A Treatise On The Principles And Practice Of Harbour Engineering

CHAPTER IX. ENTRANCE CHANNELS. Variation in Conditions—Features of a Tidal Régime—Blind Channels—Variable Channels —Fixed Channels—Accretion and Réclamation—Navigable Routes—Bars and their Origin — Training Works for Channels — Groynes—Walls—Fascines—Wave Traps— Height and Extent of Training-walls—Dredging Appliances — Mechanical Eroders — Rock-cutting —Suction Dredgers—Sluicing—Instances of Channel Regulation Works at the mouth of the River Weser, Germany ; at Tampico Harbour, Mexico ; at Westport Harbour, New Zealand ; at the mouth of the Richmond River, New South Wales ; and at the Ports of Ostend, Belgium, and of New York, U.S.A. Variable Conditions of Entrance Channels. —The régulation, and, where necessary, the rectification of entrance channels, are matters of extreme moment to all ports which are situated otherwise than upon the open sea- board, and especially do they call for attention in connection with harbours located within an estuary or upon the banks of a river within tidal range. In the case of ports lying either upon a river flowing into a tideless sea, or upon a tidal river above the limits of tidal access, the agencies determining the form and direction of the river bed are, comparatively speaking, fixed and constant. The stream follows certain well-defined laws which, if not thoroughly understood, are at least clearly enunciated and expressed. It is a matter of general knowledge that in pursuing its sinuous course to the sea, the current of a river, as is indicated in fig. 202, impinges alternately against each bank, scouring the concave side of a bend and being thence diverted to a similar concavity on the opposite side at the next deflection of the river- bed: all this being in accordance with the principles of centrifugal force and action. The tendency, therefore, is for the navigable channel to form and maintain itself along the line of current, and there are few or no conflicting agencies to interfere with or modify this tendency. Features of a Tidal Régime.—Within the limits of tidal influence, however, the conditions are of a totally different character, and the dispositions arising therefrom become much more intricate and complex. The current does not always flow in a seaward direction. For a very considerable portion of the time it is completely reversed, and a very large bulk of sea-water is directed inland with a velocity which is sufficient to overcome the fresh-water discharge, and to raise the level of the surface for a very considerable distance. The main body of the incoming current, moreover, does not by auy means necessarily pursue the same course as that of the downward stream; neither 222