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<br />Table 4. Summary of Discharges <br /> <br /> Peak Flows from Regional Curves <br />Flooding Source and Drainage Area (ds) <br />Location (square miles) 10-year 50-year 100-year 500-year <br />Lake Fork of the Gunnison <br />below Henson Creek 238 3000 4800 5800 9500 <br />Lake Fork of the Gunnison <br />above Henson Creek 147 2000 3100 3600 5600 <br />Henson Creek 91 1400 2000 2300 3300 <br /> <br />3.2 Hydraulic Analyses <br /> <br />Based on the hydrologic report, this study was made to estimate <br />water-surface profiles for the 10-, 50-, 100-, and 500-year floods <br />and to determine a one-foot increase floodway for the 100-year flood. <br />Data supplied by field surveying was used to construct a river model of <br />the study reach. The basis for this model was the HEC II, step- <br />backwater water modeling program of the COE (Reference 3). <br /> <br />Roughness coefficients (Mannings "n" values), were estimated by field <br />observation at each cross section. <br /> <br />Cross sections for this study were surveyed by the Colorado Water <br />Conservation Board in August 1982. <br /> <br />Topographic mapping for this study was provided by the Colorado Water <br />Conservation Board. The mapping was prepared in 1975 by Merrick and <br />Company of Denver, Colorado at a scale of 1:200, with a contour <br />interval of 5 feet (Reference 1). The datum is mean sea level. <br /> <br />Locations of selected cross sections used in the hydraulic analyses are <br />shown on the Flood Profiles (Exhibit 1). <br /> <br />Flood profiles were drawn showing the computed water-surface elevations <br />for floods of the selected recurrence intervals. These elevations are <br />shown for clear water conditions. Debris accumulations on narrow <br />streams such as these could cause blockage, particularly at bridges <br />where the channel narrows, causing a rise in water-surface elevations <br />above those shown. <br /> <br />9 <br />