A Treatise On The Principles And Practice Of Harbour Engineering

ENTRANCE CHANNELS. 231 warding the current off the bank to which they are connected, and of urging it towards the centre of its bed, and so producing a contraction in width. This contraction or concentration of the flow results in increased scouring action, and consequently in a deepening of the river bed. As regards the river sides, the spaces between the groynes become gradually warped and acereted, and the accumulation of material intercepted by the groynes leads to the formation of continuous embankments marked by a series of crescent-shaped embayments. These embayments are due to the eddying action of the current, which also has a tendency to denude and undermine the outer ends of the groynes. The extremities, therefore, should be specially protected, or, at least, constructed in a very secure and substantial manner. Groynes have been extensively used both in this country and abroad : notably on the Clyde, the Tyne, the Tees, and the Danube. They constitute a useful initiatory measure, in that they do not enforce too rigid repression upon a stream. Constructed primarily in short lengths, capable of extension by easy stages, they deflect the current gradually and with an absence of those violent changes of environment which are so liable to produce untoward results. When accretion has been proceeding for some time, and the curreut has been induced to occupy its intended channel, the outer ends of the groynes will advisedly be connected by means of a continuous wall so as to form an unbroken front. This leads us on to the second class of work. (2) Training by means of Walls.—The term “wall,” though in common use in this connection, is not strictly applicable to the whole class of structures included within its category. In the majority of cases the so-called walls are merely mounds of rubble stone ; sometimes the rubble does no more than form a rough surface paving or pitching to a slope, from 2 to 3 feet thick, or even less ; at other times it stands up to some height to a wedge-shaped section with a broad base. Moreover, the wall, whether a pitched slope, revetment, or upright mound, is far from being universally constructed of stone. Fascine mattresses, either singly or in layers, have been most success- fully adapted to all the functions of a training-wall. Slag and clay are also used, and, in minor cases, bags of sand. The formation of a stone training-wall, though apparently a simple process, is attended by certain difficulties. Rubble, when deposited in a heaped mass, has every disposition to subside in a foundation of soft, saturated sand and mud, particularly when the action is fostered and assisted by the scour of a current along the base. The loss incurred in this way has to be made good, and further material deposited until a firm bearing is obtained ; and this result is not achieved, in many cases, without considerable outlay in supplies of stone. In general practice, the rubble is thrown or tipped overboard by hand from punts and barges; but the process is slow, and, if the undertaking be at all extensive, it will prove a more expéditions and economical course to discharge from hopper barges. When dealt with in this way, the stone