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• As documented in Exhibit 19, Drainage and Sediment Control Plan, total <br />runoff volume for the 10-year, 24-hour storm event for the mine area is <br />9.1 acre feet. Comparing this runoff volume with a calculated total <br />runoff volume of 213 acre feet for the receiving drainage of Newlin Creek, <br />it is evident that total runoff from the disturbed area, even under storm <br />conditions, constitutes a relatively small portion of total watershed <br />runoff. <br />Increases in storm runoff from disturbance areas are effectively mitigated <br />by operation of sedimentation ponds which retain disturbed area runoff to <br />allow settlement of suspended solids and controlled discharge of retained <br />water to approximate normal streamflow conditions. In this way, the <br />effect of increased runoff volumes cart be buffered, preventing significant <br />increases in downstream peak flows, channel erosion or other adverse <br />affects. Upon completion of mining and related activities, surface <br />disturbance areas will be reclaimed, with the end result being runoff and <br />infiltration characteristics similar to those which existed prior to <br />• mining. <br />Sedimentation ponds are an integral part of EFCI's plans for mitigating <br />mining related impacts on the surface hydrologic system. However, because <br />they retain surface runoff, they have the potential to reduce downstream <br />flows. Because the sedimentation ponds are designed to gradually release <br />impounded runoff following required retention for sediment control, most <br />of the runoff is returned to the surface drainage system with only a short <br />time lag corresponding to the design retention time for each pond. <br />However, the ponds have been designed such that discharge structures are <br />above the design sediment level. Any water impounded between the actual <br />sediment level and the discharge elevation will be retained in the pond <br />as dead storage. <br />For the mine and loadout areas, cumulative dead storage for all <br />sedimentation ponds is 4.01 and 1.73 acre feet respectively. This volume <br />represents the reduction in flows to the surface water system. With <br />• annual surface water flows in Newlin and Oak Creeks of 1950 and 2830 acre <br />feet respectively, the corresponding dead storage volumes for mine and <br />loadout areas are less than 0.2 percent and represent a negligible portion <br />2.05.6-48 <br />