A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 227 upon the area of settlement, and this cannot be the case with a channel limited to one part of it. The River Seine constitutes a typical illustration of the effects of training a channel through an estuary. Fifty years ago the outlet exhibited all the usual vagaries of estuarine channels in regard to alteration in position and irregularity of depth. From that time regulation works have been in hand, and the channel is now clearly defined from Rouen to some distance beyond Berville (fig. 207). At the outset, the probable results were entirely mis- calculated; little or no considération seems to have been taken of the question of silting, or rather, its potentialities were so under-estimated as to be deemed negligible. It was not long, however, before the conséquences began to make themselves felt. Huge volumes of alluvium settled in the external vicinity of the training-walls, and the quantity increased rapidly as the capacity of the estuary to receive tidal water was diminished. Land reclamation followed as a natural sequel. But these processes, though beneficial in some respects, and by no means disadvantageous to the port of Rouen situated 74 miles up the river, became seriously prejudicial to the port of Havre at its mouth. The entrance channels of this latter port began to shoal, sandbanks formed in the approachés, and Havre, as a port, was threatened with extinction. The training-works were arrested for a time. The gain to Rouen had been undoubtedly great; a serviceable channel was promoted and assured, so that, whereas fornierly vessels of between 100 and 200 tons navigated the distance from the sea with difficulty, vessels of ten times tliat tonnage now effected the journey with ease. Moreover, the gain of land had appreciable advantages from a national point of view. Still, it was manifestly mistaken policy to consider that these benefits out- weighed a depreciation in the prosperity of the port of Havre. The difficulty was met by providing Havre with a sheltered deep-water approach direct from the open sea, entirely beyond the influence of accretion in the estuary of the Seine. With this step, involving the construction of two breakwaters of considerable extent, inclosing a new harbour and the formation of an entrance facing south-west, and outside the estuary altogether freedom has been gained for prosecuting the training-works of the Seine, and these seem destined to be continued to the river’s mouth. Navigable Routes.—It must be pointed out, from a navigational point of view, that the vagaries of a shifting channel do not always entail an entire change of route for shipping. Deep gullies and guts may be excavated on the site of former shoals, and adjacent gullies may be silted up; but vessels entering and leaving a port do not necessarily follow the line of greatest depth. Such a line may, in fact, be associated with the blind channels already alluded to. A navigable channel, as a rule, consists of a series of deeps separated by intervening ridges or shoals, and the serviceability of the channel is governed by the depths of the latter. When any one of the ridges becomes unduly high for the draught of passing vessels, then, in the absence of remedial measures, it becomes necessary to lay down another