River and Canal Engineering, the characteristics of open flowing streams, and the principles and methods to be followed in dealing with them.

by E. S. Bellasis

It does not, of course, follow that lowering the bed is always the best plan and straightening the worst. Any one of the processes may be more or less impracticable because, for instance, of the hardness of the material to be removed, or the expense—including compensation—of removing obstructions.

A particular kind of widening consists in digging a new channel and keeping both the new and the old channel open.

If a channel contains a weir, or a local raised portion of bed forming a kind of submerged weir, or a contracted place or narrow bridge, the upstream water-level can be lowered by simply removing or reducing the obstruction. The lowering of the water-level will be greatest at the site of the obstruction, and will be zero at some point far upstream (Hydraulics, Chap. VII., Art. 5). If the raised portion forms a long shoal, its removal—supposing its height above the general bed to be the same—will have more effect than if it were short. If the height of the raised portion is small compared to the depth of water, or the amount of contraction small compared to the width of the stream, the removal may have much less effect than might appear ([Chap. I., Art. 4]).

In soft soils one advantage of the straightening system for lowering the water-level is that short-cuts can be dug to a small section, and left to enlarge themselves ([Chap. VII., Art. 1]).

Another advantage is that after any diversions have enlarged themselves to the size of the rest of the channel—or have originally been so excavated—the whole channel may scour, and the water-level continue to fall. This, of course, should be allowed for if likely to occur.

The same thing may occur in the case of the removal of a weir, shoal, or contracted piece of channel. The scour will act at first close to the site of the obstruction, but it may work upstream.

In widening or deepening a channel for the purpose of mitigating floods, it is a good plan to begin work at the downstream end, because the lowering of the water-level will extend upstream beyond the reach in which work is done, and this may facilitate work further upstream. As regards any tendency for a widened reach to silt up again, any such silting is not likely to be great in a short period of time, and need not prevent the carrying on of work in various reaches, if this is convenient.

7. Flood Embankments.—A flood embankment may be close to the edge of the river or it may be set back. If set back it need not follow all the windings of the stream. The setting back of an embankment gives an increased waterway to the stream during floods, and therefore a lower flood-level, but the effect of this is trifling in cases where the depth of the water on the flooded land is small, especially if such land is covered with vegetation, or is otherwise much obstructed. Setting back is generally necessary in cases where the stream is liable to erode the banks to any considerable extent. In such a case the embankment should not be so near to the river as to be in much danger from erosion, but the ground, as already stated ([Chap. IV., Art. 9]), generally falls, in going away from the river, so that when an embankment is set well back it is in lower ground, more expensive and more liable to breach. The most suitable alignment is a matter of judgment, and depends largely on the extent to which the river is likely to shift.

Embankments should, where possible, be made in straight or properly curved reaches. A flood embankment, at least at its upstream end, should terminate in ground which is above flood-level. The top of an embankment should be, in the case of a large river, 2 or 3 feet above the high flood-level of the river. It should, of course, be graded parallel to the general high flood-level, but neither the gradient nor the height of the flood is usually known with accuracy ([Chap. II., Arts. 1] and [2]). There is generally a record or mark of some high flood, and this is taken provisionally as the flood-level. Or the level is calculated approximately from the flood readings on the nearest river gauge. If experience shows that the embankment is too low, it is raised. The cross-section of an embankment depends on the soil, on the extent of damage which results if a breach occurs, on the funds available, and on the value of the land which the embankment has to occupy.

Where an affluent enters the river it will probably be necessary to run out branch embankments. Sometimes cross embankments are run from the main embankment to high land. Their object is to localise the damage if a breach occurs. Along the back of the embankment there may be a drain and it can be made to discharge its water, when the river is not in flood, through the embankment by means of sluices or by pipes closed by flap valves which will not allow flood water from the river to pass through. There may be sluices in the embankment for the purpose of irrigating the land at the back.