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~. maintain inlet control at maximum discharge (» 1 cfs), the outlet pipe must be at a <br />minimum slope 0.007 (0.7%). Under these conditions, the pipe will be flowing nearly full. <br />Any slope greater than this will shorten the length of pipe required and increase the <br />discharge capacity. The orifice elevation is at 7494.5 ft. <br />There are two trash rack structures which surround the west pond low flow orifice. <br />The first (outside rack) is an 18" piece of corrugated metal pipe (CMP) placed vertically <br />on end over the low flow pipe. The CMP has'/z' holes drilled it every 2" in a vertical <br />direction. There are 12 holes around the circumference of the pipe. The second rack <br />(inside rack) is a braided wire basket welded to the outlet pipe immedia#ely over the <br />orifice. The basket is 5'/"x5'/<x4" and the openings between the wire constitute about <br />80% of the surface area. The hydraulics of both racks were modelled using an orifice <br />equation. <br />The discharge at the maximum head for the 10yr-24hr event was used to assess the <br />relative capacities of the outlet orifice, the inside rack and the outside rack. For this case <br />the orifice discharges 0..37 cfs, the inside rack 3.45 cfs and the outside rack 1.42 cfs. <br />As seen the trash racks do not restrict flow relative to the orifice. The excess capacities <br />of the trash racks will allow for some blockage. <br />Outlets for the east pond also include a spillway and low flow orifice. The spillway is <br />a rectangular sharp crested weir with a notch length of 5 ft. and a depth of 0.7 ft. A 2.0" <br />• orifice located at elevation 7492 is required- for the low flow outlet. The structure and <br />trash racks should be the same as described for the west pond. <br />7. Topsoil Storage Pile Drainage <br />The topsoil storage pile is located above the mine waste bank in the eastside drainage. <br />This pile must be protected from loss of material due to surface erosion on the face of <br />the pile. A berm or ditch, approximately 12 inches in height should be constructed on the <br />downslope sides of the pile. This berm-will intercept surface flow and thus any <br />suspended or rolling material originating from the pile. <br />t'• 14 <br />January 1998 <br />