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<br />Table 1. Summary of Discharges <br /> <br />Drainage Area <br />(Square Miles) <br /> <br />Peak Discharges <br />(Cubic Feet per Second) <br />10-Year 50-Year lOa-Year 500-Year <br /> <br />Flooding Source <br />and Location <br /> <br />Gypsum Creek <br />At Confluence <br />With Eagle River <br /> <br />100 <br /> <br />1450 <br /> <br />1775 <br /> <br />1950 <br /> <br />2150 <br /> <br />Eagle River <br />Downstream of <br />U.S. Highway 24 <br /> <br />944 <br /> <br />5904 <br /> <br />7564 <br /> <br />8223 <br /> <br />9727 <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in the community <br />were carried out to provide estimates of the elevations of floods <br />of the selected recurrence intervals along each stream studied in <br />the community. <br /> <br />Water-surface elevations for floods of the selected recurrence <br />intervals were computed using the u.S. Army Corps of Engineers <br />HEC-2 step-backwater computer program (Reference 8). <br /> <br />Cross sections for the backwater analyses of Gypsum Creek were obtained <br />from topographic maps at a scale of 1:1200, with a contour interval of <br />2 feet (References 9 and 10). All bridges were field checked to <br />obtain elevatioD,data and structural geometry. Cross sections for <br />Eagle River were field surveyed. <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 />Starting water-surface elevations for Gypsum Creek and Eagle River <br />were calculated using the slope-area method. <br /> <br />Roughness factors (Manning1s "n") used in the hydraulic computations <br />were chosen by engineering judgement, based on field observations of <br />the streams and flood plain areas. A channel roughness value of <br />0.035 was selected for Gypsum Creek; flood plain roughness values <br />ranged from 0.040 to 0.090. For the Eagle River, a roughness factor <br />of 0.040 was used for the channel, and roughness values for the <br />overbanks ranged from 0.050 to 0.070. <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 />6 <br />