Instead of running out at right angles to the bank a spur may be inclined somewhat downstream. This somewhat reduces the eddying and scour round the end. The ends of a system of spurs should be in the line which it is intended that the edge of the stream shall have ([fig. 9]). The tops of short spurs are usually above high flood level. Sometimes spurs are made to slope downwards ([fig. 10]), and they then cause less disturbance of the water and less scour than if built to the form shown by the dotted line. Such spurs are sometimes combined with a low wall running across the bed of the stream, the whole forming a “profile” of the cross-section to which it is intended to bring the channel. Regarding such walls, see [Chap. V., Art. 6]. When a spur is long it may have small subsidiary spurs ([fig. 11]) to reduce the rush of water along it; or its end may have to be protected in the same manner as the advancing end of a closure dam ([Chap. VII., Art. 2]).
The following is a curious case of misconception of the action of spurs. In 1909 the river Indus was eroding its right bank and threatening to destroy the town of Dera Ghazi Khan. A clump of date palms formed a promontory and resisted erosion to some extent. A suggestion was made—by an engineer of eminence who had formerly been consulted in the case—to the effect that the date palms be removed, the reason given being that they caused disturbance and scour. On this principle spurs would have to be made not to protect a bank but to cause it to be eroded.
3. Continuous Lining of the Bank.—The lining or protection of a bank may be of stone or brick pitching (figs. 12 and 13), loose stone ([fig. 14]), fascines ([fig. 15]), turfing, plantations, brushwood, or of other materials laid on the slopes. Before protecting a bank it is best to remove irregularities and bring it to a regular line. This can generally be done most easily by filling in hollows, but sometimes it is done by cutting off projections. It is also necessary to make the side slope uniform. Where the slope is as shown by the dotted lines in figs. 12 to 14, filling in can be effected, but cutting away the upper part of the slope is also feasible. Such cutting away has been proposed as a remedy in itself in cases where the steep upper part of the slope was falling in, but it is not much of a remedy.
Stone pitching may rest, if boats are required to come close to the bank, on a toe wall of concrete, as in [fig. 13],[9] or otherwise on a foundation of loose stone, as in [fig. 12]. When concrete is used the bed is dredged to such a depth as will provide against undermining by scour. Sloping boards attached to piles are placed along the front face and the concrete is thrown in under water. The slope of stone or brick pitching is usually from 2 to 1 to 1 to 1, but it may be as steep as ½ to 1. The earth behind the pitching must be well rammed in layers. In order to prevent the earth from being eaten away by the water which penetrates through the interstices of the stone or brick, a layer, 3 to 6 inches thick, of gravel or ballast is placed over the earth and rammed. When loose stone is used, dredging is not necessary, but the stone is allowed to gradually sink down and more is added at the top. A certain proportion of the stones should be of large size.
Fig. 12.
Fig. 13.
When fascining is used, long twigs are made into bundles and tied up at every 2 feet so as to form fascines about 4 to 6 inches thick, and these are laid on the slopes and secured by pegs driven in at short intervals, between the fascines.
