2015-06-22_PERMIT FILE - C1996083 (2)

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saddles or other means. Within the embankment, the outlet pipe should have cutoff collars • placed on it to increase the flow path along the outlet pipe and discourage piping. These collars should be approximately 4 inches greater diameter then the outlet pipe and be placed on 5-foot centers. The sediment pond can be rapidly dewatered using this S-inch pipe. If the sediment pond was storing water and sediment generated by the 10-year, 24-hour event, the outlet pipe has the capacity to transport flows at 4.45 cubic feet per second (cfs, or 2,000 gpm at average depth), dewatering the pond in 1.7 hours. Maximum flows are not recommended, however, due to high outlet velocities and erosion of the highway ditch. If outlet flow rates are maintained near 4.45 cfs, riprap protection requirements for the highway ditch are the same as the runoff control ditch at the toe of the waste bank. Details of this slope protection are located on Drawing 95266-04. The second dewatering device is the emergency spillway. This spillway is located on the west embankment, as shown in Drawing 95266-04. This spillway will transport flows generated by a 25-year, 24-hour storm event (Rule 4.05.6(5)(bll. Side slopes for the spillway will be 2:1, with a 3-foot bottom width and 2.0-foot depth. Flows generated by the storm will produce 1-foot of depth on the 1 percent bottom grade (leaving 12-inches of dry • freeboardl, at a flow rate of 3.1 fps. Suggested riprap protection should be 12-inch maximum rock size (OSM, 1984) with filter as described on Drawing 95266-04. Riprap placed below the spillway on the downstream embankment face should be the gradation used for the spillway riprap, but will be grouted in place to ensure stabilization on the 2:1 slope. The protection should extend 1.5 feet beyond the spillway on both sides, and extend 2 feet past the downstream toe of the embankment. 4.2.6 Maintenance Maintenance of the sediment pond will be concentrated on the removal of sediment accumulated by small magnitude storms. The frequency of the sediment removal will have to be determined in the field, depending on rainfall conditions. A calibrated pole or other device should be set within the sediment pond to track sediment depth so proper remediation can be employed when the sediment is too deep to adequately hold the design storm. Vehicle access for sediment removal is provided on the east end with a road ramp into the pond. The grass slope stabilization should also be maintained to prevent erosion. • 86266/1424.RPT Coel Mine Wes[e Benk Finel Design for tha Bowia No. 2 Mine June 1998 WESTEC 19 M[:pnACY-Ol