4. Heavy Stone Pitching with Apron.—On the great shifting rivers of India a system of bank protection is adopted, consisting of a pitched slope with an apron ([fig. 22]). The system is used chiefly in connection with railway bridges or weirs, but it has been used in one instance, that of Dera Ghazi Khan, for the protection of the bank near a town. When, as is usual, the flood-level is higher than the river bank, an artificial bank is made. In any case the bank is properly aligned. The pitching has a slope of 2 to 1, and consists of quarried blocks of stone loosely laid, the largest blocks weighing perhaps 120 lbs. The apron is laid at the time of low water on the sandbank or bed of the stream. If necessary, the ground is specially levelled for it. It is intended to slip when scour occurs. The following dimensions of the apron are given by Spring (Government of India Technical Paper, No. 153, “River Training and Control on the Guide Bank System,” 1904). The probable maximum depth of scour can be calculated as explained in [Chap. XI., Art. 3]. If this depth, measured from the toe of the slope pitching is D, and if T is the thickness considered necessary for the slope pitching, then the width of the apron should be 1·5 D, and its thickness 1·25 T next the slope and 2·8 T next the river. It will then be able to cover the scoured slope to a thickness of 1·25 T. This thickness is made greater than T because the stone is not likely to slip quite regularly. The thickness T should, according to Spring, be 16 inches to 52 inches, being least with a slow current and a channel of coarse sand, and greatest with a more rapid current and fine sand; but since the sand is generally finer as the current is slower, it would appear that a thickness of about 3 feet would generally be suitable. Under the rough stone there should be smaller pieces or bricks. Along the top of the bank there is generally a line of rails so that stone from reserve stacks, which are placed at intervals along the bank, can be quickly brought to the spot in case the river anywhere damages the pitched slope.
For the special protection to banks required near weirs and similar works, see [Chap. X., Arts. 2] and [3].
CHAPTER VII
DIVERSIONS AND CLOSURES OF STREAMS
1. Diversions.—When a stream is permanently diverted the new course is generally shorter than the old one, and the diversion is then often called a cut-off. The first result of a cut-off is a lowering of the water-level upstream and a tendency to scour there, and to silt downstream of the cut-off. [Fig. 23] shows the longitudinal section of a stream after a cut-off A B has been made. The bed tends to assume the position shown by the dotted line. If both the diversion and the old channel are to remain open, the water-level at the bifurcation will be lowered still more, and the tendency to scour in the diversion will be reduced.
Fig. 23.
If the material is soft enough to be scoured by the stream, it is often practicable to excavate a diversion to a small section and to let it enlarge itself by scour. This operation is immensely facilitated if the old channel can be closed at the bifurcation. The question whether the scoured material will deposit in the channel downstream of the diversion must be taken into consideration; also the question whether the diversion will continue to enlarge itself more than is desirable. The velocity in the diversion will be a maximum if its section is of the “best form,” i.e. if its bed and sides are tangents to a semicircle whose diameter coincides with the water surface, but this may not ([Chap. IV., Art. 6]) be the section which will give most scour. In order to prevent the enlargement of the diversion taking place irregularly, the excavation can be made as shown in [fig. 24], water being admitted only to the central gullet. The side gullets should not be quite continuous, but unexcavated portions should be left at intervals, so that if the water in scouring out the channel breaks into the gullet, it will not be able to flow along it until it has broken in all along.
Fig. 24.
If a diversion is made, not with the object of lowering the water-level but merely in order to shorten the channel, the increased velocity caused by the steepened slope may be inconvenient. In this case a weir or weirs can be added ([Chap. VIII., Art. 4)].