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West Elk Mine - Mine Water Control Plan Mountain Coal Company <br />4.0 SEDCAD+ INPUT PARAMETERS FOR STRUCTURES <br />4.1 Ditches <br />Ditch dimensions, side slopes, and lining material were based on field visits. Slopes were <br />determined from previous analyses, or from contour mapping. A freeboard of 0.30 foot was <br />assigned to all ditches. Ditch lining materials include shotcrete (n=0.017), silts to cobbles <br />(n=0.030, allowable velocity 5.5 ft/sec), cobbles and shingles (n=0.035, allowable velocity 5.5 <br />ft/sec), riprap (n varies), and grass (n varies). <br />4.2 Culverts <br />Culvert diameter, type, and allowable headwater were based on field visits. Culverts were <br />conservatively assumed to have a projecting entrance condition (entrance coefficient = 0.90), <br />except for a few that were installed with headwalls. Culverts were further assumed to operate <br />under inlet control, with a nominal barrel slope of 5% (0.05 ft/ft). Allowable headwater was <br />typically 1.5 times the culvert diameter, and sometimes less if constrained by shallow ditch <br />depths. The SEDCAD+ model's limitation to 100 or fewer structures prevented inclusion of <br />many culverts in the overall watershed model for Pond MB -5E. Separate culvert <br />demonstrations are included for those culverts not included in the MB -5E SEDCAD+ run. <br />4.3 Muskingum Routing <br />Muskingum routing lengths were determined in AutoCAD, and entered into SEDCAD+ where <br />appropriate. Slopes were determined from previous analyses, or from contour mapping. <br />4.4 Ponds <br />Pond design information was obtained from previous SEDCAD+ runs, except for the RPE pond <br />and Pond MB -5E. The old SEDCAD+ runs for RPE were missing pond design information, so <br />the elevation -surface area relationship was determined by digitizing grading contours in <br />AutoCAD from an image of the as -built drawings. The outlet structure details were obtained <br />from the same drawing set. Details for Pond MB -5E were obtained from the bid documents. <br />In -pond sediment storage was estimated using a rainfall erosivity factor (R) of 20 or 30, over 2 <br />or 3 years, in accordance with previous studies. The mine recognizes that this is only an <br />estimate. Sediment storage capacity in each pond is monitored either through a survey, or <br />through use of a level indicator in the pond. When sediment has exceeded the recommended <br />storage capacity, the mine will drain the pond and excavate the sediment, allowing for additional <br />sediment storage. <br />West Elk Mine has gated valves on the primary spillways of all ponds. These valves are <br />typically closed. This allows the mine to increase their storage, if required, which will in turn <br />provide a longer time period for sediment to settle. The mine can thus better regulate the <br />suspended sediment concentration before water is discharged. As a result, the SEDCAD+ <br />results for the ponds are conservative. The RPE pond consists of two cells, connected via <br />trickle tubes. Pond MB -5E has two cells, operated alternatively by opening and closing gate <br />valves on the inflow pipe distribution box. The RPE pond was analyzed using only the primary <br />inflow cell, leading to a conservative design. Pond MB -5E was likewise analyzed using a single <br />cell. Pond emergency spillways were designed to pass the 25 -year, 24-hour event. <br />Tetra Tech October 2010 <br />