A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 229 created zones of slack water. Since, however, the meeting-places of such currents must necessarily vary to a considerable extent from time to time in accordance with the mutable conditions of tide, wind, and weather, the con- tention does not seem powerful enough on its own hypothesis to account for a fixed bar ; and most bars are fairly stationary. A third theory is that the widening mouth of a river, combined with a constant cross sectional area, naturally entails a reduction in depth. Against this it is to be urged that bars are almost universally abrupt mounds standing at slopes far steeper than would be the case above water, having regard, that is, to the angle of repose for the material, and that, therefore, they bear no apparent relationship to the much more gradual widening of an estuary. The opinion now most generally held is that bars are the outcome of littoral drift, and tbat the chief causes of their formation are tidal currents and storms. Of these, the former agencies are more constant in action, and therefore perhaps more influential. The flood-tide, travelling along a shore which is being subjected to secular denudation, carries or rolls along with it a quantity of gravel, sand, and shingle, the motion of which is arrested when it cornes in contact with a counter-current issuing from the mouth of a river. This theory does not altogether account for the existence of prominent bars in localities where littoral erosion is not an evident process. In this case, it is contended that the natural tendency of wave motion is to produce irregu- larities in the bed of the sea, and that these irregularities in certain places have culminated in definite ridges and depressions. But here again the explan- ation seems to be inadequate, since a bar is a special ridge peculiarly associated with river mouths, and not by any means ubiquitous ; though, at the same time, it must be admitted that there are bars in existence off the coast-line, where no river finds its outlet, as, for instance, at Portland Bill. Finally, it is to be noted that there are bars of indurated material, which are evidently of a permanent character and primeval origin, being due to the denudation of the sea floor and the attrition of its softer portions. Such ridges consist either of rock, tough boulder clay, or conglomerate, and they manifestly constitute features attributable to no transporting agency whatever. The problem is one attended by some difficulties ; and it apparently does not admit of a single solution only. In many cases there are indirect causes, some of which are obscure ; while in general, the predominant tendencies are recognisable. On the whole, it seems fairly well established that the forma- tion of the majority of bars, especially those in shallow, sandy estuaries, is attributable to the conflict between the external and internal physical agencies, and constitutes the régime under which the forces are in a state of equilibrium, more or less stable. Hence, if bars of this character be removed, there is every likelihood of their recurrence unless special preventive measures be taken. Bars of indurated material, ou the other hand, are such as to give no ground for any appréhension of this kind.