Laserfiche WebLink
ANN 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 18 -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 RN -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 -01. 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 4.05.9(2)(c)(ii). <br />TR -37 Revised 04/05 <br />