2009-04-29_PERMIT FILE - C1982056 (3)

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. Pond 1 Pond 1 has an area of 0.449 acres, measured at the top of the pond embankment. The Pond 1 embankment will be constructed from native material. The inslope is designed at 2:1, and the outslope is at 3:1. The top width of the embankment is 12 feet, allowing access by pickup trucks and small- to medium-size heavy equipment. The embankment will be stabilized with vegetation via hydromulching. The maximum height from the bottom of the natural channel to the bottom of the emergency spillway will be 7.5 ft, and 9.5 ft from the natural channel to the top of the embankment. Pond 1 will be incised a maximum of 2.5 ft at the embankment. The maximum depth of Pond 1 will be 12 ft to the highest point on the embankment (10 ft from the bottom of the incised pond to the bottom of the emergency spillway). Exhibit 3 provides design details for Pond 1. Capacity of the pond will be 1.561 ac-ft to the invert of the principal spillway as compared to the 10-year, 24-hour storm event runoff volume of 0.365 ac-ft. Pond 1 will completely contain three years of sediment storage (.009 acre-feet). The principal spillway for Pond 1 is a vertical 18 inch steel riser pipe connected to an 18 inch steel barrel that protrudes through the pond embankment. A filter diaphragm will be placed around the 18 inch barrel downstream from the key trench and downstream from the top of dam centerline. The filter diaphragm was designed to drain seepage and filter any soil particles transported by the seepage water. The filter diaphragm will be constructed from sand meeting requirements for ASTM C-33 Fine Concrete Aggregate. The filter diaphragm is a vertical zone of sand within the embankment that will connect to a horizontal sand drain placed underneath the 18 inch barrel. The sand drain will act as the pathway to . transport seepage from the filter diaphragm to its exit at the downstream toe of the embankment. Riprap will cover the sand drain where it exits the embankment to provide protection from surface erosion. To maximize Pond 1's efficiency for removing sediments from runoff during construction, the primary spillway riser pipe will be installed without perforations, as shown on Exhibit 4. During construction, outflow from Pond 1 will only occur when the water rises above the elevation of the top of the intake pipe, allowing for primary settlement of sediments in Pond 1. After construction of the shaft pad, the primary spillway riser pipe for Pond 1 will be perforated to elevation 6864 ft above mean seal level (amsl) (Exhibit 5) to allow the pond to dewater in 1.17 days. Pond 1 is designed with an emergency spillway. The invert of the emergency spillway is 8 ft above the natural stream channel. The emergency spillway is an open channel cut through native material with an invert elevation at 6872 ft amsl. The emergency spillway is 10 feet wide with 3:1 sideslopes. The emergency spillway will be a revegetated grass-lined channel that will convey discharge from Pond 1 to the receiving channel below the pond. Although the SEDCAD modeling shows that an unvegetated erodible channel will adequately handle the peak flow velocities, it is still recommended that the emergency spillway channel be revegetated. A rock mulch riprap of 2-inch minus material, erosion control matting or a similar material should be applied to the emergency spillway channel for interim stability until channel vegetation is established. Pond 2 Pond 2 is designed to further treat runoff discharged from Pond 1 during construction of the 18L shaft pad • to meet NPDES requirements for water quality. After construction of the shaft pad, Pond 2 can be reclaimed. Twentytmile Coal Company 18L Shaft Pad 2 Water & Earth Technologies, Inc. Drainage and Sediment Control System Design March 2; 2009