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2.0 Embankment and Excavation <br />• Pond 002 is the result of a portion of the old Wadge Pit highwall filling with water — there is no <br />embankment. The impoundment and associated structures are Dre- existing. The as -built <br />configuration was prepared under the supervision of others; Mr. Ronald J. Gehrke, a registered <br />engineer in the State of Colorado (P.E. No. 19959) experienced in the design and construction of <br />earthen structures certified the Pond 002 construction and pond configuration. The "original" <br />certified as -built drawing is found in the Seneca II Mine permit as Exhibit 7 -8, Wadge Impoundment <br />#002 and Cross - Section. <br />3.0 Design Analysis <br />As noted above, SCC evaluated the pond design in 1986. The evaluation demonstrated that the pond <br />met the following specifications. <br />• Provide adequate storage to treat runoff to the pond from a 10 -year, 24 -hour storm event to <br />assure compliance with receiving water quality standards in accordance with Rule <br />4.05.6(3)(a). <br />• Utilize a single open - channel spillway system configured as required by Rule 4.05.9(2)(a)(i). <br />• Provide a spillway system that safely passes runoff from the 25 -year, 24 -hour storm event in <br />accordance with Rule 4.05.9(2)(c)(ii). <br />Sage Creek Coal Company has re- evaluated the pond to assure continued compliance with the <br />applicable rules and regulations. The pond will be permitted under the Colorado Division of Water <br />Quality's (CDWQ) Colorado Discharge Permit System (CDPS) and, consequently, the pond <br />discharge must meet receiving stream effluent limitations (e.g., settleable solids concentrations) and <br />the requirements of Rules 4.05.2(7) and 4.05.6(3)(b). <br />The combination spillway has an invert elevation at 6,614.5 feet. The top of the impoundment <br />elevation is the cord elevation for the haul road (see Cross - Section C -C' on Map 2.05.3 -E2.A, <br />Sediment Pond 002 As -Built [Sheet 2]). A 2007 survey conducted by Seneca Coal Company <br />indicated the elevation of the road adjacent to the spillway is approximately 6,618 feet. The top of <br />impoundment design elevation is conservative because the haul road is lined with a MSHA berm <br />which increases the "top" elevation. <br />4.0 SEDCAD Demonstration <br />The hydrology and sedimentology of the watershed were analyzed using the SEDCAD4 computer <br />program (Schwab, 1998). Model methodology and assumptions are presented in Exhibit 2.05.3 -E2, <br />Drainage and Sediment Control Plan. Addendum 2.05.3- E2.A -1 and 2.05.3- E2.A -2 of this appendix <br />present the modeling demonstrations for the 10 -year, 24 -hour and 25 -year, 24 -hour design storm <br />• events, respectively. <br />2.05.3 -E2.A2 12/09 <br />