Appendix E Flood Analyses Report
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APPENDIX E FLOOD ANALYSES REPORT South Bay Salt Pond Restoration Project December 2007 Final Environmental Impact Statement/Report 1750.07 <This page intentionally left blank> South Bay Salt Pond Restoration Project July 2006 - Revised Flood Analyses Report ii 1751.04 TABLE OF CONTENTS Page 1. EXECUTIVE SUMMARY 1 2. INTRODUCTION 7 2.1 Purpose...................................................................................................................................9 2.2 Prior Studies......................................................................................................................... 10 2.2.1 Coastal Studies 10 2.2.2 Fluvial Studies 11 2.3 Report Organization ............................................................................................................12 3. OVERVIEW OF FLOOD PROCESSES 15 3.1 Coastal Flood Processes ....................................................................................................... 15 3.2 Fluvial Flood Processes ........................................................................................................17 4. COASTAL FLOOD ANALYSES 19 4.1 Datums.................................................................................................................................. 19 4.2 Extreme Water Levels.......................................................................................................... 21 4.2.1 Storm Surge & Tidal Dynamics 22 4.2.2 Available Data 23 4.2.3 Water Level Analysis 27 4.2.4 Results 28 4.3 Coastal Flood Hazard Management .................................................................................... 32 4.3.1 Flood Water Levels 33 4.3.2 Levee Alignment 36 4.3.3 Preliminary Levee Cross-Sections 38 5. FLUVIAL FLOOD ANALYSES 41 5.1 Hydraulic Modeling Methodology....................................................................................... 41 5.1.1 USACE HEC-RAS Program 43 5.1.2 District’s HEC-RAS Model (steady-state) 44 5.1.3 District’s UNET Model (unsteady-state) 45 5.1.4 Combined Approach 46 5.2 Existing Conditions Model Setup (steady-state).................................................................. 46 5.3 Project Alternatives Modeling Setup (unsteady-state)........................................................ 48 5.3.1 Baseline Conditions Model 61 5.3.2 Alternative A: No Project 63 5.3.3 Alternative B: 50% Tidal 65 5.3.4 Alternative C: 90% Tidal 67 5.4 Hydraulic Model Results...................................................................................................... 69 5.5 Alviso Slough Modeling Conclusions................................................................................... 73 South Bay Salt Pond Restoration Project July 2006 - Revised Flood Analyses Report i 1751.04 6. REFERENCES 75 7. LIST OF PREPARERS 79 8. FIGURES 81 ENCLOSURE (Available upon request) DVD containing the original UNET and the HEC-RAS models received from the District. The DVD also contains the HEC-RAS models prepared for this project analysis. The data output generated by the HEC- RAS program is also available on the DVD. Use the Readme file located on the DVD for additional information about the specific content of the DVD. LIST OF FIGURES Figure 1. Project Vicinity Figure 2. 20-Year Means of Mean Annual Maximum Water Level at San Francisco Figure 3. 100-Year Water Levels from USACE 1984 Tidal Stage vs. Frequency Study Figure 4. Comparison of Predicted and Measured Water Levels in South San Francisco Bay During a Storm Surge (1983) Figure 5. Comparison of Storm Surge Heights at San Francisco and San Mateo Bridge-West Figure 6. NOAA NOS COOPs Tide Gauge Locations in South San Francisco Bay Figure 7. Levee Alignment for Alternative A Figure 8. Levee Alignment for Alternative B Figure 9. Levee Alignment for Alternative C Figure 10. Preliminary Levee Cross-Sections-No Outboard Marsh Figure 11. Preliminary Levee Cross-Sections-With Outboard Marsh Figure 12. Preliminary Levee Cross-Sections-With Outboard Managed Pond Figure 13. Design Hydrograph Figure 14. Alviso Slough Levee Crests – UNET Compared to Existing Figure 15. Alviso Slough Plan Map South Bay Salt Pond Restoration Project July 2006 - Revised Flood Analyses Report ii 1751.04 Figure 16. Project Reach Water Surface Profiles Figure 17. Alternative B – Contributing Tidal Habitat Area and Levee Breach Locations Figure 18. Alternative C – Contributing Tidal Habitat Area and Levee Breach Locations Figure 19. Typical Slough Channel Cross Section Figure 20. 10-year Tide Signal and 10-year Tide Signal with Sea-Level Rise Figure 21. HEC-RAS Baseline Conditions Model Schematic Figure 22. Project Reach Water Surface Profiles - Short Term Conditions Figure 23. Project Reach Water Surface Profiles – Long-Term Conditions Figure 24. Project Reach Water Surface Profiles - Short and Long-Term Conditions Figure 25. Project Reach Water Surface Profiles – Steady State Water Level Analysis LIST OF TABLES Table 1. Approximate Levee Crest Elevations for the Preliminary Coastal Levees Table 2a. Conversions between MLLW and NAVD88 Table 2b. Tidal Datums in and near the Project Area Table 3. NOAA NOS COOP South San Francisco Bay Station Inventory Table 4. Interim Monitoring Project Data Collection Stations Table 5. Summary of Extreme Water Level Study Results Table 6. Wind Setup Values Table 7. Wave Heights & Runup Elevations Table 8a. Components of Minimum and Maximum Levee Crest Elevations Table 8b. Minimum and Maximum Levee Crest Elevations Table 9. Alviso Slough Hydraulic Simulations Table 10. Hydraulic Model Assumptions South Bay Salt Pond Restoration Project July 2006 - Revised Flood Analyses Report iii 1751.04 Table 11. Marsh Area and Tidal Prism Values Used in Long-Term Alternatives Modeling Table 12. Initial Pond Conditions Table 13. Lateral Spillways and Pond Connections Table 14. Final Storage Volume and Water Surface Elevations Table 15. Alviso Slough to Pond A8 – Peak Discharge and Net Spill Volume Table 16. Alviso Slough / Guadalupe River Water Surface Elevation Checks Table 17. Change in Water Surface Elevations from Baseline Conditions Model South Bay Salt Pond Restoration Project July 2006 - Revised Flood Analyses Report iv 1751.04 ABBREVIATIONS & ACRONYMS 1D One-dimensional 2D Two-dimensional 3D Three-dimensional BFE Base Flood Elevation BM Bench Mark CCS California Coordinate System CDFG California Department of Fish and Game CEQA California Environmental Quality Act cms Cubic Meters per Second cu-m Cubic Meters EIR Environmental Impact Report EIS Environmental Impact Statement FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map FIS Flood Insurance Study HEC-2 Hydraulic Engineering Center 2 Model HEC-RAS Hydraulic Engineering Center – River Analysis System ISP Initial Stewardship Plan LGR Lower Guadalupe River LGRP Lower Guadalupe River Flood Protection Project LiDAR Light Detection and Ranging MHHW Mean Higher High Water MHW Mean High Water m Meters NAVD88 North American Vertical Datum of 1988 NEPA National Environmental Policy Act NGVD29 National Geodetic Vertical Datum of 1929 QA Quality Assurance RDSS Read Decision Support System RS River Station Shoreline Study South San Francisco Bay Shoreline Study SBSP South Bay Salt Pond SCVWD Santa Clara Valley Water District SLR Sea Level Rise UNET Unsteady NETwork model UPRR Union Pacific Railroad USACE United States Army Corps of Engineers USF&WS United States Fish and Wildlife Service USGS United States Geological Survey WSEL Water Surface Elevation South Bay Salt Pond Restoration Project July 2006 - Revised Flood Analyses Report v 1751.04 <This page intentionally left blank> South Bay Salt Pond Restoration Project July 2006 - Revised Flood Analyses Report vi 1751.04 1. EXECUTIVE SUMMARY This report documents the coastal and fluvial flood technical analyses conducted in support of long term planning for the South San Francisco Bay Salt Pond (SBSP) Restoration Project.1 The report provides a more detailed description of the proposed flood management program presented in the Final Alternatives Report (PWA et al. 2006) and presents modeling analyses demonstrating the potential benefits of the project to reducing flood hazards at the mouths of creeks flowing through the project area. The analyses documented in this report will be used to assess potential project-related changes in flood protection at the programmatic level in the SBSP Restoration Project EIS/R. The report is organized into two main sections: • Coastal flood analyses. The purpose of the coastal analyses is to inform planning and design for the proposed SBSP coastal flood protection program, consisting of a system of shoreline levees to provide flood protection from high bay water levels and wind-waves. These coastal flood levees will connect with the levee system providing flood management along each of the fluvial channels. The coastal analyses consist of defining the proposed flood protection levee alignment and providing an initial levee design (levee cross-sections, including crest elevation) expected to be necessary to provide 100-year coastal flood protection that could be certified by the Federal Emergency Management Agency (FEMA). The coastal analyses include an evaluation of the South San Francisco Bay extreme water levels and waves. • Fluvial flood analyses. The fluvial analyses focus on demonstrating “proof of concept” for the project approach to fluvial flood risk reduction, modeling one case study in detail. The project proposes to reduce flood hazards at the mouths of creeks flowing through the SBSP complex