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<br />be flowing full, such that any tributary runoff would flow across <br />unimpeded, remaining within the same drainage basin. The ditch's <br />function as a storm runoff conveyance facility was ignored as a <br />conservative procedure. <br /> <br />Peak discharge-drainage area relationships for streams studied in <br />detail are shown in Table 4. <br /> <br />There is an extensive loss of <br />mouth of Happy Canyon Creek. <br />across low ground on the left <br /> <br />discharge 2,000 feet <br />This is because of a <br />bank of the creek. <br /> <br />upstream of the <br />flow diversion <br /> <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 />For all streams studied, the water-surface elevations of floods of <br />the selected recurrence intervals were computed through use of the <br />U.S. Army Corps of Engineers HEC-2 step-backwater computer program <br />(Reference 10). <br /> <br />Cross sections for the backwater analyses of the San Miguel River <br />were obtained from aerial photographs at a scale of 1:2,400, with <br />a contour interval of 2 feet (Reference 11). The below-water sec- <br />tions were obtained by field measurement. All bridges, dams, and <br />culverts were field surveyed to obtain elevation data and structural <br />geometry. <br /> <br />Cross sections for the Uncompahgre River were also obtained from <br />aerial photogrammetry (Reference 12). All below-water sections, <br />as well as all bridges, dams, and culverts, were measured and sur- <br />veyed in the field. <br /> <br />The cross sections for Dry Cedar Creek, Montrose Arroyo, and Cedar <br />Creek were obtained from the Colorado Water Conservation Board. <br />These cross sections were also derived from aerial photos (Refer- <br />ence 12). All bridges, dams, culverts, and below-water sections <br />were measured and surveyed in the field. <br /> <br />For Happy Canyon Creek, all cross section information was obtained <br />through field surveys. <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 />Roughness factors (Manning's "nil) used in the hydraulic computations <br />for the Montrose study reaches were chosen by engineering judgment <br />and based on field observations of the flooding sources and flood <br /> <br />19 <br />