The Yellow River of China has a large pro portion of arid and semi-arid territory, and five times in its history it has devastated its valley, notwithstanding all the defenses of man, and were built) is 15 inches thick at the top and 55 inches at the bottom.
The Three Miles Falls Dam, in Oregon (built 1914), is a multiple-arch dam, curved to a radius of 1,200 feet, and 24 feet in height. It is composed of 40 arches sprung from the buttresses as piers with a radius of 18 feet. The buttresses are placed 20 feet from centre to centre, 2 feet wide at the crest and 34 feet at the base. They are 12 inches thick at the top and 24 inches at the bottom.
The Big Bear Valley Dam, in California (built 1914-16), is 363 feet long and 92 feet high. It is a multiple-arch dam, consisting of 10 arches of 30% feet chord, sprung from 'but tresses 22 feet deep at the crest and 111 feet at the base. The buttresses are 18 inches thick at the top and 54 inches at the bottom.
The Mathis Dam, at Tallulah Falls, Ga., is 650 feet long and 90 feet high, of reinforced concrete. The deck is 18 inches thick at the top and 39 inches at the bottom, and is sup ported by buttresses 16 feet deep at the crest and feet at the base. The buttresses are changed its lower course to the sea. All the factors in flood flow will sometime conjoin, and a work that is to last through the centuries must recognize all the possibilities.
Adjuncts in the way of gates, sluices and movable structures are much favored by some designers, especially where they serve to lessen cost. Such provisions should not be carried to a point where their failure to operate or be operated will produce damages amounting to a disaster.
The location of service pipes and outlets through or beneath dams is shown by experi ence to be a source of danger, and the best practice now seeks an independent location.
The tendency is now to avoid the long race for power purposes, to build the dam at the foot rather than the head of the rapids or descent, and to make the power station and forebays a part of the structure, taking the water directly from the pool.
Outside of the main elements of a structure on which safety and permanency depend, many devices and constructions are permissible, as failure in these results in only temporary loss and inconvenience.
General Remarks.— Dam building is still in the evolutionary stage, and the resources of en therefor are still developing. Modern
practice includes the construction in the body of the darn of a more or less complete drainage system to collect any water leaking in from the reservoir. In the Kensico Dam there are also thennophones to give warning of unusual changes of temperature The Wachusett Darn (diagrams on another page) was the first in which ice pressure was taken into account in calculating the cross-section. The engineers estimated it at 47,000 pounds per lineal foot for ice 12 inches thick at full reservoir. In the Arrowrock Dam there is a system of inspection galleries, of which the control chambers form a part. These give access to the body of the dam at several elevations, the lowest being 230 feet below normal high water in the reservoir.
So far as general designs are concerned, the type structures would seem to be well defined, but every site has its own peculiar conditions, requiring elasticity in treatment, and financial limitations often blight the proper solution. A dam when once built assimilates so closely to natural resources, is so fully identified with the public welfare, and the failure is so disastrous, that its construction becomes a matter of solici tude above that of any other engineering work. Of 28 failures of dams investigated, 12 were found to be due to faulty construction, although the dimensions were correct; 11 were due to in sufficient spillway—these were mainly earthen dams, and high water overtopped them; 4 were due to faulty foundation and 1 to percolation along pipe-lines. See IRRIGATION; RESERVOIR; WATER-WORKS; WATER POWER.
Bibliography.—Bassell, B., 'Earth Dams: A Study' (New York 1907) ; Blight, W. G., 'Dams and Weirs' (Chicago 1915) ; Bright more, A. W., 'Structural Engineering' (Lon don 1913) ; Morrison, C. E., and Brodie, O. L., 'Hieh Masonry Dam Design' (New York 1916) • Ottley, J. W. 'Stresses in Masonry Dams' (London 1908) ; Prelini, C., 'Graphic Determination of Earth Slopes, Retaining Walls, and Dams' (New York 1908) ; Smith, C. W.,