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• III IIIIIIIIIIIIIIII • 999 STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Drpdrlment of Natural Resources I J I 1 Sherman 51., Roum 21 5 II~ Denver, Colorado H0203 Phone IJ031 N6b-a567 rAk: 11031 H3'~7{IU6 DEPARTMENT OF December 24, 1997 NATURAL RESOURCES Richard Mills Ruy ftnme+ Gorernnr Environmental Manager Ian,rs s. L~K hncad Twentymile Coal Company e.rr~~i„e oirenu. Michael H Lung 29515 Routt County Road #27 urvisinn oiree~or Oak Creek, Colorado 80467 Re: Minor Revision No. 97-156, Temporary Rock Check Structure Design for Ditch RDA-2 Permit No. C-82-056 Subject: Technical Adequacy Dear Mr. Mills: The purpose of this letter is to provide concerns the Division of Minerals and Geology has with regard to the referenced Minor Revision application (Twentymile Coal Company letter of December 19. 1997). 1. Rock check structures are a valid option for obtaining the objectives of the minor revision. An equivalent Manning's "n" to demonstrate the effect of rock check structures may not be appropriate. A given flow through any ditch segment of a given shape (a function of slope as well as Manning's "n", etc.) determines a velocity. Any section of the ditch between the hydraulic effect of check structures would be subject to such velocities. For example, ditch RDAs through RDAc carries a flow of 16.90 cfs. Check structures do not reduce this flow. Check structures do effect velocity from the point of pending above the check dam to the exit discharge (a point near the downstream toe-of-slope). Those segments of ditch RDA-2 between the toe of one check structure and the pond pool of the lower are still required to accommodate the 16.90 cfs at the velocities demonstrated by SEDCAD+. The check structures do not reduce flow velocities in these intermediate ditch segments (as suggested by the analysis of an equivalent Manning's "n" of 0.075). Thus the objective of capturing some sediment behind the check structures would be achieved, but the erosion of ditch channels between check structures would not be arrested. The design (sketch) of the check structures should indicate (as suggested by the SEDCAD+ demonstration) a bottom width (of the tlow channel) of 8 feet. and depths of rock should be 12" minimum up the side slopes as well as across the bottom.