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two identical siphons, each one discharging through separate 3-inch HDPE pipes to the <br />3 Dam Seep Water pipeline. The siphon events have been modeled to determine the <br />expected rate of flow. Detail 6 in the upper left corner of Drawing No. 3DS-3 shows the <br />expected flow rates and velocities plotted against time. When the siphon action begins <br />the pipe will be empty and the head differential will be maximized. The flow rate may be <br />as high as 90 gpm and a velocity of 4 feet per second for a few minutes, finally settling <br />down to a flow of about 36 gpm and 1.5 feet per second. When the siphon break occurs <br />at about 15 minutes the discharge pipeline will slowly empty. The frequency of the <br />siphon events will depend on the rate of inflow. <br />Both chambers of the tank will need to be cleaned on a schedule so that the <br />precipitate won't affect the siphon action. Free access to the vault is therefore important. <br />Drawing No. 3DS-4 shows the construction of a maintenance access road from the <br />junction of Grand County Roads 3 and 30. That route provides the shortest access to <br />all facilities that will need to be cleaned, and also simplifies the construction since the <br />pipeline corridor will also be the access corridor. The road will meet Grand County <br />driveway standards, including width (14 feet) and visibility requirements at the driveway <br />entrance. The road will be gravel surfaced. <br />Wye-lateral cleanout risers are shown at two locations along the pipe corridor, <br />one at Station 6+40 where the pipeline meets County Road 3 and the second at Station <br />9+00 before the pipeline crosses the road. A third clean-out will be located at the siphon <br />vault. <br />Vault and Piping Design (Reference Drawing No. 3DS-3) <br />The intention of the design is to avoid pumping if possible. It will not be known <br />whether pumping can be totally avoided until the lowest seepage area has been <br />exposed by trenching. An additional vault is therefore included upstream from the <br />siphon vault, and is shown on the plan view of the collection system, Detail 4 of Drawing <br />No. 3DS-2. If the elevation of the lowest seep is below the elevation that can be <br />discharged with the auto-siphon, a solar-powered pump could be installed in that second <br />vault to lift the water into the siphon vault. That vault, called the Boundary Cutoff <br />Manhole, will also provide the final opportunity for containing the seepage. The <br />Boundary Cutoff Manhole will be a perforated concrete manhole embedded in 1-inch <br />drain rock at the vertex of the lowest drain (Detail 1, Drawing No. 3DS-2) <br />Henderson Operations, Mill Site <br />No. 3 Dam Seep Mitigation, Design Report <br />Drain <br />9/7/10 <br />Page 5