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Response to DRMS Adequacy Review (2) -Cotter SR- I 1 Mine Reclamation Plan Amendment <br />4) Page ESWMP -6, Section 7.4 Stormwater Routing and Retention. <br />a. The 100 year, 24 -hour runoff volume criteria used for sizing storage in the pond is <br />acceptable. However, a spillway is necessary to pass runoff from successive storms as <br />there is no way presented in the Retention Pond design plan to drain the pond via <br />gravity. As such, the emergency spillway for the pond needs to be designed to convey <br />100 year peakflow, assuming the ponds are full (to the spillway invert elevation) at the <br />onset of the design storm. Please provide analyses and designs to demonstrate the <br />spillway has the capacity to pass the peak flow resulting from the 100 year, 24 -hour <br />design storm. (NOTE - The DRMS checked with the Colorado Division of Water <br />Resources (DWR) regarding the status of the Dolores River appropriations. The DWR <br />maintains that the Dolores River is not currently over appropriated and as such, DWR <br />has no current requirement to release retained stormwater within 72 hours. This status is <br />however subject to change.) <br />The retention pond has been redesigned, as requested, to provide "retention" <br />storage of the 10 -year event with an armored spillway capable of passing the 100 - <br />year event. In addition, based on your comments and those from the Division of <br />Water Resources, the pond will have a small drain pipe with screened inlet in the <br />bottom to allow the impoundment to drain fully within the 72 -hour timeframe after <br />a storm event. New storm routing and pond volume calculations are included in <br />Attachment #3. <br />b. Please provide grading plans, details, and specifications for embankment <br />design/compaction; spillway hydraulic analyses, revetment design; and outlet works <br />associated with the retention pond. <br />Revised drawings with grading plans, details, and specification are included in <br />Attachment #4 of this document. <br />S) 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 a <br />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 removed at <br />the final reclamation stage to allow stormwater to pass through the pond with no <br />retention of surface runoff. Revised drawing Sheet #8 is included in the Attachment <br />#4 to indicate this. <br />