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Response to DRMS Adequacy Review (2 drainage) — Cotter SM -18 Mine Reclamation Plan Amendment <br />Offsite Basin 20 just below the existing rock outcrop and before the confluence with the <br />Pond 20 outlet. This section is labeled 20 -3.5 and is a part of the regrading and <br />armoring proposed at the confluence. Capacity and velocity analysis worksheets are <br />provided for section 20 -3.5 in Attachment 4. In addition, the sections downstream of <br />this confluence have been reevaluated to carry the combined discharge of the 100 -year <br />event from Pond 20 and the Offsite Basin 20 channel (3.64 cfs total). Worksheets for <br />capacity and velocity analysis of these sections (20 -4 and 20 -5) are shown in Attachment <br />5. <br />S. Page ESWMP -6 and 7, sections 7.4 and 7.5 and Retention Ponds Drainage Design Plans. <br />The narrative response to this comment is adequate. However, there are some anomalies <br />with the weir calculations I Attachment 6 and some design omissions in Attachment 7. <br />a. Attachment 6, weir calculations: There are three anomalies with each of the three weir <br />calculation worksheets: 1) The tailwater elevation is higher than the crest elevation, <br />suggesting the weirs operate under submerged conditions. This is contrary to the designs <br />shown in Attachment 7; 2). The crest breadth is S. 0 feet for all three weirs. Based on <br />information in the Attachment 7 drawings, the crest breadth at the top of riprap <br />is 11. 0, 13.4, and 8 feet for Ponds 10, 20, and 30, respectively; 3) The broad <br />crested weir worksheets assume a rectangular cross section for the analyses. The <br />drawings in Attachment 7 indicate the weirs have 2H.•1 V side slopes. Please provide a <br />narrative addressing each of these anomalies, explaining the reasons for these <br />assumptions and/or making corrections. <br />Setting the tailwater elevation above the crest was meant to be a conservative "worst <br />case" analysis. A new analysis is now included for each weir in Attachment 6 for a non - <br />submerged condition. The differences are generally negligible. The original weir <br />analysis of each weir is included for comparison. The intended crest breadth is 5' for <br />all broad - crested weirs and is now labeled as such on the drawings (Sheets 6, 7, and 8 of <br />Attachment 7). The weir analysis does assume a rectangular opening for all weirs. This <br />is a conservative analysis as the sloped openings will discharge a slightly greater <br />amount for the same vertical flow depth. However, the weirs were also analyzed as <br />trapezoidal channels for a comparison of velocities and other parameters. In all cases, <br />the channel analysis shows negligible differences when compared with the rectangular <br />weir worksheets. All worksheets are included in Attachment 6 for comparison. <br />b. Attachment 7, design drawings: The design drawings do not indicate whether or not the <br />spillway channels downgradient of the weir section are lined with riprap or not. Please <br />indicate on the plan views of each pond spillway these spillway channels are lined with <br />riprap to at least 10 feet beyond the toe of the embankment, and dimension the <br />width of the riprap -lined portion of the spillway channel. <br />3 <br />