WSP01354 - Laserfiche WebLink

Home Browse Search

( z.(t) Impacts of drawing down Lake Powell to allow exposure of Glen Canyon 1 - With respect to David Brower's concept to drain Lake Powell completely, the following can be noted: a - The spillway discharge tunnels, to which are connected the river bypass tunnels used during construction, have a "flip bucket" elevation of 3158 feet, approximately 14 feet below the original streambed elevation of 3 172 feet at the dam, and a tunnel bottom elevation of 3134 feet near the flip bucket. The flip buckets would likely be removed to allow free flow of both water and sediment through the tunnel. Thus with the bypass tunnels open, the lake would be completely drained, even the dead storage which cannot be released from the hollow jet outlet tubes that were used for the spike flow test. b - The river bypass tunnels have a diameter of 41 feet. With the concrete plugs removed, the dam would function much like during the initial construction period when the tunnels were used to bypass the entire Colorado River flow, Under an assumption of non-pressurized, open channel flow, the tunnels have a combined release capacity of about 100,000 cfs, Greater amounts could be discharged if Lake Powell refilled enough to pressurize the tunnels (about 10 to 20 feet of head), c - Peak flows on the Colorado River at Lees Ferry typically occur in Mayor June and have a broadly based shape, Statistical analysis revealed that the 100-year peak flows are about 155,000 cfs, the 20-year peak flows are about 125,000 cfs, and the average peak flows are about 65,000 cfs, This means that the rive bypass tunnels probably could pass all but about 10% of the flows in all years, Only years such as 1952, 1957, 1983, and 1984 would there be more water than the tunnels could pass. In those cases, there would be some filling (10 to 20 feet) of the reservoir, creating some head on the tunnel resulting in river flows of about 120,000 to 130,000 cfs, d - High spring runoff is highly sediment laden, With the tunnels open, sediment would be bypassed downstream under a natural flow and sediment regime, It is likely that the Grand Canyon would be brought back near to pre-dam conditions, e - One would expect that under this action, the "Law of the River" would likely be revisited, however, only the 8,23 MAF minimum objective release and the obvious equalization provisions might need to be eliminated, With an average annual natural inflow during the historic critical hydrologic period of about 12 MAF and current upper basin use of4,2 MAF, the difference of7,8 MAF is enough to satisfY the Colorado River Compact obligation of75 MAFfIO years to the lower basin without needing the storage of Lake Powell, In addition, recovered evaporation losses from Lake Powell would help to meet any potential deficiency