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• INTRODUCTION <br />Sedimentation Pond 009 will be constructed in the SE1y SWk of Section 11, Township 5 North, <br />Range B8 West in a tributary to Sage Creek. The proposed pond and embankment will be <br />constructed as a temporary sedimentation structure to control runoff and sediment from a <br />watershed disturbed by proposed mining activities. This structure will be constructed <br />upon approval in 1991. This sedimentation structure will be maintained until reclamation <br />of this structure is approved by Colorado Mined Land Reclamation Division (CMLRD), <br />SITE DESCRIPTION <br />Land Use. Sedimentation Pond 009 has a 438.6-acre watershed that is tributary to Sage <br />Creek (see Exhibit 13-2). This watershed will be disturbed by mining activities as shown <br />on Exhibits 12-2 and 20-1 and contemporaneously reclaimed as shown on Exhibit 19-1. The <br />watershed will be approzi mately 73 percent undisturbed and 27 percent disturbed. Design <br />of the pond was conservatively based on the maximum disturbance at any one time within the <br />proposed mining area. <br /> Embankment and Excavation. The proposed embankment will be a homogeneous earth fill <br />• structure. The top width of the proposed embankment will be approximately 15 feet with <br /> upstream and downstream slopes of approximately 3h:ty each. The principal spillway will <br /> consist of an 18-inch diameter drop inlet culvert; the emergency spillway will be a <br />trapezoidal channel 15 feet wide (see Exhibit 13-7). A portion of the pond capacity will <br />be a result of excavating topsoil and material below natural ground level. Excavated <br />suitable material will be used in embankment construction. The ponding area's side slopes <br />will be 3h:1v, typically. <br />DESIGN ANA LVSIS <br />General. Sedimentation Pond 009 was designed to meet or exceed the design and performance <br />requirements of CMLRD while utilizing as much of the existing topography as possible. <br />Sedimentation Pond 009 was designed to adhere to the following specifications: <br />1. Provide storage and treatment for the runoff from the 10-year, 24-hour storm event <br />without experiencing any discharge through the emergency spillway; <br />2. Provide an emergency spillway capable of passing runoff from the 25-year, 24-hour <br />storm event; <br />3. Provide adequate storage for sediment generated as determined from the Revised <br />• Universal Soil Loss Equation as shown on Page 13-5-5. <br />13-5-1 <br />