A Treatise On The Principles And Practice Of Harbour Engineering

238 HARBOUR ENGINEERING. up themselves during the quiescent period of retention. To obviate this, the basin is, in some cases, as at Honfleur, only filled about the time of high water, when the influent is comparatively clear. In other cases, as at Kamsgate and Dover, the basin has been divided into two compartments, one of which is used periodically to cleanse the other. Some harbours are equipped with a natural sluicing basin. Such is the case at Santa Ana, Curaçao, which is probably one of the finest natural harbours in the world. The Schottegat lagoon, behind it, forms a tidal basin 21 miles in length, with a depth of 50 to 60 feet. At Yarmouth there is a magnificent backwater, receiving various tributaries and forming an immense reservoir of fresh and salt water, which serves to keep the harbour fully open, and even deepens the approaches. In cases where the sluicing basin is fed with fresh water, 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 the lighter liquid to flow over the denser; and this phenomenon, which is a matter of ordinary observation, detracts some- what from the scouring effect of fresh water. A coastal inlet or estuary may be transformed into an automatic sluicing basin by the construction, as at the mouth of the Liffey, of a low retaining wall, which becomes submerged above half-tide level. When the tide falls again below this level, the ebbing water converges to a contracted outlet, which sluices the harbour entrance. Compared with dredging, sluicing is an agency not nearly so powerful or so effective. The head or pressure under which it acts is rapidly dissipated by the resistance which it encounters, and at some little distance from the source its scouring effect is greatly reduced, and rendered but slightly appré- ciable. Indeed, it may be said that sluicing, as a means of channel mainten- ance, has practically been entirely superseded by dredging. So far as it is possible to deal in a single chapter with a subject which is capable of being expanded to an entire volume, the foregoing represents an attempt at a fair review of the methods by which the entrance channels of ports are improved and maintained. It only remains to append a few examples of work actually carried out in various parts of the world, in order to afford some illustration of the manner in which general principles åre applied to particular cases, and the modifications which have to be introduced to meet special local conditions Instances of Channel Regulation Works. Regulation Works at the Mouth of the River Weser.1—The estuary of the Weser has been undergoing a course of improvement since the year 1891, when Herr Franzius designed works consisting chiefly of two training-walls for the removal of a bar, caused by a division of the current, which had existed for about thirty years, and had finally attained a length of 1 Franzius and Thierry on River Regulation Works in Germany, Min. Proc. Inst. C.E., vol. CXXXV.