2008-12-04_PERMIT FILE - C1994082 (14)

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.~ / ~ no longer appropriate or applicable (e.g., land condition, sediment delivery). The TR-37 hydrology and sedimentology demonstration utilizes a watershed condition based on the September 2007 landscape. This is an appropriate approach because all coal resource removal activities have ceased and most disturbed areas within the watershed are reclaimed and successfully revegetated (see Yoast Mine, 2007 Annual Reclamation Report, Chronological Seeding Map). Consequently, the current sediment delivery is greatly reduced from the worst-case conditions evaluated in the 2004 design and as-built evaluations. The hydrology and sedimentology in the TR-37 demonstrations were analyzed using the SEDCAD4 computer program (Schwab, 1998). Model methodology and assumptions are presented in Attachment 13-12, Yoast Mine Drainage and Sediment Control Plan. Appendix 13-19.1-A and 13-19.1-B present the SEDCAD4 demonstrations for the 10-year, 24-hour and 25-year, 24-hour design storms. Input consists of precipitation (inches), watershed area (acres), time of concentration (hours), runoff curve number (CN), representative particle-size distributions of soils in the area, sedimentology information, spillway dimensions, etc. The drainage area(s) for the watershed, as well as, slope lengths and distances for time of concentration estimates were measured on an approved permit topographic map (see Exhibit 13-12.1A, Permanent Channel and Pond Watersheds). The 25-year, 24-hour evaluation conservatively assumed the pond is 100 percent full to the emergency spillway invert elevation (e.g., principal spillway is inoperable). SCS Curve Number and "k" Factor. The curve numbers used for the TR-37 SEDCAD demonstration are presented in Attachment 13-12. The "k" factors were determined based on soil survey information and National Resource Conservation Service soil interpretations. The curve numbers and "k" factors were then "weighted" for each sub-watershed as presented in the Hydrology Input Parameter tables found at the back of Appendix 13-19.1-A. Precipitation and Storm Type. The precipitation amounts at the Yoast Mine area for the 10-year and 25-year precipitation events were obtained from NOAA Precipitation Frequency Atlas - Frequency Atlas of the Western United States - Volume II - Colorado (Miller, et al., 1973}. A standard SCS Type II storm was utilized for each analysis. • 10-year, 24-hour event: 1.7 inches • 25-year, 24-hour event: 2.1 inches sedimentology. To determine the sediment loading to the pond, the average annual "R" method was used. The amount of sediment storage is determined for a defined length of time and is based on the design storm R, design storm TR-37 13-19.1-2 Revised 08/08