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• velocities are controlled and runoff is collected as close to the source <br />as possible to decrease potential infiltration. In addition, EFCI has <br />effectively minimized the area of surface disturbance by keeping all <br />surface facilities within a relatively limited area. Total surface <br />disturbance in both the mine and loadout areas is a relatively small <br />percentage of drainage basin area for effected drainages. Consequently, <br />the effect of localized increases in surface runoff will be small relative <br />to total drainage flows within the basin. <br />The effects of increased surface runoff during active operations will be <br />buffered by operation of sedimentation ponds which offer the flexibility <br />of controlled release of runoff retention volumes so as to most closely <br />approximate natural surface flow patterns in receiving drainages. As soon <br />as operationally feasible following completion of mining and related <br />activities, EFCI will reclaim disturbed areas, with reclamation practices <br />effectively restoring normal infiltration and runoff characteristics. <br />• Operation of the drainage and sediment control network has the potential <br />to alter flow patterns in receiving drainages since runoff from <br />disturbance areas will be retained in sedimentation ponds to allow <br />settlement of suspended solids. Because the Southfield operations are <br />located in a relatively arid climate, runoff from most of the drainages <br />occurs only as a result of snowmelt and large storm events. Operation of <br />the sedimentation structures should not significantly effect surface <br />flows. Total storm retention volume far all sedimentation ponds in the <br />mine area is approximately ]3.18 acre feet. This volume compares with a <br />calculated storm drainage volume (10-year, 24-hour) for Newlin Creek of <br />213.5 acre feet. Although storm drainage volume figures for Chandler and <br />Oak Creeks are not available, similar considerations are applicable with <br />comparative retention capacity of the sedimentation structures being a <br />small percentage of storm flows for the corresponding drainage basins. <br />It must be recognized that with the exception of dead storage below the <br />. outlet level, all runoff retained in sedimentation ponds will normally be <br />discharged within approximately 24 to 36 hours from the time it enters <br />the ponds. With outlet control, EFCI has the flexibility to control <br />discharge from the sedimentation ponds to most closely approximate natural <br />2.05.6-30 <br />