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<br />Peak discharges for the 10- and 100-year floods for the City Park, <br />Nissen Reservoir, Gay Reservoir, and West Lake Basins were obtained <br />from a 1979 UDFCD report (Reference 2). In the UDFCD report, the <br />10- and 100-year flood discharges were computed using the Storm <br />Water Management computer model (Reference 3). The rainfall figure <br />used was based on Volume III of the Prec1Ditation-FreQuencv Atlas <br />of the Western United States (Reference 4). <br /> <br />The study contractor determined the 50- and 500-year flood <br />discharges by plotting the 10- and 100-year flood discharges on <br />log-probability paper and interpolating or extrapolating as <br />appropriate. The 50-year flood discharge was determined by <br />interpolation; the 500-year flood discharge was determined from <br />extrapolation. <br /> <br />Peak discharge-drainage area relationships for all flooding sources <br />are shown in Table 2. <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 <br />elevations for floods of <br />computed using the U.S. <br />backwater computer program <br /> <br />by detailed methods, the water-surface <br />the selected recurrence intervals were <br />Army Corps of Engineers HEC-2 step- <br />(Reference 5). <br /> <br />Cross sections used in the backwater analyses for all streams were <br />obtained by aerial photogranunetry (Reference 6). The below-water <br />sections of all cross sections were obtained by field measurement. <br />All bridges, dams, and culverts were field surveyed to obtain <br />elevation data and structural geometry. <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 />Roughness factors (Manning's Un") used in the hydraulic <br />computations for the detailed study streams were chosen by <br />engineering judgment and based on field observations. Roughness <br />values for the main channels of the streams studied ranged from <br />0.02 to 0.065; overbank roughness values ranged from 0.02 to 0.10. <br /> <br />10 <br />