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• 29 <br />The 10-year and 100-year peak discharges calculated using MULTSED are rea- <br />sonable using the data presented in Chapter II and the Preliminary Data Base <br />Report. The peak discharges calculated by MULTSED compare well with values <br />estimated using SRwARB for both the 10-year and 100-year events. The TM-1 <br />results compared cell for the 100-year event, however, they rare sloth higher <br />than both MULTSED and SRWARB for the 10-year event. The results for the 10- <br />year event are better represented by MULTSED and SRWARB. It is meaningful that <br />MULTSED compares best with SRwARB since this latter methodology vas developed <br />. for the area being analyzed. The results calculated by the MULTSED model were <br />used to assess the adequary of existing structures and establish design criteria <br />of proposed structures. <br />L <br />5.2 Comparison of Suspended Sediment Concentrations Entering and Leaving the <br />~- Lease Area <br />The suspended sediment concentration resulting from the runoff rithin the <br />lease area is a main concern. The State of Colorado regulations require a peak <br />concentration of 35 mg/1 (35 ppm) or less in the effluent from the lease area. <br />However, the most practical and environmentally sound approach is to not in- <br />crease the suspended sediment concentration in the effluent leaving the lease <br />area above the level entering. This is the best possible management practice <br />for several reasons. The undisturbed area above the mine site produces sedi- <br />went concentrations on the order of 9000 ppm oz greater. If the rater leaving <br />the mine vas treated to the 35 ppm effluent standard, severe downstream impacts <br />could occur. Among them would be degradation of the Magpie Creek channel bed, <br />increased bank instability and failure, and the progression of headcuts uo trib- <br />utaries in the Magpie Creek drainage below the point of treated water release. <br />For present-mining conditions and the 10-year storm event, the suspended <br />sediment concentration leaving the lease area is approximately four times as <br />large as the level entering the lease area. This is due primarily to the degra- <br />dation of the major diversion channels (CH-15 and CH-17) and is not a consequence <br />of disturbed or reclaimed areas. The water runoff through the diversion channels <br />is from the undisturbed areas above the mine site and the disturbed area within <br />• the mine site does not contribute. Also, the reclaimed areas do not contribute <br />any runoff to the diversion channels. However, to bring the system into a <br />hydrologic balance, the diversion channels should be stabilized. Section 6.4 <br />discusses alternatives to reduce the increase in suspended sediment concentration <br />