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<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 <br />because several of the original design assumptions ar_d permit conditions are <br />no longer appropriate or applicable (e.g., land condition, sediment <br />delivery). <br />The TR-37 hydrology and sedimentology demonstration utilizes 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). Consequently, the current sediment delivery is greatly reduced <br />from the worst-case conditions evaluated in the 1997 design and as-built <br />evaluations. <br />The hydrology and sedimentology in the TR-37 demonstrations were analyzed <br />using the SEDCAD4 computer program (Schwab, 1998). SEDCAD4 is an enhanced <br />version of the SEDCAD+ computer program that was used for the 1997 design and <br />as-built demonstrations. Model methodology and assumptions are presented in <br />Attachment 13-12, Yoast Mine Drainage and Sediment Control Plan. Appendix <br />13-4A.1-A and 13-4A.1-B present the SEDCAD4 demonstrations for the 10-year, <br />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, Permanent 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 13-4A.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 - Frequency Atlas of the Western <br />TR-37 13-4A.1-2 Revised 02/08 <br />