2008-09-16_PERMIT FILE - C1982056A (4)

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10-year, 24-hour storm. The outslope of the East Rockfall Hazard Trench consists of 2.25 acres of fill and associated construction disturbance, which is predicted to result in a peak flow of 2.16 cfs following a 10-year 24-hour storm event. The outslope will be treated with 2 tons/acre mulch, and the mulch in combination with a 20-foot wide • vegetative filter will result in the achievement of effluent limitations following a 10-year 24-hour storm event. The SEDCAD+ demonstrations are provided in Exhibit 8. TCC plans to collect a limited number of discharge water samples at rock filters during a future precipitation event to support current field observations. Samples will be compared to applicable stormwater limitations for settleable solids. Sandstones Power Substation -Sediment control for this area (less than one acre) will consist of a perimeter containment berm/temporary diversion ditch and gravel filter(s) sufficient to treat the estimated 0.06 acre-feet of runoff from the 10-year, 24-hour event. The location of this SAE is shown on Map 39 (Substation and Powerline Location). This SAE was expanded slightly in Fall 1993 to allow the drilling of a replacement borehole into the mine, which was in turn, used for drilling of a water supply/monitoring well underground (FCM No. 2). The expansion of the SAE entails extension of the berm to the west to encompass the very limited (approximately 50 x 50 feet) of additional disturbance. Any topsoil stripped will be windrowed/temporarily stockpiled adjacent to the pad. Following completion of the well work, a portion of the expanded area may be reclaimed, with remaining topsoil being added to the existing topsoil stockpile. The topsoil and reclaimed area will be seeded and stabilized using the pastureland see mixture. Rockdust Tank - An SAE for sedimentation control has been granted for the rockdust tank and truck access/unloading area. The total disturbance will be approximately 0.5 acres. Of this area, 0.3 acres will be filled with pit-run gravel, which acts as an infiltration screen for any overland flow associated with the rockdust tank and access/unloading area. Estimated peak flow from the area for the 10-year, 24-hour event is approximately 0.04 acre-feet. Runoff from the topsoil pile is controlled by a berm/swale/rock filter system. In addition to the infiltration screen, flow from the tank and unloading area is conveyed by means of swales and berms to a gravel filter. These controls, in conjunction with existing vegetation, will be sufficient to control runoff from the area as • observed during previous inspections. Former CYCC Office Building Area (TR OS-451 Sediment control for the Colorado Yampa Coal Company (CYCC) Office building area (a.k.a. Brown Palace), was previously addressed under Permit C-81-071. As a historic disturbance within the present Permit Area, TCC is responsible for continuing maintenance, permit compliance and final reclamation of this area and the associated sediment control structures. Note that Map 29 indicates the quantity and location of the topsoil pile specifically designated for future reclamation of this area. Effective drainage and sediment control for the estimated 0.01 of of runoff (from a 10-year, 24-hour event) from this area is achieved through use of the following components: 1)Asphalt-surfacing of parking areas; 2) An established vegetative filter on the down-gradient outslope; and 3) Drainage ditches on both sides of the Brown Palace access road (connecting with Haulroad C), with a series of rock check-dams to reduce flow velocities and collect any contained sediment prior to discharge to Foidel Creek. The small area exemption is shown on Map 24. Asbestos abatement was completed in early 2005, and the building was later demolished during Summer-2005. During demolition, drainage and sediment control for the site was provided by the existing drainage control features and sediment fencing and/or straw wattles. Following demolition, the site was stabilized by grading and reseeding disturbed areas. Calculations documenting the adequacy of the design drainage features are presented in Exhibit 8Z, CYCC Office Building Drainage Design Documentation. Pond T Topsoil Stockpile -This SAE covers about 0.5 acres between the Pond T embankment face and railroad spur. Runoff from this area flows downslope between the railroad embankment and Pond T embankment outslope • to the Pond T weir, and is then conveyed under the track by means of a culvert to Foidel Creek. Vegetative filtering is provided by the establishment of good vegetation on the topsoil pile and in the conveyance ditch below the topsoil stockpile. Additional filtering of sediment is provided by a rock filter between the stockpile and the culvert, benches, and cut and fill slopes of the facility itself. TR08-65 2.05-80 08/20/08