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Floodplain Development Permit Package,July 2019 <br /> 3. BASE FLOOD ELEVATION HYDRAULIC INVESTIGATION <br /> 3.1. INTRODUCTION <br /> The purpose of this hydraulic investigation is to model the with-project conditions to understand the <br /> overall effect on river stage with respect to the base flood elevation (BFE).To quantify the localized effect <br /> the project has on river stage, identical model geometries depicting with-and without-project conditions <br /> were executed using HEC-RAS v5.0.5 (ACOE, 2015) in both a project-specific 2-D model and in the 1-D <br /> Colorado Hazard Mapping Program (CHAMP) Model (CWCB, 2017c), which has not yet been approved in <br /> the project area but is being used to revise FEMA flood mapping information in Weld County. <br /> 3.2. MODELING APPROACH AND METHODS <br /> Project Specific 2-D Model <br /> In addition to examining existing and with-project conditions using the 1-D CHAMP model, a 2-D model <br /> developed for the Big Thompson Confluence(BTC) project design was expanded to accommodate the BFE <br /> hydrology and inundation area, Figure 3. The model domain extends from the South Platte River at 351n <br /> Avenue in Evans, CO upstream to County Rd 27-1/2 on the BTR and to the BNSF RR Bridge on the South <br /> Platte, upstream of where floodwaters merge from the 2 rivers. Both model domains are identical and <br /> 1\ <br /> Figure 3: Modified WES-BTC 2-D model domain (blue) showing project area(red) and 1-D CHAMP <br /> model cross sections(grey). Results for cross sections shown in green are in Table 1. <br /> Headwaters Corporation 4 of 7 <br />