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Encyclopedia Britannica - Main :: JEE-JUN |
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JETTY . The term jetty, derived from Fr. jetee, and therefore signifying something " thrown out," is applied to a variety of structures employed in river, dock and maritime works, whichare generally carried out in pairs from river banks, or in continuation of river channels at their outlets into deep water; or out into docks, and outside their entrances; or for forming basins along the sea-coast for ports in tideless seas. The forms and construction of these jetties are as varied as their uses; for though they invariably extend out into water, and serve either for directing a current or for accommodating vessels, they are sometimes formed of high open timber- work
object. Jetties for regulating Rivers.Formerly jetties of timber- work
contract a wide channel, and by concentration of the current to produce a deepening of the central channel; or sometimes mounds of rubble stone, stretching down the foreshore
great
Jetties at Docks.Where docks are given sloping sides, openwork timber jetties are generally carried across the slope, at the ends of which vessels can lie in deep water (fig. I); or more solid structures are erected over the slope for supporting coal-tips. Pilework jetties are also constructed in the water outside the entrances to docks on each side, so as to form an enlarging trumpet-shaped channel between the entrance, lock or tidal basin and the approach channel, in order to guide vessels in entering or leaving the docks. Solid jetties, moreover, lined with quay walls, are sometimes carried out into a wide dock, at right angles to the line of quays at the side, 'to enlarge the accommodation; and they also serve, when extended on a large scale from the coast of a tideless sea under shelter of an out-lying breakwater, to form the basins in which vessels lie when discharging and taking in cargoes in such a port as Marseilles (see Dock). Jetties at Entrances to Jetty Harbours.The approach channel to some ports situated on sandy coasts is guided and protected across the beach by parallel jetties, made solid up to a little above low water of neap tides, on which open timber-work is erected, provided with a planked platform at the top raised above the highest tides. The channel between the jetties was originally maintained by tidal scour from low-lying areas close to the coast, and subsequently by the current from sluicing basins; but it is now often considerably deepened by sand-pump dredging. It is protected to some extent by the solid portion of the jetties from the inroad of sand from the adjacent beach, and from the levelling action of the waves; whilst the upper open portion serves to indicate the channel, and to guide the vessels if necessary (see HARBOUR). The bottom part of the older jetties, in such long-established jetty ports as Calais, Dunkirk and Ostend, was composed of clay or rubble stone, covered on the top by fascine-work or pitching: but the deepening of the jetty channel by dredging, and the need which arose for its enlargement, led to the reconstruction of the jetties at these ports. The new jetties at Dunkirk were founded in the sandy beach, by the aid of compressed air, at a depth of 22 ft. below low water of spring tides; and their solid masonry portion, on a concrete foundation, was raised 51 ft. above low water of neap tides (fig. 2).Jetties at Lagoon Outlets.A small tidal rise spreading tidal water over a large expanse of lagoon or inland back-water causes the influx and efflux of the tide to maintain a deep channel through a narrow outlet; but the issuing current on emerging from the outlet, being no longer confined by a bank on each side, becomes dispersed, and owing to the reduction of its scouring force, is no longer able at a moderate distance from the shore effectually to resist the action of the waves and littoral currents tending to form a continuous beach in front of the outlet. Hence a bar is produced which diminishes the available depth in the ap- proach channel. By carrying out a solid jetty over the bar, however, on each side of the outlet, the tidal currents are concentrated in the channel across the bar, and lower it by scour. Thus the available depth of the approach channels to Venice through the Malamocco and Lido outlets from the Venetian lagoon have been deepened several feet over their bars by jetties of rubble structure (fig. 3), carried out across the foreshore
south
drift
I WO ST o.1T been regulated by jetties; and by extending the- south
drift
harbour was constructed, it has been given the form and strength logs, but subsequently of rubble stone, and by the two converging of a breakwater situated in shallow water (fig. 6). (L. F. V.-H.) rubble jetties carried out from each shore of Dublin bay for deepening the approach to Dublin harbour. Jetties at the Outlet of Tideless Rivers.Jetties have been constructed on each side of the outlet of some of the rivers flowing into the Baltic, with the objects of \\\\\\ prolonging the scour of the river M W e .. and protecting the channel from %\\\~ being shoaled by the littoral drift 'M along the shore The most inter- . esting . \\\\\\ application of parallel jetties is in lowering the bar in 0.1alsq+ ~v^%.~~ ~41 Ala _ front of one of the mouths of a ~~~~~~ i~:10 ..t.A0,eeles ,se.V. N-Po,pq ,, deltaic river into a tide- - less sea, by extending the scour ~~~~~+~i s,!~~r~s1i6 iii ~s ~i +! >jjs i~ 1/ r ~lflttr rtl~r _ of the river out to the bar by r~~=~%+~`r~~:~.!~+~:~Ii~%i ~s~.~~~,i'~~Ieis~i-~-a~~_ i~.11~1.~~_~: ~tit'!+- ' - - a virtual prolongation of its banks. Jetties prolonging the Sulina branch of the Danube into the Black Sea, and the FIG. 6.River Nervion Outlet, Western Jetty. south pass of the Mississippi into the Gulf of Mexico
and concrete blocks, and fascine mattresses weighted with stone and surmounted with large concrete blocks respectively, have enabled the discharge of these rivers to scour away the bars ob- structing the access to them; and they have also carried the sediment-bearing waters sufficiently far out to come under the influence of littoral currents, which, by conveying away some of the sediment, post- pone the eventual formation of a fresh bar farther out (see RIVER ENGINEERING). Jetties at the Mouth of Tidal Rivers.Where a river is narrow near its mouth, and its discharge is generally feeble, the sea is liable on an exposed coast, when the tidal range is small, to block
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