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of water. Even though the 6MN Storage Reservoir has been designed and will be operated as a closed reservoir, a
<br />single open-channel emergency spillway, with an invert of 6,714.9 feet, has been incorporated into the design in
<br />compliance with applicable regulatory requirements for impoundments. The design of the reservoir also
<br />incorporates a compacted soil liner (approximately 4,800 cy with an estimated permeability of 2 x 10"7 cm/in) to
<br />minimize infiltration and water loss. Due to the capacity and embankment height for the reservoir, reservoir design
<br />and construction must also meet the requirements of the Mine Safety and Health Administration (MSHA, 30CFR
<br />77.216). Reservoir design and construction specifications meet all applicable design and regulatory requirements.
<br />Separate applications for approval were submitted to this agency, with a copy of final approval provided to the
<br />CDRMS.
<br />TCC's intent is to consistently maintain approximately 9.55 million gallons, or 29.3 acre-feet, of water in the 6MN
<br />Reservoir, to supply the mine water system. As designed, the Reservoir has sufficient additional capacity 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, and 1.9 acre-feet (equivalent to the runoff from a
<br />100-year, 24-hour storm event) of runoff from the limited contributing watershed. At maximum water storage level,
<br />taking into account the incremental storage capacities outlined (34.3 acre-feet), reservoir freeboard, at the maximum
<br />pool 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. Hydrologic design information for the 6MN Storage Reservoir is provided in Exhibit 8CC, 6MN
<br />Mine Water Storage Reservoir - Drainage Designs. The discharge pumping system for the 6MN Storage Reservoir
<br />has been designed with the capacity to dewater the design storm-water storage volume in less than 12 hours. Any
<br />discharge of stored mine or stormwater from the 6MN Storage Reservoir, will be directly to the mine water system,
<br />with no discharge through the emergency spillway to natural drainage. While it will not be utilized during normal
<br />operations or under design storm conditions, the open-channel spillway, and the lower portion of the stormwater
<br />diversion channel are designed to safely pass the peak discharge (approximately 125.5 cfs), and the total runoff,
<br />respectively from the design storm event. d Ic
<br />As part of the 6MN Facilities project, TCC will establish a new pipeline running approximately 7,750 feet from the
<br />10-Right Dewatering pipeline to the 6MN Storage Reservoir. The pipeline will be constructed of 10-inch HDPE
<br />pipe, buried to a depth of approximately six feet, with one sub-grade crossing (bored under) of Fish Creek. Details
<br />on the Fish Creek pipeline crossing are presented in Exhibit 490, 6MN Mine Water Storage Reservoir - Design
<br />Drawings. Pipeline construction will involve placement of sediment control materials (straw wattles or silt fence)
<br />downslope of the disturbance at any location where the pipeline corridor is within 50 feet of a drainage or stream
<br />buffer zone; removal and windrowing of available soil material (total soil material volume for the pipeline project is
<br />estimated at 11,150 cy) to one side of the 50f-oot pipeline construction corridor; excavation of the pipeline trench
<br />using a track-hoe, with temporary placement of the excavated material on the opposite side of the trench; placement,
<br />connection, and bedding of the pipe; backfilling and compaction of the excavated material around the pipe; re-
<br />spreading of the soil material; and re-seeding using the standard rangeland seed mixture. Because the soil materials
<br />will be replaced immediately after pipeline placement, as construction advances, stabilization measures are not
<br />necessary.
<br />1 {` ,
<br />Where the pipeline crosses under Fish Creek, jacking pits will be excavated adjacent to the creek, while maintaining
<br />a minimum 5-foot buffer between the near edge of the pits and the crest of the stream bank, and the pipeline will be
<br />bored or jacked under the creek. Protective measures (BMP's) at the crossing location will include placement of
<br />stream buffer zone markers at any point where the pipeline corridor is within 50 feet of the stream buffer zone prior
<br />to any activity; completion of the work during low-flows; placement of silt fence or anchored straw bales at least 50
<br />feet downstream to catch any sediment released prior to initiation of work; placement of sediment control materials
<br />(straw wattles or silt fence) around the entire side and near stream perimeters of the jacking pit disturbance areas;
<br />restrictions on storage or transfers of petroleum products within the buffer zone, and reclamation of the affected
<br />areas immediately following completion of pipeline installation. Construction and sediment control details are k
<br />shown by the design drawings in Exhibit 490.
<br />With construction of the 6MN Mine Water Storage Reservoir, the temporarily direct connection from the 10-Right
<br />Mine Dewatering Pipeline to the 6MN Million-Gallon Tank will be replaced with an inlet line from the 10-Right
<br />Mine Dewatering Pipeline to the 6MN Reservoir, and a pumped outlet line from the Reservoir to the 6MN Million-
<br />Gallon Tank. The 6MN Storage Reservoir will be used to provide back-up storage for TCC's 10-Right Mine
<br />Dewatering Well (also previously identified as the EMD Dewatering Facility), allowing the well to pump
<br />continuously at full capacity, providing buffer storage for pump maintenance or failure, and limiting the distance
<br />MR09-234 2.05 - 45.6g 02/03/09
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