2025-11-14_PERMIT FILE - M2025056 (32)

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JD-8 Mine-Drainage Design Plan 10 (• Table 7. Planned Dimensions for Storm Water Retention Pond 1 Area of Storm Water Pond 4,583 ft2 Area of Floor(5H:1V slopes) 1,037 ft2 Perimeter of Floor 224.2 ft Total Depth 2.6 ft Volume(1.6 ft. deep) 3,100 ft3 0.071 acre-ft 23,190 gallons Freeboard(10-year 24-hour) 1.0 ft Freeboard(100-year 24-hour) 0.6 ft Runoff from the eastern areas of the mine is presently routed to Retention Pond 2 (Sub-Basin 6 in Figure 1) which has a current working capacity of approximately 0.08 acre-ft (3,400 ft3). Retention Pond 2 can be expanded in the existing area to a maximum volume of 7,940 ft3 (at 1.6 ft. deep) using the available 8,562 ft2 footprint shown in Figure 1 with 5H:1V side slopes and a 2.6 ft total depth (Table 8). Freeboard will be reduced from 1.0 ft during the 10-year 24-hour event to 0.5 ft during the 100-year 24-hour event. Table 8. Planned Dimensions for the Expanded Storm Water Retention Pond 2 Area of Storm Water Pond 8,562 ft2 Area of Floor(5H:1V slopes) 3,766 ft2 Perimeter of Floor 299 ft Total Depth 2.6 ft Volume(1.6 ft. deep) 7,940 ft3 0.182 acre-ft 59,390 gallons Freeboard(10-year 24-hour) 1.0 ft Freeboard(100-year 24-hour) 0.5 ft 3.4 100-year 24-hour Routing Capacity Calculation parameters for the 100-year 24-hour anticipated peak flow rate (Qloo) generated by the existing mine site sub-basins and entering the retention ponds are shown in Table 9. Table 9. Mine Site Current Conditions- Peak Discharge (Q1oo) Sub-Basin Sub-Basin Parameter 1-2-3 4-5-6 Tsh(sheet flow)(hr) 0.16 0.14 Tsc(shallow concentrated)(hr) 0.03 0.04 Tch(channel)(hr) 0.00 0.00 Tc(hr) 0.19 0.18 Q„(cfs) 490 520 Q100(cfs) 1.5 3.7 ,• Excess flows from Retention Pond 1 will be conveyed across the spillway shown in Figure 1. 4122A.120702 Whetstone Associates•