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<br />time of completion of this study. Maps and flood elevations will be <br />amended periodically to reflect future changes. <br /> <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 3). <br />These analyses followed the standard log-Pearson Type III method <br />as outlined by the U.S. Water Resources Council (Reference 4). <br /> <br />Peak discharge-drainage area relationships for the Uncompahgre <br />River are shown in Tahle 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 eleva- <br />tions of floods of the seleCted recurrence intervals. <br /> <br />Water-surface elevations for floods of the selected recurrence <br />intervals Were computed USing the U.S. Army Corps of Engineers <br />BEC-2 step-backwater computer progr am (Reference 5). <br /> <br />Cross section data for the backwater analysis were derived from <br />topographic maps at a scale of 1:2,400 (Reference 6). The below- <br />water sections were obtained by field measurement. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1) and on the Flood Boundary <br />and Floodway Map (Exhibit 2). <br /> <br />Roughness factors (Manning's On") used in the hydraulic computations <br />were estimated based on field investigation. The roughness factor <br />used for the channel was 0.072, overbank factors ranged from 0.08 <br />to 0.10. <br /> <br />Starting water-surface elevations were based on critical depth. <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus considered <br />valid only if hydraulic structures remain unobstructed, operate <br />properly, and do not fail. <br /> <br />Approximate elevations were determined by field reconnaissance, <br />historical accounts of flooding provided by local residents, and <br />engineering judgment. <br /> <br />8 <br />