Laserfiche WebLink
IRIS <br />MITIGATION AND DESIGN, INC. <br />SCHWARTZWALDER MINE PHASE 1 - UPSTREAM CUTOFF WALL AND LOW FLOW BYPASS <br />PIPELINE - BIOLOGICAL ASSESSMENT <br />• Install a concrete washout (within stabilized staging area) as shown in Attachment E of the Pre - <br />Construction Notification (PCN) (SWMP /GESC Plan). <br />• Pour the concrete cutoff wall and place the section of 42 -inch steel pipe that will penetrate the <br />wall. Additional sections of the pipeline will be constructed in Phase 2. <br />• Install approximately 3,500 feet of 18 -inch diameter flexible pipe for the low flow (8 cfs) bypass. <br />The layout of this flexible pipe will be along the existing access road and anchored or staked as <br />needed. There is limited earthwork that is anticipated with installing the low flow bypass, <br />primarily where the pipe must be buried under access road crossings. <br />• Install approximately 125 feet of riprap downstream from the cutoff wall to reduce the potential <br />for erosion and scour when flows exceed pipeline design capacity and overtop the cutoff <br />wall /road as weir flow. <br />• Install temporary irrigation system and construct Phase 1 mitigation areas. <br />• Seed all disturbed areas and mitigation areas. <br />• Leave temporary BMPs in place for the duration of the Phase 1 work. Temporary BMPs will be <br />removed once the cutoff wall area meets final stabilization criteria. The staging area, stockpile <br />areas, and other areas away from the immediate vicinity of the cutoff wall may remain in place, <br />with appropriate BMPs, for future phases. <br />2.2.2 Upstream Cutoff Wall Installation <br />As discussed above, the cutoff wall will be the first Project feature constructed after mobilization, <br />establishment of staging areas, and installation of stormwater BMPs. To aid in water management, a <br />coffer dam will be constructed upstream of the cutoff wall, and construction dewatering will commence <br />to draw the water table down to bedrock. Excavation for installation of the cutoff wall will then <br />commence (see Figures 4 to 6). All construction activities in the vicinity of the cutoff wall will be <br />accomplished according to the Stormwater Management Plan (SWMP) /Grading, Erosion and Sediment <br />Control (GESC) Plan (see Attachment E of the PCN). <br />The cutoff wall has been located in an already disturbed unpaved road crossing of Ralston Creek. <br />There are two culverts and a headwall in this location allowing Ralston Creek to flow beneath the <br />Page 7 August 2011 <br />