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<br />Special consideration was given to Plateau Creek at Collbran because <br />of the presence of Vega Dam. Because of the lack of hydrologic <br />data for the watershed and hydraulic data for the reservoir and <br />dam, a routing procedure was not practical. Instead, an analysis <br />of gaging station data before and after the construction of Vega <br />Dam in conjunction with the regional floodflow frequency analysis <br />was used to develop the regulated discharges at Collbran. U.S. <br />Geological Survey Gage No. 09096500, which is located 6 miles <br />east of Collbran and 2 miles downstream of the dam, was used in <br />the analysis. <br /> <br />Peak discharge-drainage area relationships for Plateau Creek and <br />Grove Creek are shown in Table 1. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of the flooding sources <br />studied in the community were carried out to provide estimates <br />of the elevations of floods of the selected recurrence intervals <br />along each of these flooding sources. <br /> <br />For Plateau and Grove Creeks, the water-surface elevations for <br />floods of the selected recurrence intervals were computed through <br />use of the U.S. Army Corps of Engineers HEC-2 step backwater computer <br />program (Reference 9). <br /> <br />Cross sections for the backwater analyses of Plateau and Grove <br />Creeks were obtained by aerial photogrammetry (Reference 10). <br />The below-water sections were obtained by field measurement. <br />All bridges, dams, and culverts were field surveyed to obtain <br />elevation data and structural geometry. On Grove Creek, several <br />small private driveway bridges were not considered sturdy enough <br />to withstand peak floodflows and were therefore considered to <br />be washed out in the computations. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (EXhibit 1). For stream segments <br />for which a floodway is computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />Roughness factors (Manning's "n") used in the hydraulic computations <br />for Plateau and Grove Creeks were chosen by engineering judgment <br />and based on field observations of the flooding sources and flood <br />plain areas. Roughness values for the main channels of Plateau <br />and Grove Creeks ranged from 0.035 to 0.080, and overbank roughness <br />values ranged from 0.050 to 0.100. <br /> <br />Flood profiles were drawn showing computed water-surface elevations <br />to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). <br /> <br />7 <br />