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

Fig. 42.

The “dam” across the Ravi, at the head of the Sidhnai Canal in the Punjab, also consists of sluice openings without a weir. The piers are connected by horizontal beams ([fig. 42]), against which, and against a sill at their lower ends, rest a number of nearly vertical timber “needles,” fitting close together, which can be removed when necessary by men standing on a foot-bridge. In floods the needles are all removed and laid on the high-level bridge (not shown in the drawing), the foot-bridge being then submerged. With needles the span between two piers can be greater than would be possible with a gate. Needles can be used up to a length of 12 or 14 feet, excluding the handle which projects above the horizontal beam. They can be of pine, about 5 inches deep in the direction of the stream, and 4 inches thick.

Where a branch takes off from a canal in India there are usually no fixed weirs but two sets of piers—one in the canal and one in the branch,—with openings and gates like those at the canal heads, or else with wider openings and needles. These works are called regulators. The gates are worked by travellers or by fixed windlasses or racks and pinions. Very small gates for distributaries are often worked entirely by screw gearing. For the smaller branches the gates are replaced by sets of planks or timbers lying one above another and removed by means of hooks. They are replaced by means of the hooks or by being held in position some little height above the water, and dropped. They are finally closed up by ramming.

In the case of either planks or needles, leakage can be much reduced by throwing shavings or chopped straw into the water upstream of them.

Needles can be provided on their downstream sides with eye-bolts just above the level of the beam against which their upper ends rest. They can then be attached by chains or cords to the beam or to the next pier, and cannot be lost when released. They can be released by a lever which can be inserted under the eye-bolt. By pushing the head of a needle forward and inserting a piece of wood under it, a little water can be let through. In this way, or by removing needles here and there, the discharge can be adjusted with exactness.

At a needle weir in an Indian canal all the needles in one opening are reported to have broken simultaneously. A possible explanation is that one needle broke and that the velocity thus set up in the approaching stream caused the others to break. On another occasion when a canal was dry all the needles were blown down.

Sometimes the beam or bar against which the upper ends of the needles rest is itself movable. At Ravenna, in Italy, the bar between any two piers has a vertical pivot at one pier and can swing horizontally. Its other end is held by a prolongation of the next bar, near to its pivot. If the end bar of the weir is released, each bar in turn is released automatically.

At Teddington on the Thames the oblique weir, 480 feet long, has thirty-five gates, which extend over half the length of the weir. They are worked by travellers which run on a foot-bridge. The openings do not extend down to the river bed, but are placed on the top of a low weir. The other half of the weir is fixed. The gates are raised to let floods pass.

At Richmond on the Thames the arrangements are similar, the gates being counterbalanced to admit of easy and rapid raising. When raised they are tilted into a horizontal position so as not to obstruct the view.

In Stoney’s sluice gates a set of rollers is interposed between the gate and the groove. The rollers are suspended from a chain, one end of which is attached to the top of the gate and the other end to the groove. The rollers thus move up or down at half the rate of the gate, and some of them are always in the proper position for taking the pressure. Escape of water between the gate and the groove is prevented by a rod which is suspended on the upstream side of the gate close to its end, and is pressed by the water against the pier. Stoney’s sluice gates, with spans ranging up to 30 feet, have been used on the Manchester Ship Canal for the sluices by which the water of the river Weaver is passed across the canal, and at locks for passing the flood waters of the Irwell and Mersey down the canal. The gates are balanced by counterweights.