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• Introduction <br />Sediment Pond 003, also known as the Northwest Impoundment was established in <br />1979. This pond is located in the SE/4 SW/4, Section 27, Township 6 North, <br />Range 87 West (see Exhibit 13-2A, Permanent Postmining Features) in the Little <br />Grassy Creek drainage, which is tributary to Grassy Creek. The contributing <br />watershed is approximately 30.1 acres, of which approximately 13.5 acres are <br />disturbed by mining activities. The pond receives runoff from the <br />shop/warehouse area; the Shop Diversion Ditch drains directly to Pond 003. <br />Pond 003 is an earthen embankment pond (see Exhibit 7-11, Northwest Impoundment <br />#003). The top width of the embankment is approximately 20 wide, with upstream <br />and downstream slopes of approximately 3H:1V, respectively. The principal <br />spillway is an 16-inch diameter straight CMP pipe capped with an end plate; a <br />6-inch CMP pipe, with an invert elevation of 6,577.10 feet, is fitted through <br />the end plate. The emergency spillway is a trapezoidal open channel with a <br />bottom width of 10-feet and 3H:1V side slopes. The emergency spillway channel <br />is well vegetated, with additional protection provided by riprap. The <br />principal spillway is located at the 100 percent sediment storage level. <br />The Colorado Division of Minerals and Geology, in its Proposed Decision and <br />• Findings Document for RM-03, dated June 6, 1997, indicated that Pond 003 was an <br />approved permanent impoundment. <br />Design Analysis <br />Seneca Coal Company evaluated the pond design in 1986 as part of RN-O1. The <br />evaluation was completed to assure that the pond contained sufficient storage <br />capacity, detention time and efficiency to satisfy the regulations of the <br />Colorado Mined Land Reclamation Board's Rule 4.05.6. The design evaluation is <br />presented in Appendix 7-6b. The certified as-built drawing is presented as <br />Exhibit 7-11. <br />Pond 003 was constructed to meet the following specifications: <br />1) Provide adequate capacity to treat runoff to the pond from a 10-year, 24- <br />hour storm event to assure compliance with receiving water quality <br />standards in accordance with Rule 4.05.6(3)(a). <br />2) Provide storage for the 10-year, 24-hour storm event without discharging <br />through the emergency spillway in accordance with Rule 4.05.9(2)(b). <br />3) Provide an emergency spillway that safely passes runoff from the 25-year, <br />24-hour storm event in accordance with Rule 4.06.9(2)(c)(ii). <br />i <br />TR-37 Revised 09/05 <br />