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

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West Elk Mine - Mine Water Control Plan Mountain Coal 4.0 SEDCAD INPUT PARAMETERS FOR STRUCTURES • 4.1 Ditches Ditch dimensions, side slopes, and lining material were based on field visits. Slopes were determined from previous analyses, or from contour mapping. A freeboard of 0.30 foot was assigned to all ditches. Ditch lining materials include shotcrete (n=0.017), silts to cobbles (n=0.030, allowable velocity 5.5 ft/sec), cobbles and shingles (n=0.035, allowable velocity 5.5 ft/sec), riprap (n varies), and grass (n varies). 4.2 Culverts Culvert diameter, type, and allowable headwater were based on field visits. All culverts were conservatively assumed to have a projecting entrance condition (entrance coefficient = 0.90), though a few were actually installed with headwalls. Culverts were further assumed to operate under inlet control, with a nominal barrel slope of 5%. Allowable headwater was typically 1.5 times the culvert diameter, and sometimes less if constrained by shallow ditch depths. 4.3 Muskingum Routing Muskingum routing lengths were determined in AutoCAD, and entered into SEDCAD where appropriate. Slopes were determined from previous analyses, or from contour mapping. 4.4 Ponds Pond design information was obtained from previous SEDCAD runs, except for the RPE pond, • where the old runs were missing pond design information. For that pond, the elevation-surface area relationship was determined by digitizing grading contours in AutoCAD from an image of the as-built drawings. The outlet structure details were obtained from the same drawing set. In-pond sediment storage was estimated using a rainfall erosivity factor (R) of 30 or 20, over 3 years, in accordance with previous studies. The mine recognizes that this is only an estimate. Sediment storage capacity in each pond is monitored either through a survey, or though use of a level indicator in the pond. When sediment has exceeded the recommended storage capacity, the mine will drain the pond and excavate the sediment, allowing for additional sediment storage. West Elk Mine has gated valves on the primary spillways of MB-1, MB-2R, MB-3, MB-5, and RPE. These valves are typically closed. This allows the mine to increase their storage, if required, which will in turn provide a longer time period for sediment to settle. The mine can thus better regulate the suspended sediment concentration before water is discharged. As a result, the SEDCAD results for the ponds having gated spillways are conservative. Ponds MB- 4, MB-5, and SG-1 utilize trickle tubes for the primary outlet, and open channel spillways for the secondary outlet works. Ponds MB-1 and RPE each consist of two cells, connected via trickle tubes. They were each analyzed using only the primary inflow cell, leading to a conservative design. Ponds MB-1 and MB-2R were previously designed to pass the 100-year, 24-hour event through the emergency spillways. All other emergency spillways were designed to pass the 25-year, 24- hour event. Tetra Tech October 2008