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. Pond 2 has an area of 0.256 acres, measured at the top of the pond embankment. Like Pond 1, the Pond 2 <br />embankment will be constructed from native material, with an inslope designed at 2:1, and the outslope at <br />3:1. The top width of the embankment is 10 feet, allowing access by pickup trucks and small- to <br />medium-size heavy equipment. The embankment will be stabilized with vegetation via hydromulching. <br />The maximum depth of the pond will be 9 ft to the highest point on the embankment. Pond 2 has an <br />emergency spillway only, and because there is no conduit through the embankment, there is no need for a <br />filter diaphragm and seepage drain system. The emergency spillway is an open channel cut through <br />native material with an invert elevation at 6854 ft amsl. The invert of the emergency spillway is 3 ft <br />above the natural stream channel. The emergency spillway will be constructed with a 10 ft bottom width <br />and 3:1 sideslopes. A revegetated channel will convey discharge from the emergency spillway to the <br />receiving channel below the pond. Although the SEDCAD modeling shows that an unvegetated erodible <br />channel will adequately handle the peak flow velocities, it is still recommended that the emergency <br />spillway channel be revegetated. A rock mulch riprap of 2-inch minus material, erosion control matting <br />or a similar material should be applied to the emergency spillway channel for interim stability until <br />channel vegetation is established. Capacity of the pond will be 0.839 ac-ft to the invert of the <br />emergency spillway as compared to the 10-year, 24-hour storm event runoff volume of 0.401 acre-feet. <br />Sediment storage in Pond 2 was not considered, because the life of Pond 2 will only be through the end of <br />construction for the intake shaft installation. <br />SEDCAD Modeling <br />• The intake shaft installation was modeled for hydrology and sedimentology under two different scenarios: <br />during construction and after construction is complete. Sedimentation occurs at a higher rate during <br />construction due to both the additional area of disturbance within the area that contributes runoff and due <br />to the conditions of the soil at that phase in the project. Hydrology and sedimentology parameters were <br />selected based on existing site conditions and mapping, aerial photo coverage, NOAA Atlas 2 - <br />Precipitation-Frequency Atlas of the Western United States 1973, and data from a January 15, 2009 soils <br />report entitled, "Twentymile Coal Company: Western Mining District 18-Left Shaft Baseline Soil <br />Investigation" prepared by Habitat Management, Inc. Additionally, RUSLE variables used in the <br />SEDCAD modeling were corroborated with Dr. Richard Warner, SEDCAD author at the University of <br />Kentucky (phone conference with Dr. Richard Warner January 28, 2009). <br />Scenario 1: During Construction <br />Both ponds were modeled for the During Construction scenario using the 10-year, 24-hour and 25-year, <br />24-hour storm events. The total watershed reporting to Pond 1 is 8.3 ac, which includes 4.0 acres of <br />disturbance. The total watershed area reporting to Pond 2 is 9.9 ac. During construction, runoff from the <br />disturbed area is pre-treated by a silt fence prior to entering Pond 1 (Exhibit 6). Pond 1 discharges into <br />Pond 2 for additional treatment. The following values were used for Scenario 1 modeling parameters <br />(Table 1). <br />Table 1. Scenario 1 SEDCAD Inputs <br />Parameter Value <br />Watershed Area (ac)* 9.9 total, 4.0 disturbed <br />10- ear, 24-hour storm event (in) 1.8 <br />Twentytmile Coal Company 18L Shaft Pad 3 Water & Earth Technologies, Inc. <br />Drainage and Sediment Control System Design March 2, 2009