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III 1111111111111111 <br />999 <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Deparimenl of Natural Resources <br />1313 Sherman St., Rrwm 215 <br />Denver, Colorado 80201 <br />Phone: 130}I 8663567 <br />FAX: (303)832-8106 <br />September 23, 1999 <br />Carla J. Lenkey <br />Colorado Department of Public Health and Environment <br />WQCD-WQP-B2 <br />4300 Cherry Creek Drive South <br />Denver. CO 80246-1530 <br />Re: Information, Dischazge Point OIOA, Colowyo Coal Company L.P. <br />Deaz Ms. Lenkey: <br />DIVISION O F <br />MINERALS <br />GEOLOGY <br />RECLAMATION <br />MINING•SAFETY <br />Bill Owens <br />Governor <br />Greg E. Walther <br />Executive Dveaor <br />Michael8 Long <br />Division D~ceaor <br />This information is provided to you in response to your telephone call earlier this month <br />concerning pond design information, and information in inspection reports azound April 12, 1999, <br />with regard to Discharge Point Ol OA at the Colowyo Coal Company L.P. <br />Dischazge Point OI OA is associated with the East Taylor Pond (Pond). Design information is <br />contained in Appendix J, Exhibit 7, of Petmit C-81-O19. The Sediment, Erosion, Discharge by <br />Computer Aided Design (SEDCAD) program by Civil Software Design was used to model the <br />pond. A "worst-case" situation was selected to model the pond. This "worst case" represented <br />"closure" of the West Pit, a time when the watershed would consist of [he maximum area of <br />disturbance and the greatest amount of azea unprotected by vegetation. The demonstration <br />included a watershed of 918.6 acres, of which 126 consisted of bare topsoil that had been seeded <br />(initial reclamation) and the rest with some sort of vegetation cover. Storm events of 10-year, 24- <br />hour (1.8 inches of rain) and 25-yeaz, 24-hour (2.3 inches of rain) were used in the design. The <br />demonstration is for a pond that dewaters automatically. In automatic dewatering mode, the pond <br />is designed to dewater through weep holes down to an elevation of 6944.0 feet (static pool level). <br />The design maximum level of collected sediment in the pond is up to 6939.78 feet. The elevation <br />of the invert of the primary spillway riser is at 6952.0 feet. The following table summarizes the <br />demonstration with regard to Peak Sediment Concentrations (mg/I) and Peak Settleable <br />Concentrations (ml/I). <br />] 0-Year, 24-Hour Event <br /> Peak Sediment Concentration (mg/I) Peak Settleable Concentration (ml/l) <br /> <br />In: 314640 141.77 <br />Out: 78491 ~ 0.00 <br /> <br />25-Yeaz, 24-Hour Event <br />In: 319047 144.55 <br />Out: 93576 0.01 <br /> <br />