A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 233 are requisite ; otherwise the stream will exhibit a tendency to spread, and the channel to shoal. At the mouths of rivers, double retaining walls may be either parallel or splayed, and the splay may be inwards or outwards, so that the walls either converge or diverge as they approach the sea. Parallel retaining walls serve to maintain the downstream current unimpaired in streugth and velocity ; but if they are carried up to any height in tidal estuaries, they lead to an accretion which obstructs the flood-stream and excludes a considérable portion of the water which would otherwise enter the estuary. Another danger attaching to such walls is the likelihood of shoaling in the neighbour- hood of the entrance, due to the arrest of littoral drift by the walls. This drawback has manifested itself in a number of cases, and at Dunkirk, for instance, the jetties have been extended outwards from time to time, in order to reach deep water and to scour away the intermediate deposit which threatened to destroy the accessibility of the port. Moreover, parallel walls do little or nothing towards the dissipa- tion of storm-waves passing in from the x^. sea. It is from this point of view that I^ converging walls have been designed, \ £ the inclosed area being of the nature \ » of a basin containing a relatively larger \’ mass of water, upon which external agita-\ tion has less effect. These walls, in fact, y are sometimes adapted so as to form s ea compartnients called wave-traps (fig. Flo 213 213). The drawback of the system is the same as that mentioned in connection with parallel walls, viz., the reduction in volume, and consequently in scouring efficacy, of the influent waters. This objection, of course, only applies to tidal seas. From this last standpoint, divergent walls are preferable, for with their splayed arms they admit the flood-tide freely and the outward flow of the ebb maintains the channel in that gradually widening form which is the ideal regime of an estuary. The contraction of the sides must not be too rapid, or there will be a tendency to throttle the inward flow, and pile up the tidal wave until it forms something of the nature of a “ bore ”—the term applied, in certain rivers, to an influx of water possessing a steep face and moving with considerable rapidity. This is dangerous alike to navigation and to the stability of the banks. It must be admitted that no great uniformity is exhibited in the expansive ratios of natural estuaries. They fluctuate exceed- ingly, and range, in parts, from something like 2550 feet to the mile in the Humber to little more than 100 feet to the mile in the Severn. On the whole, however, it may be said that a ratio of 2000 feet to the mile constitutes a suit- able standard for adoption.