2 Movable Weirs

The addition of a foot-bridge facilitates the raising and lowering of these weirs and the regulation of the discharge, but it makes the weir more costly than the ordinary needle weir. Moreover, where large quantities of drift come down with sudden floods, the frames of the bridge are liable to be carried away and therefore boats must be relied on for working the weir as on the upper reaches of the Ohio river. In the United States the type is known as the Chanoine wicket. The Chanoine shutter is adapted for use both on overfalls and in navigable passes.

Bear-trap Weirs.

The earliest example of that form of the shutter weir known as the bear-trap, one of the most commonly used types in North America, was constructed in 1818 on the Lehigh river (Pa.). It consists of two timber—or, in recent types, steel—gates, each hinged on a horizontal axis, inclined towards one another and abutting together at an angle in the centre when the weir is raised. The up-stream leaf or gate serves as the weir and the down-stream one forms its support. Both gates fall flat on the sill floor when the weir is opened. The weir is raised by admitting water beneath the gates through culverts in connection with the upper pool and controlled by valves, and is lowered by letting the water under the gates escape into the lower pool. In its original form the bear-trap was open to several important objections such as the twisting or warping of a wide gate in raising or lowering, the friction between the leaves, and lodgement of driftwood or stones under them. Since about 1896 many improvements in the design of bear-traps have been made and there are numerous examples on North American rivers with leaves over 120 ft., wide, and with lifts up to about 17 ft.

Drum-weirs and Sector Gates.

The drum-weirs invented by Desfontaines and erected on the river Marne between 1857-67 comprise a series of upper and under wrought-iron paddles or blades which can make a quarter of a revolution round a central axis laid along the sill of the weir. By means of valves the pres sure of the water due to the head of the upper pool can be applied to either side of the lower and larger paddle, which is contained in a masonry chamber, or drum, below the weir apron, and thus the upper paddle can be raised against a head of water or lowered as required. The disadvantage of the arrangement is the high cost of constructing the deep chambers below the weir sill, and its use is generally restricted to over-fall weirs. Several examples of the type in modified forms and on a large scale have, however, been erected in Germany, one at Charlottenberg having an upper paddle 33 ft. long and 91 ft. high.

A form of drum-weir invented by an American engineer, H. M. Chittenden, has been used in the United States. An early example was erected about 1895 in a weir on the Osage river near its confluence with the Missouri where a hollow, wooden sector of a cylinder having a radius of 9 ft. rotates on a horizontal axis

and is housed when lowered in a drum chamber below the weir sill. The weir is raised by admitting water from the upper pool into a wedge-shaped space left below the sector when it is lowered. Provision is also made for rendering the sector buoyant by forcing air into it so that it can be raised when the head of water in the upper pool is insufficient to lift it.

The sector-gate applied to weirs may be said to be a develop ment of the Chittenden drum-weir. Two large gates, each ion ft.

long and about 16 ft. high, have been constructed across the Genesee river near Rochester, N.Y. The gates are, when lowered, housed in concrete chambers formed between the abutments below the level of the fixed-weir crest. The steel sector-frame which forms the gate is hinged on its axial line, and the plating fixed to the cylindrical face forms the water barrier when the sector is raised. The gate is operated by admitting water from the higher pool under the sector by means of culverts and valves. Somewhat similar gates are in use on the river Drac in France; on the Weser at Bremen; on the Chicago drainage canal; in Norway, where one gate is 163 ft. long; and in Perak.

Automatic Crest Gates or Weirs.

Several forms of gates or shutters which operate automatically have been devised, par ticularly in recent years, for use on the crests of solid weirs to give increased height. The common feature of these devices is the automatic lowering of the gate as soon as the water rises to a certain level. When the water falls below this level, the gate rises again to its normal position as a barrier.

The Stickney crest gate consists of two leaves joined together at about right angles and hinged to a masonry base on a horizontal axis. Under the lower leaf is a quadrant-shaped chamber formed in the concrete or masonry of the weir which is in communication by means of water openings with the up-stream pool. The areas of the leaves are so proportioned that the pressure against the under side of the lower leaf preponderates until the water rises above its normal level when the gate falls. These gates have been made in sections of over loo ft. in length and for heights of about 8 ft. For long weirs several crest gates, separated by piers, may be used. Several are in use on the New York State barge canal system.

Another form of automatic gate consists of single leaves hinged horizontally at the crest level of the solid weir and balanced by counter-weights carried by pivoted levers or by chains and pulleys on piers raised above the weir at intervals of about so ft., the spans between the piers being occupied by the hinged gates. Various ingenious devices at the fulcrum of the lever or in con nection with the pulley wheels are employed to vary automatically the balance as the gate rises and falls (Plate, fig. 4).