OCR

Bureau of Reclamation and the work was engineered by them also. The dam was constructed by a joint venture composed of Morrison-Knudsen Co., Inc.; Winston Brothers Co.; and Utah. Winston. Brothers Co. was the sponsoring company for this project which also included a powerhouse and the project was started in 1936. During construction, miners descended 84 feet through shafts left in the rising dam to clear and grout faulted rock. Approximate cost of this project was $4,360,000. Imperial Dam - This dam was the third of five dissimilar dams built on the Colorado River. Though lacking the spectacular dimensions of Hoover one of the world's highest, and Parker, the world's deepest, Imperial Dam is unique among engineering designs. Its water-purifying function has no counterpart. Water discharged from upstream Hoover and Parker Dams (Davis Dam later was built between them.) was practically clear, but soon picked up its maximum load of silt from the stream bed. The silt-laden water, diverted into the All-American Canal that serves the fabulously rich Imperial Valley in California, had caused a yearly dredging expense of $1, 000, 000. Imperial Dam's permanent desilting works, costing only half as much more, stops that high annual expense. At the California end of the dam, three separate settling basins, each 540 feet wide, 770 feet long and over 12 feet deep, were built with wedge- shaped influent channels running through their centers . Twelve thousand cubic feet of water a second flows into the desilting basins through unique vertical slots in the influent channel walls . As the water passes slowly through the basins it drops its silt, which is swept from the bottom of the desilting basins by 72 hugh revolving scrapers. These motor-driven rotating scrapers, each 125 feet in diameter, are of two types weighing from six to seven tons each. The mud is swept from the basins into central underground galleries and flushed back into the river bed below the works. In this way 70,000 tons of silt are removed daily from the water which flows, clear, into the All-American Canal. At Imperial's Arizona end water entering the Gila Canal system to irrigate over 500, 000 acres is desilted in a concrete -lined settling basin, 115 feet wide and 1,090 feet long. Water runs into the basin through 35 x 14 feet radial gates. Flow from the basin is controlled by 16 fixed- wheel gates, built in tow tiers of eight each. Periodically the lower eight gates are raised, allowing water to flush the deposited silt into the river channel below the dam. Today's completed dam gives little evidence of the construction involved. To dry the broad river bed required one of the largest well-point unwatering operations in dam building history. At one time in the con- struction, 2, 800 well-points, connecting 60 pumps, were blotting up the seepage at the rate of 45 cubic feet of water a second. The river bed was unwatered in sections, surrounded by sheet piling, which each unit of the job proceeded.