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<br />Drainage area and average annual precipitation over that area are the two factors required <br />to estimate floodflows. <br /> <br />Results of the regional hydrologic analysis were compared with statistical analyses of <br />historical discharge records at two gaging stations in Lawson and Golden, Clear Creek <br />County, operated by the USGS (References 5,6,7,8 and 9). The Lawson gage, which <br />provided the major portion of the data, had a 25-year length of record. <br /> <br />Analyses of these two stations followed the standard log-Pearson Type III method outlined <br />by the U.S. Water Resources Council (Refer~nce 10). <br /> <br />Peak discharge-drainage area relationships for Clear Creek are shown in Table I, <br />"Summary of Discharges." <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the sources studied were carried <br />out to provide estimates of the elevations of floods of the selected recurrence intervals. <br />Users should be aware that flood elevations shown on the FIRM represent rounded <br />whole-foot elevations and may not exactly reflect the elevations shown on the Flood <br />Profiles or in the Floodway Data tables in the FlS report. Flood elevations shown on the <br />FIRM are primarily intended for flood insurance rating purposes. For construction and/or <br />floodplain management purposes, users are cautioned to use the flood elevation data <br />presented in this FlS in conjunction with the data shown on the FIRM. <br /> <br />Water-surtace profiles for all streams studied in detail were developed using the USACE <br />HEC-2 backwater computer program (Reference 5). <br /> <br />Cross sections for the hydraulic analyses were developed from topographic mapping at a <br />scale of I inch equals 100 feet with a contour interval of 2 feet (Reference 6). The <br />mapping was developed along Interstate 70 for. the Colorado Department of Transportation <br />and was furnished to the study contractor by Clear Creek County. Bridges and culverts <br />were field surveyed in September 1995. Plans for the Cherokee Street replacement bridge <br />were provided by the Town of Silver Plume. Stream bed elevations taken from the <br />topographic mapping were adjusted according to field survey data obtained at selected <br />locations to more accurately follow the actual stream bed profile. <br /> <br />Roughness coefficients (Manning's "n" values) for the restudy were determined according <br />to field observations and surface photography. The method given in the report <br />Determination for Roughness Coefficients for Streams in Colorado (Reference 7) was used <br />to help determine "n" values. Manning's "n" values varied from 0.042 to 0.150 for the <br />stream channel and 0.065 to 0.200 for the overbanks. A limited number of high water <br />mark measurements on Clear Creek and a discharge measurement were made at Silver <br />Plume during high flows in June 1995. At that time, the stream was flowing bankfull, <br />This information was used to verify the selected channel roughness values. <br /> <br />The starting water-surface elevations for the water-surface profiles were based on the <br />slope-area method of HEC-2. Channel slopes were obtained from topographic mapping. <br /> <br />Locations of selected cross sections used in the hydraulic analyses are shown on the Flood <br />Profiles (Exhibit I). For stream segments for which a floodway was computed <br /> <br />4 <br />