1988-04-18_PERMIT FILE - C1981008A (4)

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INTRODUCTION Sedimentation Pond 007 is proposed to be constructed close to the southwest corner of the Nucla East Mine permit area. Pond 007 will be a temporary sediment structure to control runoff and sediment from the disturbed areas of the Nucla East Mine permit area. Pond 007 will be constructed in a tributary to Calamity Draw. Pond 007 is located in Section 6 of Township 46 North, Range 15W, west of the town of Nucla (see Exhibit 13-5). SITE DESCRIPTION Land Use Sedimentation Pond 007 has a 457.1-acre watershed that is tributary to Calamity Draw. This watershed is classified as approximately 40 percent juniper-grass, 29 percent disturbed, 28 percent pasture or range, and 3 percent meadow (riparian). Embankment The proposed embankment for Pond 007 will be a homogeneous earth fill compacted in lifts to design specifications. The top width of the Embankment will be approximately 30 feet wide with upstream and downstream slopes of approximately 3h:ly, respectively. The principal spillway will be a 7-inch diameter orifice cut in a 30-inch diameter drop inlet pipe. The emergency spillway will be the 30-inch diameter drop inlet CMP. DESIGN ANALYSIS General Sedimentation Pond 007 was designed to meet or exceed the design and performance requirements of the Colorado Mined Land Reclamation Division. Sediment Pond 007 was designed to adhere to the following specifications: 1. Provide storage for the runoff from the 10-year, 24-hour storm event without experiencing any discharge through the emergency spillway (16.67 ac-ft.) 2. Provide an emergency spillway capable of passing runoff from the 25-year, 24-hour storm event (21.41 ac-ft.) 3. Provide adequate storage for sediment generated from the 10-year, 24-hour (MUSLE) storm event, (4.7, 10-year, 24-hour MUSLE storm events or 6.68 ac-ft). 4. Provide a nonclogging dewatering device and principal spillway that will provide adequate detention time for the 10-year, 24-hour storm event and still allow a 1