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<br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for each flooding source studied in detail <br />affecting the community. <br /> <br />Uncompahgre River floodflow frequency data were based on statistical <br />analyses of streamflow records covering a 54-year period at gaging <br />stations operated by the U.S. Geological Survey (Reference 5). <br />These analyses followed the standard log-Pearson Type III method <br />as outlined by the U.S. Water Resources Council (Reference 6). <br /> <br />Peak discharge-drainage area relationships for Uncompahgre River <br />are shown in Table 1. <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 ele- <br />vations of floods of the selected recurrence intervals. <br /> <br />Cross sections for backwater analysis of the streams in the study <br />area were obtained from field surveys and aerial photographs (Refer- <br />ence 7). <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). Selected cross section <br />locations are also shown on the Flood Boundary and Floodway Map <br />(Exhibit 2). <br /> <br />Channel roughness factors (Manning's On") used "in hydraulic com- <br />putations were chosen by engineering judgment and based on field <br />observations of the streams and flood plain areas. The locations <br />and values obtained are shown in the Technical Addendum (Refer- <br />ence 8). <br /> <br />Water-surface elevations of floods of the selected recurrence <br />intervals were computed through use of the U.S. Army Corps of <br />Engineers HEC-2 step-backwater computer program (Reference 9). <br />Flood profiles were drawn showing computed water-surface elevations <br />for floods of the selected recurrence intervals. Starting water- <br />surface elevations for Uncompahgre River were calculated using <br />normal-depth methods. <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 />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NCVO). Elevation reference marks used in this <br />study are shown on the maps. <br /> <br />6 <br />