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iii iiiiiiiiiiiiiiii <br />999 <br />DIVISION OF MINERALS AND GEOLOGY <br />Deparlmenl o(Natural Resources <br />1313 Sherman 51 ,Room 215 <br />Denver, CO 80203 <br />Phone: 13031 866-3567 <br />FAX: 13031 8 7 2-B I Oh <br />October 22, 1993 <br />Juan Garcia <br />Hydrologist/Environmental Engineer <br />Colowyo Coal Company <br />5731 State Highway 13 <br />Meeker, CO 81641 <br />RE: Colowyo Mine, Permit C-81-019, Technical Revision No. 25 <br />Dear Mr. Garcia: <br />pF Cp~ <br />~`v 4 <br />tip. <br />~~ <br />~ r8i6 ~ <br />Roy Romer <br />Governor <br />Michael B Long <br />Division Diredo~ <br />The Division has reviewed items submitted by Colowyo regarding Hydrology/Sedimentology <br />Pond Design Update for the West Pit Pond, Technical Revision No. 25. Although the Division <br />disagrees with the CP factor utilized in Colowyo's sediment volume calculation, the Division <br />does agree that the West Pit Pond appears adequately sized to treat runoff from designed <br />events. <br />In determining a CP value to be used in the Sediment Volume Calculation, Colowyo reduced <br />the value with a factor of .4. This value was taken from the Sedcad+ manual, which allows for <br />that reduction when land surfaces are characterized as "Very rough, major effect on runoff and <br />sediment storage, depressions greater than 6 inches". Field observations of areas at the <br />Colowyo Mine, where topsoil has been removed, have not included depressions greater than <br />6 inches. Our observations have been that after removal by scrapers, the subsoil is relatively <br />smooth. We believe that the CP factor should be multiplied by .9, which would represent <br />"Smooth, minor surface sediment storage, depressions less than 2 inches. Please have the <br />operator either correct this factor to .9 in their submittal, or provide additional information to <br />substantiate the use of .4. <br />If you have further questions, please either contact me or Susan Burgmeier at the Division. <br />Sincerely, <br />anet Binns <br />viro ental Protection Specialist <br />STATE OF COLORADO <br />cc: Larry P. Routten, DMG <br />