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Peabody <br />July 28, 2009 <br />Twentymile Coal Company <br />29515 Routt County Road #27 <br />Oak Creek, CO 80467 <br />970.879.3800 <br />???1VE® <br />Mrs. Janet Binns, Environmental Protection Specialist F, <br />Colorado Division of Reclamation, Mining, and Safety ??09 <br />1313 Sherman Street, Room 215 <br />Denver, Colorado 80203 d Satetio??. <br />and Safety <br />(303) 866-4944 M?n'n9 <br />RE: Foidel Creek Mine (Permit No. C-82-056) - Technical Revision (TR06-57) for Modifications <br />to the 6-Main North Sedimentation Pond <br />Dear Mrs. Binns: <br />With this submittal, Twentymile Coal Company (TCC) is submitting revised information, and requests <br />CDRMS approval of the pending Technical Revision (TR06-57) to our existing approved Permit, for <br />modification of the 6-Main North (6MN) Sedimentation Pond, to convert the pond from a containment <br />pond to a discharging structure with a single open-channel spillway. High spring runoff, due to heavy <br />snowpack, resulted in greater flows than anticipated during the initial years of use, and the consequent <br />need to pump and truck water from the pond in order to maintain adequate storm runoff storage capacity. <br />Given that the 6MN ventilation shaft is a long-term mining installation, TCC has determined that <br />conversion of the 6MN Sedimentation Pond to a discharging structure is a more practical operational <br />approach. In addition, the pond was originally modeled and designed for worst-case construction <br />conditions. Gravel surfacing and revegetation of portions of the pad area have significantly improved <br />runoff and sediment conditions, resulting in lower runoff and sediment contributions to the pond. <br />The modified pond will have adequate capacity to totally contain 3-years of sediment and a permanent <br />pool of approximately 0.46 acre-feet, and is designed to retain and treat the runoff from the 10-year, 24- <br />hour design storm event with 1.0 foot of freeboard between the 10-year, 24-hour water surface and the <br />spillway invert. The open-channel spillway is designed to safely pass the peak flows from the 25-year, <br />24-hour design storm event. In operation, the pond would adequately control sediment and retain, treat, <br />and discharge the runoff from a 10-year, 24-hour storm event even if the pond was full to the spillway <br />invert at the start of the 10-year, 24-hour event. <br />Conversion of the 6MN Sedimentation Pond to a discharging structure will involve excavation and <br />placement of riprap to stabilize the single open-channel spillway and short outlet segment, and minor <br />earthwork and subsequent revegetation to assure that an existing natural swale has adequate capacity and <br />freeboard as a discharge channel. The open-channel spillway and outlet segment have been designed to <br />safely pass design flows for the peak runoff from a 25-year, 24-hour design storm event. With the <br />exception of a short segment of the discharge channel, the pond modifications (spillway and outlet <br />segment) will be constructed within existing disturbance areas, so this revision will result in minimal new <br />surface disturbance. While it is assumed that minor earthwork will be required to assure that the existing <br />natural swale provides adequate flow capacity and freeboard to safely handle design flows, disturbed <br />areas will be revegetated with the permanent reclamation seed mixture, with the assumption that <br />redisturbance for final reclamation will not be necessary (the area is bonded to provide contingency funds <br />in the event that re-seeding is necessary). As an existing disturbance area, much of the disturbed area <br />runoff is already captured and controlled by the existing drainage and sediment control system.