2009-09-21_PERMIT FILE - C1980007A (5)

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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. Culverts were conservatively assumed to have a projecting entrance condition (entrance coefficient = 0.90), except for a few that were installed with headwalls. Culverts were further assumed to operate under inlet control, with a nominal barrel slope of 5% (0.05 ft/ft). Allowable headwater was typically 1.5 times the culvert diameter, and sometimes less if constrained by shallow ditch depths. The SEDCAD+ model's limitation to 100 or fewer structures prevented inclusion of many culverts in the overall watershed model for Pond MB-5E. Separate culvert demonstrations are included for those culverts not included in the MB-5E SEDCAD+ run. 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 and Pond MB-5E. The old SEDCAD+ runs for RPE were missing pond design information, so 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. Details for Pond MB-5E were obtained from the bid documents. In-pond sediment storage was estimated using a rainfall erosivity factor (R) of 20 or 30, over 2 or 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 through 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 all ponds. 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 are conservative. The RPE pond consists of two cells, connected via trickle tubes. Pond MB-5E has two cells, operated alternatively by opening and closing gate valves on the inflow pipe distribution box. The RPE pond was analyzed using only the primary inflow cell, leading to a conservative design. Pond MB-5E was likewise analyzed using a single cell. Pond emergency spillways were designed to pass the 25-year, 24-hour event. • Tetra Tech October 2008