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I• <br />• <br />• <br />For the DWDA the model predicts the peak runoff to be 12.76 inches with 4.2 inches of <br />runoff (zero infiltration). The required ditch size to convey that flow is a trapezoidal channel <br />with a bottom width of 3.0 ft, a height of 1.5 ft, side slopes of 1:1, and a minimum slope of <br />1.0 %. The minimum pipe diameter needed to convey the 100 year 24 hour peak flow is 24 <br />inches at a 1.1% slope. The pipe should be installed such that there is a minimum of 1.2 feet <br />of material from the top of the pipe to the top of the bank /berm (i.e., the lowest point on <br />any top of berm/ top of bank around the DWDA shall be no less than 1.2 feet above the top <br />of the 24 inch outlet pipe). These calculations assume the pipe projects from fill and flow is <br />inlet controlled. <br />An erosion estimate was done for the DWDA and as a result a sediment basin of 250 cubic <br />yards is recommended. The sediment basin should be installed at the inlet, the entire <br />volume of which is below the pipe invert. This sediment basin will require regular <br />maintenance (i.e., sediment removal). The size of the sediment basin can be reduced if New <br />Elk employs some type of cover management, as this value assumes the only management <br />used will be equipment tracking. As an example, if mulch is applied and maintained at 2.0 <br />tons per acre the sediment basin can be reduced to 5 cubic yards. <br />The analysis results for the clean water diversion basins, ditches, and culverts can be found <br />on the attached figure and modeling results. <br />Attached please find the output from the model and other calculations. <br />If you have questions, concerns, or would like to discuss this further please don t hesitate to <br />call. 1 n <br />Thanks, <br />Marc Kenney <br />CO PE #41215 <br />CPSWQ #0623 <br />CPESC #5695 <br />mjk/ <br />attachments <br />1 41215 <br />SAPROJECM1134 Agapito - New Elk Mine \Design \StormNET\Revised DWDA Calc Summary.doc <br />TR -53 Page E 39 -3 <br />Page 2 <br />Revised 1/3/11 <br />