Chapter XV: River Bars

1. =Deltaic Rivers.=--When a river flows into a tideless sea its silt deposits and forms a shoal or bar. This shoal may in time extend and rise up to the water-level. The current of the river makes its way through it in various directions, and in this way a delta is formed and constantly extends seawards. This flattens the slope of the lower portions of the river, and causes raising of the bed in the reaches upstream, and this again may cause the water to break out further upstream and form fresh channels to the sea. The bars at the mouths of deltaic rivers are generally formed with great rapidity, and they are apt to form a complete hindrance to navigation. They are sometimes partly scoured away by floods in the river, but in this case the scoured material may deposit on the outer slopes of the bar. If a river which carries silt has no delta, it is probably because there is a littoral current, which prevents the silt from depositing. On the other hand, if a river brings down very heavy sediment, a delta may be formed even when tidal flow is not wholly absent. This occurs in the case of the Ganges.

The bars at the mouths of deltaic rivers cannot usually be kept down by dredging except at great expense. The usual method of dealing with them is to run out two parallel jetties, in continuation of the river banks, so as to bring the mouth of the river out to the bar. The river then scours a channel through the bar and, if the walls are not too far apart, the depth will probably become as great as in the river and sufficient for navigation. The river, however, tends to at once form a new bar further out. The rapidity with which the new bar forms will be greater or less as the specific gravity of the materials carried by the river is greater or less, and as the strength of any littoral current is less or greater. Clay is spread far out while sand quickly sinks. All deposits are, however, swept away if there is a strong littoral current. The steeper the slope of the bed of the sea away from the bar the longer the new deposit will take in forming a fresh bar. Also the less the discharge of the river the less the deposit will be. The branch of a deltaic river selected for improvement by having the bar at its mouth removed, should be one which has a small discharge and whose mouth is in a position where there is a strong littoral current. In the case of the Rhone, the branch selected was the eastern one, whose mouth was not exposed to any littoral current. Moreover, the other branches of the river were closed, and this increased the discharge of the branch which was left open. The work did not succeed. In other cases, the parallel jetty method has succeeded, and notably in the case of the Mississippi. In this case willow mattresses weighted with stones were used. The question of keeping down the discharge does not, however, appear to have always received sufficient attention. In the case of the Mississippi the “South pass” was selected for improvement. In order to remove a shoal its upper end was narrowed and its discharge reduced. The upper ends of the other “passes” were then obstructed so as to restore the discharges of all the passes to their former amounts. The wisdom of this step is questionable. It is desirable to keep down the discharge of the branch which is to be improved to the lowest limit consistent with free navigation.

If the width of the river near its mouth is greater than is desirable for the width between the jetties, the latter are sometimes made to converge though their outer ends are made parallel.

In the case of the Mississippi the jetties were made with a slight curve to the right. It would seem desirable always to make the jetties with quite a considerable curve. The jetty which was convex to the channel could then probably be shortened. In a case where there is a littoral current, say to the right, the curve of the jetties could be to the right, so that the stream on issuing would tend to merge into the current and assist it.

2. =Other Rivers.=--It often happens that the materials--sand, gravel, and shingle--of which a sea beach is composed shift gradually along the shore. This is known as “littoral drift.” It is by some supposed to be due to the action of the tides, and by others to the action of waves, the drift taking place in the direction of the prevailing winds, excluding those which are off shore. The latter cause is the more probable.

Most rivers have bars at their mouths. In the case of deltaic rivers the bar, as already stated, is caused by the heavy silt carried by the river, though it may be assisted by littoral drift. In the case of non-deltaic rivers flowing into tideless seas, the quantity of silt is not enough to form a bar, and the same is generally true in the case of tidal rivers where the volume of tidal water is usually much greater than that of the upland water. In both these classes of rivers the formation of the bars is due chiefly to littoral drift or to sediment brought in by the sea water. The bar, as in the case of deltaic rivers, may be partly scoured away by a flood in the river, and the scoured material may deposit on the seaward slope of the bar. Generally, the navigation channel across a bar of this kind can be kept sufficiently deep by dredging, but sometimes jetties, like those mentioned in the preceding article, have been constructed, and in this case there is the great advantage that the bar is not liable to form further out. If littoral drift tends to accumulate, the jetties, or at least the one on the side whence the drift comes, can be lengthened. This was done, as mentioned by Harcourt (_Rivers and Canals_, CHAP. IX.), in the case of the rivers Chicago, Buffalo, and Oswego, which flow into the Great Lakes of America. The same writer states that the jetties at the Swine mouth of the tideless river Oder were made to curve to the left, the convex or left-hand jetty being the shorter, but that this exposed the mouth to littoral drift coming from the left. The river, upstream of the jetties, had a slight curve towards the left, but this could have been corrected or, at all events, the jetties made to curve to the right.

A case (fig. 69) where parallel jetties were recently constructed in a tidal sea is that of the mouth of the Richmond River, New South Wales (_Min. Proc. Inst. C.E._, vol. clx.).

In the case of a bar at the mouth of an estuary, parallel jetties would be too far apart. In such cases converging breakwaters (fig. 70) are sometimes made, especially if the tidal capacity of the estuary is small. The entrance is generally 1000 to 2500 feet wide. If made narrow, it would reduce the tidal flow too much. The space inside the breakwaters adds to the tidal capacity, and thus induces scour at the bar. The case is similar to that of the Mersey estuary (CHAP. XIV., _Art. 5_), the breakwaters assisting scour at the bar, though perhaps slightly interfering with the tidal flow in the estuary.

Converging breakwaters also tend to stop littoral drift, and the space inside them acts as a harbour of refuge in storms and as a sheltered place where dredgers can work (_Rivers and Canals_, CHAP. XI.). They have to be heavily built and are very expensive, and they are generally adopted only when there is an important seaport, and when they can be put to all the uses above indicated.