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Root Boulder Creek Hydrologic Analysis

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Root Boulder Creek Hydrologic Analysis Final Report Boulder Creek Hydrologic Analysis Prepared for Colorado Department of Transportation August 2014 9191 S. Jamaica Street Englewood, CO 80112 Contents Section Page Acronyms and Abbreviations .............................................................................................................. v Executive Summary ....................................................................................................................... ES-1 1.0 Purpose and Objective ........................................................................................................ 1-1 1.1 Background ...................................................................................................................... 1-1 1.2 Project Area Description .................................................................................................. 1-1 1.2.1 Middle Boulder Creek and North Boulder Creek ................................................ 1-1 1.2.2 Fourmile Creek .................................................................................................... 1-1 1.2.3 Boulder Creek ..................................................................................................... 1-2 1.3 Mapping ........................................................................................................................... 1-2 1.4 Data Collection ................................................................................................................. 1-2 1.5 Flood History .................................................................................................................... 1-3 1.5.1 Historical Flood Events........................................................................................ 1-3 1.5.2 September 2013 ................................................................................................. 1-4 2.0 Hydrologic Analyses ............................................................................................................ 2-1 2.1 Previous Studies ............................................................................................................... 2-1 2.1.1 Floodplain Information: Boulder Creek and South Boulder Creek, Volume II, Boulder Metropolitan Region, Colorado (USACE, 1969) ..................................... 2-1 2.1.2 Water and Related Land Resources Management Study, Metropolitan Denver and South Platte River and Tributaries, Colorado, Wyoming, and Nebraska, Volume V – Supporting Technical Reports Appendices, Appendix H – Hydrology (USACE, 1977) and Review Report, Boulder Creek (USACE, undated) ............... 2-2 2.1.3 Hydrology Verification Report for Boulder Creek (Anderson Consulting Engineers, 2009) ................................................................................................. 2-3 2.2 Stream Gage Analysis ....................................................................................................... 2-4 2.3 Peak Flow Estimates ........................................................................................................ 2-5 2.4 Rainfall-Runoff Modeling ................................................................................................. 2-6 2.4.1 Overall Modeling Approach ................................................................................ 2-6 2.4.2 Summary of Modeling Approaches Considered ................................................. 2-7 2.4.3 Basin Delineation ................................................................................................ 2-8 2.4.4 Basin Characterization ........................................................................................ 2-9 2.4.5 Model Development ......................................................................................... 2-10 2.4.6 Model Calibration ............................................................................................. 2-13 2.4.7 Predictive Model Implementation .................................................................... 2-15 3.0 Hydrologic Model Results .................................................................................................... 3-1 3.1 Estimate of Design Flow Magnitudes .............................................................................. 3-1 3.2 Comparison to Previous Studies, FEMA Effective Hydrology, and Stream Gage Analyses ........................................................................................................................... 3-2 4.0 Conclusions and Recommendations..................................................................................... 4-1 4.1 Design Flow Magnitudes .................................................................................................. 4-1 4.2 Assessment of September 2013 Event ............................................................................ 4-1 5.0 References .......................................................................................................................... 5-1 iii CONTENTS, CONTINUED Tables ES-1 Comparison of Modeled Discharges to Observed Discharges ES-2 Estimate of September 2013 High-Flow Recurrence Interval 1 Data Collected for Boulder Creek 2 Summary of Flooding Events Documented for Boulder Creek 3 Rainfall Distribution for Effective Rainfall-Runoff Model (per USACE, 1977) 4 Discharge-Frequency Based on Flood Frequency Analysis of Gage Records 5 Critical-Depth Method Peak Discharge Estimates and Locations 6 Comparison of Modeled Discharges to Observed Discharges 7 Comparison of Modeled Time-to-Peak to Observed Time-to-Peak 8 Comparison of Modeled Runoff Volume to Observed Runoff Volume (24-hour Calibration Period 9 Modeled 10, 4, 2, 1, and 0.2 Percent Chance Peak Discharge 10 Modeled 10, 4, 2, 1, and 0.2 Percent Chance Unit Peak Discharge 11 Comparison of Modeled 1 Percent Annual Chance Flow to Other Estimates 12 Proposed 10, 4, 2, 1, and 0.2 Percent Chance Peak Discharge 13 Estimate of September 2013 High-Flow Recurrence Interval Figures ES-1 Boulder Creek Overview Map B-1 Vicinity and Watershed Map (Appendix B) B-2 Peak Discharge Estimate and Gage Locations (Appendix B) B-3 Land Use Map (Appendix B) B-4 AWA 10-Day Precipitation (Appendix B) B-5 Boulder Creek SPAS Rainfall and CDWR Gage Measurements (Appendix B) B-6 NRCS 24-Hour Type II Unit Hyetograph (Appendix B) B-7 Soil Data Map (Appendix B) B-8 Connectivity Map (Appendix B) B-9 Muskingum-Cunge Eight-Point Routing Cross Sections (Appendix B) B-10 September 2013 10-Day Calibration Results – Time-lag Provided by Barker Reservoir (Appendix B) B-11 Hydrographs at Key Design Locations (Appendix B) B-12 Boulder Creek Peak Discharge Profiles (Appendix B) B-13 1 Percent Annual Chance Peak Unit Discharge versus Subbasin Area (Appendix B) Tables – Appendix B: B-1 Boulder Creek Subbasin Area B-2 Boulder Creek Rainfall Depths B-3 5-Minute Dimensionless NRCS Type II Cumulative Rainfall Distribution B-4 Boulder Creek Lag Time Parameters B-5 Barker Reservoir Elevation-Storage Data during September 2013 High-Flow Event B-6 Boulder Creek Curve Numbers B-7 Boulder Creek Land Use Conditions B-8 Boulder Creek Hydrologic Modeling Detailed Results Summary Appendices A FEMA Information B Hydrologic Analysis and Parameters C Ayres Associates and CH2M HILL Flood Frequency Analyses D Project Correspondence and Response to Review Comments E Digital Data iv Acronyms and Abbreviations AMC Antecedent Moisture Condition AWA Applied Weather Associates CDOT Colorado Department of Transportation CDWR Colorado Division of Water Resources cfs cubic feet per second CN curve number CWCB Colorado Water Conservation Board DARF Depth-area Reduction Factor DRCOG Denver Regional Council of Governments ESRI Environmental Systems Research Institute FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map FIS Federal Insurance Study GIS geographic information system HEC-HMS Hydrologic Engineering Center’s Hydrologic Modeling System LiDAR Light Detection and Ranging NED National Elevation Dataset NLCD National Land Cover Dataset NOAA National Oceanic and Atmospheric Administration NRCS Natural Resources Conservation Service QC Quality Control SCS Soil Conservation Service SH State Highway SPAS Storm Precipitation Analysis System SWMM Storm Water Management Model USACE U.S. Army Corps of Engineers USDA U.S. Department of Agriculture USEPA U.S. Environmental Protection Agency USGS U.S. Geological Survey v Executive Summary In September 2013, the Colorado Front Range experienced an extensive rainstorm event spanning approximately 10 days from September 9 to September 18. The event generated widespread flooding as the long duration storm saturated soils and increased runoff potential. Flooding resulted in substantial erosion, bank widening, and realigning of stream channels; transport of mud, rock, and debris; failures of dams; landslides; damage to roads, bridges, utilities, and other public infrastructures; and flood impacts to many residential and commercial structures. Ten fatalities were attributed to the floods. During and immediately following the rainstorm event, the Colorado Department of Transportation (CDOT) engaged in a massive flood response effort to protect the traveling public, rebuild damaged roadways and bridges to reopen critical travel corridors, and engage in assessments and analyses to guide longer-term rebuilding efforts. As part of this effort, CDOT collaborated with the Colorado Water Conservation Board (CWCB) to initiate hydrologic analyses in several key river systems impacted by the
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