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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 (Refe~ence 2). In the UDFCD report. the <br />10- and. 100-year flood discha~ges were computed using the Storm <br />Water Management computer model' (Reference 3). The rainfall figure <br />used was based on Volume III bf the Precipitation-Freauency Atlas <br />of the Western United States (R~ference 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 ipterpolating or extrapolating as <br />appropriate. The 50-year fload discharge was determined by <br />interpolation; the 500-year flood discharge was determined from <br />extrapolation. <br /> <br />Peak discharge-drainage area r~lationships for all flooding sources <br />are shown in Table 2. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic ch~racteristics of flooding from the <br />sources studied were carried oqt 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 baGkwater analyses for all streams were <br />obtained by aerial photograrnmi!try (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 $eometry. <br /> <br />Locations of selected cross sections used in <br />are shown on the Flood Profil~s (Exhibit 1). <br />for which a floodway was cOmPuted (Section <br />section locations are also sho~ on the Flood <br />Map (Exhibit 2). <br /> <br />the hydraulic analyses <br />For stream segments <br />4.2), selected cross <br />Boundary and Floodway <br /> <br />Roughness factors (Manningls "n") used in the hydraulic <br />computations for the detail~d study streams were chosen by <br />engineering judgment and baseb on field observations. Roughness <br />values for the main channels' of the streams studied ranged from <br />0.02 to 0.065; overbank roughn~ss values ranged from 0.02 to 0.10. <br /> <br />10 <br />