2008-12-04_PERMIT FILE - C1994082 (14)

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~~ Introduction This pond storage capacity report amends the As-Built Report dated September 2004. The purpose of this report is to update the original as-built report with revised SEDCAD hydrology and sedimentology demonstrations that account for the current watershed conditions. The sediment storage capacity has been recalculated using watershed conditions as of September 2007. This is an appropriate approach because previous SEDCAD modeling demonstrations evaluated sediment delivery to the pond based on worst-case disturbance conditions. The worst-case disturbance approach resulted in a much higher amount of sediment delivery than the amount delivered in the current (reclaimed) condition. Consequently, the amount of storage volume needed to effect compliance with Rule 4.05.2(7) and Rule 4.05.6(3) has greatly decreased. Therefore the top of sediment elevation can be increased and the pond cleaned less frequently than calculated in the original as-built report. Furthermore, this runoff demonstration uses the same hydrologic inputs utilized- for the postmine channel design demonstrations for YPM-9 and YPM-13 to assure consistency throughout the PAP. The capacity of Pond O11A is attributed partially to the embankment and partially to the pond excavation. The embankment design, construction and resulting stability analyses were previously certified and have not changed. Structure elevations input into the modeling conducted for this evaluation, and shown on Exhibit 13-5C, As-Built Pond OZ1A, have been verified by Physical surveying. Existing Condition AnalVSis The TR-37 hydrology and sedimentology evaluation demonstrates that Pond 011A meets or exceeds the design and performance requirements of the Regulations of the Colorado Mined Land Reclamation Board for Coal Mining. Specifically, the as-built configuration adheres to the following requirements: • Provide adequate 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 and treatment for the runoff from the 10-year, 24-hour storm event without experiencing any discharge through the emergency spillway (Rule 4.05.9(2)(b)); • Provide an emergency spillway capable of passing runoff from the 25- year, 24-hour storm event in accordance with Rule 4.05.9(2)(c)(ii). The contributing watershed is approximately 45.7 acres - mining activities disturbed approximately 99 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-37 because several of the original design assumptions and permit conditions are 13-19.1-1 Revised 08/08 TR-37