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• 2.05..6(b)(iv.) DETERMINATION OF TflE PROBABLE HYDROLOGIC <br />CONSEQUENCES OF MINING <br />1.0 Surface Water <br />At this mine site there are three primary areas of concern <br />for impacts on the stream system. They are: general increases <br />in suspended sediment during mining and reclamation activities; <br />changes in the stream channel characteristics of Lewis Gulch <br />resulting from the proposed fill for the truck turnaround and <br />sediment pond; and, possible damage to the water-supply pipeline, <br />for the City of Rockvale which crosses the haul road. <br />f <br />The first is a problem at most surface mine facilities and <br />ie usually given careful consideration in a drainage control <br />system of ditches and ponds. The Twin Pines No. 2 application <br />contains detailed information for their sediment control plan. <br />Their proposal ie well conceived and detailed so that no <br />• recommendations are needed for structure sizing and construction <br />criteria. <br />Of more importance is the disposition of the proposed fill <br />in Lewis Gulch. This is essentially a 20-foot high earth dam. A <br />240-foot, 60-inch diameter CMP culvert would be used to convey <br />flow in Lewis Gulch under the fill. The mayor effects of any <br />impoundment on flow characteristics include: attenuation of flow <br />peaks, deposition of suspended solids at the upstream section, <br />concentrating the flow at the outlet, and lowering of the base <br />level below the outlet. In general, culverts have maintenance <br />requirements which include cleaning trash racks, unclogging the <br />barrel, and cleaning or upgrading riprap at the downstream <br />section. <br />It is certain that the effects on the stream channel from a <br />permanent impoundment and culvert discussed above will occur. In <br />a short-term framework, most could be mitigated by infrequent <br />maintenance. Over a long-term period, however, it ie difficult <br />JFSA-65 <br />