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JD-8 Mine - Drainage Design Plan 9 <br />• 3.2 Stormwater Retention and Routing Capacity <br />The preliminary dimensions for the westernmost stormwater retention pond are shown in Figure 3 and <br />Table 9. The proposed volume of 16,890 ft3 exceeds the calculated runoff from the 10-year 24-hour storm <br />by 31% (without freeboard) and would retain 63% of the 100-year 24-hour storm. A riprap spillway would <br />be constructed with a riprap apron to convey excess flows associated with the 100-year 24-hour storm. <br />Table 9. Preliminary Dimensions for the Operational and Post-Mining Stormwater <br />Retention Pond <br />Area of Catchment 8627.316 ftZ <br />Area of Floor (511:1 V slopes) 3,375 ftZ <br />Perimeter of floor 300.6 ft <br />Depth (avg) 3.0 ft <br />Volume 16,890 ft3 <br />Volume 0.388 acre-ft <br />Volume 126,340 gallons <br />• <br />E <br />4122A.110601 Whetstone Associates