(a)(b) a) Discharge affected by Regulation or Diversion.

b) Discharge due to snowmelt, hurricane, ice-jam or debris dam breakup.

Source: (USGS, 2023c); the drainage area is 5198 mi2.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-27 November 2025 Table 2.4.3-6 Subbasin Drainage Areas and NWS Rainfall Used in Basin Runoff Model Calibration Model Subbasin Drainage Area (mi2)

NWS Subbasin Rainfall File Name(a) 11 86 NBRT2 12 130 GBCT2 13 456 Weighted Average (SEGT2, GNLT2U, & GNLT2M)(b) 14 355 Weighted Average (WMBTU &

WMBT)(b) 15A 57 KYET2 15B 24 LLGT2U 16 48.5 LLGT2U 17 46.5 LLGT2U 18 82 LLGT2U 19 143 LLG2T 20 82 LLG2T 21 23 GNLT2 22A 112 LULT2U 22B 277 LULT2 23 108 GNLT2 24 69 CUET2U 25 483 DLWT2 26 209 CUET2U 27A 390 Weighted Average (WHOT &

WHOT2U)(b) 27B 159 WHOT2M 27C 162 CUET 28A 132.5 VICTU 28 132 VICT2 29 357 SCDT2 30 133 CKDT2 31 148 DUPT2 a) NWS subbasin names as defined in Figure 2.4.3-4.

b) Weighted average is based on drainage areas.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-28 November 2025 Table 2.4.3-7 Subbasin Runoff Parameters 1998 Calibration Results Model Sub-Basin Initial Loss (in.)

Constant Loss Rate (in/hr)

Synders Lag(a)

(hr.)

Synders Cp(a)

Base Flow STRTQ(a)

(cfs/mi2)

QRCSN(a)

Recession Coefficient(a) 11 1.5 0.15 3.0 0.6 0.3 0.02 0.887 12 0.5 0.15 5.0 0.625 0.4 0.08 0.887 13 0.5 0.10 10 0.5469 0.3 0.05 0.887 14 1.8 0.45 2.7 0.6 0.05 0.04 0.887 15A 1.0 0.08 2.0 0.6 0.05 0.01 0.887 15B 1.0 0.08 2.0 0.6 0.05 0.1 0.887 16 1.9 0.10 3.0 0.6 0.05 0.01 0.750 17 1.9 0.10 3.0 0.6 0.05 0.01 0.750 18 2.5 0.10 4.0 0.6 0.05 0.01 0.750 19 2.0 0.10 4.0 0.6 0.05 0.01 0.700 20 2.0 0.10 6.0 0.6 0.05 0.01 0.750 21 2.0 0.10 2.5 0.5469 0.05 0.1 0.887 22A 1.65 0.05 5.0 0.6 0.01 0.04 0.788 22B 1.5 0.05 6.0 0.6 0.01 0.04 0.788 23 0.8 0.10 6.0 0.6 0.1 0.05 0.887 24 0.5 0.08 4.0 0.6 0.3 0.05 0.887 25 0.5 0.11 25 0.5 0.1 0.6 0.1 26 0.5 0.05 15 0.5 0.3 0.05 0.887 27A 1.5 0.10 44.0 0.75 0.1 0.001 0.55 27B 1.5 0.10 43.0 0.75 0.1 0.001 0.55 27C 1.5 0.10 40.0 0.75 0.1 0.001 0.55 28A 0.5 0.10 15.0 0.5469 0.1 0.05 0.887 28 0.5 0.10 15.0 0.5469 0.1 0.05 0.887 29 2.5 0.12 14.0 0.55 0.1 0.1 0.75 30 2.0 0.12 6.0 0.55 0.1 0.01 0.75 31 2.0 0.12 6.0 0.55 0.1 0.01 0.75 a) Definitions are given in HEC-HMS Users Manual (USACE, 2006a).

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-29 November 2025 Table 2.4.3-8 Channel Elements Muskingum K and X 1998 Calibration Results Model Channel Elements Channel Reach Location(a)

Muskingum Channel Routing K (hr.)

X Number of Sub-Reaches SMCNRB thro Sub 11 3

0.2 3

NBRSMR thro Sub 13 30 0.35 20 WMBKYE thro Sub 15A KYESMR thro Sub 15B SCSBLC (b)

BLCYRK thro Sub 18 6

0.3 5

YRKLLG thro Sub 20 3.3 0.2 2

LLGPLM thro Sub 21 4

0.1 2

LCPSM thro Sub 22B 25 0.3 10 PLMGR thro Sub 23 10 0.3 5

SMRGNL (c) 2 0.3 1

GNLPCH thro Sub 24 10 0.3 6

PCHSAN thro Sub 26 22 0.3 11 WHOGR thro Sub 27C 20 0.4 7

SANCUE (d) 2 0.2 1

UPPERVIC thro Sub 28A 4

0.4 9

CUEVIC thro Sub 28 4

0.4 9

Vic-Coleto (e) 3 0.3 3

Coleto-1 thro Sub 30 7

0.25 3

Coleto-2 thro Sub 31 7

0.25 3

Coleto-Vic (f) 1 0.2 1

a) Refer to Figure 2.4.3-3.

b) On San Marcos River below SCS #3 and 5 reservoirs to its confluence with Blanco River.

c) On Guadalupe River below its confluence with San Marcos River to USGS Gage at Gonzales, Texas.

d) On Guadalupe River below its confluence with Sandies Creek to USGS Gage at Cuero, Texas.

e) On Guadalupe River below USGS Gage at Victoria, Texas.

f) On Coleto Creek below Sub 31 to its confluence with Guadalupe River.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-30 November 2025 Table 2.4.3-9 Subbasin Parameters 2004 Calibration Results Model Sub-Basin Initial Loss (in.)

Constant Loss Rate (in/hr)

Synders Lag(a)

(hr.)

Synders Cp(a)

Base Flow STRTQ(a)

(cfs/mi2)

QRCSN(a)

Recession Coefficient(a) 11 1.3 0.11 4.0 0.6 2

0.05 0.887 12 0.5 0.15 5.0 0.625 0.4 0.08 0.887 13 0.5 0.10 13.0 0.5469 0.3 0.05 0.887 14 0.42 0.09 3.8 0.625 6

0.1 0.92 15A 2.0 0.20 6.0 0.6 0.3 0.1 0.887 15B 1.0 0.08 3.0 0.625 0.3 0.1 0.887 16 1.8 0.05 4.0 0.6 0.3 0.05 0.788 17 1.8 0.05 4.0 0.6 0.3 0.05 0.788 18 2.5 0.05 5.0 0.6 0.3 0.05 0.788 19 2.0 0.05 5.0 0.6 0.3 0.05 0.788 20 2.0 0.05 8.0 0.6 0.3 0.05 0.788 21 1.8 0.08 3.0 0.6 0.3 0.05 0.887 22A 0.5 0.08 5.8 0.35 0.1 0.24 0.600 22B 0.5 0.15 6.0 0.6 0.1 0.1 0.788 23 0.8 0.10 6.0 0.5469 0.1 0.05 0.887 24 0.5 0.08 6.0 0.5469 0.3 0.05 0.887 25 1.5 0.15 20 0.55 0.1 0.6 0.887 26 0.5 0.05 8.0 0.5469 0.3 0.05 0.887 27A 1.55 0.12 43.0 0.6 0.1 0.05 0.75 27B 1.55 0.12 35.0 0.6 0.1 0.05 0.75 27C 1.55 0.12 33.0 0.6 0.3 0.05 0.75 28A 2.5 0.15 8.0 0.5469 0.1 0.05 0.887 28 2.5 0.15 8.0 0.5469 0.1 0.05 0.887 29 1.8 0.09 5.0 0.52 0.1 0.07 0.75 30 1.5 0.10 6.0 0.58 0.1 0.01 0.75 31 1.5 0.10-6.0 0.55 0.1 0.01 0.75 a) Definitions are given in HEC-HMS Users Manual (USACE, 2006a).

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-31 November 2025 Table 2.4.3-10 Channel Elements Muskingum K and X Values 2004 Calibration Results Model Channel Elements Channel Reach Location(a)

Muskingum Channel Routing K (hr.)

X Number of Sub-reaches SMCNRB thro Sub 11 4

0.25 3

NBRSMR thro Sub 13 39 0.2 20 WMBKYE thro Sub 15A KYESMR thro Sub 15B SCSBLC (b)

BLCYRK thro Sub 18 8

0.15 5

YRKLLG thro Sub 20 4

0.25 2

LLGPLM thro Sub 21 4

0.1 2

LCPSM thro Sub 22B 20 0.2 10 PLMGR thro Sub 23 15 0.2 5

SMRGNL (c) 2 0.2 1

GNLPCH thro Sub 24 12 0.2 6

PCHSAN thro Sub 26 22 0.2 11 WHOGR thro Sub 27C 18 0.2 7

SANCUE (d) 2 0.15 1

UPPERVIC thro Sub 28A 18 0.2 9

CUEVIC thro Sub 28 18 0.2 9

Vic-Coleto (e) 2 0.2 3

Coleto-1 thro Sub 30 3

0.25 3

Coleto-2 thro Sub 31 3

0.25 3

Coleto-Vic (f) 1 0.2 1

a) Refer to Figure 2.4.3-3.

b) On San Marcos River below SCS #3 and 5 reservoirs to its confluence with Blanco River.

c) On Guadalupe River below its confluence with San Marcos River to USGS Gage at Gonzales, Texas.

d) On Guadalupe River below its confluence with Sandies Creek to USGS Gage at Cuero, Texas.

e) On Guadalupe River below USGS Gage at Victoria, Texas.

f) On Coleto Creek below Sub 31 to its confluence with Guadalupe River.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-32 November 2025 Table 2.4.3-11 PMF Basin Runoff Model Muskingum Channel Routing Coefficients K and X Model Channel Elements(a)

Muskingum Channel Routing K (hr.)

X Number of Sub-reaches NFGage (b)

(b)

(b)

HNTJNC (b)

(b)

(b)

GRAJNC (b)

(b)

(b)

JNCCOM (b)

(b)

(b)

COMFORT (b)

(b)

(b)

COMFCAN (b)

(b)

(b)

SMCNRB 3

0.2 3

NBRSMR 30 0.35 20 WMBKYE KYESMR SCSBLC BLCYRK 6

0.3 5

YRKLLG 3.3 0.2 2

LLGPLM 4

0.1 2

LCPSM 25 0.3 10 PLMGR 10 0.3 5

SMRGNL 2

0.3 1

GNLPCH 10 0.3 6

PCHSAN 22 0.3 11 WHOGR 20 0.4 7

SANCUE 2

0.2 1

UPPERVIC 4

0.4 9

CUEVIC 4

0.4 9

Vic-Coleto 3

0.3 3

Coleto-1 3

0.2 3

Coleto-2 3

0.2 3

Coleto-Vic 1

0.2 1

a) Refer to Tables 2.4.3-8, 2.4.3-10, and 2.4.3-14 for locations of the channel reaches.

b) Refer to Table 2.4.3-14.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-33 November 2025 Table 2.4.3-12 Canyon Dam Watershed Subbasins Subbasin Descriptions Drainage Area (mi2) 1 North Fork of Guadalupe River 179.14 2

South Fork of Guadalupe River 102.00 3

Area between North & South Forks of Guadalupe and mouth of Johnson Creek 31.69 4

Johnson Creek 131.82 5

Area between Johnson/Guadalupe confluence to Comfort 382.53 6

Area between Comfort and Head of Canyon Lake 473.43 8

Area adjacent to north side of Canyon Lake 64.07 9

Area adjacent to south side of Canyon Lake 32.04 10 Canyon Lake Surface 20.14 Total 1417.85 Source: (USACE, 2005a)

Note: Subbasin 7 is intentionally omitted.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-34 November 2025 Table 2.4.3-13 Canyon Dam Watershed 1-Hour Unit Hydrographs for Subbasins (Sheet 1 of 2)

Time (hr.)

Subbasin Hydrographs (cfs) 1 2

3 4

5 6

8 9

10 0

45 235 357 300 5

2 19,049 12,120 12,995 1

270 1215 11,644 1560 2275 47 16463 7347 12,995 2

1530.0 2979 4522 3840 13,650 70 4561 1076 12,995 3

18,000 20,482 2321 26,400 60,900 512 1012 131 12,995 4

27,000 11,917 1190 15,360 40,950 1163 208 2

5 18,000 8428 417 10,865 27,300 2558 44 6

11,250 6311 0.0 8142 19,474 12,276 8

7 7200 4283 5520 14,788 21,762 8

5760 2607 3360 11,830 27,714 9

4590 931 1200 9555 30,059 10 3870 559 720 8008 28,272 11 3150 470 600 6734 23,622 12 2700 451 576 5733 17,298 13 2250 431 552 4823 14,322 14 1980 412 528 4095 12,276 15 1710 392.

504 3458 10,602 16 1440 372 480 2821 9254 17 1260 353 456 2366 8277 18 1080 333 432 1911 7440 19 900 314 408 1638 6789 20 720 294 384 1365 6185 21 540 274 360 1138 5673 22 360 255 336 865 5208 23 0

235 312 637 4790 24 216 288 455 4418 25 196 264 273 4092 26 176 240 182 3720 27 157 216 91 3488 28 137 192

73.

3302 29 118 168 55 3023 30 98 144 36 2790 31 78 120 18 2604 32 59 96 0.0 2372

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-35 November 2025 33 39 72 2186

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-36 November 2025 Table 2.4.3-13 Canyon Dam Watershed 1-Hour Unit Hydrographs for Subbasins (Sheet 2 of 2)

Time (hr.)

Subbasin Hydrographs (cfs) 1 2

3 4

5 6

8 9

10 34 20 48 2000 35 24 1814 36 1674 37 1535 38 1395 39 1256 40 1116 41 977 42 884 43 791 44 698 45 605 46 512 47 419 48 372 49 326 50 279 51 233 52 186 53 140 54 93 55 47 Source: (USACE, 2005a)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-37 November 2025 Table 2.4.3-14 Canyon Dam Watershed Channel Elements Guadalupe River Storage-Discharge Relationships Channel Element Location of Channel Reach Storage (ac-ft)

Discharge (cfs)

NF Gage North Gauge to North and South Fork Confluence 66,112 800,000 HNTJNC North & South Confluence to confluence with John Creek 66,112 800,000 JNCCOM John Creek-Guadalupe confluence to Comfort 264,448 800,000 COMFORT Comfort to a point d/s of Comfort 595,008 800,000 COMFCAN A point d/s of Comfort to head of Canyon Lake Reservoir 330,560 800,000 Source: (USACE, 2005a)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-38 November 2025 Table 2.4.3-15 Canyon Dam Elevation-Storage-Discharge Relationship Elevation (ft.)

Storage (ac-ft)

Discharge (cfs) 909 378,899 0.0 910 387,248 0.0 912 404,216 0.0 915 430,423 0.0 918 457,703 0.0 920 476,495 0.0 925 525719 0.0 930 578,588 0.0 935 635,221 0.0 940 695,624 0.0 943 733,602 0.0 944 746,545 2500 944.5 753,095 4750 945 759,645 7000 946 772,895 14,000 947 786,285 23,000 948 799,820 34,000 949 813,485 47,000 950 827,295 62,000 951 841,265 77,000 952 855,395 95,000 953 869,685 110,000 954 884,135 130,000 958 943,400 210,000 959 958,610 235,000 962 1,005,275 310,000 966 1,070,010 410,000 968 1,103,500 470,000 970 1,137,730 525,000 975 1,226,445 640,000 Source: (USACE, 2005a)

Note: Elevations in Table 2.4.3-15 are given in terms of National Geodetic Vertical Datum of 1929 (NGVD 29). To convert to NAVD 88, add 0.34 ft. to the values shown in the table.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-39 November 2025 Table 2.4.3-16 X and Y Coordinates of Storm Center and Optimized Orientation for PMP Estimates Storm Center X(a)

Y(a)

Optimized Orientation (°)

South-east corner of Subbasin 13 459.0 2599.5 155 Centroid of Subbasin 23 461.2 2607.0 155 Centroid of Subbasin 24 469.1 2598.6 155 Centroid of Subbasin 27B 433.7 2608.0 140 Lower end of Subbasin 5 373.82 2633.5 279.5(b) a) X and Y coordinates are based on Texas State Plane system, with units in mi.

b) Not optimized; obtained from USACE (USACE, 2005a).

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-40 November 2025 Table 2.4.3-17 PMF Development Starting Reservoir Water Levels Reservoir 40% PMP Run Full PMP Run Starting Reservoir Water Level (elevation in ft.)(a)(f)

Starting Reservoir Storage (ac-ft)

Starting Reservoir Water Level (elevation in ft.)(a)(e)(f)

Starting Reservoir Storage (ac-ft)

Canyon Dam 909.0(b) 378,899(b) 947.73(b) 796,166(b)

SCS#3 611.0(c) 127(c) 613.3 190.6 SCS#5 616.2(c) 161(c) 632.8 1023 Coleto Creek Dam 98.5(d) 32,640(d) 98.9 33,863 a) Elevations in Table 2.4.3-17 are given in terms of NGVD 29. To convert to NAVD 88 for Canyon Dam, SCS #3, and SCS #5, add 0.34 ft., 0.30 ft., and 0.31 ft., respectively, to the values shown in the table.

b) (USACE, 2005a); USACE used 50% PMP as the preceding storm.

c) (USACE, 2004) d) From HEC-HMS calibrations of the 1998 and 2004 floods.

e) These are the water levels at the respective reservoirs 3 days after the cessation of the 72-hr. 40% PMP, except as noted.

f) For Coleto Creek Dam, subtract 0.29 ft. from the values shown in the table.

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-41 November 2025 Table 2.4.3-18 PMF Water Surface Profile along Guadalupe River STA Discharge (cfs)

Water Level (ft. NAVD 88) 7.2467 1,300,000 28.87 8.7744 1,300,000 29.97 11.1811 1,300,000 31.50 12.0915 1,300,000 32.30 12.9444 1,300,000 32.76 14.5044 1,120,000 33.01 16.0078 1,120,000 33.22 17.6557 1,120,000 33.69 18.6485 1,120,000 34.83 20.7087 1,120,000 35.92 22.0501 1,120,000 37.12 23.4397 1,120,000 39.22 25.0028 1,120,000 41.58 26.7812 1,120,000 42.95 29.5984 1,120,000 44.64 29.5984 1,120,000 44.64 30.8097 1,120,000 46.02 32.2088 1,120,000 47.73 37.1142 1,120,000 50.78 41.6305 1,100,000 54.25 46.127 1,100,000 59.42 49.2913 1,100,000 63.12 52.1817 1,100,000 66.74 54.702 1,100,000 72.70 56.1333 1,100,000 103.69 60.9682 1,100,000 104.34 63.8964 1,100,000 105.52 66.2563 1,100,000 108.32

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-42 November 2025 Table 2.4.3-19 All-Season PMP Precipitation Depths in Inches for the Basin with Storm Center at the Long Mott Generating Station Site Basin Size (mi2) 6 hr.

12 hr.

24 hr.

48 hr.

72 hr.

10 32 38.7 47.1 51.8 55.7

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-43 November 2025 Table 2.4.3-20 Depth Duration Precipitation Values for Envelope Basins in Inches with Storm Center at the Long Mott Generating Station Site 6-hr. Time Period 6 hr.

12 hr.

18 hr.

24 hr.

30 hr.

36 hr.

42 hr.

48 hr.

54 hr.

60 hr.

66 hr.

72 hr.

10-mi2 cumulative (in.)

32 38.7 42.9 47.1 48.3 49.5 50.6 51.8 52.8 53.8 54.7 55.7 10-mi2 increment (in.)

32 6.7 4.2 4.2 1.2 1.2 1.1 1.2 1

1 1

1 10-mi2 increment (in/hr) 5.33 1.12 0.7 0.7 0.2 0.2 0.18 0.2 0.17 0.17 0.15 0.17

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-44 November 2025 Table 2.4.3-21 Precipitation Depths in Inches for Each Realigned 6-Hour Time Period According to the Standardized Temporal Distribution from HMR 52 6-hr Increment Number 12 11 10 9

7 6

5 3

1 2

4 8

6-hr Time Period from Error!

Reference source not found.

72 66 60 54 42 36 30 18 6

12 24 48 Time Period 0-6 hr.

6-12 hr.

12-18 hr.

18-24 hr.

24-30 hr.

30-36 hr.

36-42 hr.

42-48 hr.

48-54 hr.

54-60 hr.

60-66 hr.

66-72 hr.

10-mi2 Incremental PMP (in.)

1 1

1 1

1.1 1.2 1.2 4.2 32 6.7 4.2 1.2 10-mi2 Incremental PMP (in/hr) 0.17 0.17 0.17 0.17 0.18 0.2 0.2 0.7 5.3 1.1 0.7 0.2

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-45 November 2025 Table 2.4.3-22 Curve Number Combinations Land Use Description Hydrologic Soil Group A

B C

D Water 100 100 100 100 Medium Residential 57 72 81 86 Forest 30 58 71 78 Agricultural 67 77 83 87 Source: (USDA-SCS, 1986)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-46 November 2025 Table 2.4.3-23 Calculation of Wind-Driven Waves and Wind Setup Approaching the Long Mott Generating Station Site Point of Interest Average Depth Along Fetch (ft.)

Fetch Length (ft.)

Design Wind Speed (mph)

Wind Speed Duration (min.)

Hmo (ft.)

Tp (s)

Wave length (ft.)

Wind Setup (ft.)

H1%

(ft.)

Wave Runup (ft.)

Embankment around the Plant 3.9 10,023 50 80 1.19 2.09 19.16 0.56 1.99 2

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-47 November 2025 Figure 2.4.3-1 Guadalupe and San Antonio River Basin Stream Gages

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-48 November 2025 Figure 2.4.3-2 West Coloma Creek Watershed

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-49 November 2025 Figure 2.4.3-3 Subbasin Delineation U.S. Army Corps of Engineers

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-50 November 2025 Figure 2.4.3-4 Subbasin Delineation U.S. National Weather Service

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-51 November 2025 Figure 2.4.3-5 1998 Flood Observed and Computed Hydrographs, Guadalupe River above Comal River (USGS No. 8168500)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-52 November 2025 Figure 2.4.3-6 1998 Flood Observed and Computed Hydrographs, Blanco River at Wimberley (USGS No. 8171000)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-53 November 2025 Figure 2.4.3-7 1998 Flood Observed and Computed Hydrographs, Plum Creek at Lockhart (USGS No. 8172400)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-54 November 2025 Figure 2.4.3-8 1998 Flood Observed and Computed Hydrographs, San Marcos River at Luling (USGS No. 8172000)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-55 November 2025 Figure 2.4.3-9 1998 Flood Observed and Computed Hydrographs, Sandies Creek near Westhoff (USGS No. 8175000)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-56 November 2025 Figure 2.4.3-10 1998 Flood Observed and Computed Hydrographs, Guadalupe River at Cureo (USGS No. 8175800)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-57 November 2025 Figure 2.4.3-11 1998 Flood Observed and Computed Hydrographs, Guadalupe River at Victoria (USGS no. 8176500)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-58 November 2025 Figure 2.4.3-12 1998 Flood Observed and Computed Hydrographs, Coleto Creek at Road Crossing near Schroeder (USGS No. 8176900)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-59 November 2025 Figure 2.4.3-13 1998 Flood Observed and Computed Hydrographs, Coleto Creek near Victoria (USGS No. 8177500)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-60 November 2025 Figure 2.4.3-14 2004 Flood Observed and Computed Hydrographs, Guadalupe River above Comal River (USGS No. 8168500)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-61 November 2025 Figure 2.4.3-15 2004 Flood Observed and Computed Hydrographs, Blanco River at Wimberley (USGS No. 8171000)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-62 November 2025 Figure 2.4.3-16 2004 Flood Observed and Computed Hydrographs, Plum Creek at Lockhart (USGS No. 8172400)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-63 November 2025 Figure 2.4.3-17 2004 Flood Observed and Computed Hydrographs, Plum Creek near Luling (USGS No. 8173000)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-64 November 2025 Figure 2.4.3-18 2004 Flood Observed and Computed Hydrographs, San Marcos River at Luling (USGS No. 8172000)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-65 November 2025 Figure 2.4.3-19 2004 Flood Observed and Computed Hydrographs, Guadalupe River at Gonzales (USGS No. 8173900)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-66 November 2025 Figure 2.4.3-20 2004 Flood Observed and Computed Hydrographs, Peach Creek at Dilworth (USGS No. 8174600)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-67 November 2025 Figure 2.4.3-21 2004 Flood Observed and Computed Hydrographs, Sandies Creek near Westhoff (USGS No. 8175000)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-68 November 2025 Figure 2.4.3-22 2004 Flood Observed and Computed Hydrographs, Guadalupe River at Cuero (USGS No. 8175800)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-69 November 2025 Figure 2.4.3-23 2004 Flood Observed and Computed Hydrographs, Guadalupe River at Victoria (USGS No. 8176500)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-70 November 2025 Figure 2.4.3-24 2004 Flood Observed and Computed Hydrographs, Coleto Creek at Road Crossing near Schroeder (USGS No. 8176900)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-71 November 2025 Figure 2.4.3-25 2004 Flood Observed and Computed Hydrographs, Coleto Creek near Victoria (USGS No. 8176500)

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-47 November 2025 Figure 2.4.3-26 HEC-RAS Cross Section Locations

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-73 November 2025 Figure 2.4.3-27 HUC-12 Subwatershed Boundaries Adjacent to the Long Mott Generating Station Site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-74 November 2025 Figure 2.4.3-28 Tributary Areas to the Long Mott Generating Station Site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-75 November 2025 Figure 2.4.3-29 Direct Watershed Upstream of the Long Mott Generating Station Site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-76 November 2025 Figure 2.4.3-30 Sparks Watersheds, Hydraulic Structures, and Flow Direction

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-77 November 2025 Figure 2.4.3-31 Drainage Ditches North of Site Tributary Area

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-78 November 2025 Figure 2.4.3-32 Drainage Ditches North of Site Tributary Area - Blowup

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-79 November 2025 Figure 2.4.3-33 Topography of West Coloma Creek Watershed below Sparks Road

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-80 November 2025 Figure 2.4.3-34 PMP Hyetograph for PMF over West Coloma Creek

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-81 November 2025 Figure 2.4.3-35 Watershed Land Use

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-82 November 2025 Figure 2.4.3-36 Watershed Hydrologic Soil Group

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-83 November 2025 Figure 2.4.3-37 HEC-RAS 2D Model Outline and Boundary Conditions

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-84 November 2025 Figure 2.4.3-38 HEC-HMS Model Input/Output for Sparks Watershed

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-85 November 2025 Figure 2.4.3-39 Computational Mesh Overview

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-86 November 2025 Figure 2.4.3-40 HEC-RAS Unsteady Computation Options and Tolerances

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-87 November 2025 Figure 2.4.3-41 HEC-RAS 2D Model Results - Maximum Flow Depth

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-88 November 2025 Figure 2.4.3-42 HEC-RAS 2D Model Results - Maximum Flow Depth at Long Mott Generating Station Site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-89 November 2025 Figure 2.4.3-43 HEC-RAS 2D Model Results - WSEL Upstream of Long Mott Generating Station Site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-90 November 2025 Figure 2.4.3-44 HEC-RAS 2D Model Results - Maximum Flow Velocity

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-91 November 2025 Figure 2.4.3-45 HEC-RAS 2D Model Results - Maximum Flow Velocity at Long Mott Generating Station Site

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-92 November 2025 Figure 2.4.3-46 HEC-RAS 2D Model Results - Maximum Fetch

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-93 November 2025 Figure 2.4.3-47 Guadalupe River Basin

Long Mott Generating Station Preliminary Safety Analysis Report Section 2.4 Hydrology 2.4.3-94 November 2025 Figure 2.4.3-48 San Antonio River Basin