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<br />respectively, of being equaled or exceeded during any year. Although <br />the recurrence interval represents the long term averaqe period between <br />floods of a specific magnitude, rare floods could occur at short intervals <br />or even within the same year. The risk of experiencing a rare flood <br />increases when periods greater than 1 year are considered. For example, <br />the risk of having a flood which equals or exceeds the lOO-year flood (1 <br />percent chance of annual exceedencel in any SO-year period is approximately <br />40 percent (4 in 10), and, for any 90-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein reflect <br />flooding potentials based on conditions existing in the community at the <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 county. <br /> <br />Uncompahgre River floodflow frequency data were based on statisti- <br />cal analyses of streamflow records covering a 54-year period at <br />gaging stations operated by the llSGS (Reference 4). These analyses <br />followed the standard log-Peanon type III method as outlined by <br />the U.S. Water Resources Council (Reference 5). <br /> <br />Peak discharge-drainage area relationships for the Uncompahgre <br />River are shOlm 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 eleva- <br />tions of floods of the selected recurrence intervals. <br /> <br />Water-surface elevations for fJ.oods of the selected recurrence <br />intervals were computed using the U.S. Army Corps of Engineers <br />BEC-2 step-backwater computer program (Reference 6). <br /> <br />Cross section data for the backwater analyses were derived from <br />topographic maps at a scale of 1.2,400 (Reference 7). 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 factors <br />used for the channel ranged from 0.051 to 0.072; overbank factors <br />ranged from 0.08 to 0.10. <br /> <br />8 <br />