The jetties at the entrances to the Venetian lagoon at Lido and Malamocco (see HARBOURS) are of rubble stone surmounted by a small masonry superstructure brought up above water level. Those at the Charleston (S.C.) harbour entrance were originally built of fascine mattresses weighted with stone, but are now formed entirely of rubble. The converging jetties carried out from each shore of Dublin bay for deepening the approach to the river Liffey and Dublin harbour are also of rubble.
Jetties have been constructed at the outlets of many rivers flowing into tide less (or nearly tideless) seas as at Swinemiinde, on the Baltic, and Tampico in the gulf of Mexico, with the objects of prolong ing the scour of the river and protecting the channel from being shoaled by littoral drift. The most interesting application of parallel jetties is in connection with the mouths of deltaic rivers flowing into tideless seas. In such cases the construction of jetties, by a virtual prolongation of the river banks, extends the scour of the river out to the bar. Jetties prolonging the Sulina branch of the Danube into the Black sea, and the south and south-west passes of the Mississippi into the Gulf of Mexico (fig. 1) have concentrated the discharge of these rivers so as to scour the bars obstructing the access to them and have effected considerable increase in depth in the navigable channels. The sediment-bearing waters are moreover carried by this concentra tion of discharge sufficiently far out to come under the influence of littoral currents, which by conveying away some of the sedi ment, postpone the eventual formation of a fresh bar further out. It is, however, very seldom that jetties alone suffice to secure the maintenance of a sufficient depth of water for modern re quirements, and recourse has been had to intensive suction dredg ing both at Sulina and the Mississippi passes. (See RIVER AND RIVER ENGINEERING.) Jetties at the Mouths of Tidal Rivers.—Rivers whose dis charge is generally feeble and which debouch on an exposed coast subject to littoral drift are liable to have their outlets blocked during severe storms. This is specially the case when the river is narrow near its mouth and the tidal range is small. Sea action piles up sand and shingle to the obstruction of the outlet and the river is thus forced to seek another exit at a weak spot of the beach which, along a low coast, may be some distance off. The new outlet in its turn may be blocked up, so that the river from time to time shifts the position of its mouth. This incon venient cycle of changes may be stopped by fixing the outlet of the river at a suitable site, by carrying a jetty on each side of this outlet across the beach, thereby concentrating its discharge in a definite channel and protecting the mouth from being blocked up by littoral drift. This system was long ago applied to the shifting outlet of the river Yare to the south of Yarmouth. Later it was successfully employed for fixing the wandering mouth of the Adur near Shoreham, and of the Adour flowing into the bay of Biscay below Bayonne. Timber-piled jetties filled with rubble stone have often been employed in such cases.
When the new channel was cut across the Hook of Holland to provide a straighter and deeper outlet channel for the river Maas, forming the approach channel to Rotterdam, jetties formed of fascine mattresses weighted with stone were carried across the foreshore on either side of the cut to protect the channel from littoral drift and confine the discharge of the river. (See HAR
BOURS and RIVER AND RIVER ENGINEERING.) Jetties in Docks, Rivers, etc.—Openwork timber or rein forced concrete jetties are often constructed in docks (q.v.) with sloping sides, being carried across the slope so that vessels may lie alongside them in deep water. Similar structures are also employed in open basins, harbours and rivers as well as in docks for supporting coal-loading tips and hoists, and for berthing ves sels carrying oil in bulk. Continuous quayage is not essential in these cases and for oil berths nothing more is required than ade quate dolphin and fendering structures (against which vessels may lie) and comparatively light structures connecting with the shore for carrying the necessary pipes, etc., for loading and dis charging the oil. (See plans of Colombo and other harbours in HARBOURS.) Examples of reinforced concrete jetties carrying coal hoists will be found among the illustrations in the article DOCKS. Long and wide structures projected from the sides of docks and basins, designed for berthing vessels on either side, are virtually continuous wharves or quays and are more properly called piers. A convenient distinction is to restrict the term jetty, as applied to berthing, to those structures intended for berthing ships across their ends or in front of a T-shaped head and to openwork island structures only connected with the shore by bridges or approach viaducts. Long openwork structures thrown out into a river and curving round so as to provide ship ping berths more or less parallel with the river bank, such as the Tilbury river jetty and the coal-staiths at Dunston on the Tyne and at Blyth, are correctly termed jetties. Jetties of open steel and iron construction are also used, both for cargo and passenger traffic, particularly in tropical ports (see DOCKS).
Jetties of pilework and occa sionally of solid character are constructed outside the entrances to docks on each side of the channel from the river or sea ap proach so as to form a funnel-shaped passage leading to the lock entrance. These jetties serve to guide vessels entering or leaving the docks, to protect them from the effect of tidal or river cur rents and, in some cases, as convenient lay-bys where a vessel may, if necessary, tie up when waiting for the gates to be opened. The entrance jetties at Avonmouth are solid constructions, found ed on concrete monoliths, with timber face work; those at the King George V. dock, London, and at Immingham are of open timber work. Timber-piled jetties filled in with rubble stone are also employed, as at Swansea. In this instance they serve as minor breakwaters sheltering the entrance to the river and docks and prevent, to some extent, sand from entering the dredged channel.
Timber and, in recent times, reinforced concrete jetties are frequently employed as adjuncts to breakwater and pier structures, serving as landing places and for other purposes, as for instance oil-bunkering and watering. The word jetty is also used to describe a timber fendering struc ture or outwork constructed in connection with swing and other bridges in navigable waters to protect the bridge piers from damage by vessels passing through the navigation openings.
(N. G. G.)