CHAPTER VIII
THE TRAINING AND CANALISATION OF RIVERS
1. =Preliminary Remarks.=--When a stream is trained or regularised it is generally made narrower, but sometimes narrow places have to be widened. Deepening has also very frequently to be effected. The object of training is generally the improvement of navigation, but it may be the prevention of silt deposit. Some natural arms of rivers which form the head reaches of canals in the Punjab are wide and tortuous, and they are sometimes trained. Training often includes straightening or the cutting-off of bends, as to which reference may be made to CHAP. VII.
2. =Dredging and Excavating.=--When a flowing stream is to be deepened, the work is usually done by dredgers. Dredgers can remove mud, sand, clay, boulders, or broken pieces of rock. The “bucket ladder” dredger is the commonest type. The “dipper” dredger is another. Both these can work in depths of water ranging up to 35 feet. The “grab bucket” dredger can work up to any depth and in a confined space. The “suction dredger” drawls up mud or sand mixed with water. A dredger may be fitted with a hopper or movable bottom, by means of which it can discharge the dredged material--this, however, involves cessation of work while the dredger makes a journey to the place where the material is to be deposited--or it can discharge into hopper barges or directly on to the shore by means of long shoots. For small works in comparatively shallow water the “bag and spoon” dredger, worked by two men, can be used.
When rock has to be removed under water it is blasted or broken up by the blows of heavy rams provided with steel-pointed cutters.
In widening a channel the excavation can be carried down in the ordinary way to below the water-level, a narrow piece of earth, like a wall, being left to keep the water out. If the channel cannot be laid dry, the work can be finished by dredging.
Regarding methods by which the stream is itself made to deepen or widen its channel, reference may be made to CHAP. V.
3. =Reduction of Width.=--If a channel which is to be narrowed is not a wide one, the reduction in width can be effected by any of the processes described under bank protection (CHAP. VI.). But in a wide channel, reduction of the width by any direct process is generally impracticable. The expense would generally be prohibitive. Earth, if filled in, is liable to be washed away unless protected all along. Reduction in the width of a large channel is nearly always effected either by groynes (fig. 26) or by training walls (fig. 27). Spurs or short groynes for bank protection have been already described (CHAP. VI., _Art. 2_). Groynes for narrowing streams are made in the same way and of the same materials, but are longer. They are at right angles to the stream or nearly so. Groynes in the river Sutlej have been mentioned in CHAP. V., _Art. 5_, and are shown in fig. 6, p. 55. Whether groynes or training walls are used, the object is to confine the stream to a definite zone and to silt up the spaces at the sides. These spaces when partly silted can be planted with osiers or with anything which will grow when partly submerged, and this will assist in completing the silting.
A training wall can be made of any of the materials used for groynes. In order to silt up the spaces between each wall and the adjacent bank of the stream, other walls are run at intervals across them. Usually the training walls and cross walls are carried up only to ordinary water-level, sometimes only to low-water level. Floods can thus spread out and submerge the walls and deposit silt. If the walls are carried up too high it may be necessary, in order to give room for floods, to space them too far apart, and this, as will be seen below, is objectionable.
The difference between training walls and groynes is one of degree rather than one of kind. The material most commonly used is, in either case, loose stone--with pitching, if desired, above low-water level,--but it may be wattled stakes. If the water of the stream contains silt at all stages of the supply, gaps can be left in training walls so that silt deposit may occur at all times and not only in floods. If the walls are of wattled stakes, water will pass through them, and it may not be necessary to leave any gaps. Groynes are frequently made with =T=-heads (fig. 26), and they are thus equivalent to training walls with long gaps in them. The edge of the narrowed channel usually forms somewhat as shown in the figure. If the groynes are placed so near together as to give a regular channel, the cost is not likely to be much less than that of training walls.
The alignment of training walls or groynes should be such as will give the best channel consistent with economy in cost. The best channel is generally that which is most free from sharp bends. It is assumed for the present that no cuts or diversions of such lengths as to materially alter the gradient are to be made, but that a certain amount of choice of alignment is afforded by the reduced width of the trained channel and by small diversions or easings of bends. It is sometimes said that straight reaches are objectionable because the stream will tend to wander from side to side and cause shoals, whereas in a bend there will be no such tendency. The difficulty as to shoaling will be greatest at low water, but it is likely to be serious only when the width between the training walls is too great. If the width cannot be reduced to such an extent as to do away with the trouble, it may be better to adopt a curved course. The width between the training walls should generally be the same throughout, whether the reaches are straight or curved, but in view of the preceding remarks it may be desirable, where a reach cannot be otherwise than straight and where shoaling is feared, to give the straight portion a reduced width with of course a greater depth, and similarly to reduce the width at reverse changes of curvature. In curves which are at all sharp the curvature should be rather sharper in the middle of the curve than at the ends (CHAP. IV., _Art. 8_).
4. =Alteration of Depth or Water-Level.=--When the width of a stream is altered, the depth of water--the gradient being supposed to be unchanged--must alter in the opposite manner. A narrowing of the channel by training necessitates an increase in the depth of water, and the same remark applies if an arm of the stream is closed. The increase in depth may be effected either by raising the water-level or by lowering the bed--as may be convenient--or both. If the bed is to be lowered and is of hard clay, it may be necessary to dredge it and, when this has been done, training may be unnecessary. If the bed is of soft mud, a dredged channel is likely to fill up again, and training alone will be the method to adopt. If the bed is moderately hard, say compact sand, it may be suitable to train the channel first and then to dredge if necessary. In any case, shoals of hard material may have to be dredged or rocks, whether these form shoals or lateral obstructions, to be blasted or otherwise broken up (_Art. 2_). In cases where it is desired to raise the water-level without any lowering of the bed, training is of course necessary. In any case in which the bed is likely to scour to a lower level than is desired, or if the bed is to be raised, the measures described in CHAP. V., _Art. 6_, may be adopted, but they are hardly likely to be suitable and satisfactory in all cases.
5. =Training and Canalising.=--The steps so far described, together with any of those described in CHAPS. V. and VI., exhaust the list of what can be done so long as only the cross-section of a stream is dealt with. This is often called the “regulation” of a stream, though “training” is a more satisfactory term.[10]
A mere alteration of the cross-section of a stream will not always afford a solution of the problem to be solved. Frequently a change of gradient is required. The gradient can be steepened by means of straightenings, or flattened by introducing weirs, or perhaps by adopting a course somewhat more circuitous than was intended. This extended scope of operations is known as canalising in the case of a river, and remodelling in the case of a canal.
Suppose that it is desired to alter the cross-section of a stream, at ordinary water-level, so as to reduce the width and increase the depth (fig. 28). If the mean depth is doubled, the new width will be about equal to 1/(3·2) of the old width (_Hydraulics_, CHAP. VI., _Art. 2_). If this gives too narrow a channel, it may be desirable to flatten the gradient. If it gives too wide a channel, the gradient can be steepened or a greater depth adopted. While the width and depth of the stream will be fixed so as to be suitable for the navigation, the ratio of depth to velocity should be so arranged, if this is possible, as to minimise trouble connected with silting or scour (CHAP. IV., _Art. 6_). A remodelled channel is, in short, designed in exactly the same way as a new channel. The depth of water exercises the greatest effect on the discharge, and the gradient the least. The weak point in a scheme which includes weirs is the difficulty of dealing with floods. A scheme perfect in all other respects may be vitiated because of the obstruction, caused by weirs, to the passage of floods. The difficulty is got over by means of movable weirs. The whole subject of weirs is dealt with in CHAP. X.
Training or canalising should not be effected in any reach of a stream without regard to other reaches. A mere local lowering of the water-level by dredging may accentuate the effect of a shoal at the upper end of the reach.
When the water-level is raised by a weir or by narrowing the channel--though in the latter case the raising may not be permanent--it is generally best to commence the work from the upstream end. The raising of the water-level will then not interfere with the execution of the rest of the work. But in a case of widening, where the water-level upstream of the work is lowered, the work can conveniently be begun at the downstream end, and the remark applies also to a case of straightening, provided that the new channel is not so small that it at first causes no lowering. In any case in which there is a doubt whether the whole of the scheme will be carried out, the reach to be dealt with first can be decided on according to circumstances. There is no general reason for selecting an upstream or downstream reach, except that any raising or lowering of the water-level will extend upstream of the reach and not downstream of it (CHAP. I., _Art. 4_).
Training walls and groynes, if made with stakes or fascines or any materials except stone, require careful watching and maintenance.