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Response to DRMS 3rd General Stormwater Comments of June 10, 2013 Cotter SR -13A Mine <br />b. Provide a cross - section of the channel through the breach... The side berms for the <br />spillway channel are not ideal, as these berms will eventually erode away. However, given <br />that the spillway channel is to be aligned along the natural drainage, the discharges should <br />be confined to the existing drainage way and the proposed riprap should minimize the <br />expected erosion. The response to this comment is adequate. <br />No response required. <br />c. Flatten the inlet channel to competent rock or armor it... The note added to Drawing 3A <br />points to the spillway approach, not the inlet channel cutffilled in east side of the pond. <br />Please add the note the inlet channel as previously requested. <br />A note indicating the inlet channel should be flattened to competent rock or to add <br />riprap to prevent head cutting has been added to the drawing. It has been relocated <br />to make it more evident that it refers to the inlet channel and can be seen on Sheet 4 of <br />6 in Attachment 3. <br />d. Commit to delaying the breaching of the embankment... The May 24, 2013 response to this <br />comment is adequate. <br />No response required. <br />Drawings: <br />12. Sheet 3 and hydraulic analysis. The analysis provided for the spillway chute does not <br />include an evaluation for the maximum roughness. Based on previously provided hydraulic <br />analyses for riprap -lined channels, there should be an analysis for n = 0.045. this would result <br />in an expected flow depth of 0.33 feet. As, such, the required 0.5 feet of freeboard would not be <br />available with a 0. 76 foot deep channel. Please provide an analysis for the maximum <br />Manning's roughness (for capacity) and a design detail on the drawings with adequate <br />freeboard. <br />The 10% slope of the spillway chute for the working pond has been analyzed for capacity <br />and velocity using maximum and minimum n values, respectively. The results of this <br />show a maximum flow depth of 0.33 feet and a maximum velocity of 5.17 fps. These <br />results can be seen on the worksheets in Attachment 1. As a result of the flow depth <br />analysis, the channel depth has been revised to a minimum of 1.0 foot. This is shown on <br />the revised Section D -D, Sheet 6 of 6 in Attachment 3. In addition, a riprap sizing <br />analysis was performed using the FHA analysis discussed previously and shows a <br />minimum D50 requirement of 3.64". The revised Section D -D indicates a minimum D5o of <br />6" is adequately proposed. The results of this analysis can be seen in Attachment 2. <br />ra <br />