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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 />2.2.3 Boulder Riprap and 18 - Inch Low Flow Bypass Pipeline Installation <br />Once the cutoff wall has been installed and grading is done, the next construction activity will be to <br />install and grout boulders across the restored unpaved road crossing and graded terrain down to the <br />creek. Grading east of the restored road crossing will drain flows to the creek and will be armored with <br />grouted boulders, including a boulder drop and boulder dissipation structure (following Urban Drainage <br />Flood Control District [UDFCD] design guidelines) with loose riprap transitioning back to creek grade <br />(see Figures 4 and 5). The proposed armoring and boulder work is needed to handle anticipated <br />higher flows along Ralston Creek that may occur when the Temporary Diversion Pipeline capacity is <br />exceeded and the cutoff wall /road acts as a weir to convey flood flows, which range up to <br />approximately 2,500 cfs for the 100 -year event. The proposed work includes necessary erosion control <br />protection, well- defined staging and access areas to limit Site disturbance, and final stabilization of the <br />disturbed construction area (see Appendix A — Agency Letters and Responses and Figure 3). Once <br />the boulder work is done, the area will be cleaned, and any bare areas remaining will be seeded and <br />mulched as further described in the mitigation portion of this BA. <br />The 18 -inch low flow bypass pipeline will be installed largely aboveground on the north side of the <br />existing access road for approximately 3,500 linear feet to the east. The initial portion of the pipeline <br />will be buried due to elevation constraints and the pipeline also will be buried at road crossings. The <br />low flow bypass pipeline will discharge to Ralston Creek below the proposed location of the <br />downstream cutoff wall (Phase 2). During construction, treatment of this discharge will be provided via <br />filter bags, as needed, to remove sediment as detailed in Attachment E of the PCN and as shown on <br />Figure 3. <br />2.2.4 Best Management Practices (BMPs) <br />A SWMP /GESC Plan has been prepared for the Project, including Phase 1. This plan has been <br />prepared in accordance with the requirements of the Colorado Department of Public Health and <br />Environment (CDPHE) General Permit for Stormwater Discharges Associated with Construction <br />Activities and Jefferson County GESC requirements. This plan must be reviewed and approved by <br />Jefferson County prior to the start of construction as a part of the grading permit application. The <br />SWMP /GESC describes stormwater BMPs that will be used to control sediment and other stormwater <br />pollutants on the construction Site. The SWMP /GESC is provided in the PCN as Attachment E. <br />Page 12 August 2011 <br />