CO7 Permanent Repair Project MP 19 to 33 (CO72 to Lyons) 60% Draft River Rehabilitation Design Report Prepared for: Colorado Department of Transportation (CDOT Region 4) RS&H (1/25/2021) CO7 Permanent Repair Project 60% Draft MP 19 to 33 (CO72 to Lyons) 60% Draft River Rehabilitation Design Report 3665 JFK Parkway, Bldg. 2, Suite 100 Fort Collins, CO 80525-3152 970.223.5556 www.AyresAssociates.com Ayres Project No. 36-4691 Contents Page No. 1. Introduction ......................................................................................................................................... 1 Project Location ....................................................................................................................................... 1 Project Description ................................................................................................................................... 1 River Rehabilitation Limits ....................................................................................................................... 1 Survey and Topographic Data ................................................................................................................. 2 Federal Emergency Management Agency (FEMA) Special Flood Hazard Area Designations ............... 3 Compliance .............................................................................................................................................. 3 Criteria ................................................................................................................................................ 3 Design Variances ................................................................................................................................ 3 Permitting ............................................................................................................................................ 4 Peer Review ....................................................................................................................................... 4 2. Hydrology ............................................................................................................................................ 5 Flooding Background ............................................................................................................................... 5 Regulatory Flows ..................................................................................................................................... 5 Low Flow Hydrology ........................................................................................................................... 6 Design Discharge / Effective Discharge .................................................................................................. 7 Methods .............................................................................................................................................. 7 Results ................................................................................................................................................ 8 Discussion ........................................................................................................................................ 11 3. River Rehabilitation Design .............................................................................................................. 13 Purpose and Overview ........................................................................................................................... 13 Geomorphology ..................................................................................................................................... 14 Multi-stage Channel .......................................................................................................................... 14 Channel Classification ...................................................................................................................... 15 Bedforms and Channel Features ...................................................................................................... 15 Alignment .......................................................................................................................................... 16 Large Woody Material ............................................................................................................................ 16 Bed Grain Size ....................................................................................................................................... 17 4. 1D Hydraulic Modeling ...................................................................................................................... 18 Effective Modeling .................................................................................................................................. 18 Existing Conditions (Corrected Effective) Modeling .............................................................................. 18 Topography ....................................................................................................................................... 18 Alignment .......................................................................................................................................... 18 Manning’s n Values .......................................................................................................................... 18 Bridges .............................................................................................................................................. 19 i Boundary Conditions ........................................................................................................................ 19 Modeling Segments .......................................................................................................................... 19 Floodway .......................................................................................................................................... 20 Proposed Conditions Modeling .............................................................................................................. 20 Overview ........................................................................................................................................... 20 Design Features ............................................................................................................................... 20 Alignment .......................................................................................................................................... 20 Manning’s n Values .......................................................................................................................... 20 Bridges .............................................................................................................................................. 20 Boundary Conditions ........................................................................................................................ 21 Modeling Segments .......................................................................................................................... 21 Floodway .......................................................................................................................................... 22 Floodplain Permitting ............................................................................................................................. 22 FEMA and Boulder County Requirements ....................................................................................... 22 No Rise Floodplain Permits .............................................................................................................. 22 Temporary Floodplain Impacts ......................................................................................................... 22 5. 2D Hydraulics ................................................................................................................................... 23 Existing Conditions Modeling ................................................................................................................. 23 Topography ....................................................................................................................................... 23 Modeling Segments .......................................................................................................................... 23 Manning’s n Values .......................................................................................................................... 23 Bridges .............................................................................................................................................. 24 Boundary Conditions ........................................................................................................................ 24 Proposed Conditions Modeling .............................................................................................................. 25 Overview ........................................................................................................................................... 25 Design Features ............................................................................................................................... 25 Alignment .......................................................................................................................................... 26 Manning’s