Irrigation works

by E. S. Bellasis

On inundation canals the rules regarding outlets have to be modified. Great numbers of watercourses take off directly from the canals. In such cases, especially near the head of a canal, the ground to be watered is often 5 to 8 feet above the canal bed and it is wholly unsuitable to place the outlet at bed level. The cost of the tail wall would be excessive. The floor level in such cases must be at about the lowest probable cleared bed level of the watercourse, say, in order to be safe, a foot or half a foot below the usual cleared bed of the watercourse, so that water need never be prevented from entering the watercourse. The irrigators should be consulted as to the floor level and their wishes be attended to as far as possible. For lift outlets the floor should be at the bed level of the canal or distributary. If this bed is to be raised in the course of remodelling, the floor should be at the old bed level until the bed has actually been raised, unless there is a weir which raises the water. It is necessary that lift outlets should work however small the canal supply may be. In a distributary or small canal, the head wall should be built up to F.S. level but in a canal with deep water the head wall should reach up to just above the roof of the outlet and be submerged in high supplies. The stepping of the head wall should be set back if the channel is to be widened and should project into the channel if the channel is to be narrowed. The centre line of the channel near the outlet site must always be laid down and the outlet built at right angles to it and also at the correct distance from it.

Occasionally there is a wide berm, say 20 ft. or even 50 ft., between a channel and its bank. In such a case the outlet should be built to suit the bank. The long open cut is however objectionable because the people clear it and heap the spoil in Government land. Sometimes the bank, especially if it is crooked, can be shifted so as to come close to the channel at the outlet site. Sometimes the outlets on inundation canals are large. For outlets of more than 2·5 square feet in area, grooves should be provided so that the cultivators can use a gate if necessary.

12. Masonry Works.

—The positions and descriptions of all the masonry works of a proposed canal or distributary are of course shown on the longitudinal section of the channel and from this the discharges and water levels are obtained. The principles of design to be followed[19] for bridges, weirs, falls, regulators and syphons, are discussed in River and Canal Engineering. It is mentioned that there is no special reason for making the waterway of a regulator exactly the same as that of the stream, and that the waterway may be such as to give the maximum velocity considered desirable, and that the foundations of a bridge should be made so deep that it will be possible to add a floor, at a lower level than the bed of the stream—with the upstream and downstream pitching sloping up to the bed—so as to increase the waterway and so save pulling down the bridge in case the discharge of the channel is increased. It remains to consider certain points affecting Irrigation Canals.

[19] So far as concerns their capacity for dealing with flowing water.

The span of a bridge, where there are no piers, is generally made as shown by the dotted lines in [Figure 14], so that the mean width of waterway is the same as that of the channel. The arches, in Northern India, used at one time to be 60° as shown by the upper curved line, but in recent years arches of 90° as shown by the lower curved line, have frequently been adopted, the springing of the arch being below the F.S. level, so that the stream is somewhat contracted. The 90° arch gives a reduced thickness and height of abutment. It causes increased disturbance of the water, and this may necessitate more downstream protection. An advantage of having the springing not lower than the F.S. level is that this admits of a raising of the F.S. level in case the channel is remodelled, and this arrangement is still common on distributaries.

Fig. 14.

When a fall and bridge are combined, the bridge is placed below the fall as this gives a lower level for the roadway. The side walls of the fall are produced downstream to form those of the bridge.

The roads in India are generally unfenced and the banks of canals close to bridges, on both sides of the canal and both above and below the bridge, are generally more or less worn down by cattle, which, when being driven home in the evening and out to graze in the morning, go down to the stream to drink. In order to prevent this damage the banks are sometimes pitched, above the bridge as well as below it, but the cattle generally make a fresh “ghát” further away. The best plan is to allow a “ghát” on one bank either above or below the bridge and to protect the other three places.