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Sediment-laden runoff water from the mine pits will flow through the system of <br />collection ditches to the sediment ponds. Two basic types of sediment ponds <br />• will be constructed. For Pit 4, the sediment pond will be constructed within <br />the tirst box cut made as part of pit development. These sediment ponds will <br />be constructed entirely by excavating material. No fill placement will be <br />necessary to form the embankment for the pond at Pit 4. figures 8 and 9 show <br />details of the sediment pond for Pit 4. <br />A 6-inch diameter plastic pipe will be attached to the end of the 30" diameter <br />primary spillway discharge pipe. The plastic pipe will be attached in a <br />manner that diverts all low tlows while not diminishing the ultimate capacity <br />of the pipe, as shown on the attached sketch. The 6" diameter plastic pipe <br />will extend directly down the natural slope, discharging into Grassy Creek. <br />Intermittent anchors will be provided to structurally secure the pipe. This <br />will eliminate the erosion potential under all conditions less severe than the <br />10-year 24-hour precipitation event. <br />The existing riser pipe (principal spillway) crest elevation is 7522.50. The <br />emergency spillway crest is 7524.50. The riser pipe crest will be extended 1 <br />foot to elevation 7523.50. An area/capacity curve for the existing sediment <br />pond was developed based on actual field measurements. This area/capacity <br />curve is attached. The curve shows that with the extended riser the entire <br />volume of the 10-year 24-hour event can be stored. A small amount of water <br />would be discharged through a 5-inch diameter hole in the riser which is <br />discussed in response to the next comment. <br />A small hole will be cut into the riser pipe providing gradual release of <br />• stored water. The hole will be 5" in diameter with a centerline elevation of <br />7514. Calculations supporting these dimensions are shown beginning on page <br />III-31. <br />The dead storage of the pond is nominally 2 acre-feet or less. The water which <br />is occasionally retained by the pond is released within 24 hours. For this <br />reason, no water rights are required. <br />The original rainfall value of 1.8 inches was taken from a larger scale NOAA <br />atlas for Colorado rather than the smaller scale map shown in "Procedures for <br />Determining Peak Flows in Colorado". The watershed shape factor was not used <br />in the original calculations since the watershed will deviate trom its <br />"natural condition" throughout the course of Pit #4 mining activities. <br />However, to remain in compliance with the Division, the calculations have been <br />revised to reflect the 1.6 inch rainfall value and watershed shape tactor. The <br />calculations are attached and specific points are addressed below. <br />a. Maximum storage volume required = 6.4 acre-feet. Available storage <br />volume from top to 1 year sediment accumulation to top of riser = 10 <br />acre-feet. <br />b. Principal spillway crest elevation will remain the same as previously <br />identified; 7523.5. <br />c. Dewatering aperture diameter = 5 inches. This will provide 24-hour <br />• detention time. <br />12/87 III-20 <br />