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<br />of the 10- through 500-year flood events. The water-surface <br />profiles were transferred to topographic maps to establish the 100- <br />year floodplain boundaries. <br /> <br />A hydrologic analysis was made to determine peak flow rates for <br />various storm frequencies at several locations in the City of <br />Sterling for both Sand Creek floods and Pawnee Creek overflows. <br />Synthetic hydrographs were computed for 10-, 50-, 100-, and 500- <br />year frequency storms. Flood peaks were determined as suming the <br />simultaneous occurrence of a 10-year flood on the South Platte <br />River. <br /> <br />Peak discharge-drainage area relationships for South Platte River, <br />Pawnee Creek Overflow, and Sand Creek are shown in Table 1. <br /> <br />TABLE 1 - SUMMARY OF DISCHARGES <br /> <br />FLOODING SOURCE <br />AND LOCATION <br /> <br />DRAINAGE AREA <br />SQ. Miles <br /> <br />PEAK DISCHARGE (cfs) <br />10-year 50-vear 100-vear 500-year <br /> <br />South Platte River <br />Just upstream of <br />Sterling <br /> <br />18,000 <br /> <br />18,000 55,000 84,000 <br /> <br />207,000 <br /> <br />Pawnee Creek Overflow <br />At State Highway 138 <br /> <br />38.7 <br /> <br />1,250 2,400 5,000 <br /> <br />11,000 <br /> <br />Sand Creek <br />At Mouth <br /> <br />31.6 <br /> <br />250 3,700 4,400 <br /> <br />12,000 <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />riverine sources studied were carried out to provide estimates of <br />the elevations of floods of the selected recurrence intervals. <br /> <br />Cross sectional data along the South Platte River were obtained <br />from aerial photographs taken on December 19, 1976, by Blevins <br />Mapping Company of Denver, Colorado. These maps had a scale of 1 <br />inch equal to 200 feet and a contour interval of 2 feet. <br /> <br />Cross sectional data along Sand Creek and Pawnee Creek Overflow <br />were provided in HEC-2 format by the City of Sterling. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit I), and are also shown on <br />the Flood Insurance Rate Map (Exhibit 2). <br /> <br />Channel <br />methods <br />0.04 to <br /> <br />roughness factors for all streams studied by <br />ranged from 0.06 to 0.07 in the overbank areas <br />0.05 in the channel. <br /> <br />detailed <br />and from <br /> <br />6 <br />