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/^' r 4 s P Mr. Paul Storrs August 21, 1978 Page 2 r & Mr. Dean Plassey • The new location will be at the 7418 elevation line. The capacity of the diversion channel i.s quite adequate in this location, since flood runoff calculations were based on the water shed area upstream from the 7400 foot contour line. In addition, the reduction in water shed area controlled by the settling ponds by locating the channel at a lower elevation adds an additional factor of safety in that part of the system. All surface runoff from the uppermost bench will be routed to the settling ponds as proposed in the mine plan. Water flowing in the diversion channel will be treated as originally stated in the mine plan. The diversion ditch will contain a low-flow channel which will be riprapped throughout its entire length. A maximum flow velocity in the low-flow channel will be approximately 6 fps when it is flowing full. The ditch design work was performed by Charles F. Leaf, P.E. according to Stevens, et al (1976) 1/, the most widely accepted reference i.n channel des~.gn. Accor3ingly, the riprapped mixture in the low-flow channel will have stones at least 6 inches in diameter. This will be adequate for flow conditions up to 6 fps bottom veloCi.ty. At high flow conditions, velocities are expected to exceed 10 fps. In order to maintain stability for high-flow conditions riprap size will be at least 18 inches at both ends of the channel for a distance of at least 75 feet upstream from the outfall. One-half of this riprap will be 14 inches or greater. All outside bends in the large channel will a~so be riprapped with rock of this sire. As specified by BLM, gabions and riprap will be used to protect the receiving water shed at the East end. Riprap (at least 18 inches maximum diameter) will also be placed in the receiving water shed at the West end of the channel. Gabions and/or riprap will extend at least 35 feet beyond each end. Also, both the bottom and sides of the high-flow channel will be riprapped near each end. Plate No. 1 (attached) shows the new location of the diversion ditch. We have also completed the field work necessary to begin construction of the sedimentation ponds. Site plans for each of the settling ponds may he found on Plate No. 2 (enclosed). Access roads to the settling ponds and the waste storage area are shown on Plate No. 1. Steel sheet piling will be used at the ends of tl~e bin walls for seepage control. A water level recorder will be installed on Dam I4o. 4 to monitor spills. Each dam will be equipped with a 15 inch outlet gate.