Elbert Town ofCounty, Silverthorne Colorado Drainage Master Plan July 2020 (970) 945-7755 Town of Silverthorne Drainage Master Plan July 2020 Prepared for: Town of Silverthorne Public Works Department 264 Brian Avenue Silverthorne, CO 80498 Prepared by: Wright Water Engineers, Inc. 818 Colorado Avenue, Suite 307 Glenwood Springs, CO 81601 Town of Silverthorne Drainage Master Plan July 2020 Contents EXECUTIVE SUMMARY .......................................................................................................................... 1 1. INTRODUCTION ............................................................................................................................. 3 1.1 PURPOSE AND SCOPE ........................................................................................................... 3 1.2 MAPPING AND SURVEYS ...................................................................................................... 4 1.3 DATA COLLECTION ................................................................................................................ 4 1.4 ACKNOWLEDGEMENTS ........................................................................................................ 5 2. STUDY AREA .................................................................................................................................. 6 2.1 PROJECT AREA ....................................................................................................................... 6 2.2 LAND USE .............................................................................................................................. 7 2.3 FLOOD HISTORY .................................................................................................................... 7 2.4 KNOWN DEFICIENCIES ........................................................................................................ 10 3. HYDROLOGIC ANALYSIS .............................................................................................................. 12 3.1 PREVIOUS STUDIES ............................................................................................................. 12 3.2 MODELING SOFTWARE ....................................................................................................... 13 3.3 SUBBASIN PARAMETERIZATION ......................................................................................... 13 3.4 GAGE ANALYSIS ................................................................................................................... 16 3.5 RAINFALL ............................................................................................................................. 19 3.6 SNOWMELT ......................................................................................................................... 21 4. HYDRAULIC ANALYSIS ................................................................................................................. 24 4.1 ROUTING METHOD ............................................................................................................. 24 4.2 PARAMETERIZATION OF ROUTING ELEMENTS .................................................................. 24 5. MODEL VERIFICATION AND CALIBRATION ................................................................................. 26 5.1 AREAL REDUCTION FOR SNOWMELT RUNOFF .................................................................. 26 6. IDENTIFICATION OF INFRASTRUCTURE DEFICIENCIES ............................................................... 27 6.1 STORM DRAINS AND MAJOR CROSSINGS .......................................................................... 27 6.2 OPEN CHANNEL INFRASTRUCTURE .................................................................................... 28 6.3 WATER QUALITY ................................................................................................................. 30 6.4 MAINTENANCE .................................................................................................................... 31 7. CROSSINGS ALONG THE BLUE RIVER PARKWAY ........................................................................ 32 8. PROJECT IDENTIFICATION AND PRIORITIZATION ....................................................................... 35 8.1 COST ESTIMATES ................................................................................................................. 35 Page i Town of Silverthorne Drainage Master Plan July 2020 8.2 IDENTIFIED PROJECTS ......................................................................................................... 35 8.3 PROJECT PRIORITIZATION ................................................................................................... 40 9. RECOMMENDED PLAN ................................................................................................................ 42 9.1 PRIORITIZED IMPROVEMENT PROJECTS ............................................................................ 42 9.2 UPDATES TO EXISTING DRAINAGE DESIGN CRITERIA ........................................................ 42 9.3 STORMWATER MAINTENANCE PLAN ................................................................................. 42 9.4 WATER QUALITY RECOMMENDATIONS ............................................................................. 43 9.5 STORMWATER UTILITY FEE................................................................................................. 43 9.6 POTENTIAL AREAS OF FUTURE STUDY ............................................................................... 44 9.7 DIGITAL FILES ...................................................................................................................... 44 9.8 DISCLAIMER ......................................................................................................................... 44 10. REFERENCES ............................................................................................................................ 45 Page ii Town of Silverthorne Drainage Master Plan July 2020 Tables Table 1. Summary of Previous Studies ................................................................................................ 12 Table 2. Horton Infiltration Parameters by Hydrologic Soil Groups (MHFD, USDCM, 2018) ............ 14 Table 3. Depression Storage Values by Land Use (MHFD, USDCM, 2018) ......................................... 15 Table 4. Manning’s ‘n’ Values for Overland Flow by Land Use (EPA SWMM Reference Manual) .... 16 Table 5. Straight Creek Flood Frequency Summary Table .................................................................. 18 Table 6. Willow Creek Flood Frequency Summary Table ................................................................... 18 Table 7. NOAA Atlas 14, 6-Hour, Point Precipitation Depths for the Town of Silverthorne .............. 20 Table 8. NOAA Atlas 14, 6-Hour, Second-Quartile 10% Temporal Distribution ................................. 21 Table 9. Unit Rates of Snowmelt Runoff (Town of Vail Criteria) ........................................................ 22 Table 10. Manning’s ‘n’ Values for Closed Conduit and Open Channel Flow (EPA SWMM Reference Manual) ................................................................................................................................................ 25 Table 11. Unit Rates of Snowmelt Runoff with Areal Reduction Factor ............................................ 26 Table 12. Summary of Existing Storm Drain Capacity Evaluation ...................................................... 28 Table 13. Maximum Permissible Depth and Encroachment for Curbs and Gutters (Town Criteria) 29 Table 14. Summary of Blue River Parkway Crossings ......................................................................... 32 Table 15. Project Prioritization ............................................................................................................ 41 Figures Figure 1. Study Area Vicinity Map ......................................................................................................... 9 Figure 2. Seasonal Trends in Daily Discharge Records for the Straight Creek Gage .......................... 19 Figure 3. Observed Snow vs. Elevation During Runoff Season ........................................................... 23 Page iii Town of Silverthorne Drainage Master Plan July 2020 Maps (Located at end of report) Map A: Model Overview and Inputs Map A.1 Overall Study Area Map A.2 Land Use Map A.3 Soils Map B: FEMA Flood Hazard Zones Map C: Hydrologic Routing Map D: Modeling Results Map D.1 Hydraulic Results Map D.2 The Blue River Parkway Crossing Capacities Map D.3 Identified Deficiencies Map E: Recommended Plan Appendices Appendix A. Prioritization Table Appendix B. Model Output Summary Tables Appendix C. Cost Estimates Appendix D. Model Calibration Memorandum Appendix E. Bulletin 17B Flood Frequency Analyses Appendix F. NOAA Atlas 14 Rainfall Data Appendix G. Soils Report Appendix H. EPA SWMM Model Reports (electronic appendix provided on flash drive) Page iv Town of Silverthorne Drainage Master Plan July 2020 EXECUTIVE SUMMARY ES.1 Purpose and Objectives The Town of Silverthorne Drainage Master Plan (Plan) was updated to assess the function of the stormwater infrastructure throughout the Town of Silverthorne (Town) and identify potential multi- objective improvement projects and recommendations. The Plan provides the Town with a comprehensive analysis of the conveyance capacity and flood risk associated with major drainageways