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25-year, 24-hour Shaft Pad During Construction - Pond 1 is assumed to contain three years of <br />accumulated stored sediment and has a starting initial pool elevation at the invert of the emergency <br />spillway, and the principal spillway is assumed to be non-operational (to simulate worst-case conditions <br />where the entire 25-yr, 24-hr peak flow is routed through the emergency spillway). Pond 2 also has a <br />starting initial pool elevation at the invert of the emergency spillway. This analysis was completed to <br />show that the emergency spillways for Pond 1 and Pond 2 can safely pass discharge from the 25-year, 24- <br />hour storm event with at least 1 ft of freeboard. The SEDCAD results show that the peak water surface <br />elevation for flows over the emergency spillway at Pond 1 is 6872.3 ft amsl, 1.7 feet below the top of dam <br />elevation of 6874 ft amsl. At Pond 2, the top of dam elevation is 6855.5 ft amsl, providing 1.3 feet of <br />freeboard above the modeled peak water surface elevation of 6854.2 ft amsl. <br />10-year, 24-hour Shaft Pad After Construction - Pond 1 is assumed to contain three years of <br />accumulated stored sediment and has a starting initial pool elevation at the invert of the principal spillway <br />(lowest perforation in the riser pipe). This analysis was completed to show that given these conditions, <br />discharge from Pond 1 will meet NPDES requirements for water quality. The inflow TSS concentration <br />for Pond 1 is 862 mg/l, while the SEDCAD modeled outflow has a peak TSS concentration of 54 mg/l. <br />For settleable solids, Pond 1 inflow had a SS concentration of 0.56 ml/1, with an outflow SS concentration <br />of zero. <br />25-year, 24-hour Shaft Pad After Construction - Pond 1 is assumed to contain three years of <br />accumulated stored sediment and has a starting initial pool elevation at the invert of the emergency <br />spillway, and the principal spillway is assumed to be non-operational (to simulate worst-case conditions <br />• where the entire 25-yr, 24-hr peak flow is routed through the emergency spillway). This analysis was <br />completed to show that the emergency spillway in Pond 1 can safely pass discharge from the 25-year, 24- <br />hour storm event with at least 1 ft of freeboard. Freeboard of 1.6 feet is available given the top of dam <br />elevation of 6874 ft amsl and the peak stage modeled at 6872.4 ft amsl. <br />Summary of SEDCAD Modeling <br />The runoff volume into Pond 1 during construction resulting from the 10-year, 24-hour storm is 0.365 ac <br />ft. By subtracting this number from the capacity of the pond at maximum pool (1.561 ac ft at the invert of <br />the principal spillway), it is clear that unless 1.196 ac ft is already impounded when the inflow occurs <br />(i.e., Pond 1 is already impounding water to a depth of 9.15 ft at elevation 6870.15 ft) Pond 1 has <br />sufficient capacity to completely contain runoff from the 10-year, 24-hour storm event without <br />discharging. <br />Likewise, the runoff volume into Pond 2 during construction resulting from the 10-year, 24-hour storm is <br />0.429 ac ft and the capacity of the pond at maximum pool is 0.839 ac ft at the invert of the emergency <br />spillway. Therefore, unless Pond 2 is already impounding more than 0.410 ac ft (elevation 6851.60 ft <br />amsl, pond depth 5.10 ft), Pond 2 will contain the entire runoff from the 10-year, 24-hour storm event <br />without discharging. <br />The following tables summarize information from the SEDCAD modeling (Table 5,Table 6). <br />• <br />Twentytmile Coal Company 18L Shaft Pad 6 Water & Earth Technologies, Inc. <br />Drainage and Sediment Control System Design March 2, 2009