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TWENTYMILE COAL COMPANY -FOIDEL CREEK MINE <br />DESIGN BASIS FOR 6MN MINE WATER STORAGE RESERVOIR <br />The 6MN Storage Reservoir is designed as a closed reservoir (containment pond under Rule 4.05.9(2)), with no <br />discharge, and provision for adequate operating freeboard to accommodate operational mine water storage, <br />storage of the design storm runoff from a small upslope drainage area, and buffer storage to address any pump <br />outage or malfunction. Most runoff from the upgradient drainage basin will be captured and diverted around the <br />reservoir by a designed temporary diversion ditch. The design of the reservoir also incorporates a compacted <br />soil liner (approximately 4,800 cy with an estimated permeability of 2 x 10-' cm/in) to minimize infiltration and <br />water loss. <br />The Reservoir is designed to consistently maintain approximately 9.55 million gallons, or 29.3 acre-feet, of <br />operational mine water storage, to supply the mine water system; 1.9 acre-feet (equivalent to the runoff from a <br />100-year, 24-hour storm event) of storwater runoff from the limited contributing watershed; and to provide <br />buffer storage for another 1.0 million gallons, or 3.1 acre-feet, of emergency mine water storage to address any <br />maintenance or emergency shut-downs of the pumping system. At maximum water storage level, taking into <br />account the incremental storage capacities outlined (34.3 acre-feet), reservoir freeboard, at the maximum pool <br />level, will be approximately 0.5 feet to the invert of the open-channel spillway, and 2.6 feet to the top of the <br />embankment. <br />TCC will remotely monitor and control reservoir water level, and mine water discharge to and from the 6MN <br />Storage Reservoir, to maintain the water level at or below design storage capacities, and to maintain the design <br />freeboard, at all times. Water level and pump control interfaces will provide for integrated control of pumping <br />systems to balance inflow and discharge and shut-off the deep well pump if the pond level reaches a certain pre- <br />set threshold level(s), and an integrated alarm system will remotely notify the operator of any system failure, <br />malfunction, or water level concern. These controls will also assure that adequate storm-water storage capacity <br />is maintained at all times. The discharge pumping system for the 6MN Storage Reservoir has been designed <br />with the capacity to dewater the design storm-water storage volume in less than 12 hours. Any discharge of <br />stored mine or stormwater from the 6MN Storage Reservoir, will be directly to the mine water system, with no <br />discharge through the emergency spillway to natural drainage. <br />Even though the 6MN Storage Reservoir has been designed and will be operated as a closed reservoir, a single <br />open-channel emergency spillway, with an invert of 6,714.9 feet, has been incorporated into the design in <br />compliance with applicable CDRMS regulatory requirements for impoundments. While it will not be utilized <br />during normal operations or under design storm conditions, the open-channel spillway, and the lower portion of <br />the stormwater diversion channel are designed to safely pass the peak discharge (approximately 125.5 cfs), and <br />the total runoff from the design storm event, as well as the pumped discharge. The lower portion of the <br />stormwater diversion channel is designed to safely pass the maximum possible flow volume, which would be <br />comprised of stormwater discharge from both the upgradient drainage and reservoir areas, and the pumped <br />discharge volume. <br />Hydrologic design information for the 6MN Storage Reservoir is documented by the following hydrologic and <br />hydraulic designs, completed using the SEDCAD+ hydrologic design software: <br />6MN Mine Water Storage Reservoir - 6MN Storage Reservoir SEDCAD Runs <br />6MN Mine Water Storage Reservoir -Evaluation with Reservoir Level at Invert of Emergency Spillway <br />(Exhibit 8CC) <br />6MN Mine Water Storage Reservoir -Evaluation of Spillway Channel Flow with Pumped Discharge <br />(Exhibit 8CC) <br />6MN Mine Water Storage Reservoir -Stage Storage Curve (Exhibit 8CC) <br />TR07-61 08/18/08 <br />