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SCS Unit Hvdrograph Method <br />• T'he SCS unit hydrograph procedure (also lmown as the TR-20 runoff method) generates a runoff <br />hydrograph by the following basic steps: (For brevity, this is a simplified description.) <br />A) 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 />standazdized duration of 24 hours. <br />B) 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 all <br />times during the storm. <br />C) 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 aze used to determine the <br />portion of that burst that will appeaz as runoff. <br />.I D) 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 hydrograph <br />for a single burst. <br />E) Individual hydrographs aze 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 <br />bursts. If each burst involves a unit hydrograph of 100 coordinates, then 140,000 coordinates must <br />be summed to produce the composite hydrograph. <br />Because of the computational requirements of the unit hydrograph procedure, the SCS derived a <br />simplified tabulaz method as published in Technical Release 55 JR-55). <br />•) <br />Whirlwind Mine June 07 D-30 <br />