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MINE ENGINEERING <br />MINE RECLAMATION <br />November 2, 2000 <br />'J. E. STOVER & ASSOCIATES <br />2768 COMPASS DRIVE, SUITE 101 <br />GRAND JUNCTION, COLORADO 81506 <br />PHONE: (970) 245-4101, FAX 242-7908 <br />F'-i.r~\/fin <br />Dan T. Mathews <br />Division of Minerals and Geology <br />2148 Broadway, #C5 <br />Grand Junction, CO 81503 <br />Re: Powderhorn Coal Company <br />Permit C-81-041 <br />Revision Order TR-32 <br />Dear Mr. Mathews: <br />N 0 V C ~ 2000 <br />III IIIIIIIIIIIIIIII <br />999 <br />CIVIL ENGINEERING <br />CONST. MANAGEMENT <br />Division of Minerals 3 ~,....,... -.---~ ~ • ~-•+~+~` ^" ""6 ^ <br /> <br />NOV - 3 2000 <br />•c <br />The DMG's letter dated August 10, 2000 transmitted its adequacy response for the <br />referenced technical revision. Following are Powderhorn Coal Company's responses to <br />the DMG's questions and concerns: <br />1-5 No response required. <br />6. Concerns regarding the proposed CRDA-2 upland diversion. <br />a. Both ditch legs will be constructed at least partially in refuse and will receive <br />some surface run-off from the reclaimed surface of the refuse pile. I <br />estimate approximately 0.5 acres of the reclaimed surface will drain into the <br />upland diversion. The shape of this drainage area makes modeling to <br />estimate erosion and resultant contribution of sediment into the diversion <br />ditch problematic. The disturbed area modeled was 1690 feet long by 12.8 <br />~ ~~,~ feet wide at a 2:1 slope. The peak flow was 0.4 cfs. I modeled a trapezoidal <br />channel 1690 feet wide to determine the velocity of flow along the short flow <br />path. The results indicated a flow velocity of only 0.02 fps. Very little if any <br />erosion would occur at this flow velocity. Therefore, very little if any sediment <br />would be transported into the diversion ditch. Therefore, no sediment <br />control should be required for this long narrow drainage area that contributes <br />untreated run-off into the upper diversion ditch for CRDA-2. Pages CRDA- <br />13n - 13s present these calculations. The trapezoidal channel was actually <br />modeled at a 66.7 percent slope 1.5H:1 V. I could not get SedCad to model <br />a 50% slope 2H:1V. The program kept giving me an error message. <br />However, the 66.7 percent slope will give a higher flow velocity than the 50 <br />percent slope so the calculation is conservative. <br />