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<br />.1.1 ...J,." <br /> <br />Findings and Recommendations <br /> <br />This site-specific PMP study for the Cherry Creek basin has produced PMP <br />values that can be used to determine the Probable Maximum Flood for the <br />Cherry Creek Reservoir. The PMP values are basin average rainfall values, i.e. <br />the average rainfall over the 386 square mile basin. These PMP values were <br />derived using Colorado storms together with physically consistent conservative <br />assumptions. The following PMP values were computed. The NWS values are <br />provided for comparison. <br /> <br />Cherry Creek Basin Average PMP values <br /> <br />AWA study <br />NWS study <br />Difference <br /> <br />24-hour <br />15.8 inches <br />21.1 inches <br />25% lower <br /> <br />72-hour <br />17.4 inches <br />24.7 inches <br />30% lower <br /> <br />The A W A study computed only basin average rainfall amounts. It is <br />useful to compare 1 a-square mile values since they approximate point rainfall <br />amounts. The basin average values from this study are adjusted to 1 a-square <br />mile values using the within storm curves for Colorado storms. <br /> <br />Cherry Creek 1 a-square mile PMP values <br /> <br />AWAstudy <br />NWS study <br />Difference <br />NWS study without "k-factors" <br /> <br />24-hour <br />24.7 inches <br />27.0 inches <br />9% lower <br />24.5 inches <br /> <br />72-hour <br />27.2 inches <br />30.5 inches <br />11 % lower <br />27.7 inches <br /> <br />If the A W A study finding of no orographic rainfall increase within the <br />Cherry Creek basin is applied, removing the + 10% "k-factor" rainfall increase <br />used in the NWS study gives 1 a-square mile values almost identical to the <br />A W A study. However, the "k-factor" alone does not account for the <br />differences in the basin average PMP values. These differences are probably <br />due to difference in the spatial rainfall distributions between Colorado storms <br />and the storms used in HMR 52 and HMR 55A. <br />