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Response to Exhibit C, Comment 8 The transitional flow from the natural channel to the mined out pit with a transition gradient of 10% will produce a velocity of over 15 feet per second during bank-full flow. This will scour the channel and could cause headcutting outside the upstream permit boundary. To avoid this, a sill needs to be installed above the slope transition. The sill should be installed along the southern permit boundary and tie to the riprapped bank on the west side of Phase I. When Phase I is completed, the sill should be extended to the north along the east side of the permit boundary to tie into the riprapped section along the east bank as shown on Exhibit E. Because of the small size of the proposed mining operation and the likelihood for refilling of the mined out pits within a fairly short time period, it is felt that the sill ne~d not be deigned as a permanent structure. Research by Nandlal and Sikka suggests that permeable and semi-flexible erosion control structures would be applicable because of their simplicity, cost effectiveness, and environmental compatibility. The type of sill, or semi-flexible dike, proposed consists of a wire/rail combination. Driven rails and above-ground noire netting have been used for many years on the Uncompahgre River to divert flood flows and protect against erosion. The attached calculations and sketches show a rail/wire system employing 85-pound rails installed at 10-foot centers across the channel and at 20-foot centers parallel to it. Ten gage chain link fencing between the rails acts to create the reinforced sill and prevent head cutting. The sill will be buried so that the top of the chain link fencing is at the existing ground surface. After installation, cobble and gravel backfill will be placed on the upstream side and riprap on the downstream side. The sill will be placed evenly on both sides of the wire to re-establish a uniform ground surface. The wire will prevent downcuttinq by forming a barrier which bedload material will have to cross over. 4Nandlal, S., 1985, Lightweight Semi-Flexible Dike," Office ~azette of the U.S. Patent Office, Vol., 1055 Sikka, D.R., 1973, "Highlights of Flood Protection Works Done on River Narmada at Hoshangabad in Madhya Pradesh State-India," in Flood Investigation, Vol. II, Proceedings of the International Association for Hydraulic Research Symposium on River Mechanics, Bangkok, Thailand Page 5