intended, but in view of the increase in depth continually demanded by modern shipping, a concrete floor to a basin is a feature which cannot be considered free from inconveniences. No deepening of the basin is possible without its removal, which must prove a costly and troublesome undertaking.

Sluicing is carried on daily at the Canada Basin, but the maximum effect is obtained at low water of spring tides, a time when the basin is very shallow, and when the inner docks can afford to part with a considerable amount of the water impounded on the flood tide. The water in the docks is always levelled with the incoming tide two hours before high water, within which period the operations of docking and undocking are carried on.

Sluicing on a large scale is a prominent feature of ports bordering on the English Channel. The method usually adopted is that of impounding a quantity of water during the flood tide, in a basin specially constructed for the purpose. At high water the sluice gates of this basin are closed, and the contents retained until a suitable period about low water, when the gates are opened again. The discharge of a large volume of water is found to be absolutely essential to the maintenance of entrance channels so subject to the introduction of sand by a littoral current, with its attendant deposition. The rate of discharge provided at Dunkirk and Calais is about 500 cubic yards per second, and the effective duration about three-quarters of an hour.

The recent improvement works at the port of Ostend (fig. 175) comprise a considerable enlargement of area in the sluicing enclosure there, concerning which M. Van der Schueren* makes the following obser

"Ships drawing much water will be able to enter the port by favour of *Van der Schueren on "Travaux exécutés récemment et en cours d'exécution au port d'Ostende," Int. Nav. Cong., Paris, 1900.

SLUICES AT OSTEND.

245 the tide, to reach the quay of the new outer harbour, and to remain there afloat; but, to this end, it is necessary to maintain a draught of 26 feet at low water.

"If it were considered essential to obtain this result by means of dredging, it is to be feared that the cost of the undertaking would be considerable, even excessive, and that the cumbersome appliances necessarily employed for its execution would be found only too often usurping, in front of the quay walls, berths destined for commercial vessels.

"Dredging, therefore, would constitute a drawback-a serious danger even-for navigation at the port; and the maintenance of great depths could with difficulty be assured by this means alone.

"In regard to ports on the Belgian littoral, the rapidity with which deposits of mud accumulate, in channels withdrawn from the action of natural or artificial currents, is well known.

"Under these circumstances, the utility of a sluicing basin would appear to be incontestible. The sluices are designed to supplement the action of the upper waters and of tidal currents, with a view to maintaining uninterruptedly, along the tidal quay of the new outer harbour, the assigned depth of 26 feet, without having recourse to continual dredging.

"The sill of the sluice is located 13 feet below datum, differing in this respect from existing sluices, the sills of which are level with, or not below, low water datum.

"The arrangement adopted is justified in respect of the efficacy of the current. Calculation, in accordance with observed results, enables it to be determined to what degree the useful work of the sluice is increased in this way.

"In his inquiry into the improvement of ports on a sandy beach the late M. Mey demonstrates, in effect, that in ordinary conditions, relative to the dimensions of the sluice and the reservoir basin, the useful effect of the effluent varies in the ratio of about 1 to 65 when the sill of the sluice, assumed primarily at the level zero (low water), is lowered afterwards to 13 feet below this datum."

The following are particulars of the sluicing arrangements at Ostend :

:

To prevent the sluicing basin itself from being silted up, it is in some cases allowed to be filled only on the top of high water, when the influent is comparatively clear. This is the case at Honfleur. Elsewhere, as at

Ramsgate and Dover, the basin has 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 Liffey 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 serious 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 ineffectual. The same results were experienced at Dunkirk, where the sill of a former sluicing basin was found undermined to a depth of 13 feet.

The

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. 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. Inst. C. E., vol. lxvii., p. 461.