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STATE OF COLORADO rut irn i.4 MM, rlnvc~nm <br />DEPARTMENT OF NATURAL RESOURC III IIIIIIiiiIIII III <br />D. Monte Pascoe, Execusive Director <br />999 <br />July 9, 1981 ,n` <br />n~(V <br />P1T. E. LeRoy Black \~L~.//\ ' d ~ +,3--~ <br />Ute Engineering and Surveying Company ~ n _ ' <br />P.O. Box 674 Q/V`.Gw'/`'Yn~/t, ~ <br />Delta, Colorado 81416 <br />Re: Grand Mesa Coal Company Revised Sediment Control Calculations <br />Our File No. C-034-81 <br />David C. Shelton <br />Director <br />Dear Mr. Black: <br />MLNIJD LANll NEI~LAMA'I'lON <br />423 Centennial Building, 1313 Sherman Street <br />Denver, Colorado 80203 Tel. (30$) 839-3567 <br /> <br />The revised sediment control calculations are all completed in a very pro- <br />fessional and accurate fashion. The staff at Ute Engineering is to be <br />complimented on a job well done. There remain only a few concerns regarding <br />the plan with regard to ditch lining, culvert inlet protection and spillway <br />design. <br />Ditches: Many of the ditches at the site have design velocities which may <br />be erosive. AZ1 ditches which have design velocities over 9 fps should have <br />the native ditch lining material described and in cases where the velocities <br />are erosive for the native lining, sufficient riprap must be provided. GMCC <br />may wish to break the ditches into sections of Zike slope and provide lining <br />material only where the velocity becomes erosive. Of greatest concern are <br />ditches which carry undisturbed runoff at velocities exceeding 4 fps; <br />ditches 3 and 6 in the unaffected area. <br />Culverts: The provision of embankment protection at culvert inlets should <br />extend sufficiently at the sides to prevent excessive erosion. It is <br />recommended that where riprap is required, it be placed to either side of <br />the installation to a distance equal to the design headwater. <br />Emergency Spillway Design: As it is clear that an emergency spillway is not <br />needed to safely pass the 25-yr., 24-hr. storm event, GMCC need not install <br />one. However, if the life of the mine exceeds 25 years, GMCC may wish to <br />design and install an emergency spillway for the purpose of safety. Also, if <br />the embankment height from the bottom of the dam to the crest of the principal <br />spillway exceeds 10 ft-, an emergency spillway designed for the 100-yr. event <br />is required (4.05.6(8)(c)). As the principal spillway in the existing design <br />is the main facility providing dam safety, a trash rack and anti-vortex <br />baffle must be incorporated in the design. <br />Sincerely, <br />Richard Do 'ngue <br />Reclamation Specialist <br />