1986-12-11_PERMIT FILE - C1981017

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-17- Mine No. 5 Sediment Control The disturbed area at Mine No. 5 is 8.0 acres. There are two separate bench areas which cont H bute to the one sediment pond. Approximately 2.8 acres is on excavated bedrock with the remaining 5.2 acres on fill or excavated cobbly soils. Runoff from undisturbed areas will be diverted by ditches and culverts around the disturbed area. The No. 5 sediment pond is designed to contain three years of sediment using the universal soil loss equation and the runoff from a 10-year, 24-hour precipitation event. The sediment pond is designed with an emergency spillway capable of passing the 25-year, 24-hour peak flow. Rock Tunnel Sediment Control Three sediment ponds in series having a total storage capacity of 11.1 acre-feet have been constructed to control runoff from the disturbed area at the Rock Tunnel . All sediment accumulated over a three year period can be stored in the upper two ponds. The total runoff volume from a 10-year, 24-hour precipitation event can be stored in the lower two ponds, assuming the upper pond is full of sediment. All three ponds are equipped with 36-inch CMP emergency spillways which will pass the 25-year, 24-hour peak flow. The lower two ponds are equipped with 6-inch, CMP gated dewatering pipes. The middle pond has the dewatering device located above the maximum sediment storage level. The dewatering device on the lower pond is located 2.5 feet above the bottom of the pond. The 2.5 feet of storage is for sediment in excess of three years accumulation or for sediment which is not settled out in the upper two ponds. Even with this amount of additional dead storage, the sediment ponds are capable of containing the runoff from a 10-year, 24-hour precipitation event, as designed. Preparations Plant Sediment Control Eleven sediment ponds draining approximately 51 acres of disturbed area have been constructed at the preparations plant site. All the sediment ponds are sized to contain the runoff from a 25-year, 24-hour precipitation event, plus 0.1 acre-feet of sediment per acre of disturbed drainage area. In fact, all the sediment ponds are shown by the applicant to have more storage than required. Two additional sediment ponds are planned to be constructed, and one sediment pond will be expanded. The sediment ponds are to be constructed to contain the runoff from the pad and fill area for an underground bin system and associated conveyor system. The sediment pond to be enlarged is also to drain the above system. These sediment ponds are designed to contain the runoff from a storm in excess of the 10-year, 24-hour event plus 0.2 acre-feet of sediment per acre of disturbed drainage area.