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• 2005 Spillway Design — A pond inspection in April 2005 noted that the principal spillway was <br />deteriorating and should be replaced. A spillway abandonment/replacement plan was inserted <br />into the Seneca II permit as part of TR -37 in June 2005. The 2005 st)illwav <br />abandonment/replacement plan is found in the Seneca H permit in Appendix 7 -6c. <br />• 2009 As -built Report Demonstration — This report presented 2007 storage capacity <br />calculations that use the existing as -built (2005) configuration and the August 2009 watershed <br />area updates. The 2009 as -built report demonstration is located in the Seneca II permit as <br />Appendix 7 -6c.1. <br />The original principal spillway consisted of a 36 -inch straight corrugated metal pipe (CMP), with a 6- <br />inch opening. During the April 2005 inspection following snow and ice melt, SCC representatives <br />determined that the 6 -inch discharge barrel was deteriorated such that the existing principal spillway <br />warranted replacement. SCC submitted a technical revision which presented the design and <br />replacement plan for a new principal spillway in June 2005. <br />The replacement principal spillway construction was initiated and completed during August 2005. <br />The replacement principal spillway is an 8 -inch diameter straight plastic corrugated pipe set at an <br />invert elevation of 6,948.2 feet. A screened cage is fitted to the inflow end to collect trash and debris. <br />The 2005 redesign did not affect or include the pond embankment configuration or the existing <br />• emergency spillway. The pond embankment has a top width of approximately 15 feet, with upstream <br />and downstream slopes of approximately 3H:1V, respectively. The emergency spillway is a <br />trapezoidal open channel with a bottom width of 12 -feet and 4H:1 V side slopes. The crest is a <br />vegetated channel, while the outfall channel is rock riprap lined with a D 50 of 6- inches. <br />1986 Design Analysis — Pond 004 was originally designed to meet or exceed the design and <br />performance requirements of the Regulations of the Colorado Mined Land Reclamation Board for <br />Coal Mining and adhere to the following specifications: <br />• Provide storage and treatment for the runoff from the 10 -year, 24 -hour storm event to assure <br />compliance with receiving water quality standards in accordance with Rule 4.05.6(3)(a). <br />• Provide storage for the 10 -year, 24 -hour storm event without discharging through the <br />emergency spillway in accordance with Rule 4.05.9(2)(b). <br />• Provide an emergency spillway capable of passing runoff from the 25 -year, 24 -hour storm <br />event in accordance with Rule 4.05.9(2)(c)(ii). <br />SCC evaluated the pond design in 1986 in response to Seneca H Mine permit renewal comments from <br />• the Colorado Mine Land Reclamation Division (a precursor to CDRMS). The 1986 design evaluation <br />is located in the Seneca II Mine permit in Appendix 7 -6c. The SEDIMOT H hydrology and <br />2.05.3 -E2.C2 11/09 <br />