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<br />NW MAINS VENT SHAFT PAD SAE <br />RAG Twentmile Coal Company (TCC) is requesting an SAE for the proposed NW MainsVent Shag Pad <br />Area located at the Twen[ymile Mine site as shown on Figure 1, Vent Shaft Pad SAE. Drainage and <br />sediment control for the Vent Shaft Pad site is provided by an up-gradient diversion ditchand access road ditch, <br />a short containment berm at the southern limit of the pad, gravel surfacing, and the combination of a gravel <br />filter (2' Gravel berm) atone end of the pad and the natural vegetative filter existing between the shaft site and <br />Fish Creek. As a relatively small area (approximately 1.8 acres), and given the effectiveness of the drainage <br />and sediment control measures, the additional disturbance that would be involved in constructing a sediment <br />pond is not warranted and oft site sedvnent contributions can effectively be controlled by these measures under <br />a Small Area Exemption, consistent with applicable provisions of Rule 4.05.2(3)(6). There will be no <br />discharge or mixing of mine water with surface drainage from the site. Given the limited site azea, a "D" soil <br />type (Binco silty clay loam), the drainage and sediment control measures, and a distance of approximately 500' <br />between the shaft site and Fish Creek, TCC believes an SAE is justified. The estimated peak flow and runoff <br />from the site for the l0-year 24-hour storm event is 1.65 cfs and 0.13 ac-tt, respectively. The estimated <br />discharge peak settleable concentration is 0.52 mVl. This sediment level is consistent with natural sediment <br />levels for runoff from undisturbed lands, given similar conditions, and would be significantly reduced as the <br />runoff passes over the vegetated area between the shaft site and Fish Creek. <br />The up-gradient diversion ditch will be a "V"ditch designed [o safely divert the peak flow from the 10- <br />year, 24-hotu storm with 0:3' of freeboard and located as shown on Figure 1, Vent Shaft Pad SAE. The <br />shaft pad will be constructed such that it slopes toward the gravel filter. The containment berm will consist <br />of topsoil (seeded with [he approved interim seed mix) stripped from the site and will be a minimum of 2' <br />high. Although very little mnoff is expected from the topsoil berm, a small `w"ditch will be bladed along <br />the outside perimeter of the topsoil berm and sloped toward the gravel filter. The check dam and grass <br />filter efficiency was modeled using the SEDCAD4 check dam and grass filter utility. he following is <br />submitted to demonstrate that the check dam and grass filter can effectively treat the discharge from the <br />ventshaftpad. <br />ASSUMPTIONS: <br />1. The drill pad location and dimensions are as shown on Figure 1, Vent Shaft Pad SAE <br />2. Hydrology and sedimen[ology inputs are similar to parameters used on previously approved grass filter <br />and check dam submittals by TCC <br />DESIGN PARAMETERS: <br />Fish Creek Vent Shalt Pad Diversion <br />Watershed Area - 22.8 ac <br />l0-year, 24-hour storm = 1.8in <br />CN = 63 (Sagebrush - C) <br />All other design input parameters are shown on the attached SEDCAD4 printouts. Ditch design also <br />attached. <br />Vent Shaft Pad SAE <br />Watershed Area - 1.8 ac <br />10-year, 24-hour storm = 1.8in <br />CN = 89 (Gravel) <br />. All other design input parameters are shown on the attached SEDCAD4 printouts. <br />!7N/G /ecerve~ $~20~0/ <br />