DAMS AND BESEKVOIBS. 761 DAMS AND BESEKVOIRS. with asphalt. The Lower Otay Dam, near Sau Diego, Cal., comjjleted in IS'JT, is a roek-till dam with a steel core. The dam was started in masonry, but being carried to a height of -tO feet above its lowest point, when its top length was only 83 feet, it was decided to change the design. An inverted T-iron ( thus, _l ) was bolted to the masonry and steel plates one-third of an inch thick, 17 Vi feet long, and 5 feet high were riveted first to the T-iron, then to each other until three courses had been placed. The plates were dimin- ished in thickness as they neared the top. The dam is IGl feet high above its lowest point, 130 feet high above the natural earth, and is of rock fill for 121 feet. The steel plates were protected by a coat of hot Alcatraz asphalt, then a layer of burlap, then harder asphalt, and finally one foot of Portland cement concrete, on each side. A part of the rock fill was deposited in place by the force of a very heavy blast and the rest was transported from the quarry by a cableway (q.v.) 948 feet long. Nearly 180,000 cubic yards of stone were used. The stream flow must be passed around and not over rock-fill dams. .Steei, Dams have recentlj' been built. One of the first of these, known as the Ash Fork, was built about 1897 or 1898, by the Atchison, To- peka and Santa Fe Kailway Company to supply face ci- Dam-r Fig. i. — CROSS-SECTION OF STEEL, DAM, Xah. Fork, '..riz. its engines and incidentally to furnish water to the village of Ash Fork, Ariz. It forms a reser- voir of 30,000,000 gallons capacity. The steel portion of the structure is 184 feet long at the top and its greatest lieight is 46 feet. It con- sists of a series of triangular steel frames, against the upper sides of which rest the riveted steel plates, three-eighths of an inch thick. These plates are curved so as to form a series of clian- nels from the top to the bottom of the inclined face of the dam, with flat strips between. Water flows over the top of the steel dam and between two masonry abutments having a combined length of llfi feet. k 40'0 Fig. 5. — CROSS-SECTION op timber and stone dam, Bangor, Me. Timber Dams include a great variety of struc- tures built of framed timber, logs, and crib-work of either timber or logs filled with stone. They are generally comparatively low, overflow dams. They frequently have a sloping up-stream face and eitlier a sloping or stepped down-stream face, with an ajjron below the toe of the latter, to break the force of falling water. Sometimes piles are used in tlicir construction and often earth is filled against their upper side. The once famous timber dam at llolyokc, Mass., built in 1849 for water-power, 1017 feet long and with a ma.ximuni heiglit of 30 feet, has been replaced by a masonry structure. ilovADLE Dams are those which can be lowered or raised at will, according to the stage of water in the river. They are generally aids to naviga- tion, placed at stretches where there are shallows or rapids, but where permanent structures to raise the water-level might do damage by causing floods at times of high water. They may also be used on any ordinary dam, or on waste weirs. It is more common, however, to call movable de- vices connected with ordinary dams flashboards or floodgates. Flashboards are generally com- paratively low and are of fragile construction, or have supports designed to give way in time of freshet. There are a great variety of movable dams, but they fall more or less closely into three groups — needle, iciclcet or shutter, and hrnr- tiai). The latter have some decided points of superiority, being raised and lowered by the force of the water itself, on its being turned under or discharged from chambers beneath or within the dam. Needle dams were developed in France about the close of the eighteenth cen- tury. They are an outgrowth of the earlier I'reneh and English needle dams and consist of horizontal beams, or stop planks, dropped into grooves built in the two abutments of a pass through the dam. These beams could be lifted out at times of high water. Later, to facilitate handling, they were set vertically, or nearly so, resting against a sill below and a beam above. A chain was finally substituted for this beam to make greater lengths of dam feasible. In this way movable dams 40 feet wide were developed on the Yonne. in France. In 1834 M. Poiree in- creased the width of one of these dams, or passes, to 72 feet by substituting iron bars for the chains. The bars were short and were supported by means of vertical, iron frames, placed at right angles to the length of the dam. To throw down the dam, it was only necessary to remove the needles one by one, detach the horizontal bars, tlien lower the frames into recesses in the top of the masonry portion of the dam. The needle dams were somewhat modified subsequently and used in various parts of France, in Belgium, in Germany, and in the United States. The first needle dam in this country was built in 1891-97 by the United States Government across the Big Sandy River at I..ouisa, Ky. The whole improve- ment includes a lock 52 feet wide, a navigable pass 130 feet long, and an overflow weir 140 feet long. The sill of the pass is 13 feet and that of the weir is 7 feet below the normal height of water in the pool, and the sill of the pass is one foot below low-water mark of recent years. The steel frames supporting the pass needles are four feet between centres. The horizontal bars connecting the frames and supporting the upper ends of the needles are hinged at one end and hooked at the other. The frames have a sheet- iron floor, forming a foot-bridge, which falls with them, and are connected by a chain. The pass needles are of white pine. 12 inches wide. SI/2 inches thick at the bottom, and 4% inches thick
