Laserfiche WebLink
• d. Outflow velocities during dewatering (8-inch diameter pipe) range from <br />maximum of 11.3 fps to minimum of 0 fps. Riprap will be provided at <br />discharge point to Grassy Creek. <br />e. A baffle type device will be placed around the dewatering aperture. <br />See attached sketch. <br />Hydrographs were developed for both the 10-year and 25-year, 24-hour storms <br />and routed through the sediment pond. Peak outflows from both the principal <br />spillway and dewatering orifice were determined. A pipe size was calculated <br />to convey the peak discharge from either storm down the slope to Grassy Creek <br />thereby reducing the erosion potential. The calculations are attached and <br />specific points addressed below. <br />a. Total volume of runoff for 25-year, 24-hour storm event = 9.9 acre- <br />feet. The available storage above the 1-year sediment accumulation <br />to the crest of the emergency spillway = 12.5 acre-feet. Therefore, <br />the total outflow volume trom principal spillway = 9.9 acre-feet. <br />Total outflow volume from emergency spillway = 0 acre-feet. <br />b. There will be no discharge from the emergency spillway during the <br />25-year, 24-hour event. All discharges will be from the principal <br />spillway. <br />c. An 8-inch diameter corrugated plastic pipe will convey the discharge <br />from the 25-year, 24-hour storm to Grassy Creek. Peak discharge = 4.3 <br />• cfs with maximum velocity = 12 fps. As indicated in response l.d., <br />riprap will be placed at the discharge point to Grassy Creek to reduce <br />erosion. <br />A single sediment pond will be constructed for Pits 5 and 6. The sediment <br />pond, located in the natural drainage between Pits 5 and 6, will be tormed <br />using the existing haul road embankment. Collection ditches will be <br />constructed within the pits and along the haul roads so that all runoff from <br />disturbed areas is conveyed to the sediment ponds. Diversion ditches will be <br />used to convey runoff from disturbed areas around the sediment pond to Grassy <br />Creek. The general arrangement for this sediment pond is shown on the Mining <br />Plan Map. The site plan and details of this sediment pond are shown on <br />Figures 12 and 13. <br />The sediment ponds were sized to store the volume of a 10-year 24-hour preci- <br />pitation event together with a one year accumulation of sediment without <br />discharging water. This means that the ponds provide at least a 24-hour <br />retention time for the 10-year 24-hour precipitation event. The principle <br />spillway has been designed to pass the peak flow resulting from the 10-year <br />24-hour event. An emergency spillway has been included in each sediment pond. <br />The emergency spillway is capable of passing a 25-year 24-hour precipitation <br />event while maintaining at least 1 foot of freeboard below the crest. This <br />arrangement is shown on Figures 9 and 11. <br />12/87 <br />• III-20a <br />