2008-12-04_PERMIT FILE - C1994082 (32)

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. Introduction Sediment Pond 010 is located in the NE/4 of Section 8, Township 5 North, Range 87 West, o£ the 6"' P.M. along Annand Draw, an ephemeral tributary to Grassy Creek (see Exhibit 20-2, Poatmining Topography Map). The purposo of this pontl and embankment is control of runoff and sediment from the mining disturbance area located almost 5,000 feet to the south. The embankment was constructed with a horizontal drainage system. A toe drain was installed that will collect the water and discharge it below the toe of the embankment. Pond 010 was originally designed and constructed in 1996. The SEDCAD+ computer model was used to estimate storm event runoff from the 10-year, 25- year and 100-year, 24-hour storm events. The original design evaluated a land condition based on the maximum amount of disturbance (approximately 41 percent of watershed disturbed). The original (1996) design met or exceeded the design and performance criteria of Rules 4.05.6 and 4.05.9. Specifically, the design adhered to the following requirements: • Provide storage and treatment for runoff from the 10-year, 24-hour storm event to assure compliance with receiving water quality standards ' in accordance with Rule 4.05.6 (3)(a). • • Provide storage for the 10-year, 24-hour storm event without discharging through the emergency spillway in accordance with Rule 4.05.9 (2)(b). • Provide an emergency spillway capable of passin g runoff from the 25- year, 24-hour storm event in accordance with Rule 4.05.9 {2)(e){ii). • In addition, the design included a non-clogging dewatering device and principal spillway that will provide adequate detention time for the 10-year, 24-hour storm event and still allow a method to dewater the pond down to the 100 percent sediment storage level in a reason able amount of time. The contributing watershed is approximately 940 acres - mining activities disturbed approximately 41 percent of the total area (see Exhibit 12-1, Operations Plan and Exhibit 20-2, Postmining Topography Map). Seneca Coal Company (SCC) re-evaluated sediment storage requirements as part of TR-30 because several of the original design assumptions and permit conditions are no longer appropriate or applicable (e.g., land condition, sediment yield). The TR-30 hydrology and sedimentology demonstration utilizes a watershed condition based on the August 2006 landscape (see Exhibit 13-4B, Pond 010 Existing Watershed Conditions August 2006). This is an appropriate approach because all coal resource removal activities have ceased and most disturbed areas within the watershed are reclaimed and successfully revegeta ted (see TR-30 13-3A.1 Revised 11/06