2013-01-29_PERMIT FILE - C1981022A (3)

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Setrim - 2.05.6 appropriate surfacing materials; uses ditches to collect nutoff as close as possible to its source and • route it to sedimentation ponds for retention and settlement of suspended solids prior to discharge to natural drainages; and designs, constmcu, and maintains diversion collection ditches to minimize erosion and increased sediment loading. On completion of mining operations, disturbed areas will be reclaimed. Reclamation will involve backfilling and grading, reestablishment of natural drainage patterns, soi]/substitute replacement, and revegetation. It is anticipated that reclamation will effectively restore infiltration and runoff patterns to approximate the baseline conditions currentlyexisting for the surface disturbance areas. The calculated total average annual runoff volume for all existing and proposed mine disturbance areas is approximately 65-acre feet. Comparing this runoff volume with a calculated total average annual rtmoff volume of 470,000 acre feet for the receiving drainage of the North Fork of the Gunnison River, it is evident that total runoff from the disturbed area constitutes a relatively small portion (0.01) percent of total watershed runoff. Increases in runoff from disturbance areas may result in temporary minor increases in stream flows for the receiving drainages, however, any increases will not be significant given the very limited area of surface disturbance relative to total drainage basin areas for the potentially effected drainages. Potential flow increases will ako be partially mitigated by operation of sedimentation ponds that will retain disturbed area runoff providing some buffering effect. Although sedimentation ponds are integral to mitigating mining-related impacts on the surface hydrologic system, operation of sedimentation ponds tends to reduce discharge flow volumes and extend the period of effective flow for nuioff from both snowmelt and thunderstorm events. In effect, sedirnentarion ponds function as limited capacity flood control structures reducing the effective discharge rate for large volume flows through temporuy storage and flow routing. With the • exception of any ponds designed as anon-discharging structures, the sedimentation ponds are designed to gradually release impounded runoff following required retention for sediment control Given provisions for retention and gradual discharge of any retained storm flows, most of the runoff is returned to the surface drainage system with only a short lag time corresponding to the design retention time for each pond. Any containment ponds are so small as to have a negligible effett relative to any reduction in flow volume for the receiving drainage. Operation of the drainage and sediment control network, and specifically the sedimentation ponds that retain runoff from disturbance areas, has the potential to alter flow patterns in receiving drainages. Because the area where the OMI operations are located has asemi-arid climate, runoff contributions to most of the drainages occur only as a result of snowmelt and large storm evenu. For the intermittent and ephemeral drainages, any delayed discharge of runoff flows will actually represent a positive impact, potentially extending the period of active flow. For the North Fork, as a perennial drainage, minor alteration of the timing of runoff flows is significant only if the delay and the corresponding volume of retained runoff reduces Rows during critical low flow periods or if the deferral of runoff discharge involves significant time delays. As previously noted, the retained nuioff volume for any of the sedimentation ponds or all ponds cumulatively represents a relatively small percentage of normal flow volumes for the receiving drainages and should not adversely impact flow volumes even during low flow periods. Additionally, operation of the sedimentation structures will not result in significant discharge delays since the maxunum design detention time is only three days for the design storm runoff volume and discharge will be occurring continuously once pond levek reach the design discharge elevation for each pond. The ponds have been designed such that discharge stmctures are above the design sediment level and any mnoff accumulations in the pond below this discharge elevation will be retained in the pond as • dead storage (note that dead storage will not effect pond capacity since the design sediment storage capacity is being utilized for dead storage). The design sediment storage volume available for temporary runoff storage represenu a negligible amount relative to area stream discharge volumes and so does not constitute a significant potential flow reduction. PR04 2.05-90 Revised August 2000