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Pond Description HG 8 will replace HG 1 to control surface water from the mining block east of County Road 53. By utilizing old pits that have been partially back- filled 21.1 acre ft. of storage can be made available, 7.5 acre ft. of which will be incised. In order to prevent contramination of groundwater by seepage through the backfill mate rial a liner will be constructed on the bottom and sides of the pond using 2 ft. of the "B" overburden horizon which is a sandy-clay material and has been approved by the State of Colorado for use in the construction of HG 1. The liner will be placed by scrapers an d compacted with a standard compactor. The maximum drainage area for the pond is 2b7 acres, which does not include the active pit area. 13.6 acre-ft. of runoff storage is required for this acreage which is provided by the described pond and which will be self-draining through the primary spillway. Rain falling into the pit will be held there until the pond has been drained down to the top of the sedi- ment at which time there will be at least 3.3 acre-ft. of storage available below the primary spillway. The pond has been designed to hold 36 acre- ft. of sediment. This is the projected yield for the three years 1980 through 1982. The discharge from the pond will meet state and federal requirements for water quality. In order to maintain water quality, the ditch and ponds • presently located on the east side of the highway will be reclaimed and vegetation established to maintain an adequate drainage system. The design criteria for the control devices are based on the Office of Surface Mining's Final Regulations and the Proposed Rules and Regulations of the Colorado Mined Land Reclamation Board. Some of the basic design parameters are: 1. Sedimentation ponds. designed to contain all runoff from a 10-year, 24-hour storm and 3-year accumulation of sediment. A 24-hour detention time has been designed into the pond. 2. Dam spillways designed to safely discharge peak runoff from a 100- year, 24-hour rainfall of 22I cfs and still maintain one foot of freeboard. 3. Temporary diversion structures designed to safely pass the peak runoff from a precipitation event with a 10-year recurrence interval. 4. All ditches have been designed to carry the peak flow from a 10-year, 24-hour event. Rip rap will be used to prevent erosion where necessary. Maximum velocity for an unlined ditch is approximately 4 feet/second. 5. Soil losses were calculated using the universal soil loss equation and sediment delivery ration of 1.0. Reference materials used in the development of this plan include the • following: .