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III IIIIII1111111111 999 STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Dcparlmcnl of Nanlral Resources 1313 Sherman SL, Room 215 Denver, Colorado RO203 Phnne: (3(131 RG6 3567 FA% (lU l) R72-R106 DATE: NOVEMBER 21, 1996 TO: DAVID BERRY FROM: JOE DIIDASH ~~ D II~~~ DEPARTMENT OF 1~1ATURAL RESOURCES Roy Romer Governor lames 5. lochhead E•ecmive Deector Michael B. Long Division Dnedor RE: COAL MINE WASTE BANK PRELIMINARY DESIGNS BANBORN CREEK MINE, PERMIT NO. C-81-022, OSBOW CARBON AND MINERALB~ INC. I have finished my review of the surface water hydrology for the above-mentioned submittal. I have the following questions or comments: 1. The Division suggests that, in order to avoid confusion, the October 1996 report by Westec, entitled "Coal Mine Waste Bank Design For The Sanborn Creek Mine" be purged of possibly unneeded information. In Appendix B, the reach summaries for the 10 year-24 hour event and for the 25 year-24 hour event appear to be unnecessary, since the ditch designs are based solely on the 100 year-24 hour event. Unless there is a reason for including all of this material in the submittal, please consider removing this material from the submittal. Another area of concern is having 100 year-24 hour ditch designs in Appendix B which are superseded by differing 100 year-24 hour ditch designs in Appendix C. The Division assumes that the sole purpose of having the 100 year-24 hour ditch designs in Appendix B is that it shows the combined peak discharge for those ditch reaches which have combined subwatershed flows. However, this information already is shown in the hydrograph summary table in Appendix B. Unless needed, please consider removing the 100 year-24 hour reach summary in Appendix B, or adding a note to that summary that the ditch designs are being superseded by ditch designs in Appendix C. 2. For the sake of clarity, the Division requests that an explanation be included, on the page in Appendix B of the Westec report that has the hydrograph summary for the 2.8 inch 100 year-24 hour storm event, which describes which subwatershed peak flows contribute to each hydrograph number and to each ditch reach.