The standard units of the overflow weir will be 78 feet 6 inches long with four buttresses at 20-foot centers, leaving cantilever slab overhangs at both ends. The slabs are monolithic with the buttresses and are designed as continuous beams, the fixed end moments approximately balancing the moments from the overhanging cantilevers, thus relieving the buttresses of any appreciable bending

moment.

The reinforced concrete design for the dam has been based upon the following:

Concrete stress-650 pounds per square inch. Steel stress=16,000 pounds per square inch. Ratio of moduli of elasticity, N=10.

Laboratory tests showed an internal friction factor of 0.42 for the foundation material. The sliding factor of the dam on its foundation has been limited to 0.30 and an additional safety factor is obtained by tying the upstream apron to the floating dam by steel reinforcement.

The sluiceway will be adjacent to the All-American Canal headworks and will have twelve 16- by 7-foot radial gates with sills set 16.5 feet below normal water surface. The gates will, therefore, require top seals. The total discharge of the 12 gates under normal water level conditions will be 31,000 second-feet, and at maximum flood stage the discharge will be 42,500 second-feet. These sluice gates will be automatically controlled by a float switch, so as to maintain the normal water surface in the reservoir. The sluiceway will discharge into a sluiceway channel into which the sludge from the All-American desilting works will also flow. The sluice water will thus help to carry the sludge downstream.

The All-American Canal headworks will have four 75by 22-foot roller gates controlled from two gate houses, or from the central control house near the west end of the

Rotating arm of one of the 72 clarifying units in the desilting basin.

dam. A trashrack, 575 feet long with 26 inches clear opening between the bars, will be constructed upstream from the headgates. The location of the trashrack with the concrete wall upon which it is founded has been made on the basis of model tests giving the best hydraulic conditions and excluding the maximum amount of silt from the headgates.

Three gate structures are provided on the Arizona end of the dam for the Gila Canal diversion. Each gate structure will have three 35-foot 8-inch by 14-foot 6-inch radial gates provided with top seals. For the initial diversion only one gate structure will be used. From the gate structure the water will pass into a desilting basin through which it will flow at a velocity of 0.7 foot per second and where it will drop a large percentage of the silt load. Periodically the diversion gates at the lower end of the basin will be closed and the sluice gates opened, to increase the velocity through the basin to about 15 feet per second and thus sluice the deposited silt out into the river channel below the dam.

DESILTING WORKS

Although the water issuing from Boulder and Parker Dams will be practically free from silt, it will quickly pick up its full load from the bed of the stream. For the initial diversion of 12,000 second-feet the silt load will be some 60,000 tons per day. This silt would soon obstruct the flow in the canals and have to be excavated therefrom if the desilting works were not built. It has been estimated that the desilting works costing $1,500,000 will save a yearly expenditure of $1,000,000 for excavation from the canals.

The desilting works will initially consist of three basins, each of which is approximately 500 by 800 feet in plan and

12.5 feet deep with an influent channel through the center. Provision is being made for the addition of a fourth basin if found necessary when the ultimate diversion of 15,000 second-feet is reached. Effluent channels leading to the All-American Canal will be built between basins and at both ends of the row of three basins.

From the headworks the diverted water will flow through the inlet canal, which is to be divided by concrete sheet piles into four channels. This channelization was deemed necessary in order to maintain nonsilting velocities at times of partial diversion when one or two of the basins are closed down. The inlet canal will deliver water through twin 21- by 17-foot radial gates to the tapering influent channels extending down the centers of the basins. Similar gate installations will make it possible to bypass the flow directly to the effluent channels and All-American Canal. Normally, however, the water will enter the influent channel from which it will be distributed uniformly to the half basins on either side through unique vertical slots along the walls. These slots will serve the threefold purpose of:

(1) reducing the velocity and therefore the turbulence of the water entering the basins; (2) distributing the inflow into the basins uniformly both as to depth and width of basins; and (3) recovering nearly one-half of a foot of head from the high-velocity water in the channels.

Each half basin will be provided with 12 center drive rotating scrapers 125 feet in diameter. Each scraper, with a peripheral speed of 30 feet per minute, will continually feed the settled silt into a central collecting trench from which it will flow through sludge collecting pipes into the river. Provision is being made for sampling and measuring the sludge discharge. An interesting feature of the rotating scrapers is the overload limiting arrangement. If the silt load becomes too great the hinged arms of the scraper will automatically rise so as to scrape less deeply into the silt. At the same time there will be a visual warning of the overloaded condition. The 72 motor-driven-rotating scraper mechanisms may be controlled individually from three control houses, one at each basin. They may also be controlled in groups of 24 from the central or main control

ment, All-American Canal headworks, and sluiceway on the west side of the river, the Gila Canal headworks, Arizona abutment, and dike on the east side of the river, all back of cofferdams. Then by opening the cofferdam at the Gila headworks and building another cofferdam across the river the flow will be diverted through two gate structures of the Gila headworks during the construction of the overflow weir. Excavation for and construction of the All-American desilting works is being carried on simultaneously with the construction of the dam. The accompanying tabulation gives the estimated cost of the work and the accompanying drawings show the general plans of the dam and desilting works.

Driven end of roller gate No. 2 in the All-American Canal headworks.

house near the west end of the dam. Scraper overload indicators, water level indicators, and gate controls, all at the main control house, will centralize the complete operation of the diversion and desilting works.

CONSTRUCTION

The contract for the construction of the dam and desilting works was let in November 1935, to the Morrison-Knudsen Co., of Boise, Idaho, Utah Construction Co., of Ogden, Utah, and Winston Bros. Co., of Minneapolis, Minn., at their joint bid of $4,374,240. Work was started in January 1936. The contractors extended the railroad from the gravel plant to the dam site and built their camp, concrete mixing plants, cofferdams, and trestle bridge across the river.

The plan for construction is to build the California abut