Irrigation works

by E. S. Bellasis

1. Headworks.

—In the design of head works no very precise rules can be laid down. Some general ideas can however be given as to the chief points to be attended to and some general and approximate rules stated. In every case a large scale plan of the river is of course required and also a close examination of it and study of its character. An attempt to forecast its action is then possible. Gauge readings for several years and calculations of discharges are of course necessary. If the bed of the river, in course of time, rises upstream of the weir or scours downstream of it, a large amount of protection to the bed and banks will become necessary. Some description of headworks and weirs, with a plan of the headworks of the Sirhind Canal, India, has been given in River and Canal Engineering, Chapters IV. and X. Remarks regarding the collection of information for such works are given in Chapter II. of the same work. It is also explained how, by keeping the gates of the under-sluices closed, a “pond” is formed between the divide wall and the canal head so that heavy sand deposits in the pond and does not enter the canal. By closing the canal and opening the under-sluices the deposit is scoured away.

The best site for the headworks of a canal depends on the stability and general character of the bed of the river but in deciding between any two proposed sites, the question of the additional cost of the canal, if the upper site is adopted, has to be taken into account. Such cost may, in rugged country, be considerable.

In the case of Indian perennial canals, the head is often close to the hills where the river bed is of boulders and shingle and fairly stable, but it is often at a distance from the hills and in such cases a gradual rise in the bed of the river, even in the absence of a weir, is more probable than scour. Such a rise may necessitate a raising of the crest of the weir and of the bed of the canal.

Fig. 5.

In the general arrangement of a headworks a great deal depends on local conditions. Sometimes the river runs in a fairly straight and defined channel and the weir can then be run straight across it. Sometimes, as in the case of the Ganges Canal, there is a succession of islands and various short weirs are required in the different channels. At the heads of the Eastern and Western Jumna Canals, the river, on issuing from the hills, widens out ([Fig. 5.]) and the weir is built obliquely and not in a straight line. Its crest is higher at the east than at the west side. There are under-sluices at both sides. The upstream end and west side of the island are revetted. The old head of the Western Jumna Canal, as shown in the figure, existed long before the advent of the British, and a temporary weir, made of gabions filled with stones, was constructed across the river every year during the low water period and swept away during the floods. To have carried the weir along the line shown dotted, the head of the Western Jumna Canal being of course brought up to it, would apparently have been feasible and cheaper, but the off-take would have been in shallow water because of the curve in the river, and there would have been no current along the face of the head regulator of the canal.

The level of the floor of the under-sluices is generally about the same as that of the bed of the canal. The sill—made to exclude shingle and sand as far as possible—of the canal head regulator may be 3 feet higher and the crest of the weir 6 to 9 feet higher. The top of the weir shutters is 1 to 2·5 feet above the F.S. level of the canal which may be 5 feet or more above the bed of the canal. If the weir is provided with falling shutters the width of the waterway of the under-sluices may be about ¹⁄₁₂th of the width of the waterway of the weir alone, otherwise about ¹⁄₈th.

In nearly all cases the weir has a flat top and flat slopes both upstream and downstream. In a case where the river bed is of sand, the depth of water on the crest of the weir in floods may be 15 feet and the velocity 14 or 15 feet per second. The downstream slope of the weir may be about 1 in 15, and the upstream slope 1 in 6. Where the river bed is of boulders the velocity may be still higher. The faces of the weir are usually of hammer-dressed stone. A lock for the passage of rafts is added if necessary.

Unless the banks of the river are high, it is necessary to construct embankments to prevent the river water, when headed up by the weir during the floods, from spilling over the country with possible damage to the canal. If the river has side channels they have to be closed. The stream may also have to be trained, by means of guide banks or spurs, so as to remain in one channel and flow past the canal head and not form shoals against it. Where the river is unstable, it may shift its course so as to strike the weir obliquely and this may cause excessive heading up at one side of the weir. In such cases it is usual to divide the weir into bays or sections, each about 500 feet long, by “divide walls” running at right angles to the weir.