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<br /> <br />May 1973. Rainfal I generally ranging to as much as 6 <br />inches fell on the plains east of the Rocky Mountains beginning on <br />5 May. The Dry Creek basin received amounts ranging from 2 to 4 <br />Inches. Runoff from this general storm contributed to consider- <br />able flooding on the South Platte River. <br /> <br />18 <br /> <br /> <br />FUTURE FLOODS <br /> <br />Floods of the same or larger magnitude than those that <br />have occurred could occur in the future. To determine the flood <br />potential of the study area, the la-year, 50-year, lOa-year, and <br />500-year floods were analyzed. The results of this analysis are' <br />presented In this report as a means of demonstrating the effects <br />of large floods. <br /> <br />Discharge probabll ity relationships for Dry Creek were <br />developed using the Environmental Protection Agency's "Storm Water <br />Management Mode I . " The bas I n was d I v I ded Into 64 subcatchment <br />areas that average 200 acres In size. The National Oceanic and <br />Atmospheric Administration's technical publication, Atlas 2, Volume <br />III, Colorado, was used to select the rainfal I values for the 10-, <br />50-, and lOa-year floods. The 500-year flood was determined by <br />extrapolating the 10-, 50-, and lOa-year flood data. Maximum 1- <br />hour rainfall values for the 10-, 50-, 100-, and 500-year floods <br />are 1.58, 2.03, 2.36, and 2.90 inches, respectively, with maximum <br />2-hour rainfal I of 1.74,2.36,2.73, and 3.43 inches, respectively. <br />A soil infiltration rate of 0.50 inch per hour was used for the <br />pervious area of the Dry Creek basin. Table 2, entitled "Dry Creek <br />Basin Drainage Areas," presents basin drainage areas at various <br />locations along the study reach. Table 3, entitled "Flood Plain <br />Reference Data, Dry Creek," presents discharge data used in this <br />report. <br /> <br />19 <br />