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<br />Flood elevations for the 10-, 50-, and lOa-year floods on the Cache <br />La Poudre River were computed by the COE, Omaha District <br />(Reference 6). Water-surface elevations for the 500-year flood <br />were estimated based on rating curves prepared for each cross <br />section using the COE information. <br /> <br />Cross section data for Black Canyon Creek were taken from <br />topographic maps at a scale of 1:1,200 with a contour interval of 2 <br />feet (Reference 19) and were supplemented with field-surveyed cross <br />sections. Roughness coefficients were estimated by field <br />inspection. The channel value was 0.04 and the overbanks were 0.06. <br />Water- surface profiles were computed using the COE HEC-2 computer <br />program (Reference 22). <br /> <br />Cross section data for the Fall River were field-surveyed. A water <br />supply dam located in the upper watershed failed in July 1982 and <br />changed the channel. Roughness coefficients were estimated by <br />field inspection. The channel roughness ranged from 0.04 to 0.05 <br />and the overbank roughness ranged from 0.03 to 0.06. Water-surface <br />profiles were computed using the COE HEC-2 program (Reference 22). <br /> <br />Approximate flood elevations far the Little Thompson River were <br />determined from a COE Flood Plain Information report <br />(Reference 27). Approximate flood elevations for the Cache La <br />Poudre River were determined from two COE Flood Plain Information <br />reports (References 6 and 2B). <br /> <br />Approximate flood elevations for Boxe1der Creek and Cooper Slough <br />in the vicinity east of the City of Fort Collins were computed <br />using the COE HEC-2 computer program (Reference 22), as reported in <br />the study prepared by Simons, Li & Associates, Inc. (Reference 9). <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 Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />Starting water-surface elevations for the Big Thompson <br />upstream of Lake Estes and Fish Creek were the maximum <br />operating level of Lake Estes. Starting water-surface <br />for Dry Creek were computed assuming critical depth. <br />River, Black Canyon Creek, Boxelder Creek, and Cooper <br />starting water-surface elevations were computed using <br />area method. <br /> <br />River <br />normal <br />elevations <br />On Fall <br />Slough, <br />the slope- <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood e1evation& shown on the profiles are thus <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />20 <br />