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The entire width of valley was not necessary for a flood spillway section, and the dam was divided into two parts, a concrete spillway section and an earth dike. A balance between costs incident to height of headworks, bridge piers, and height of earth dike against costs incident to length of spillway section and highway bridge fixed the relative lengths of the two sections at 650 feet and 1,650 feet, respectively. The spillway section is 28 feet wide with a fore apron 24.75 feet wide, and was designed to rest on rock. A gravity ogee section was adopted as against the Ambursen cellular type in spite of slightly less favorable cost estimates, principally because of the greater suitability of the solid section to the rigors of the northern mountain climate. The fore apron is intended to contain
the hydraulic jump and prevent undermining of the main section. The dike section has a maximum height of 25 feet with a 5-foot freeboard at maximum flood stage.
Examination of the cobblestone bed under the dike section made it appear certain that an impervious sheetpiling cut-off would be very difficult to construct because of the cobblestones which ranged up to 18 inches in diameter. Excavation for a puddle trench was also considered very expensive because of trouble with ground waterIt was finally decided that the dike section with a moderate cut-off trench near the upper third of the base would be satisfactory. Absolute imperviousness was not necessary and piping was not feared in view of the coarseness of the cobblestone bed. Events have proven the correctness of this conclusion as no piping has occurred in 13 years of operation.
During normal high water, leakage of about 5 second-feet through low-lying gravel appears in the borrow pit. In the early years of operation the downstream face of the dike also showed dampness indicating that the assumed hydraulic gradient was about correct. Silt and windblown sand have now accumulated on the upstream slope, tightening it so that the dampness no longer shows on the lower slope. Prevailing winds confine wave action to the spillway section, and no breaching of the upstream face of the dike ever occurred.
Flow into the canal is regulated by six structural-steel radial gates, 10 feet wide and 10 feet high, set in a headworks front 70 feet wide and 22 feet 6 inches high. The gates have a top seal against curtain walls 12 feet 6 inches high. They are lifted by a cable at each side of each gate winding on drum hoists. Counterweights to reduce externally applied hoisting forces are housed in slots in the 24-inch partition walls which separate the gates. In front of the headgates and 4 feet lower than the gate sills is a concrete silt basin, about 70 by 70 feet in plan, upon which the silt load of the river drops before the water enters the canal. At full capacity of the canal the water will flow across the upper end of the basin at a velocity of 3 feet per second.
At the side of the silt basin and in line with the axis of the spillway is a battery of four 10- by 12-foot radial gates of the same general design as the headgates. The sills of these gates are at the level of the silt basin which is 2 feet above the normal low water below the dam and 9 feet above the adjacent fore apron. The full canal discharge will flow across the silt basin at a velocity of 8 feet per second when the canal headgates are closed and the sluicegates opened wide. This is calculated to sweep the basin clean. Both sets of gates are operated by a gasoline engine moving on a track above them.
In 13 years of operation the pond above the dam has practically silted full. The silt basin is scoured clean annually, usually in October after the close of the irrigation season. Even these scouring periods must be short as canal storage runs the power plant but a few hours. The scoured out silt lodges below the dam to be removed by next season's flood. While grade retrogression is general to the river, none has occurred at the dam to date, possibly because no severe floods have occurred since its construction. As a result tailwater remains high, the jump at the foot of the ogee section is drowned, and scour below the fore apron, if any, is filled up as high water recedes. The apron is always found covered with a tight layer of gravel.
Development has not yet required operation at full canal capacity. A principal operating difficulty is slush ice, which is combatted most successfully by taking water out of the silt basin above canal grade, through hand operated wooden gates placed in the stop plank grooves,
and sluicing the ice as much as possible out of the pond through the logway gate opening.
The logway is a section of flat ogee spillway 10 feet wide, the crest of which is 10.75 feet above the sluiceway gate sills and 5 feet below the normal spillway crest. The section is located adjacent to the spillway. The forebay is separated from the silt basin by a concrete training wall which sets on the edge of the silt basin floor, and has the same top elevation as the spillway crest. Timbers and iron pipe set in the wall afford additional guidance to floating timber. When not in operation the logway is closed by a wooden gate which brings the crest to the level of the spillway crest.
Work was started by Government forces in July 1921, and completed May 1923. The working period included one flood season and two winter seasons. The first winter season was unusually severe and caused work interruption of 10 weeks. Before the arrival of the flood season, the headworks, silt basin, sluiceway, logway, and one 27-foot section of the spillway were completed, or above high water so that work could be continued. Also, there was completed that part of the dike south of the old river channel, which was used as a bypass during construction. After the flood, work was prosecuted continuously to completion except for minor delays occasioned by railway strikes and impassable roads, both of which stopped delivery of materials. The second winter was so mild that concrete work in favorable locations was possible with ordinary winter pouring procedure.
The main construction equipment was gasoline-driven draglines which were used on the project principally for canal excavation. Two machines with l?-s-cubic yard buckets and 45-foot booms, and one with 2)4-cubic yard bucket and 75-foot boom were interchanged between canal excavation and dam construction. These machines cut diversion channels, stripped foundations, acted as traveling cranes for moving forms and light equipment, loaded the dike materials into wagons and concrete aggregate on screening equipment, all as required. The concrete mixer was a ^-cubic yard machine, spotted at various points as work progressed. Distribution was mainly by J-i-cubic yard cars, operated by hand on a narrow gage track built on a trestle sufficiently high to permit chuting to most of the work.
The cobblestone admixture was not run through the mixer, but placed as plum rock brought to the trestle from the screening plant or dike building job in ^-cubic-yard cars pulled by a gasoline hoist. Generally, excavations were opened wide enough to avoid the use of sheeting. Unwatering was by means of gasoline-driven centrifugal pumps and hand-power diaphragm pumps. The largest pump used
was a 6- by 8-inch centrifugal, driven by a 15-horsepo\ver engine.
The principal item of rock excavation was the cut-off trenches, which were !)'" feet wide and 3 and 5 feet deep below the respective spillway and fore apron bottoms. To avoid shattering the adjacent rock, line drilling on both sides of each trench was necessary. Holes were spaced about 6 to 8 inches on centers and a channelling effect was produced with a special tool fitted to a pneumatic hammer. Light blasting in center holes broke out the channeled material. Excavation in two lifts was found best for the 5-foot trench which had one side 7 feet deep. Compressed air was furnished by two 7- by 6-inch portable compressors driven by 20-horsepower gasoline engines, and one 8- by 8-inch, twocylinder, stationary compressor driven by tractor.
Consideration was given to building the dike with clay material from the mesa to the south, but this was abandoned in favor of using a mixture of soil and cobblestones found in
the river bottom. The material was loaded by dragline, from borrow pits on the downstream side of the dike or from spillway excavation, into 3-cubic yard dump wagons hauled by 10-ton caterpillar tractors, two or three wagons constituting a train according to lift. The borrow pits were wet, and as a result of the method of excavating and loading the top sand and lower cobbly gravel, arrived on the job wet and thoroughly mixed. The mixture was spread in layers by a standard road grader pulled by horses.
Large cobblestones were worked to the outside faces by horse-drawn stone boats. The travel of placing and levelling produced sufficient compaction and a dike was produced which was more rodent proof than if the clay material had been used. The deficiency of riprap on the two faces was made up with cobblestones, hauled from the river bed and the concrete aggregate screening plant. The latter was operated without crushing apparatus as the river bed material ran sufficiently high in moderate sizes to make crushing