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<br />~II ~I~III~I~~~~I ~I~ <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Deparlmcnl of Nal oral Nr~.uurt c. <br />1313 Sherman SL. Nunin ! I i <br />Denver, Colorado Y.o!u I <br />Phone: 1 SO3) Hhb- t ~~ r. <br />FA%. O(RI Al'~h loo <br />October 16, 1996 <br />Mr. Jim Kiger <br />Colowyo Coal Co. L.P. <br />5731 State Highway 13 <br />Meeker, CO 81641 <br />RE: Colowyo Coal Mine (C-81-019) <br />TR-39; Culvert Hydrology Modification <br />Dear Mr. Kiger, <br />~~ <br />DEPARTMENT OF <br />NATURAL <br />RESOURCES <br />Roy Romer <br />Governor <br />lama S Lochhead <br />E.e~ wwe Director <br />Michael B Long <br />Division Dneaor <br />The Division of Minerals and Geology received your technical revision application (TR-39) <br />for the Colowyo Coal Mine. The revision modifies the haul road culvert designs and <br />includes a perimeter sediment control ditch located east of the 015 Auxiliary Coal Crusher. <br />The revision was deemed complete for purposes of filing on September 24, 1996. <br />The Division has a number of concerns that should be addressed prior to approval of the <br />revision; <br />1. Colowyo remodeled watersheds and corresponding culverts A, B, C, D, E, F, G, H, <br />I, J, K, L, M, N, O, and P to refine the maintenance requirements of each culvert. <br />Colowyo proposes to allow the culverts to acquire a certain amount of sediment <br />before they are cleaned (a 300% factor of safety). In determining this factor of <br />safety, Colowyo modeled all of the culverts assuming they were clear of sediment and <br />had all of the characteristics of an ordinary culvert. If the culverts are partially full <br />of sediment during a 10-year, 24-hour storm event, the factors used in SEDCAD+ <br />would not necessarily apply. The following factors should be reevaluated in the <br />SEDCAD+ model; <br />a. Colowyo used an entrance loss coefficient of 0.90 which is for corrugated <br />metal pipe. The entrance loss coefficient would change with sediment <br />accumulation in the pipe. Please revise the entrance loss coefficient in the <br />SEDCAD+ model to account for sediment accumulation in the culvert. <br />