A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 237 The pumping principle is represented by one type only—the suction dredger. The Suction Dredg’er has proved itself to be unquestionably one of the most remarkable contrivances ever devised for the removal of subaqueous material, both in regard to the enormous extent of its output and the low cost of its operations. It is to some extent, of course, a special machine. There are, naturally, conditions and circumstances to which it is not applic- able ; but they are few. It would be useless to expect it to dredge hard rock or to lift massive boulders. In all other cases, the efficacy of the suction dredger has been demonstrated beyond question. The suction dredger consists essentially of a continuous pipe or tube, through which, by means of suitable pumping machinery, material is sucked up and discharged, either into a hopper forming part of the vessel itself, or into a scow ranged alongside, or through a shoot or tube leading to an adjacent bank or shore, which last arrangement lends itself very conveniently to land reclamation purposes. In the case of sand and light material, no preliminary treatment is necessary, but clay and marl have to be disintegrated by the cutters already alluded to, before they are in a condition to be drawn up the tube. In exposed situations, such as prevail along the seacoast, the suction dredger possesses a marked advantage over apparatus of other types, the working of which is often materially interfered with by the motion of the waves. Equipped with telescopic pipes and flexible joints, the suction dredger readily adjusts itself to the rise and fall of the sea, and is quite independent of variations in level, either momentary or prolonged. There is a great deal to be said, in extension of the foregoing remarks, on the relative advantages of the various types of dredgers, and there are many inter- esting features in connection with their working which might usefully claim our attention ; but space will not permit us to pursue the matter further here. We turn now to the last item in our series. (4) Training by means of Sluices.—The principle of sluicing is based on that of the ebb-tide current, which, flowing out of a coastal indentation, scours its passage as it goes. The application of sluicing, however, is restricted to channel deepening and maintenance. It is rarely, if ever, employed in channel-making. In practice, a large basin or receptacle is provided, within which the tidal water, entering up to the time of high water, is impounded and sub- sequently discharged through sluices or outlets at or about low water. The most effective period for sluicing is during spring-tides, when the flood waters are large and the ebb level is low. The method has been largely used in ports bordering on the English Channel and the North Sea, such as Dunkirk, Calais, Boulogne, Dieppe, Ostend, etc., where the discharge of a large volume of water in this way has been found highly serviceable in keeping the harbour entrance channels free from silt. The system has its drawbacks. The retaining basins tend to silt