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6 <br />The runoff hydrograph was computed for the 24 hr precipitation with <br />a return period of 10 years. This event is 3 inches at the mine site. <br />The depth of runoff from areas A and B from this storm is 0.35 inches. <br />The procedure used to compute the hydrograph was to compute a unit <br />hydrograph from estimates of the time of concentration and lag time. <br />The time distribution of excess rainfall was then computed using the <br />SCS-Type II distribution of a 24 hr precipitation event. Each 5 minute <br />increment of rainfall excess was routed to the sediment pond and the <br />results superimposed to obtain the hydrograph shown in Figure 1. The <br />surprisingly high peak runoff rate is due to the Type II time distribution <br />which exhibits a period of intense rainfall beginning about 11.5 hours <br />after the storm is initiated. For a CN = 61 and a 3-inch storm, excess <br />• rainfall starts just as the period of most intense rainfall begins. <br />CALCULATION OF DETENTION STORAGE REQUIREMENT ~~ <br />The detention storage requirement is that storage in the sediment <br />pond which will yield the desired detention time. The detention time is <br />defined as the time interval between the centers of mass of the inflow <br />and outflow hydrographs. Thus, the inflow hydrograph must be routed <br />through the sediment pond. Detailed routing of the inflow through the <br />sediment pond to produce the outflow hydrograph is a tedious task and <br />results of sufficient accuracy for preliminary design can be obtained <br />by assuming that both the inflow and outflow hydrographs are triangular, <br />and this procedure was used herein. <br />A triangular inflow hydrograph with a peak discharge of 0.176 in/hr, <br />time to peak = 0.5 hours, a time base of 3.98 hours, and representing a <br />• depth of runoff of 0.35 inches was selected to approximate the inflow <br />hydrograph. For this hydrograph, the time to center of mass is 1.46 <br />