2008-12-11_PERMIT FILE - C1980007A (16)

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West Elk Mine - Mine Water Control Plan Mountain Coal Company 5.0 SEDCAD OUTPUT / RESULTS 5.1 Ponds Water and sediment inflows to ponds, for the 10-, 25-, and 100-year design events, are summarized in Table 42E. Pond volumes are summarized in Table 45E. 5.2 Subwatersheds Subwatershed runoff volume and peak discharge, for the 10-, 25-, and 100-year design events, is summarized in Table 43E. 5.3 Ditches and Culverts Ditch and culvert design flows and descriptions are summarized in Table 44E. 5.4 Small Area Exceptions and Other Exemptions This effort did not include reanalysis of existing Small Area Exemptions, as their location and configuration has not changed appreciably. The following text has been excerpted from the previous report by Greystone (1993). Additional SAE demonstrations included in the SEDCAD output also include their own narratives. "There are three areas that have been categorized as small area exemptions. One area is located in the Sylvester Gulch watershed at the fan house located at the top of the road. This structure was able to pass the requirements for a SAE since there was no increased erosion. • Water that fell onto this structure flowed off of the metal roof and onto rock or concrete. Once the water was on the ground it flowed directly into the roadway and into the ditch which is exempt from the requirements to pass disturbed area drainage through a sediment pond." "The second small area exemption is located in the drainage leading to the swale between freshwater pond (FW-1) and sediment pond MB-1. This area is well vegetated, has slope shape conducive to the reduction of sediment levels. A SEDCAD+ run is enclosed which demonstrates that the discharge from a 10-year 24-hour storm even meets applicable discharge limitations. An alternative run documents that the entire area could be graveled for use as a future on-coal storage bench, and would still meet applicable effluent limitations. A third small area exemption has been designated west of the incised sediment pond MB-5. This area is reclaimed and has excellent cover. Discharge from this area meets effluent limitations as documented by the SEDCAD+ run." "Due to no runoff collection into the fresh water pond (FW-1) and method of operation, no spillway discharge is included in this storm water analysis or design." Tetra Tech October 2008 8