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<br />INFLOW DESIGN FLOOD <br /> <br />1. Methodoloqv <br />Inflow Design Flood (IDF) values <br />Reservoir were based on an analysis of <br />probable Maximum Precipitation. <br /> <br />for Cherry Creek <br />runoff from the <br /> <br />2. Probable Maximum Precipitation <br />The Probable Maximum Precipitation (PMP) is described as <br />the most severe combination of meteorological occurrences <br />considered reasonably possible in a particular region. It is <br />considered to be the upper limit of flood-producing rainfall <br />and is generally used as a design storm when virtually no risk <br />of flooding can be tolerated. The PMP has historically been <br />used in dam design to ensure the adequacy of spillways and <br />top-of-dam elevations for high hazard dams. Construction of <br />the PMP for this study is based on procedures outlined in <br />Hydrometeorological Report No. 55A, which is used for regions <br />between the continental Divide and the 103rd Meridian. <br />The general 1-, 6-, 24-, and 72- hour index PMP <br />estimates were determined for the Cherry Creek drainage basin <br />upstream of Cherry Creek Reservoir. Then the PMP estimates <br />were adj usted by the appropriate areal reduction factors. <br />Depth-duration curves were plotted so that intermediate <br />values could be interpolated and the incremental depths <br />computed. The resulting IDF rainfall distribution values are <br />shown in table 3. <br /> <br />3. Infiltration Losses <br />A uniform inf il tration rate of 0.5 inch per hour over <br />the entire basin was applied to the rainfall values to <br />evaluate the IDF runoff. This value was obtained from Soil <br />Surveys from the US Soil Conservation Service. with this loss <br />rate, the IDF runoff values are shown in table 3. <br /> <br />4. Inflow Desiqn Flood <br />According to ER 1110-8-2 (FR), rainfall to runoff <br />conversion and loss rates are derived to correspond to <br />patterns favorable for rapid concentrations of runoff from the <br />drainage basin. The resulting 16.25 inches of runoff, when <br />applied to the unit hydrograph, produced an IDF hydrograph <br />with a peak discharge of 632,000 c.f.s., a total volume of <br />334,000 acre-feet, and a maximum water surface elevation of <br />5,653.8 feet. The IDF hydrograph is shown in Figure 6. <br />Reservoir inflow unit hydrographs for IDF determinations are <br />then peaked to account for the fact that unit hydrographs are <br />usually derived from smaller floods. Hydrograph peak <br /> <br />7 <br />