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The resulting embankment structure will be approximately 14 feet high at its highest point, with the dam crest at an
<br />elevation of 6,717 feet. Actual constructed height will take into account anticipated embankment settlement, which
<br />is estimated at a maximum of 5 percent (constructed top elevation 6,717.7 feet). The surface area of the maximum
<br />pool (operational mine water storage + pump buffer storage + design stormwater storage)at an elevation of 6,714.4
<br />feet will be approximately 5.1 acres, resulting in a maximum design storage volume of approximately 34.3 acre-feet
<br />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'' 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
<br />provide buffer storage for another 1.0 million gallons, or 3.1 acre-feet, of emergency mine water storage to address
<br />any maintenance or emergency shut-downs of the pumping system, and 1.9 acre-feet (equivalent to the runoff from
<br />a 100-year, 24-hour storm event) of runoff from the limited contributing watershed. At maximum water storage
<br />level, taking into account the incremental storage capacities outlined (34.3 acre-feet), reservoir freeboard, at the
<br />maximum pool level, will be approximately 0.5 feet to the invert of the open-channel spillway, and 2.6 feet to the
<br />top of the embankment. Hydrologic design information for the 6MN Storage Reservoir is provided in Exhibit 8CC,
<br />6MN Mine Water Storage Reservoir - Drainage Designs. The discharge pumping system for the 6MN Storage
<br />Reservoir has been designed with the capacity to dewater the design storm-water storage volume in less than 12
<br />hours. Any discharge of stored mine or stormwater from the 6MN Storage Reservoir, will be directly to the mine
<br />water system, with no discharge through the emergency spillway to natural drainage. While it will not be utilized
<br />during normal operations or under design storm conditions, the open-channel spillway, and the lower portion of the
<br />stormwater diversion channel are designed to safely pass the peak discharge (approximately 125.5 cfs), and the
<br />total runoff, respectively from the design storm event.
<br />As part of the 6MN Storage Reservoir project, TCC will establish a new pipeline running approximately 6,000 feet
<br />from the 10-Right Dewatering pipeline to the 6MN Storage Reservoir. The pipeline will be constructed of 10-inch
<br />HDPE pipe, buried to a depth of approximately six feet, with one sub-grade crossing (bored under) of Fish Creek.
<br />Details on the Fish Creek pipeline crossing are presented in Exhibit 490, 6MN Mine Water Storage Reservoir -
<br />Design Drawings. Pipeline construction will involve placement of sediment control materials (straw wattles or silt
<br />fence) downslope of the disturbance at any location where the pipeline corridor is within 50 feet of a drainage or
<br />stream buffer zone; removal and windrowing of available soil material (total soil material volume for the pipeline
<br />project is estimated at 11,150 cy) to one side of the 50 foot pipeline construction corridor; excavation of the
<br />pipeline trench using a track-hoe, with temporary placement of the excavated material on the opposite side of the
<br />trench; placement, connection, and bedding of the pipe; backfilling and compaction of the excavated material
<br />around the pipe; re-spreading of the soil material; and re-seeding using the standard rangeland seed mixture.
<br />Because the soil materials will be replaced immediately after pipeline placement, as construction advances,
<br />stabilization measures are not necessary.
<br />Where the pipeline crosses under Fish Creek, jacking pits will be excavated adjacent to the creek, while
<br />maintaining a minimum 5-foot buffer between the near edge of the pits and the crest of the stream bank, and the
<br />pipeline will be bored or jacked under the creek. Protective measures (BMP's) at the crossing location will include
<br />placement of stream buffer zone markers at any point where the pipeline corridor is within 50 feet of the stream
<br />buffer zone prior to any activity; completion of the work during low-flows; placement of silt fence or anchored
<br />straw bales at least 50 feet downstream to catch any sediment released prior to initiation of work; placement of
<br />sediment control materials (straw wattles or silt fence) around the entire side and near stream perimeters of the
<br />jacking pit disturbance areas; restrictions on storage or transfers of petroleum products within the buffer zone, and
<br />TR07-61 2.05-45.7 08/18/08
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