River and Canal Engineering, the characteristics of open flowing streams, and the principles and methods to be followed in dealing with them.

by E. S. Bellasis

Frame weirs,[13] used chiefly on rivers in France but also in Belgium and Germany, are a modification of the needle and plank arrangements above described. For the masonry piers there are substituted iron frames or trestles, which are hinged at the floor-level so that, when the timbers have been removed, the frame can be turned over sideways and lie flat on the floor, thus leaving the waterway absolutely clear from side to side of the stream. The foot-bridge which rests on the frames is removed piece by piece. The frames are raised again by means of chains attached to them. In order that the frames may not be too heavy they are spaced 3 to 4 feet apart, or very much nearer than when masonry piers are used. Horizontal planks can thus be used of shorter lengths than the needles, and they can be made up into greater widths so that the leakage is less.

A further modification consists in placing the bridge platform above flood-level, and in hinging the frames to it instead of to the floor. The frame turns about a horizontal axis parallel to the length of the weir. A weir of this kind can be used for greater depths of water than the ordinary frame weir.

In some cases the horizontal planks are connected together by hinges so that they form a “curtain.” The curtain is raised by rolling it up by means of a traveller. It admits of rapid and accurate adjustment of the water-level, but there is considerable scouring action below a curtain when it is somewhat raised.

Fig. 43.

6. Falling Shutters.—In Thénard’s system, first used in France, a shutter ([fig. 43]) is hinged at its lower edge and is held up by a strut. When the lower end of the strut is pushed aside it slides downstream and the shutter falls flat. To enable the shutter to be raised again an upstream shutter, which ordinarily lies flat and is held down by a bolt, is released, and it is then raised by the current to the extent permitted by a chain attached to it. The downstream shutter is then raised. Thénard’s system was not much used in France because the river had to fall to a level somewhat too low for navigation before the shutters could be raised. The sudden jerk on the chain of the upstream shutter is also liable to do damage. The system has been adopted on some of the long weirs which cross Indian rivers downstream of the heads of irrigation canals. To prevent damage by shock, a hydraulic brake was designed by Fouracres. It consists of a piston which travels along a cylinder and drives water out through small holes. The shutters are placed on the top of the fixed weir, where they usually lie flat, except in the low water season, any adjustments of the river discharge being effected by means of the under-sluices.

In the Chanoine system of falling shutters ([fig. 44]), used first in France, the shutter is hinged at a point rather higher than the centre of pressure. The hinge is supported by a vertical trestle, which is hinged at its lower end and is supported by a strut which slides in a groove and rests against a stop. When the water rises to a certain height above the top of the shutter, it is turned by the force of the water into a horizontal position. The struts can then be pushed sideways out of the stops by means of a “tripping bar,” which lies along the floor parallel to the line of shutters and is worked from the bank. The struts, trestles, and shutters then fall flat. To close the weir the shutters are first raised into the horizontal position which they occupied before falling, by means of a hook worked from a boat or by chains attached to a foot-bridge running across the river upstream of the weir. They can then be easily closed by a boat-hook. The water closes them of itself if it falls low enough.

Fig. 44.

When the shutters fall a great rush of water occurs. To obviate this a valve is made in the upper half of the shutter. It consists of a miniature shutter on the same principle as the main shutter. The pivot of the main shutter is made at such a height that the shutter will not turn over when only a small depth of water flows over it. Instead of this the valve comes into operation. The valve also facilitates the raising of the shutter. Again, instead of the tripping bar, which would sometimes have to be of great length or be liable to damage owing to stones jamming in its teeth, the shutter can be released by pulling the strut upstream so that it falls into a second groove, down which it slides. When a tripping bar is used, its teeth can be so arranged that the shutters are released a few at a time, first singly, then in twos and threes. Sometimes there are gaps of a few inches between one shutter and the next, and the gaps can be closed by needles if necessary.