Final Report Little Thompson River Hydrologic Analysis Phase 2: Little Thompson River above Big Thompson River Prepared for Colorado Department of Transportation June 10, 2015 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 1.3 Effective Flood Insurance Studies .......................................................................................... 1‐1 1.4 Mapping ................................................................................................................................. 1‐2 1.5 Data Collection ....................................................................................................................... 1‐2 1.6 Flood History .......................................................................................................................... 1‐3 1.6.1 Historic Flood Events ................................................................................................ 1‐3 1.6.2 September 2013 ....................................................................................................... 1‐3 2.0 Hydrologic Analyses .................................................................................................................... 2‐1 2.1 Previous Studies ..................................................................................................................... 2‐1 2.1.1 Hydrology Studies for the Big Thompson River and Little Thompson River (USACE, 1974) ........................................................................................................... 2‐1 2.1.2 Floodplain Information. Report Little Thompson River. Boulder and Larimer Counties (USACE, 1977a) ............................................................................. 2‐1 2.1.3 Summary of Hydrologic Analysis, Little Thompson River, Larimer County Colorado, Flood Insurance Study/Map Revision. Technical Support Data Notebook (Ayres, 2010) ............................................................................................ 2‐2 2.1.4 Little Thompson River Hydrology Analysis – I‐25 Frontage Road Mile Marker 249.847, Weld County, Colorado (CDOT, 2011) ........................................................ 2‐2 2.1.5 Little Thompson River Hydrology Analysis – Report of September 2013 Little Thompson River Flooding (CDWR Dam Safety, 2014) .............................................. 2‐3 2.1.6 Little Thompson River Hydrologic Analysis [Phase I] (CH2M HILL, 2014) ................. 2‐3 2.2 Stream Gage Analysis ............................................................................................................. 2‐3 2.3 Peak Discharge Estimates ...................................................................................................... 2‐4 2.4 Rainfall‐Runoff Modeling ....................................................................................................... 2‐6 2.4.1 Overall Modeling Approach ...................................................................................... 2‐6 2.4.2 Summary of Modeling Approaches Considered ....................................................... 2‐6 2.4.3 Basin Delineation ...................................................................................................... 2‐7 2.4.4 Basin Characterization .............................................................................................. 2‐7 2.4.5 Model Development ................................................................................................. 2‐8 2.4.6 Model Calibration ................................................................................................... 2‐11 2.4.7 Predictive Model Implementation .......................................................................... 2‐13 3.0 Hydrologic Model Results ........................................................................................................... 3‐1 3.1 Estimate of Design Flow Magnitudes .................................................................................... 3‐1 3.2 Comparison to Previous Hydrologic Studies .......................................................................... 3‐2 3.2.1 Previous Hydrologic Studies Calibrated to September 2013 Event .......................... 3‐2 3.2.2 Previous Predictive Hydrologic Studies .................................................................... 3‐2 4.0 Conclusions and Recommendations ............................................................................................ 4‐1 4.1 Design Peak Discharge Magnitudes ....................................................................................... 4‐1 4.2 Assessment of September 2013 Event .................................................................................. 4‐2 5.0 References .................................................................................................................................. 5‐1 WT0213151026BOI III CONTENTS, CONTINUED Tables ES‐1 Little Thompson River Comparison of Modeled Discharges to Observed Discharges ES‐2 Estimate of the Recurrence Interval of the September 2013 Event 1 Data Collected for the Little Thompson River 2 Discharge‐Frequency Based on Flood Frequency Analysis of Gage Record 3 Little Thompson Physical‐Based Peak Discharge Observations 4 Depth‐Area Reduction Factors 5 Little Thompson River Comparison of Modeled Discharges to Observed Discharges 6 Comparison of Modeled Time‐to‐Peak to Observed Time‐to‐Peak 8 Little Thompson River Modeled 10‐,4,‐2,‐1, and 0.2‐Percent Chance Unit Peak Discharge 9 Calibrated Model Comparison 10 Comparison of Modeled 1 Percent Annual Chance Flow to Other Estimates 11 Proposed 10, 4, 2, 1, and 0.2 Percent Chance Peak Discharge 12 Estimate of the Recurrence Interval of the September 2013 Event Tables – Appendix B B‐1 24‐Hour Little Thompson Models: Hydrologic Zone, Subbasin Area, and Curve Number (CN) B‐2 Little Thompson River Rainfall Depths at Subbasin Centroids B‐3 5‐Minute Dimensionless 24‐Hr NRCS Type II Cumulative Rainfall Distribution B‐4 24‐Hour Little Thompson Models: Unit Hydrograph Parameters B‐5 24‐Hour Little Thompson Models: Routing Reach Parameters B‐6 Little Thompson River Land Use Conditions B‐7 Little Thompson River Proposed Model Results Summary Figures ES‐1 Little Thompson River Overview Map ES‐2 Little Thompson River Peak Discharge Profiles 1 Vicinity and Watershed Map 2 Little Thompson River Peak Discharge Estimate and Gage Locations 3 Phase 2 Connectivity Map 4 Phase 2 AWA 10‐Day Precipitation 5 NRCS 24‐Hour Type II Unit Hyetograph 6 24‐Hour Depth‐Area Reduction Factor (DARF) Curves 7 Phase 2 Land Use Map 8 Phase 2 Soils Map 9 Phase 2 Muskingum‐Cunge Eight‐Point Routing Cross Sections 10 Little Thompson September 2013 Rainfall 11 Hydrographs at Key Design Locations 12 Little Thompson River Peak Discharge Profiles 13 Comparison of Little Thompson River 1 Percent Chance Discharge Estimates vs. Drainage Area 14 1 Percent Annual Chance Peak Unit Discharge versus Subbasin Area Appendices A FEMA Information B Hydrologic Analysis and Parameters C Ayres Associates Flood Frequency Analysis D Project Correspondence and Response to Review Comments E September 2013 Peak Discharge Estimates Documentation F Drainage Area Reduction Factor Documentation G Digital Data (electronic only) IV WT0213151026BOI 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 FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Map FIS Flood Insurance Study GIS geographic information system HEC‐HMS Hydrologic Engineering Center’s Hydrologic Modeling System I‐25 Interstate 25 LiDAR Light Detection and Ranging LULC Land Use and Land Cover NED National Elevation Dataset NOAA National Oceanic and Atmospheric Administration NRCS Natural Resources Conservation Service PMR Physical Map Revision QC quality control SCS Soil Conservation Service SH State Highway SPAS Storm Precipitation Analysis System US 36 U.S. Route 36 USACE U.S. Army Corps of Engineers USDA U.S. Department of Agriculture USGS U.S. Geological Survey WT0213151026BOI V I hereby affirm that this report and hydrologic analysis for the Little Thompson River was prepared by me, or under my direct supervision, for the owners thereof, in accordance with the current provisions of the Colorado Floodplain and Stormwater Criteria Manual, and approved variances and exceptions thereto. Signature: CH2M HILL June 10, 2015 Morgan Lynch, P.E. Registered Professional Engineer State of Colorado No. 44653 (seal) This Document Prepared by: Management Review Preparation Team Technical Support Cory Hooper, PE Morgan Lynch, PE Brad Bettag, PE CH2M HILL CH2M HILL CH2M HILL