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The sedimentation ponds are sized to retain the entire runoff from a <br /> 10-year/24-hour precipitation event plus a sediment volume of 0.2 <br /> acre-foot per disturbed acre and to handle the runoff from a 25-year/ <br /> 24-hour rainfall without overtopping. The ponds will be unlined, and <br /> the runoff collected from the 10-year/24-h6ur storm will drain by <br /> natural seepage. It is estimated the water level will drop between <br /> 0.2 and 0.4 feet per day in the unlined ponds. The entire runoff <br /> from the specified storm event should drain in 5 to 10 days. The <br /> computation of the required capacities of the sedimentation ponds <br /> for the Preparation Plant Area is shown' in Table 5. <br /> A horizontal, 12-inch minimum diameter corrugated metal pipe (C'iP) <br /> spillway with a vertical "T" inlet will be installed in each pond. <br /> The storage capacity of each pond below the invert elevation of the <br /> spillway will accommodate the runoff from a 10-year/24-hour storm- <br /> and the required sediment storage without any spillway discharge. <br /> Runoff from storms of larger magnitude will require some spillway <br /> discharge if sediment storage is at the maximum. The amount of water <br /> discharged through the spillway from a 25-year/24-hour storm would be <br /> relativelysmall. Figure 4 shows a typical section and gives the <br /> required dimensions of the sedimentation ponds. <br /> 14 <br />