Laserfiche WebLink
Environmental Protection Plan, Schwartzwalder Mine 15 -6 <br />the cutoff wall is to intercept alluvial groundwater and surface flows from the watershed above the mine <br />site, up to the design flow rate of 100 cubic feet per second (cfs), and to direct the flows into the pipeline <br />for conveyance through the site. Flows exceeding the capacity of the diversion pipeline will be controlled <br />via a weir structure integrated with the cutoff wall. The overflow weir will be designed to pass any flows <br />that exceed the 100 cfs design flow rate into the Ralston Creek channel, and these excess flood flows will <br />enter the natural channel and alluvium. On an annual basis, 95% of the total flow in Ralston Creek will be <br />diverted across the site in the pipeline. The upstream and downstream cutoff walls will isolate the alluvial <br />groundwater system in the central mine area, thereby enhancing the functionality of the existing <br />groundwater pump and treat system and allowing for further investigation of potential sources in a nearly <br />dewatered condition. <br />(ii) Diversion Pipeline <br />Based on the conceptual design work performed to date (WWE, 2011), the diversion pipeline will consist <br />of approximately 4,000 linear feet of 42 -inch welded steel pipe at an average slope of approximately 2.9 <br />percent (Figure 15 -3). The pipeline will operate under non - pressurized flow conditions and will be <br />constructed primarily above grade on the north side of the canyon to avoid conflicts with traffic and <br />investigation/mitigation activities in the narrow canyon. The pipeline will be mounted on piers in some <br />locations and constructed with blocking or anchoring to resist the momentum forces from high - velocity <br />flows. Design work is still underway, and it may be determined that some portions of the pipeline may be <br />fully or partially buried. <br />The pipeline will be designed and constructed to allow wildlife access under and over the pipeline. This <br />area has been designated as critical habitat for the Preble's Meadow Jumping Mouse, and pipeline will be <br />constructed such that the mouse can safely pass below the pipeline. Deer, bear, bobcat, and other species <br />will also be accommodated by a pipeline crossing, which will be detailed in the final engineering design. <br />The design flow rate for the diversion pipeline is 100 cfs, corresponding to the anticipated 2 -year peak flow <br />rate in Ralston Creek at the mine site (WWE, 2011). The analysis and selection of design flow rate is <br />intended to balance the level of protection provided (i.e., frequency of occurrence when diversion capacity <br />is exceeded) with practical considerations including diversion feasibility, size, and cost. <br />(iii) Downstream Cutoff Walls <br />The downstream cutoff wall/diversion return will be similar to the upstream cutoff in terms of function and <br />design. An energy dissipation basin, sized in accordance with Urban Drainage and Flood Control District <br />(UDFCD) criteria, will be provided at the pipeline outfall to reduce velocities and minimize erosion at the <br />return point to Ralston Creek. The crest elevation of the downstream cutoff is likely to be higher than the <br />current channel bottom and will be determined based on a water balance for the isolated alluvium that <br />accounts for inflows (groundwater and surface flows) from tributary areas between the upstream and <br />downstream cutoff walls. The elevated cutoff will enhance the effectiveness of the current groundwater <br />treatment system, which will treat water that is isolated between the upstream and downstream cutoff walls <br />and which will discharge below the downstream cutoff wall. <br />4109C.110510 Whetstone Associates <br />