A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 223 does it essentially confine itself, more especially near the river mouth, to one definite bed or channel. Tlie influences at work upon littoral currents at the en- trances of large coastal inlets are manifold and powerful. Gales and storms arise at irregular intervals from varying points of the compass, and the pressure exerted thereby is inevitably felt by the tidal flow which is accelerated or retarded, augmented or reduced, to an ap- préciable extent. The action of the wind, moreover, produees cer- tain changes of direction. Hence, the course of the incoming tide, though generally established, is subject to some mutation, and the volume of its flow fluctuates very considerably, not only in consé- quence of irregular meteorological phenomena, but also in conformity with the natural cycle of springs and neaps. In the tidal region, therefore, there are two conflicting agencies : first, the downward stream, with its relatively uniform flow and its tendency to establish a definite bed, and secondly, the tidal current with exactly opposite characteristics. The resulting feature of the tidal estuary is accordingly unstable channels amid shallows and sand- banks. Through the latter, the river, taking the line of least ré- sistance as it presents itself at the moment, ploughs its course to the sea in routes which the succeed- ing tides break through, destroying them in succession as they are formed, before they have time to become confirmed. In the natural order of things, the fluvial current Fig. 202. —Type of Normal River Flow. Main bed of stream shown by dark lines ; shoaling shown by tinting.