A Treatise On The Principles And Practice Of Harbour Engineering

ENTRANCE CHANNELS. 245 The dredging of a channel through indurated sand affords an interesting comparison of the relative efficiency and eeonomy of a suction dredger and a bidder dredger respectively, both working in the same material. The suction dredger “ Dictys” was fitted with revolving cutting gear, without which she would have been useless, and the spoil was deposited over au adjacent wall, by nieans of pipes laid on pontoons, into a blind channel which had to be filled up. The bucket dredger “Alcides” followed in the wake of biasting operations, which were carried out cheaply and expeditiously, as the material was easily bored by a water-jet working at a pressure of about 80 Ibs. per square inch, and a hole could be put down in this soft rock at the rate of 1 foot per minute. Nobel’s Glasgow dynamite was used, and four holes were exploded simultaneously by an electric battery, the ladder dredging following and lifting the débris into hopper barges for conveyance elsewhere. The following is a tabular statement of the result of a period of working extending over a year :— Name of Dredger. Type of Dredger. Period of Test. Elfective Work done. Rate per Month. Cost per Month. Cost per Cubic Yard. Minimum Rate per Cubic Yard per Month. Alcides Dictys Single ladder, withi boring-punt, diver,I explosives, and tug- f boat J Pump with cutting gear Months. 12 15 Cubic yds. 17,580 15,163 Cubic yds. 1,465 1,011 £ 400 250 s. d. 5 5J 4 111 s. d. 3 9 3 0 The comparison as regards output lies in favour of the bucket dredger ; but as no towing of dredged material was necessary in the case of the suction pump, the cost of working by the latter system was lower. Possibly it might have been lower still, as at the commencement of the period some experi- mental work was being carried out with various kinds of blades in the cutting gear, to ascertain the most effective form, with the result that the original cutters were retained with but slight modification. The cost given in the statement includes wages, stores, repairs, etc., but excludes any interest on capital cost or charge for depreciation. Entrance Channel to the Port of Ostend.—The fairway leading to the quays of the port of Ostend is maintained by a dual system of dredging and sluicing. Up to the year 1898 sluicing alone was in vogue, and its effects were deemed satisfactory and adequate. This was perhaps more particularly the case in the interior of the channel, which was subject to silting of a very light nature, the material being chiefly mud. At the entrance and in- the external fairway, the results were not quite so pronounced, owing to the more compact and sandy nature of the deposit. When, in 1898, new works were undertaken for the development of the accommodation of the port, two out of the three existing sluicing basins were