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III IIIIIiililllilli <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Department of Natural Resources <br />1317 Sherman Si.. Room 215 <br />Denver, CO 80203 <br />Phone: 1303 866-3567 <br />FAX. (3031 832-8106 <br />September 24, 1992 <br />Mr, Larry Reschke <br />Roadside and Cameo Mines <br />Powderhorn Coal Company <br />P.O. Box 1430 <br />Palisade, Colorado 81526 <br />RE: North Decline Final Contours, Technical Revision 13 (C-81-0411 <br />Dear Mr. Reschke: <br />pF tp~ <br />~6 9 <br />~.: ;'8 <br />"~~~~ . <br />.~ ~~ <br />~ r8P6 <br />Roy Romer <br />Gacemor <br />M~ch~el B. Long <br />Drv~sion Dnector <br />After extensive analysis of the TRIHYDRO model used by Powderhorn Coal Company to <br />calculate runoff flaws and velocities for the North Decline sediment control structures, <br />OMLR's staff hydrologist has identified significant differences between this model and TR55 <br />used by the Soil Conservation Service and the SEDCAD variation employed by OMLR. <br />Enclosed is a memo from the staff hydrologist explaining the difference and OMLR's hesitation <br />in accepting runoff calculations generated by the TRIHYDRO model. Unless Powderhorn Coal <br />Company can provide more information as to the validity of assumptions put forth in the <br />TRIHYDRO model, OMLR requests the operator use a method that more closely resembles <br />Division accepted models like TR55 or SEDCAD to predict runoff flows and velocities. <br />To reiterate OMLR's letter of July 6, 1992: <br />11 The Division's calculations using SEDCAD model estimates 0.37 acre feet of <br />water would reach the West Sump following a 25-year 24-hour storm event. <br />Please assure the Division that the West Sump will be designed to totally <br />contain this volume of runoff while maintaining 1 foot of freeboard. <br />Alternately, Powderhorn may provide further demonstration supporting the <br />lower runoff volume calculation and size the sump accordingly. <br />21 OMLR's calculations using SEDCAD model for drainage to Pond 7 indicates that <br />a 10-year 24-hour storm event would consist of 0.51 acre feet of water. A 25- <br />year 24-hour event would contribute an estimated 0.68 acre feet of water to <br />Pond 7. Please assure OMLR that the pond will be sized to adequately manage <br />these events or provide more information substantiating calculations and sizing <br />developed using the TRIHYDRO model. <br />