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~~ - iii iiiiiiiiiiiii iii STATE OF COLCJxxuu MINED LAND RECLAMATION DIVISION Department of Natural Resources 7313 Sherman St.. Room 215 Denver, CO 80203 303 666-3567 Fn x: 303 832-8106 July 6, 1992 Mr. Larry Reschke Roadside and Cameo Mines Powderhorn Coal Company P.O. Box 1430 Palisade, Colorado 81526 RE: North Decline Final Contours Technical Revision 13 (C-81-041) Dear Mr. Reschke: pF ~O~p ~~~~~~~9 •. `Y; LY, r,° a $ :• ~ 1876 Roy Romer, Governor Michael B. Long. Division Dvector The Division's staff hydrologist has reviewed Powderhorn Coal Company's revision to the North Decline area pond and impoundment sizing. Some differences still remain between the Division's calculations and Powderhorn's. Two different computer models were used, the Division uses Sedcad model, while Powderhorn utilizes the Trihydro model. The Trihydro model appears to make allowances for infiltration beyond that calculated into the curve number used by Sedcad. The main discrepancies between these models are as follows: 1) West Sump: This sump is designed to contain a 25-year, 24-hour storm event and drain a 5.2 acre area. The Division's Sedcad calculations estimate 0.37 acre feet of water would reach the sump following the design event. Powderhorn's dimensions indicate the sump can contain 0.20 acre feet of water. The hydrologist's calculations indicate that increasing the depth of the sump by 1.5 feet should provide adequate storage. 2) Pond 7: Pond 7 drains 9.67 acres and has a capacity of 0.19 acre feet to the crest of the emergency spillway. Sedcad calculations indicate that a 10-year, 24-hour storm event would drain 0.51 acre feet into Pond 7. Following a 25-year, 24-hour event 0.68 acre feet of water would reach Pond 7. Please size the pond to contain the 10-year, 24-hour event, and pass the 25-year, 24-hour event while still maintaining 1 foot freeboard.