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<br />~z <br />IV. POST-MINING CONDITION <br />Using the data presented in the Preliminary Data Base Report and the changes <br />in Chapter II, the MULTSED physical process model was applied to Magpie Creek, <br />Newlin Creek, and Tipple Area watersheds for the post-mining conditions. For <br />this analysis, the 100-year design storm vas used. The post-mining condition <br />assumes complete reclamation of the G.E.C. mining area and rnmplete restoration _ <br />' back to its original ground and canopy cover. The output results for the 100- <br />year design storm and post-mining conditions are available upon request. <br />This information includes hydrographs for all planes, subwatershed and channel <br />units, total runoff wlume for all units, and suspended sediment concentrations <br />( during each time increment for the channel units. <br />1. <br />4.1 Magpie Creek Drainage <br />r For use in the MULTSED model, Simons, Li G Associates, Inc. (SLA's) reeom- <br />mended channel cross sections and alignment were used for the two major diver- <br />sion channels (PM-CH-15 and PM-CH-17). These channels are discussed in Section <br />6.4. In addition, there are areas within the Magpie Creek drainage that are <br />noncontributing due to Dorchester mining. <br />The results of the 100-year design storm are used to establish design <br />criteria for the reclamation of stream channels within the Magpie Creek <br />drainage. The total eater and sediment runoff, peak discharges, peak sedi- <br />ment concentration, and average sediment concentration for the 100-year design <br />storm are presented in Tabla 6. ' <br />Opstream of the G.E.C. mining area, the average peak sediment concentration <br />in the channel units is approximately 9400 ppm. Through the G.E.C. Minerals, Inc. <br />permit area and the reclaimed major diversion channels, the average peak sediment <br />concentration is approximately 9000 ppm; at the watershed outlet, the average <br />peak sediment concentration is 9500 ppm. By keeping the sediment concentration <br />entering and leaving the permit area approximately the same, the impacts to the <br />hydrologic balance are minimized. The sediment concentrations leaving the per- <br />mit area for the present-mining conditions are approximately four times as great <br />as the Fost-mining conditions. This is due primarily to the degradation of <br />• CH-15 and CR-17 in the present-mining condition. For post-mining conditions, <br />new channel cross sections and riprap protection were used for these two channels <br />(PM-CH-15 and PM-CH-17) which reduced the sediment leaving the permit area. The <br />channel slope, alignment, cross section and protection are discussed in Section 6.4. <br />