the water-level, which latter varies with the fullness or depletion of the reservoir. Where a water lining is desired, puddle is generally employed, often with concrete or masonry above it. Asphalt has recently been used to supplement concrete, brick, or stone lining. Occasionally the lower slope of earth dams is paved; in high dams it is frequently built with a level DAMS AND RESERVOIRS. that there were then ten earth dams 60 feet or over in height, as shown by the accompanying table. Later information shows that the fourth dam in the table, now known as the San Leandro, has a total height of 158 feet from the lowest point of the foundation to its crest, and has no heart-wall; also that it extends 125 feet above the original surface. The Tabraud dam, near Jack FIG. 1. CROSS-SECTION OF EARTH DAM, with concrete heart-wall and upper slope paved; Metropolitan WaterWorks, Southboro, Mass. place or berm part way up its height. Water is rarely allowed to come within five feet of the top of earth dams or reservoir embankments and it may be kept even lower. The minimum advisable thickness of the base will increase with the height of the dam and the gentleness of the slopes. The angle of repose, or natural slope, of ordinary earth, dumped in banks, gives a base of one and one-half feet to one foot of height, but wet earth has a less angle of repose. It is common, therefore, to give dams of ordinary earth a slope of 2 to 1 on the lower or dry face, and 22 or 3 to 1 on the wet face, and even these figures may be exceeded. Some earth dams are backed with loose stone or rock, to give greater stability. Occasionally the material composing earth dams is brought into place by means of flowing water, instead of by carts, scrapers, or buckets, running on and dumped son, Cal., completed in December, 1901, has its crest 120 feet above rock foundation, and 110 feet above the natural surface of the ground. It has no heart-wall. See Engineering News, July 10, and September 11, 1902, for illustrated descriptions of the Tabraud and San Leandro dams, respectively. Earth dams or earth embankments for reservoirs are among the oldest of engineering structures, having been built for irrigation thousands of years ago, in Egypt, India, and other Oriental countries. MASONRY DAMS, particularly of notable size, are of comparative recent origin, their construction having awaited the development of modern engineering. Moreover, while masonry dams of great height date from the sixteenth century (see table), it was only during the last half of the nineteenth century that their design accorded with the great principles of engineering—maximum strength with a minimum of material and cost. The accompanying table, taken from Wegmann, The Design and Construction of Dams (New York, 1899), will serve as a basis for tracing the development of the most notable masonry dams of the world during the last three centuries, terminating with the new Croton Dam, under construction in 1901. In 1900, or since the table was compiled, the contract was let These two dams, while without heart-walls in the dam proper, have puddle-filled trenches, 26 and 47 feet deep, respectively, from the natural surface to solid rock. from a cableway (q.v.) Hydraulic fill dams is the name applied to this rather novel class of structures. This process was used to build a part of the San Leandro and Temescal dams of the water-works supplying Oakland, Cal., and also in building earth dams at La Mesa and San Joaquin (Lake Christine), Cal., and Tyler, Tex. Earth dams vary in height from a few feet to 100 feet or more, and in length from a few score to thousands of feet, or even to miles, although most of the structures running into miles are more properly called reservoir embankments. A summary of the heights of earth dams in the United States, for water-works purposes alone, compiled from figures included in The Manual of American Water-Works for 1889-90, showed for the Wachusett Dam, which is a worthy rival of the new Croton Dam. Some figures concerning the Wachusett Dam and the Assuan Dam on the Nile (see Reservoirs, below) have been added to the table. Masonry dams are designed as though they were monolithic structures, and for this reason, as well as because of the fact that the pressure against the face of the dam tends to rupture it both vertically and horizontally, the blocks of stone are not laid in regular courses. Portland cement mortar is used to bind the stones in one homogeneous mass, or the dam may be composed of irregularly shaped masses of stone with the intervening spaces filled with concrete, or it may be made of concrete alone. (See CEMENT.) FIG. 2.-CROSS-SECTION OF A MASONRY DAM (New Croton Dam for New York City Water-Works). its own weight will not crush the material in its lower part. As a general rule the pressure should never exceed fifteen tons per square foot, and with some materials it may need to be as low as six tons. Obviously the only way to prevent excessive or crushing pressures at the bottom of very high dams is to diminish the thickness as the height increases. The action of ice and of the actual or possible current of overflowing water renders it necessary to make the top of the dam thicker than would be required to resist water pressure alone; otherwise the dam might be tapered to a knife edge at the top. The common type of cross-section for high masonry dams approaches a right-angled triangle, with the perpendicular side up-stream, but it varies from a real triangle in having both sides curved somewhat, particularly so as to give a broader base, and in having the extreme upper part built with nearly parallel sides, while the top is flat, or perhaps more or less rounded. While some of the early dams were quite bold in cross-section, most of them were far otherwise. One of the largest masonry dams (recently completed) is the New Croton Dam, built in connection with the New York City water supply to secure an increased storage reservoir capacity of about 32,000,000,000 gallons of water. This dam was originally designed by Alphonse Fteley and begun under his direction as chief engineer to the Aqueduct Commission of New York City in October, 1892, he remaining in charge until his retirement in 1899. It is located on the Croton River about 34 miles from its junction with the Hudson and about 1 mile above the Old Quaker Bridge, where the river runs almost due east and west. 761 DAMS AND RESERVOIRS. the dam. There is an 18 foot roadway on top of the dam which has a bottom thickness of 200 feet, and a greatest depth to bed rock of nearly 300 feet. It was completed in 1905, after 13 years of construction. ARCHED or CURVED DAMS have given rise to a great amount of discussion as to the possibility of utilizing the arch principle to resist a part of the thrust of the water on the dam, instead of relying wholly on gravity dams, or those with a section which gives sufficient weight to resist overturning and sliding. The most notable dams designed on this principle are the Bear Valley, Zola, and Sweetwater dams, details of which are given in the table. The first two are declared by Mr. James D. Schuyler, in his work on reservoirs (see below), to be "so slender in profile as to be absolutely unstable were they built straight." The Bear Valley Dam is only 3.17 feet thick at the top, 20 feet thick at the bottom, and is 60 feet in height. CONCRETE MASONRY DAMS are not essentially different from other masonry structures, except in their composition. (See CEMENT and CONCRETE.) Perhaps the most notable concrete dam in the world is that near San Mateo, Cal., built by the Spring Valley Water-Works Company of San Francisco. This dam, which is of the arched type, had attained a height of 13.4 feet in 1888 or 1889, but is designed to reach ultimately 170 feet, with a top width, when completed, of 25 feet and a width at the base of 176 feet. ROCK-FILL DAMS are built of large stones, or rock, loosely put in place, but with hand-laid face or slope walls. To make such dams watertight, or sufficiently so for the objects to be attained, the up-stream or wet slope may be faced with plank, concrete, concrete and asphalt, or steel. It is also possible to use earth to form either the upper or lower section; or riveted steel plates may be built in the centre of the structure. A masonry wall, with earth above and rock fill below, faced on the lower slope with stone laid in mortar, is another variation. The adoption of this form of construction is generally in the interests of economy, in localities where the transportation of cement would be very costly, where earth dams are out of the question, and where stone is abundant and easily thrown into place. The Escondido Dam, built by a Cali PHRAGM. Southern California Water Company, San Diego, Cal. Originally it was designed to consist of prac- FIG. 3.-ROCK-FILL DAM WITH STEEL HEART-WALL OR DIA. tically three parts, (1) an overflow or spillway, (2) a masonry dam, and (3) a wing wall and core wall for the embankment. In 1902, however, it was decided to replace the embankment section, which had been built by masonry, as it was believed that with such construction and such a foundation there might be a danger of failure. The embankment was therefore torn down and the masonry dam continued across the valley to the south slope. The spillway is located on the north side; it varies in height from 150 feet to about 10 feet, with a channel 1000 feet long, 50 feet wide at its upper end, and 125 feet wide where it meets fornia irrigation district of that name, is one of the most notable of the rock-fill structures. It is 76 feet high, 10 feet thick at the top, and 140 feet thick at the base, has top and bottom lengths of 380 feet and 100 feet respectively. The hand-laid dry wall on the upper or wet slope is 15 feet thick at the base and 5 feet at the top. It is covered with redwood plank and the space between the plank and the stone was rammed full of concrete. The joints in the planking were calked with oakum and daubed |