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<br />Annual peak flows for rainfall and snowmelt floods have been <br />published by the USGS (Reference 5). Additional annual peak flows <br />were obtained from inspection of the USGS stripcharts. Frequency <br />analyses were made on both peak flow data sets. The two frequency <br />curves were assumed to represent independent events, and a composite <br />flow-frequency curve was developed for each gaging. A regional <br />regression analysis was then made to determine peak flow as a <br />function of drainage area and return period (Reference 4). <br /> <br />Peak flows along Rifle Creek were based on the regional regression <br />relationship and the routing of the inflow through Rifle Gap <br />Reservoir. The hydrograph was .developed from U.S. Soil Conservation <br />Service (SCS) methodology. The inflow peaks corresponded to the <br />peak as determined from the regression relationship, and the volume <br />was checked to correspond with the excess volume as determined <br />from the SCS analyses. <br /> <br />Peak discharge-drainage area relationships for the flooding sources <br />of unincorporated Garfield County are shown in Table 1. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />sources studied were carried out to provide estimates of the eleva- <br />tions of floods of the selected recurrence intervals. <br /> <br />Water-surface elevations of floods of the selected recurrence <br />intervals were computed using the COE HEC-2 step-backwater program <br />(Reference 6). <br /> <br />Cross section data for the Colorado River at New Castle and Glenwood <br />Springs and for Alkali Creek were field surveyed. Cross section <br />data for the Colorado River at Rifle was developed from topographic <br />data furnished by the Colorado Department of Highways (References 7 <br />and 8). Cross sections for the Roaring Fork and Crystal Rivers; <br />Fourmile, Threemile, Cattle, Mitchell, Rifle, and Government Creeks; <br />and Hubbard Gulch were digitized from aerial photographs flown in <br />May 1980 and November 1980 and 1982 (References 9, 10, and 11). <br />All bridges and culverts were field surveyed to obtain elevation <br />data and structural geometry. <br /> <br />Since the cross section data for the Crystal River and Cattle Creek <br />did not include any below-water geometry, adjusted discharge values <br />were used in the HEC-2 analyses for these streams. The adjusted <br />discharge values were determined by reducing the peak discharges, <br />as shown in Table 1, by the flow in the streams at the time of the <br />aerial photography. The flow in the streams at the time of the <br />photography was obtained from USGS gaging station records. <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 /> <br />9 <br />