Bridge dams consist of a permanent bridge with the openings between the piers closed by some form of gates supported directly by the piers. The type is an evolution from the old used in mill practice, set vertically or horizontally. If vertically it is called a needle dam. The (needle( is a piece of light wood, rectangular, and shaped into a handle at top. These are placed in juxtaposition against a sill on the floor of the pass or on the weir and against a girder above water level. The needles are usually designed for one man to handle, and their application is thus limited to needles 434 inches square, and about 16 feet long, weigh ing about 100 pounds. It is properly auxil iary in some other type of movable for the purpose of regulating pool level in moderate flows of water. A needle dam has been built, however, on the Big Sandy River in which the needles are 12 inches square and 18 feet long, and weigh 863 pounds. They are handled by a derrick mounted on a boat.
The shutter or wicket dam is thus far the favorite for river work This consists of shut ters several feet in width and of the length de sired set side by side, the lower edge resting against a sill and supported by a prop behind, the shutter tipping automatically and falling to the river-bed when the depth over top exceeds a certain limit. The shutters are raised by means of a windlass from a service bridge or boat. This type of dam is used on the Ohio and Great Kanawha rivers. The dams on the Ohio River improvement are mostly of Chanoine wickets in their navigable passes, with pool regulating weirs of bear-trap type. The length of these wickets varies from 9 feet 9 inches for the lowest weirs to 18 feet for the highest passes. The wickets are 3 feet 9 inches in width, set with 3-inch openings between which they are closed by pieces of scantling in time of low water. The vertical height of the bear-trap dams varies from 9 feet to 15 feet. The usual width of the navigable passes is 600 to 700 feet, and of the bear-trap dams 91 to 94 feet, with ex tremes of 52 feet and 120 feet.
Roller dams are formed by a massive cylin der having a diameter equal to the desired height. Opening the dam is accomplished by rolling the cylinder bodily up inclined tracks laid at both ends of the dam. In a roller dam recently built at Schweinfurth, Germany, the cylinder is 13 feet 6 inches in diameter, and closes an opening 59 feet across.
Frame dams are essentially bridge dams, in which the bridge itself is movable. In its sim plest form it consists of a series of A-shaped frames set closely side by side. These are ar ranged to drop down sideways nesting one in another. They are raised by a chain connecting
thepeaks of the A-frames.
Curtain dams have an operating bridge with a series of frames hinged to the bridge at the top. These can be lowered till the other end abuts against a stop on the dam floor. The actual closure is effected by curtains of hori zontal bars of wood hinged together on their upstream side, the lower bars being heavier. A rolling (shoe( of iron is attached to the lowest bar, and an endless chain passes around the curtain at its centre. The curtain is rolled up by a windlass operating the chain. At the Port Villez Dam across the Seine 90 miles be low Paris is the original curtain dam, 700 feet long and 10 feet 6 inches high.
The Taintor gate has been much used for closing passes and chutes and has been built in sections 20 feet long and for a depth of 10 to 12 feet. This is a sector of a cylinder against the water, the centre of motion and support being below the centre of figure, so that the water pressure shall nearly balance the weight and assist in raising the gate. Some dams have been built for power purposes in which the flood flow is practically controlled by Taintor gates.
The reverse Taintor or rigid bear-trap is a sector of a cylinder in the form of an inverted trough, having one side straight, and the other curved to the radius of the straight side. The free edge of the straight side is hinged to the dam floor, so that the whole structure can re volve, sinking into a well in the floor. The curved side and ends of the ctroughp make an approximately watertight contact with the ma. sonry. Water from above the dam can be ad mitted to the under side of the sector, raising it until the water pressure balances the weight of river te. The ver flows over the crest, and the straight side. This type is in use a e power plant of the sanitary district of and is under construction in the i 'yes at Rochester, N. Y.
Under- this classification belongs also the butterfly dam in the Chicago Drainage Canal, the only structure of its kind. The swinging leaf is 184 feet long and 30 feet high, and weighs 710 tons. It consists of a girder faced with steel plates on the upstream side. It is pivoted at the centre, and so as to swing into the line of the flowing stream when open. The maximum Pressure against the bottom pivot when the dam is closed is 1,888 tons; on the top pivot 870 tons. The dam is opened by re leasing the lockbar blocking the downstream end, and then opening several shutters in the end that swings up stream: the pressure thus reduced on that end, the pressure on the other end swings the dam open—with some mechan ical aid.