AZ vs. CA:24 California Exhibits, Vol. XXIV; Miscellaneous

page 24

24 PROBLEMS OF IMPERIAL VALLEY AND VICINITY. dispense with compressed air and sink the section of the arch by open dredging from inside the vertical shafts formed by the hollow sections of the arch, which would be filled with concrete when in final position. A third method would be by adding to this main cofferdam as many lower parallel cofferdams inside the main cofferdam as the excavation showed to be necessary. 10. The determination of the best type of cofferdam can be left as a subject for further study after securing further data on the underwater conditions by additional drilling at the site of the cofferdams. The information now at hand shows that the depth to bedrock at the dam site is within practicable limits and that the diversion of the river during the construction of the foundation can be safely effected. spillwai . 11. A spillway capacity of 200,000 second-feet has been tentatively assumed and the spillway is designed to pass this quantity with a flood-water surface elevation of 1,265 with the 26,500,000 acre-foot reservoir and 1,300 with the 31,400,000 acre-foot reservoir. With an encroachment of 10 feet on the freeboard of the dam the spillway capacity increases to 300,000 second-feet. Preliminary studies indicate that these quantities can be safely passed through shafts and tunnels in the Arizona abutment where topographical conditions are favorable for a movable-crest structure. With a reservoir of either capacity proposed, it is very improbable that the spillway will ever pe subjected to a flood as great as 200,000 second-feet and it is certain that flood storage and control gates can be utilized to limit the flood to a very much smaller quantity, probably to 50,000 second-feet. IRRIGATION AND FLOOD-CONTROL OUTLET WORKS. 12. The irrigation demand will require outlet gates of about 25,000 second-foot capacity. Preliminary studies indicate that this quantity can be passed through 60-72-inch Ensign balanced valves and that these valves can be arranged at different levels so that they can be operated under a maximum head of about 150 feet for irrigation service. ' By utilizing the valves under higher heads a much larger discharge capacity can be obtained for flood-control purposes. In the plan tentatively adopted the balanced valves will be arranged to discharge into the spillway shafts and tunnels. Emergency show will be provided whicn will make the balanced valves accessible at all times. A portion of the irrigation water would at all times pass through the power plant, which leaves a large excess capacity through outlet gates for irrigation and flood-control purposes. POWER DEVELOPMENT. 13. Preliminary studies have been made for power development based on two different reservoir capacities, viz, 26,500,000 acre-feet and 31,400,000 acre-feet. 14. The accompanying tabulation (Table No. 1) shows a comparison of the power output and other related data for the two alternative reservoirs, based on different assumptions as to lands irrigated and to equated discharges for power development.