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<br />STANDARD PROJECT FLOOD <br /> <br />1. General <br />The Standard Project Flood (SPF) has often been used <br />historically as a design flood in which only a small degree of <br />risk of exceedance can be tolerated. Since the SPF is an <br />infrequent event, no specific frequency can be assigned to it. <br />It is usually used for comparison with the recommended <br />protection for a particular project, such as in the design of <br />an urban floodwall. The procedure outlined in EM 1100-2-1411 <br />is used to determine the Standard Project Storm, from which <br />the SPF hydrograph is derived. <br /> <br />2. Standard Pro;ect Storm <br />The Standard Project storm (SPS) is defined as that <br />combination of severe meteorological events that gives the <br />maximum precipitation reasonably characteristic of the <br />geographic region of interest, excluding extremely rare <br />events. The SPS is derived from the index rainfall documented <br />in EM 1110-2-1411 from which rainfall depth is determined for <br />an area of 200 square miles and a duration of 24-hours. <br />Further adjustment of this rainfall value transforms the storm <br />into a 96-hour event and provides adjustment for drainage <br />area. The HEC-1 program can be used to derive the SPS. <br /> <br />3. Calculation of SPS Usinq HEC-1 <br />When the HEC-1 program is utilized, the PS record is <br />used for automatic computation of the SPS according to EM <br />1110-2-1411. The values necessary to use this record are <br />standard project index rainfall, basin shape factor, and basin <br />area. The SPS rainfall determined by HEC-1 in this analysis <br />is shown in Table 2. The complete HEC-1 input data file is <br />shown on Figure 10. <br /> <br />4. Infiltration Losses <br />A uniform infiltration rate of 0.5 inch per hour over <br />the entire basin was applied to the rainfall values to <br />evaluate the SPS runoff. This value was obtained from Soil <br />Surveys completed by the US Soil Conservation Service. <br /> <br />4. SPF Results <br />Rainfall excess from the SPS values for the cherry Creek <br />basin were applied to the unit hydrograph for rainfall-runoff <br />transformation using the input unit hydrograph (UI records) in <br />the HEC-1 model. The resulting runoff amounts are shown in <br />Table 2. SPF inflow and outflow hydrographs and Cherry Creek <br />Reservoir water surface elevations are shown in Figure 5. The <br /> <br />5 <br />