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NECC RESPONSE: The proposed DWDA stormwater management above, around and on the <br />waste pile was evaluated for a 100 year, 24-hour storm event. The results indicate that the <br />management system already in place for the site will safely pass the storm with the <br />recommendation that the berms and ditches are field verified to be constructed as designed. <br />Stormwater management in place for the raw coal stockpile area is routed through a sediment <br />pond, through a culvert and to pond 007. The engineering evaluation is included as Exhibit 19 <br />Sediment Design Specification - Development Waste Disposal Area. <br />The catchment area is 4.09 acres and is isolated from other catchments by existing diversion <br />ditches and grade breaks. The 100 year, 24-hour storm precipitation is 4.2 inches. The model <br />predicts the peak runoff to be 12.76 inches with 4.2 inches of runoff. The required ditch size to <br />convey that flow is a trapezoidal channel of bottom width 3.0 ft, height of 1.5 ft, side slopes of <br />1:1 and a minimum slope of 1%. Existing ditches are of this recommended size and shape. The <br />minimum pipe diameter needed to convey the 100 year, 24-hour peak flow to pond 007 is 24 <br />inches. The existing culvert is 24 inches. The pipe installation should have a minimum of 1.2 <br />feet of material from the top of the pipe to the top of the bank (i.e. the lowest point on any top of <br />berm/top of bank around the DWDA shall be no less than 1.2 feet above the top of the 24 inch <br />outlet pipe. <br />An erosion estimate was done for the proposed DWDA and as a result, a sediment basin of 250 <br />cubic yards is recommended if no cover management is practiced on the DWDA. The sediment <br />basin should be installed at the inlet to the culvert, the entire volume of which is below the pipe <br />invert. The sediment basin will require regular maintenance. <br />If you have any questions please contact me. <br />Sincerely, <br />Jl? <br />Ron Thompson <br />Mine Manager <br />Attachment: <br />Exhibit 19 Sediment Design Specification - Development Waste Disposal Area. <br />rxh• bit 31