A Treatise On The Principles And Practice Of Dock Engineering

246 DOCK ENGINEERING. Ramsgate and Dover, the basin bas been divided into two separate sections by a dividing bank, and one of these sections has occasionally been used to cleanse the other. Another expedient is to feed the reservoir with inland fresh water. In this connection, it is desirable to note that the specific gravity of fresh water being less than that of salt water, there is a marked tendency for fresh water to flow over the surface of the salt water, and it has been stated that the effect of scouring with the former does not extend to depths greater than 9 feet. * At the port of Dublin a considerable area of strand of the estuary of the River Lifley is enclosed by a low retaining wall, which is submerged above half-tide level. When the tide falls below this level, the ebbing water converges to a contracted outlet, and produces a very effective scour at the mouth of the harbour. It is very necessary to emphasise the danger of excavation in front of a discharging sluice. Even when a masonry apron of considerable width has been provided, the ground immediately beyond it has been found eroded to such an extent that measures have had to be taken to prevent serions damage. A hole, 6 feet deep, was formed at the edge of a stone apron, 80 feet in width, at the low-water basin, Birkenhead, and all attempts to fill up and reduce the hole by the discharge of large blocks of rubble stone into it were ineffeetual. The same results were experienced at Dunkirk, where the sill of a former sluicing basin was found undermined to a depth of 13 feet. Scraping and Scuttling.—-This method consists in stirring up the deposit by mechanical means, to enable it to be carried away by an existing outward current. At Tilbury basin, harrows are employed for the purpose, aided by high-pressure water jets worked from a small tug during the ebbtide. The commotion caused by the revolving propeller itself of a tug with light draught in shallow water will also cause a very effective disturbance of mud. The same method with a larger vessel has been successfully employed for removing sandy bars at the mouths of rivers. Dredging.—Dredging, as a means of channel maintenance, and distinct from deepening work, is open to the objection already stated, that it obstructs the navigable way. Having in view the soft nature of the material to be dealt with and the necessity for continuous removal of shallow deposits rather than the intermittent excavation of large accumulations, suction dredgers form the most useful type for maintenance work. Grab dredgers are also valuable in confined spaces, but the bucket dredger can only be usefully employed in large and unconfined areas, where a considerable bulk of material has to be excavated. In the case of a suction dredger, the mud in the intake pipe forms a comparatively small percentage of its contents—averaging, say, from 30 to 40 per cent.—and of this a large proportion may be expected to pass out with the overflow water from the hopper into which it is discharged. The= * Min. Proc. Inet. C.E., vol. Ixvii., p. 461.