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IMPERIAL DAM AND DESILTING WORKS
ALL-AMERICAN CANAL SYSTEM
BY A. W. KIDDER, ENGINEER, BUREAU OF RECLAMATION
IMPERIAL DAM, a feature of the Boulder Canyon project, is now being constructed across the Colorado River 17 miles above Yuma, Ariz. The purpose of the dam is to provide diversion and desilting facilities for the 15,155 second-feet required in the All-American Canal on the California side of the river and ultimately 6,000 secondfeet in the Gila Canal on the Arizona side.
From the All-American Canal water will be bypassed into the Yuma Canal which, heretofore, has been supplied by water diverted at Laguna Dam, about 5 miles below the Imperial site. The Laguna Dam will, therefore, become practically obsolete except as a possible tailwater control for the new dam. Aside from supplying the Yuma Canal the All-American Canal will provide irrigation water for the Imperial Valley which at present receives water from the Imperial Canal that runs for a considerable portion of its length through Mexican territory. Provision is also made for supplying San Diego with 155 secondfeet of water from the All-American Canal.
For the present, a diversion of 2,000 second-feet to the Gila Canal is contemplated. This water is to be used for irrigation of the first unit of the Gila project, comprising 150,000 acres in the Gila Valley, Ariz. Headworks for an ultimate diversion of 6,000 second-feet are provided by the present construction.
Although the dam will create a reservoir of 85,000 acrefeet capacity and a surface area of 7,500 acres, this storage is not required by the project, nor considered a feature, as the reservoir will soon become filled with silt. The total drainage area above the dam is 187,000 square miles but the run-off from 167,000 square miles is controlled by the Boulder Dam and an additional 10,000 square miles will be controlled by the Parker Dam, now under construction. The uncontrolled drainage area contributing to the flow at the Imperial Dam will, therefore, be only 10,000 square miles. With this limited drainage area and the equalizing effect of the two immense reservoirs in the river above the Imperial site, it is estimated that the maximum flood, at this location will probably not exceed 150.000 second-feet.
Several possible dam sites for the diversion works were under consideration for many years but the Imperial site, formerly known as the Cocopah dam site No. 3, was finally selected. The alluvial flood plain at this location has an elevation of 160 feet above sea level and a width of 2,300 feet between rocky abutments. The alluvium consists chiefly of very fine sandy silt with some layers of clay and of coarse sand or gravel. It is probable that the depth of this water-borne material is about 200 feet. The rocky abutments, rising at the sides of the flood plain, consist of old formations of gneiss and schist which have been much contorted and fractured so as to show little continuity. Overlying the uneven gneiss and schist is a layer of rhyolite in evidence on both sides of the river.
The dam site has been investigated by sinking a number of test holes and pits to make sure that no strata of plastic mud lie under the location of the dam. From some of these test holes, undisturbed samples of the underlying material have been taken and tested to determine the load carrying ability and percolation factor. The percolation factor and a flow net under the dam, developed by the electric analogy method and checked by a model, indicate a measure of the probable seepage. Test piles have also been driven and tested both for compressive and uplift resistance.
The dam is of the slab-and-buttress type, with a total length of 3,430 feet, inclusive of nonoverflow sections, headworks, gate structures, sluiceway, and overflow spillway. The maximum height will be 45 feet and the normal water surface will be raised 23 feet. All gate structures have been designed with pile foundations to prevent any chance of unequal settlement. The piles will be battered, some upstream and some downstream, so as to carry the horizontal as well as the vertical loads in pure compression and tension. At both abutments several units of the dam will be founded on rock, otherwise the dam will be of the floating type, resting on the sandy bed of the river and divided into units to give some flexibility. Rubber joint seals will be used between the units. A certain amount of settlement is also
Laguna Dam, sluicegates and canal headworks, completed in 1909.
allowed by a mastic filler seal at the top of the sheet piling used as cut-offs under the structure.
The dam will be provided with an upstream apron under which will be three rows of sheet piling of sufficient depth to give a weighted path of percolation of eight times the head. The path of percolation under horizontal surfaces is assigned a weight of one-third the value of the path along vertical surfaces.
A feature of the design is the provision for drainage of percolating water under the dam. This under drainage will greatly reduce the uplift pressure under the dam and the downstream apron, resulting in a greater safety factor against sliding and a considerable saving in the required thickness of the apron. The drain pipe will be surrounded by a filter to prevent the fine sand from being washed out from under the structure. This filter will consist of four layers of sand and gravel graded in size so that the material of each layer will hold the material of the next outer layer from being washed into the drain. The drain will discharge
through ejectors in the downstream slab of the overflow weir. These ejectors, by means of the velocity of the overflow sheet of water, will maintain a water pressure under the dam considerably below the flood water level downstream.
It has been computed by means of the flow net and percolation factor of the foundation material as previously mentioned, that the maximum seepage under the dam will be about 4 gallons per minute per foot of dam. The overflow or spillway section of the dam will be 1,198 feet in length with an ogee downstream slab. With open sluice gates, the estimated maximum flood of 150,000 second-feet may be discharged with a depth of 8.5 feet of water over the crest. For design, the maximum water surface has been taken at 10 feet above the crest. The nonoverflow structures have been given an additional freeboard of 6 feet. The overflow weir is to have a heavy downstream apron with dentated sill and riprap to protect the streambed below. The design of the apron was based upon extensive model studies.