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<br />A hydraulic model of The Slough floodplain was developed using the <br />USACE HEC-2 computer program (Reference 48). The HEC-2 program is <br />used for calculating water-surface profiles for steady, gradually <br />varied flow in natural or man-made channels. <br /> <br />Channel Cross Sections A through I were established by <br />photograrnmetric methods from flights made on April 11, 1978, <br />supplied by the eWCB and Weld County. Channel Cross Sections J <br />through P were established by ground surveying that was completed in <br />1996 by the eweB. Culvert information was provided by the USACE. <br /> <br />In The Slough hydraulic model, the starting water-surface elevations <br />were based on flood routing studies through John Law Reservoir. <br />Roughness coefficients (Manning's "n" values) for this study were <br />determined by field inspection~ survey data~ photographs, and <br />engineering judgment. Assigned values ranged from 0.035 to 0.043 in <br />the channel and from 0.070 to 0.090 in the overbank areas. Higher <br />roughness coefficients were assigned to areas where structures or <br />heavy trees exist. Expansion and contraction coefficients ranged <br />from 0.1 to 0.3 and 0.6 to 0.8, respectively. Lower expansion and <br />contracticn rates were assigned to open areas and higher <br />coefficients were assigned to culverts. Near some culverts, <br />encroachments were set to eliminate the ineffective areas on the <br />downstream and upstream cross sections of the culvert to account for <br />the proper expansion and contraction near the culverts. <br /> <br />The 100- and SOD-year floodplain boundaries for The Slough were <br />delineated using the computed water-surface elevations determined at <br />each cross section. Between cross sections, the flood boundaries <br />were interpolated using USGS topographic maps at a scale of <br />1:24,000, with a contour interval of 10 feet (Reference 49). The <br />flood boundaries were also interpolated between cross sections uSing <br />a work map with 2-foot contour intervals taken from the USACE "Flood <br />Plain Information Report, The Slough, Severance, Colorado" <br />(Reference 44). <br /> <br />A Flood Insurance Study for the Town of Severance, Colorado <br />(Reference 50), is being prepared concurrently with this restudy. <br />These studies are in agreement with each other. <br /> <br />Table 1, ~~Summary of Discharges," Table 2, <br />Exhibit 1, "Flood Profiles," were also revised <br />a result of the restudy. <br /> <br />"Floodway Data," and <br />to reflect changes as <br /> <br />48 <br />