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• All riser outlets have been fitted with expanded metal trash screens <br />rather than the trash rack and antivortex device as shown on the <br />Sedimentation Ponds - Kerr Coal Mine Map (Plate I). <br />Emergency Spillway <br />In accordance with 30 CFR 816.46(i), the 100-year, 24-hour precipitation <br />event was used to size the emergency spillway for Ponds A and D. For <br />the remaining Ponds B, C, E, F, and G, the 25-year storm was utilized. <br />None of the existing ponds exceed the 20-foot OSM embankment height <br />criteria. <br />The emergency spillways are trapezoidal channels with 4H:1V sideslopes <br />constructed in either cut ground around the embankment abutment or <br />through the embankment. In all cases, the emergency spillway crest <br />elevation is set at one-tenth foot above the top of the riser elevation. <br />• The emergency spillway slopes are riprapped to protect against erosion. <br />For spillways constructed in cut ground ten (10) inches of 6 to 10-inch <br />riprap is utilized, extending from the upstream face of the embankment <br />to a minimum of ten (10) feet beyond the embankment crest centerline. <br />Mhere the spillway is cut through the embankment. 14 inches of 8 to <br />14-inch riprap is provided. extending from the upstream face to a <br />minimum of five (5) feet beyond the downstream toe. <br />The emergency spillways were designed utilizing the Sharp Crested Meir <br />Equation as follows: <br />Q = CbH1.5 <br />Mhere H = Depth of flow (ft) <br />C = assume 2.80 <br />b = width of weir (ft) <br /> <br />23-43 <br />