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• The HYDROCAD computer modeling program was used which uses the SCS Unit Hydrograph <br />Method for watershed runoff modeling. <br />SCS Unit Hydrograph Method <br />The SCS unit hydrograph procedure (also known as the TR-20 runoff method) generates a runoff <br />hydrograph by the following basic steps: (For brevity, this is a simplified description.) <br />1) A rainfall distribution is selected which indicates how the storm depth will be distributed over <br />time. This is usually a standardized distribution, such as the SCS Type II storm, and often has a <br />standardized duration of 24 hours. <br />2) The design storm depth is determined from rainfall maps, based on the return period being <br />modeled. Combined with the rainfall distribution, this specifies the cumulative rainfall depth at <br />• all times during the storm. <br />3) Based on the Time-of-Concentration, the storm is divided into "bursts" of equal duration. For <br />each burst, the SCS runoff equation and the average Curve Number are used to determine the <br />portion of that burst that will appear as runoff. <br />4) A Unit Hydro-graph, in conjunction with the Time-of-Concentration, is used to determine how <br />the runoff from a single burst is distributed over time. The result is a complete runoff <br />hydrograph for a single burst. <br />5) Individual hydrographs are added together for all bursts in the storm, yielding the complete <br />runoff hydrograph for the storm. <br />For a Tc of 7.5 minutes, the burst duration is 1 minute, so a 24-hour storm will consist of 1440 bursts. <br />If each burst involves a unit hydrograph of 100 coordinates, then 140,000 coordinates must be summed <br />to produce the composite hydrograph. <br /> <br />Whirlwind Mine 07 (rev. April 08) <br />D-33 <br />