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IRIS <br />MITIGATION AND DESIGN, INC. <br />SCHWARTZWALDER MINE PHASE 1 - UPSTREAM CUTOFF WALL AND LOW FLOW BYPASS <br />PIPELINE - BIOLOGICAL ASSESSMENT <br />unpaved road. This area will be excavated, the culverts and headwall will be removed and a trench <br />approximately 26 to 32 feet deep will be excavated to bedrock (see Figures 4 to 6). The trench will be <br />several feet wide at the bottom (to allow for construction of a 1.5 -foot thick wall) up to approximately <br />100 feet wide at the surface. The exact depth to bedrock is unknown until geotechnical work is <br />completed at the Site. Topsoil excavated from this trench will be stockpiled separately and will be <br />replaced on the ground surface during fine grading to serve as a seed bed. Excess topsoil may also <br />be used in mitigation areas as needed. The majority of excavated subsoil and topsoil will be stored <br />separately in an already impacted area at the Site (see Figures 2 and 3). <br />Once excavation is complete, forms will be placed, and a cutoff wall will be poured in place. The length <br />of the wall will be approximately 125 feet, spanning both sides of the valley and tying into bedrock on <br />either side and to bedrock below. During installation of the cutoff wall, a 42 -inch diameter steel pipe <br />(for future work in Phases 2 and 3) and an 18 -inch low flow bypass intake pipe will be installed, <br />penetrating the cutoff wall. Machinery will access the Phase 1 cutoff wall construction area via the <br />existing unpaved road. There will be impacts to the riparian area located directly upstream of the cutoff <br />wall, including installation of a riprap pad to allow for water pooling at the low flow bypass pipeline <br />intake (see Figures 3 -6). <br />The unpaved road crossing will then be replaced through backfilling the area east of the installed cutoff <br />wall with stockpiled soil. This soil will be sloped down to the riparian area to direct high flows that may <br />overtop the cutoff wall /road back into Ralston Creek (see Figure 4). Construction equipment and <br />materials will be stored in an upland area that is already heavily disturbed on the Site, and staging will <br />also occur within this already- disturbed area (see Figure 3). Once construction is accomplished, <br />approximately 6 inches of stockpiled topsoil will be spread over all graded areas. All disturbed areas <br />will then be ripped, seeded with an appropriate native seed mix (see Tables 2 to 5), and mulched with <br />a weed - free /seed -free straw and crimped. <br />Page 8 August 2011 <br />