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Response to DRMS Adequacy Review (4) — Cotter LP -21 Mine Reclamation Plan Amendment <br />The retention pond has been redesigned, as requested, to provide <br />"retention" storage of the 10 -year event with an armored spillway capable <br />of passing the 100 -year event. In addition, and based on your comments, <br />the pond will have a small drain pipe with screened inlet in the bottom to <br />allow the impoundment to drain fully within the 72 hour timeframe after a <br />storm event. Revised drawings are included in attachment 8 of this <br />document. <br />b. Page ESWMP -21 and Retention Pond 50: Grading and Details (Sheet 5 of 5). <br />The areas listed for each elevation in PondPack table on page ESWMP -21 are <br />larger than what DRMS measured on Sheet 5, leading to a greater available <br />storage volume estimate. Please re -check the input to the PondPack input table <br />and ensure the rounded corners of the proposed pond are considered. <br />Attachment 4 shows revised pond contour areas. Some revised areas were <br />larger, some smaller, than the original areas provided in the first report. <br />These new areas were carefully checked for the revised pond configuration. <br />6 Please address the reclamation /post mining plan for the retention ponds. The DRMS <br />strongly encourages breaching the embankment upon closure unless the landowner has <br />a use for the ponds (e.g., stock pond) and intends to maintain them. <br />It is now the intention to have the retention pond embankment partially <br />removed at the final reclamation stage to allow stormwater to pass through <br />the pond with no retention of surface runoff. Revised drawing Sheet 7 is <br />included in attachment 8 to indicate this. <br />7. Pages ESWMP -8 — 12, hydrographs. Peak flow computer software generated tables were <br />not provided as was the case for M -1977 -307, CM -25 mine. Please provide similar <br />tabular inputoutput information. <br />The PondPack "Master Design Storm Summary" is enclosed in Attachment <br />1, which indicates peak flows for all basins. <br />8. Page ESWMP -15. The composite area weighted CN indicates subbasin ON 30 is 2.3 <br />acres. Sheet 1, Future Onsite & Offsite Basins indicates it is 2.82 acres. Please revise the <br />incorrect data and revise the runoff analyses as appropriate. <br />The area for onsite basin 30 has been corrected to 2.82 acres and used in the <br />runoff analysis. This resulted in a slightly higher runoff for subbasin ON 30. <br />9. Pages ESWMP -15 & 16 The composite area weighted CN indicates both subbasin ON <br />30 and ON 40 are pinyon juniper cover. The reclaimed area should be herbaceous <br />cover only with a CN between 71 and 80 for HSG B (if substantiated) or between 89 and <br />93 for HSG D. Please correct the CN values for ON 30 and ON 40. <br />0 <br />