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sedimentation Pond S2 <br />Sedimentation Pond L2 is designed to retain sediment from the disturbed <br />areas and retain runoff from disturbed and undisturbed areas between the <br />Atchison, Topeka and Santa Fe Railroad and the county road, within the <br />permit boundary. The retention volume and runoff storage was based on the <br />precipitation from the 10-year, 24-hour storm event. The drainage area <br />consists of approximately 7.6 acres of which 4.9 acres are disturbed. <br />Disturbed areas created by coal storage, cut and fill and the topsoil <br />stockpile area shown on the Loadout Area Surface and Groundwater Hydrology <br />Map, (Map 13). The sedimentation pond is located near the southwest <br />corner of the permit area. This location allows for retention of runoff <br />through natural slope trends at the site and via a collection ditch <br />constructed along the northern boundary of the storage area at the toe of <br />the excavated slope. <br />The total storage available within the sedimentation pond is approximately <br />• 1.48 acre feet, 0.76 acre feet of sediment storage, and 0.72 acre feet of <br />runoff storage. As shown on the West Loadout Area Pond L2 and Drainage <br />Structure Design Map, (Map 27), the sedimentation pond is almost entirely <br />incised with the exception of a small embankment constructed with a crest <br />elevation of 5,249 feet, along the western perimeter of the pond. The <br />sedimt:,,tation pond's bottom elevation of 5,239 feet is approximately seven <br />(7) feet above the streambed located to the west of the permit area. At <br />this elevation, it is not anticipated that groundwater, nor flows within <br />the stream channel will infiltrate into the pond area. <br />Sedimentation L2 utilizes a 18-inch CMP culvert with a 10 Inch gated <br />decant as a principal spillway. The 10-inch gated decant is located at <br />sediment level and is designed to dewater the pond. Dewatering 1s <br />performed to ensure that adequate design storm capacity exists shortly <br />after each storm in case a subsequent storm event occurs. Dewatering is <br />performed only after 24 to 36 hours of detention time is achieved to allow <br />• far settling of suspended solids. The emergency spillway is a ripraped <br />trapezoidal channat ~~^signed to safely pass a 25-year, 24-hour event. <br />~`~ <br />118 R Revised '•antamficr. 1Gat <br />