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Response to DRMS General Stormwater Comments — Cotter JD -9 Mine Drainage Design Plan <br />provided by the NOAA Atlas for the area. These revisions are shown in the enclosed <br />drawings included in Attachment 5. <br />5. Please address the reclamationlpost mining plan for the retention pond. The DRMS <br />strongly encourages breaching the embankment upon closure unless the landowner has a <br />use for the pond (e.g., stock pond) and intends to maintain it. <br />As suggested, at final reclamation, it is now the intent to breach the pond embankment to <br />an elevation allowing stormwater to pass through the site without retention. This is <br />indicated on the revised drawings included in Attachment 5 of this document. <br />6 Page ESWMP -25, 72" CMP analysis; DDP Drawing 3 of 7; and Figure C2. There <br />appears to be some discrepancies in parameters such as elevations, culvert lengths and <br />outlet conditions in the three referenced pages. The two drawings indicate elevations in <br />the vicinity of 6460 vs. the elevations around 6390 on ESWMP -25. The drawings differ <br />considerably in culvert length ( 72 ft in the DDP drawing vs. —58- ft. in Figure C2) and <br />measured culvert slope ( -11 % in the DDP .drawing vs. — -2% in Figure C2). The culvert <br />outlet velocity is considerably different for these two conditions and will dictate the <br />necessary outlet scour protection to prevent erosion. There is also a photo in Appendix I <br />of the "Materials Containment Plan" (Reference photo 48 on page 4) snowing a <br />cantilevered culvert above the contact between steep fill and native terrain. This culvert <br />outlet requires an energy dissipation to prevent scour at the outfall. <br />Two sets of elevations were inadvertently used for the culvert analysis and drawings. One <br />set was based on assumed elevations used during the site topo survey and the other was <br />using adjusted elevations to match the other existing site topo provided during earlier <br />mapping. The corrected (adjusted) elevations should match those indicated on the <br />drawings, not those used in the worksheets. The correct culvert length is approximately 72 <br />feet. <br />a. Please clarity whether or not photo #8 is the 72" CMP. <br />The photo referenced in the review (photo #8 on page 4 contained in Appendix 1 of <br />the original "Materials Containment Plan ") is of the 72" CMP discussed and <br />shown in the analysis. <br />b. Please review the last two rows ofAttachmentA and clarify which is correct. <br />The bottom row of the review Attachment A is correct. <br />c. Please provide outlet protection design for the confirmed culvert condition. <br />4 <br />