Stinson Beach Watershed Program Flood Study and Alternatives Assessment Prepared for: Marin County Flood Control and Water Conservation District Prepared by: O’Connor Environmental, Inc. Healdsburg, California In Association with: Garcia and Associates, San Anselmo, California Robert Zlomke, PE, Napa, California Professor John Largier, University of California, Davis, Bodega Marine Laboratory Edited by: Edward Molnar Chris Choo Final May 2014 Stinson Beach Watershed Program Flood Study and Alternatives Assessment Table of Contents EXECUTIVE SUMMARY ................................................................................................................................... i METHODOLOGY ......................................................................................................................................... i ALTERNATIVES ANALYSIS ......................................................................................................................... iii KEY FINDINGS ........................................................................................................................................... vi NEXT STEPS ............................................................................................................................................. vii 1 INTRODUCTION ..................................................................................................................................... 1 1.1 Overview ....................................................................................................................................... 1 1.2 Scope of Technical Reports ........................................................................................................... 6 1.3 Primer on Hydrologic and Hydraulic Modeling ............................................................................. 7 2 WATERSHED PROCESSES AND SUSTAINABILITY OF FLOOD MITIGATION MEASURES .......................... 9 2.1 Alluvial Fan Processes ................................................................................................................. 10 3 FISH HABITAT ...................................................................................................................................... 14 4 ALTERNATIVES SUMMARY .................................................................................................................. 15 4.1 Alternative 1-No Action .............................................................................................................. 16 4.2 Alternative 2-Bridge Improvements ........................................................................................... 17 4.3 Alternative 3-Vegetation Management ...................................................................................... 17 4.4 Alternative 4-Channel Dredge and Sediment Management....................................................... 17 4.5 Alternative 5-Wetland Creation and Bypass to the National Park Service’s North Parking Lot . 19 4.6 Alternative 6-Wetland Enhancement (near Poison Lake) and Bypass to the National Park Service’s South Parking Lot ..................................................................................................................... 19 4.7 Alternative 7-Causeway .............................................................................................................. 19 4.8 Alternative 8-Raising Calle del Arroyo ........................................................................................ 20 4.9 Alternative 9-Combined Dredge, Wetland Enhancement, and Bypass ...................................... 20 4.10 Alternative 10-Structure Elevation ............................................................................................. 20 4.11 Infeasible Alternatives ................................................................................................................ 20 5 METHODOLOGY FOR ALTERNATIVES ANALYSIS ................................................................................. 22 6 SUMMARY OF RESULTS ....................................................................................................................... 26 7 KEY FINDINGS ...................................................................................................................................... 34 8 Alternative 1-No Action ...................................................................................................................... 37 9 Alternative 2-Bridge Improvements ................................................................................................... 40 9.1 Summary ..................................................................................................................................... 40 9.2 Description of Alternative ........................................................................................................... 46 9.3 Flood Control Benefits ................................................................................................................ 47 9.4 Preliminary Design and Estimated Construction Costs ............................................................... 48 www.oe-i.com Stinson Beach Watershed Program Flood Study and Alternatives Assessment 9.5 Permitting Issues ......................................................................................................................... 52 9.6 Operation and Maintenance Requirements and Costs .............................................................. 55 9.7 Sustainability (Short-term and Long-term) ................................................................................. 56 9.8 Feasibility, Next Steps and Additional Information Needs ......................................................... 56 10 Alternative 3-Vegetation Management .......................................................................................... 57 11 Alternative 4-Channel Dredge and Sediment Management........................................................... 58 11.1 Summary ..................................................................................................................................... 58 11.2 Description of Alternative ........................................................................................................... 63 11.3 Flood Control Benefits ................................................................................................................ 64 11.4 Preliminary Design and Estimated Construction Costs ............................................................... 65 11.5 Permitting Issues ......................................................................................................................... 76 11.6 Operation and Maintenance Requirements and Costs .............................................................. 78 11.7 Sustainability (Short-term and Long-term) ................................................................................. 79 11.8 Feasibility, Next Steps and Additional Information Needs ......................................................... 80 12 Alternative 5-Wetland Creation and Bypass to the National Park Service’s North Parking Lot ..... 81 12.1 Summary ..................................................................................................................................... 81 12.2 Description of Alternative ........................................................................................................... 82 12.3 Flood Control Benefits ................................................................................................................ 83 12.4 Preliminary Design and Estimated Construction Costs ............................................................... 84 12.5 Permitting Issues ......................................................................................................................... 87 12.6 Operation and Maintenance Requirements and Costs .............................................................. 87 12.7 Sustainability (Short-term and Long-term) ................................................................................. 88 12.8 Feasibility, Next Steps and Additional Information Needs ......................................................... 89 13 Alternative 6-Wetland Enhancement (near Poison Lake) and Bypass to the National Park Service’s South Parking Lot ........................................................................................................................................ 90 13.1 Summary ..................................................................................................................................... 90 13.2 Description of Alternative ........................................................................................................... 96 13.3 Flood Control Benefits ................................................................................................................ 97 13.4 Preliminary Design and Estimated Construction Costs ............................................................... 98 13.5 Permitting Issues ....................................................................................................................... 104 13.6 Operation and Maintenance Requirements and Costs ............................................................ 107 13.7 Sustainability (Short-term and Long-term) ............................................................................... 108 13.8 Feasibility, Next Steps and Additional Information Needs