A Treatise On The Principles And Practice Of Harbour Engineering

234 HARBOUR ENGINEERING. Height of Training-Walls. —The height to which training-walls should be raised is a moot point. If nothing more than the mere rectification of a channel be in view, the wall will only be of the nature of a low revetment, confirming and protecting the edge of a newly-formed bank, and need not be raised above low water level. It has been urged against this, that in a sandy estuary, a channel so formed would soon be silted up with sediment washed in from adjacent banks over the top of the walls. There is, however, no more reason why silting should take place under the new conditions than there is under the old, and it may be safely assumed that the stream is powerful enough to maintain its own bed. If it be desired to form an entirely fresh channel, or to radically divert an existing one, something more definite than a mere revetment becomes necessary ; scarcely anything less than a half-tide wall will suffice to confine a stream within arbitrary limits and guide it through a novel environment. The teudency to resume a long-established course must always remain a powerful influence, if ever the compelling forces be modified or removed. When land reclamation is defiuitely aimed at, training-walls will be first laid up to mean tide level, and then gradually raised until the level of highest high water is reached. (3) Training by means of Dredging.1—Of all the agencies at work for the régularisation of channels and for the removal of natural impediments, there is none so effective and so powerful as dredging, exemplified, as it is, at the present day by machines of enormous size and tremendous capabilities. Natural scour is serviceable enough in its way, but it is only effective in soft, friable material. It is quite powerless to remove indurated ground within any reasonable time, and it has no influence whatever on huge boulders and rock. To all training-works, of whatever description, dredging is a most use- ful auxiliary, and there are few ports the entrance channels of which can be maintained without the aid of continuons and systematic dredging. It is, in fact, the recognised medium for the removal of bars and shoals. Nor are its operations confined to any one class of material. Dredging in rock is as feasible as dredging in alluvium, and boulders are removed as easily as sand. At its first introduction, dredging was carried on by small and insignificant agencies, but the scope of present-day operations has become so vast and extensive as to necessitate the employment of extremely powerful plant and appliances. Such primitive expedients as “the bag and spoon,” “ the aqua- motrice,” and “ the rake,” except in very insignificant localities, have given way to large and imposing vessels, self-propelled, navigable, and specially equipped with machinery for dealing with something like from one to five thousand tons of material per hour. The conditions of dredging work are exacting. Formidable obstacles are frequently encountered. Not only is the material dealt with of a very uncertain and varied character, ranging from impalpable mud to adamantine 1 Dredging appliances are fully dealt with and illustrated in Chapter III. of Dock Engineering.