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<br />The contributing watershed is approximately 940 acres - mining activities <br />disturbed approximately 34 percent of the total area (see Exhibit 12-1, <br />Operations Plan and Exhibit 20-2, Postmining Topography Map). Seneca Coal <br />Company (SCC) re-evaluated sediment storage requirements as part of TR-37 to <br />assure the pond has sufficient long-term (more than 100 years) storage <br />capacity in anticipation of the pond being retained as a permanent, postmine <br />feature. <br />The TR-37 hydrology and sedimentology demonstrations utilize a watershed <br />condition based on the September 2007 landscape. This is an appropriate <br />approach because all coal resource removal activities have ceased and most <br />disturbed areas .within the watershed are reclaimed and successfully <br />revegetated (see Yoast Mine, 2007 Annual Reclamation Report, Chronological <br />Seeding Map). <br />The hydrology and sedimentology in the TR-37 demonstrations were analyzed <br />using the SEDCAD4 computer program (Schwab, 1998). Model methodology and <br />assumptions are presented in Attachment 13-12, Yoast Mine Drainage and <br />Sediment Control Plan. Appendix 13-3A.1-A and 13-3A.1-B present the SEDCP.D4 <br />demonstrations for the 10-year, 24-hour and 25-year, 24-hour design storms. <br />Input consists of precipitation (inches), watershed area (acres), time of <br />concentration (hours), runoff curve number (CN), representative particle-size <br />distributions of soils in the area, sedimentology information, spillway <br />dimensions, etc. The drainage area(s) for the watershed, as well as, slope <br />lengths and distances for time of concentration estimates were measured on an <br />approved permit topographic map (see Exhibit 13-12.1A, Postmine Channel and <br />Pond Watersheds). <br />This evaluation conservatively assumed the pond(s) are 100 percent full to <br />the emergency spillway invert elevation (e.g., principal spillway is <br />inoperable) during the 25-year, 24-hour precipitation event and provides <br />adequate freeboard to prevent overtopping of the embankment. <br />SCS Curve Number and `~k" Factor. The curve numbers used for the TR-37 SEDCAD <br />demonstration are presented in Attachment 13-12. The "k" factors were <br />determined based on soil survey information and National Resource <br />Conservation Service soil interpretations. The curve numbers and "k" factors <br />were then "weighted" for each sub-watershed as presented in the Hydrology <br />Input Parameter tables found at the back of Appendix 13-3A.1-A. <br />Precipitation and Storm Type. The precipitation amounts at the Yoast Mine <br />area for the 10-year, 25-year and 100-year precipitation events were obtained <br />from NOAA Precipitation Frequency Atlas - FrecTUency Atlas of the Western <br />United States - Volume II - Colorado (Miller, et al., 1973). A standard SCS <br />Type II storm was utilized for each analysis. <br />TR-37 13-3A.1-2 Revised 02/08 <br />