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<br />runoff. Three assumptions were required for the flood reconstitutions: <br /> <br />1, Surface Storace Loss- Based on calibrated data, the Urban Drainage and Flood Control <br />District recommends a surface storage loss of 0,35 inches for watersheds consisting of <br />lawns. This value was used in the flood volume reconstitutions. <br /> <br />2. Basin Percentlmoerviousness- The value for percent imperviousness for the existing <br />conditions is 40%. The calibrated storms were in the early 1970s, so the value of percent <br />imperviousness was reduced to 30% to reflect 1970s conditions. <br /> <br />3. Antecedent Soil Conditions- Simulation of the infiltration process was based on being <br />an exponential loss rate if the antecedent conditions were dry or uniform loss rate if there <br />was significant rainfall immediately prior to the storm being simulated. <br /> <br />Table 4 shows the results of the flood reconstitutions. Figures 6,7, and 8 show the historic <br />rainfall and runoff rates for the three storms, <br /> <br />Table 4 <br />Irondale Gulch Calibrated Infiltration Rates From Storm Reconstitutions <br /> <br /> Calibrated <br /> Peak 5 Observed Simulated Minimum <br /> Total min. Previous Runoff Runoff Infillration <br /> Stonn Stann Storm Volume Volume Rate <br />Stann Volume Intensity Amounts (Ae-It) (Ae-It) (inlhr) <br /> (in.) (inlhr) <br />July 24. 0.95 3.36 0.63" - 2 4.9 5.0 0.8' <br />1973 days <br /> earlier <br />Aug. 7, 1973 1.95 5.64 0.35" - 8 10.7 10.8 0.8" <br /> days <br /> earlier <br />July 30, 1.39 3.94 1,33" - 8 7,9 8.1 1.3' <br />1974 days <br /> earlier <br />'Using uniform infiltration loss rate <br />"Using exponential loss rate based on UDFCD criteria <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFf <br /> <br />7.21 <br />