4 Other Lifting Weirs

4. OTHER LIFTING WEIRS Bridge-suspended Weirs.—The difficulty of operating the Camere curtains in conjunction with movable frames hinged at the sill of a solid weir or apron led to the construction, as a part of the Poses weir on the lower Seine (completed in 1885), of a fixed overhead-bridge from which the frames and curtains are suspended. The suspended frames are hinged to the under side of the bridge and rest against a sill at the bottom when the weir is in operation. The rolling curtains are raised when necessary and the hinged-frames lifted to a horizontal position under the bridge by means of travelling winches and chains. This system, which has been employed for several weirs on the Seine, on the Moldau in Bohemia, and on the Danube canal near Vienna, has the merit of raising all the movable parts of the weir out of the water, in flood time, into a position where they are not subjected to the risk of damage, and rendering the working of the weir safe and easy. On the other hand it involves the expense of wide and high river piers and a substantial over-bridge.

A bridge-suspended weir with Camere curtains at the St. Andrew's rapids on the Red river, Manitoba, was the first of its kind to be built in America (1908-10. The bridge is of 6 spans of 120 ft. each and also carries a roadway.

The Mohawk river, forming a part of the New York State barge canal system, has been canalized by the construction of a number of dams, 8 of which are of the suspended weir type. At each of these weirs the river is crossed by a steel-truss bridge, usually of three spans, carried upon masonry and concrete piers. The clear spans vary in width from 120 ft. to 24o ft. Some of the bridges carry roadways as well as the weir-operating machin ery. The only fixed structure in the river other than the midge piers is a concrete sill level with the river bed. Against this sill bear vertical steel beams suspended from the over-bridge at inter vals of 15 ft. (see Plate, fig. 6). Steel doors 3o ft. wide mounted on rollers and arranged in two tiers can be moved up on and down on the up-stream faces of the hanging beams by means of travel ling electric winches carried by the bridge. During the navigation season the hanging beams are kept lowered and are only raised partly or entirely in cases of extreme freshets. The discharge of ordinary floods is provided for by raising the roller doors. At the close of the navigation season the hanging beams with their doors are raised to a horizontal position under the bridge floor to permit the free passage of ice.

Taintor Gates.—The Taintor gate is a form of sector gate but it differs from the type described in section 2 p. 489 in being raised, by means of hand-operated gearing and counterweights, in order to open the waterway. Many of these gates have been constructed in recent years, particularly on the New York State barge canal system. (See Plate, fig. 7.) Some of the larger gates are 5o ft. wide with a water depth of 20 ft. In common with some other forms of lifting gates they possess the advantage over low ering weirs of all working parts being accessible from above water.

Rolling Dams.

A form of movable weir consisting essen tially of a rolling gate was first employed on a tributary of the river Main near Schweinfurt, Bavaria, in 1901. It consists of a steel drum or pipe, usually cylindrical and sometimes with a pro truding rib to increase the retaining height, which closes the open ing in a fixed weir when required and which, when the weir is to be opened, can be rolled by mechanical means up inclines at the abutments, leaving a clear opening underneath. Several of these weirs have been built in Germany, Bohemia, France, Italy and the United States. They have been made up to ii5 ft. in length and with closing heights up to 20 ft.

One of the longest weirs on the Ohio river is a navigable pass, 1,200 ft. long, closed by Chanoine wickets, an overfall weir of Chanoine wickets 700 ft. in length, three bear-traps occupying 325 ft. and a fixed weir of 715 ft. making a total of 2,940 feet.

An example of a long weir with over-bridge is the Lloyd bar rage on the Indus at Sukkur in Scinde (1929) which has 66 Stoney gates each of 6o ft. span and 181 ft. in height. The total length of the weir, including the piers, is 4,620 feet.

BIBLIOGRAPHY.-See list of works at end of article CANALS AND CANALIZED RIVERS, in particular Improvement of Rivers by Thomas & Watt. Also E. Wegmann, Design and Construction of Dams, 8th edn. (1927) bibl.; W. G. Bligh, Irrigation Works, 3rd edn. (1927) ; W. L. Stronge, Indian Storage Reservoirs, 3rd edn. (1928). The annual Reports of the Chief of Engineers, U.S.A., are invaluable for informa tion as to weirs, etc., on rivers in the U.S.A., as also are the Annales des Ponts et Chaussees for French rivers. The Proceedings of the Inter national Navigation Congresses should also be consulted.

(N. G. G.)