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• SEDIMENTATION POND HYDRAULICS ANO DESIGN <br />GENERAL <br />The existing sedimentation ponds are designed to retain the runoff <br />resulting from the 10-year, 24-hour precipitation event to allow <br />settlement of suspended solids. The ponds are designed to contain a <br />three-year accumulation of sediment. The ponds range in size from 3.3 <br />to 40.0 acre-feet. The embankments range in height from 5.9 to 8.7 feet <br />using the height criteria specified in 30 CFR 816.46(q), from the <br />upstream toe to the emergency spillway crest. <br />Embankment crests and slopes of all ponds are protected by vegetative <br />cover. Sideslopes are constructed at 3H:1V upstream and 2.5H:1Y <br />downstream. The Sedimentation Ponds - Kerr Coal Mine Map (Plate I), <br />contains a Pond Schedule with information describing all seven (1) <br />• existing ponds (Ponds A through G). This map also contains a <br />cross-section of a pond embankment illustrating the typical pond <br />configuration. Figures 23-4 through 23-10, Embankment and Emergency <br />Spillway Configuration, illustrate the plan view pond configurations. <br />All ponds have a Type 3 riser which limits the outflow rate to such an <br />extent that the entire 10-year. 24-hour inflow volume is assumed to be <br />stored. Trapezoidal channels are utilized as emergency spillways <br />designed to pass the peak flow from the required design storm. <br />Flood routing through the ponds assumes an initial water surface at the <br />required sedimentation level. This elevation corresponds to the orifice <br />elevation on the riser pipe. The reservoirs will be cleaned when <br />accumulated sediment reaches 60 percent of the sediment storage rolume. <br /> <br />23-34 <br />