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Parameter <br />Sub -Basin <br />1 -2 -3 <br />Sub -Basin <br />4 -5 -6 <br />Tsh (sheet flow) (hr) <br />0.16 <br />0.14 <br />T, (shallow concentrated) (hr) <br />0.03 <br />0.04 <br />T (channel) (hr) <br />0.00 <br />0.00 <br />Tc (hr) <br />0.19 <br />0.18 <br />Q„ (cfs) <br />490 <br />520 <br />Q100 (cfs) <br />1.5 <br />3.7 <br />JD -8 Mine - Drainage Design Plan 10 <br />Table 7. Planned Dimensions for Storm Water Retention Pond 1 <br />Area of Storm Water Pond <br />Area of Floor (513:1V slopes) <br />Perimeter of Floor <br />Total Depth <br />Volume (1.6 ft. deep) <br />Freeboard (10 -year 24 -hour) <br />Freeboard (100 -year 24 -hour) <br />Area of Storm Water Pond <br />Area of Floor (5H:1V slopes) <br />Perimeter of Floor <br />Total Depth <br />Volume (1.6 ft. deep) <br />Freeboard (10 -year 24 -hour) <br />Freeboard (100 -year 24 -hour) <br />4,583 ft <br />1,037 ft <br />224.2 ft <br />2.6 ft <br />3,100 ft <br />0.071 acre -ft <br />23,190 gallons <br />1.0 ft <br />0.6 ft <br />Runoff from the eastern areas of the mine is presently routed to Retention Pond 2 (Sub -Basin 6 in Figure 1) <br />which has a current working capacity of approximately 0.08 acre -ft (3,400 ft Retention Pond 2 can be <br />expanded in the existing area to a maximum volume of 7,940 ft (at 1.6 ft. deep) using the available 8,562 <br />ft footprint shown in Figure 1 with 5H:1V side slopes and a 2.6 ft total depth (Table 8). Freeboard will be <br />reduced from 1.0 ft during the 10 -year 24 -hour event to 0.5 ft during the 100 -year 24 -hour event. <br />Table 8. Planned Dimensions for the Expanded Storm Water Retention Pond 2 <br />8,562 ft <br />3,766 ft <br />299 ft <br />2.6 ft <br />7,940 ft <br />0.182 acre -ft <br />59,390 gallons <br />1.0 ft <br />0.5 ft <br />3.4 100 - year 24 - hour Routing Capacity <br />Calculation parameters for the 100 -year 24 -hour anticipated peak flow rate (Qloo) generated by the existing <br />mine site sub - basins and entering the retention ponds are shown in Table 9. <br />Table 9. Mine Site Current Conditions - Peak Discharge (Q100) <br />Excess flows from Retention Pond 1 will be conveyed across the spillway shown in Figure 1. <br />4122A.120702 Whetstone Associates <br />