2022-04-04_PERMIT FILE - M2022013 (45)

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Floodplain Development Permit Package,July 2019 consist of 91,930 computational points with cell sizes ranging from 40 feet in the overbank areas, to 15 feet in areas of smaller channels and ditches. Breaklines were added along roads, channels, levees and project features which correlate to the underlying topography. This model was run at a 30 second computational time-step for 72 hours with an 8-hour warm-up period to achieve steady-state hydraulic conditions for both existing and with-project scenarios. Topography For existing conditions,the underlying DEM was compiled using 3 ft resolution FEMA LiDAR (FEMA, 2013 & FEMA, 2014) and a 2018 bathymetric survey of the Big Thompson and South Platte Rivers performed by Headwaters Corporation. This data was converted to 3 seamless rasters and merged together in RAS Mapper v1.0.0 to generate the terrain for the computational mesh.The with-project DEM was developed in AutoCAD Civil 3D and exported to raster and merged with the existing conditions terrain for the with- project condition. Both terrains are identical in resolution and domain and only the project area differs from baseline conditions to ensure direct comparability between existing and with-project conditions. Hydrology All hydrology data used for model inputs and BFE comparison are approved CHAMP data from the TSDN Y1 and Y2 studies available from the Colorado Hazard Mapping website. The Q100 (1%) recurrence interval flood for the Big Thompson River is 22,247 cfs at the confluence with the S. Platte(CWCB, 2017a). The South Platte River flow assumption for the BTR-1A study is 15,727 cfs from Appendix D.3 (CWCB, 2017b)which corresponds to a Q10 (10%) recurrence interval flood on the SPR. Hydraulics All hydraulic assumptions for the project 2-D model were replicated from the BTR-1A TSDN 1-D CHAMP modeling effort and applied to the project 2-D model along with a calibrated channel n-value of 0.33 for the WES-BTC project. For comparison to 2-D model results, the approved 1-D CHAMP model was also altered to reflect with-project condition geometry in the areas of overlapping cross sections and run to develop base flood elevations through the project reach.The Big Thompson River and South Platte River 1-D model data were sourced from the Colorado Hazard Mapping Website and used for comparison in this study and are included in this submittal. 3.3. HYDRAULIC MODEL RESULTS Predicted water surface elevations reported bythe CHAMP 1-D model and project 2-D model show similar patterns in the lower 19,000 feet of the lower BTR. Through the wetland restoration (excavation) area (STA 9+00 to 38+50), post-project river stage is generally lower and through the spoil area (STA 45+70 — 57+92),generally higher.The range of stage effects are-1.58 ft to+0.50 ft as reported by the CHAMP 1-D model between existing and with-project conditions. The effects reported in the project 2-D model are -0.75 ft to +0.33 ft between existing and with-project conditions.The average effects on stage according to the 1-D model are -0.18 ft between according to the 1-D model and converges with the existing conditions model at the County Rd 27-1/2 bridge. The 2-D model reports an average stage difference of +0.05 ft and converges at STA 75+00,just upstream of the spoil location. A comprehensive water surface profile and individual cross section station results are reported in Figure 4 and Table 1. Headwaters Corporation 5 of 7