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<br />The starting WSEL at the downstream end of the study reach was based on normal-depth <br />calculation. <br /> <br />Yampa River (City of Steamboat Springs) <br /> <br />Cross sections utilized by TC&B were based on new field surveys performed by MSM, <br />topography from the City of Steamboat Springs with 2-foot contour intervals <br />(Reference 22), topography from D&D Surveying with 5-foot contour intervals <br />(Reference 23), a USGS Quadrangle topographic map with 40-foot contour intervals <br />(Reference 15), and supplemented field surveys performed by Survcon. <br /> <br />The starting WSEL at the downstream end of the study reach was based on <br />normal-depth calculation. <br /> <br />A split-flow analysis was run on the Yampa River just upstream of the SH 131 bridge. <br />Overflows from the main channel of the Yampa River divert flood flows along the <br />Yampa River split flow at the SH 131 reach and the Yampa River Bypass reach. <br />Approximately 1,000 efs leaves the main river channel during the base flood and flows <br />through the Yampa River Bypass reach. Another 3,750 cfs leaves the main channel just <br />above the SH 131 bridge and flows through the Yampa River split flow at SH 131. <br />Hydraulic analyses for the bypass reach and split flow at SH 131 were based on more <br />detailed topographic information and hydraulic computations provided by Taylor, in a <br />report entitled "Yampa River Floodplain Impact Analysis for River Valley Resource," <br />dated May 8, 2008 (Reference 24). <br /> <br />For all streams included in this countywide restudy, the roughness coefficients <br />(Manning's "n" values) were assigned using digital aerial photographs of the study <br />stream reaches, field inspections, previous hydraulic studies (Reference 25), and <br />calibration to stream gage rating curves. These values are listed in Table 2, <br />Manning's "n" Values. A comparison of the computed WSELs to the stream gage <br />rating curves is included in the TC&B Technical Support Data Notebook <br />(Reference 14). <br /> <br />Digital aerial photography and field reconnaissance were used to identify the <br />locations of all hydraulic structures, such as culverts and bridges, along the study <br />reaches in this restudy. Information on hydraulic structures was obtained primarily <br />from field surveys conducted by MSM, and by Surveon (Reference 26). Survey data <br />was obtained using Global Positioning System equipment. In several cases as-built <br />construction plans, available from the City of Steamboat Springs, were used. All <br />bridges were photographed during site visits to assist in interpreting field-surveyed <br />information. <br /> <br />For this restudy, the USACE HEC-RAS backwater analysis program (Reference 27) <br />was used to determine the flood WSELs produced by the flood discharges described in <br />Table 1. WSELs of the 10-, 50-, 100-, and 500-year flood events were computed. <br /> <br />32 <br />