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• <br />DEPARTMENT OF NATURAL RESOURCES <br />David H. Getches, Executive Director <br />MINED LAND RECLAMATION DIVISION <br />DAVID C. SH ELTON, Director <br />Richard D. Lamm <br />Governor <br />DATE: May 15, 1984 <br />T0: Carol Russell <br />FROM: Bob Liddle <br />RE: R.A. Mining Company - Gypsum Pit <br />You requested that I review the above referenced Gypsum mine application <br />pertaining to drainage and water quality. The application does not contain <br />any specific drainage designs for me to review. They do indicate the use of a <br />pond, but no details are given. <br />Normally I would ask for designs on an upland diversion ditch that would be <br />left as permanent after mining has been completed. This would minimize water <br />contact with the Gypsum Pit. Typical designs on the pond including capacity <br />and spillway size should also be provided. The pond should be removed after <br />mining unless the site will not be revegetated. <br />I did some rough estimates on pond sizing for your use in evaluating the <br />operator's proposal. Drainage from the initial operation from a 25 year <br />event, assuming 2.0 inches rainfall, steep slope, and low infiltration is 0.75 <br />acre feet with a peak flow of 15 cfs. A conduit spillway of 21 inches with 1 <br />foot of freeboard should pass 15 cfs or an 15 inch CMP with 3.0 feet of <br />freeboard could be used to handle 15 cfs. <br />/ph <br />cc: Ed Bischoff <br />Doc. No. 1420 <br />423 Centennial Building, 1313 Sherman Street Denver, Colorado 80203 Tel. (30'x) 866-3567 <br />III IIIIIIIIIIIIIIII <br />. 999 <br />