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<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 profiles were developed using the U.S. Army Corps of <br />Engineers step-backwater computer model (Reference 8). <br /> <br />Cross section data for the Big Thompson River and Black Canyon <br />Creek were obtained from aerial topographic maps at a scale of <br />1:1,200, with a contour interval of 2 feet (Reference 9). All <br />bridges and culverts were measured to obtain structural geometry. <br /> <br />Cross sections along the Fall River were surveyed because of the <br />channel changes after the Lawn Lake Dam failure of July 1982. <br /> <br />Cross section data on Dry Gulch were obtained from aerial topo- <br />graphic maps at a scale of 1:2,400, with a contour interval of 2 <br />feet (Reference 10). <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 was computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Insurance Rate Map <br />(Exhibit 2). <br /> <br />Roughness coefficients (Manning's ann) for all streams were esti- <br />mated by field inspection. The roughness coefficient for the stream- <br />bed was generally 0.04, and roughness values in the overbank areas <br />ranged from 0.03 to 0.08 (Reference 11). <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 />The starting water-surface elevations for the Big Thompson River <br />and Fish Creek were the maximum normal operating level of Lake <br />Estes, obtained from the U.S. Bureau of Reclamation. Starting <br />water-surface elevations for the Fall River and Black Canyon Creek <br />were computed using the slope-area method. Starting water-surface <br />elevations for the Big Thompson and Fall River Overflows were taken <br />from the main stem. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus con- <br />sidered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />10 <br />