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JETTY

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Originally appearing in Volume V15, Page 360 of the 1911 Encyclopedia Britannica.
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JETTY . The See also:

term jetty, derived from Fr. jetee, and therefore signifying something " thrown out," is applied to a variety of structures employed in See also:river, See also:dock and maritime See also:works, whichare generally carried out in pairs from river See also:banks, or in continuation of river channels at their outlets into deep See also:water; or out into docks, and outside their entrances; or for forming basins along the See also:sea-See also: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 See also:timber-See also:work, sometimes of See also:low solid projections, and occasionally only differ from breakwaters in their See also:object. Jetties for regulating See also:Rivers.—Formerly jetties of timber-work were very commonly extended out, opposite one another, from each See also:bank of a river, at intervals, to See also:contract a wide channel, and by concentration of the current to produce a deepening of the central channel; or sometimes mounds of See also:rubble See also:stone, stretching down the See also:foreshore from each bank, served the same purpose. As, however, this See also:system occasioned a greater scour between the ends of the jetties than in the intervening channels, and consequently produced an irregular See also:depth, it has to a See also:great extent been superseded by See also:longitudinal training works, or by dipping See also:cross dikes pointing somewhat up-stream (see RIVER See also:ENGINEERING). 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 See also:lie in deep water (fig. I); or more solid structures are erected over the slope for supporting See also:coal-tips. Pilework jetties are also constructed in the water outside the entrances to docks on each See also:side, so as to See also:form an enlarging See also:trumpet-shaped channel between the entrance, See also:lock or tidal See also:basin and the approach channel, in See also:order to See also:guide vessels in entering or leaving the docks. Solid jetties, moreover, lined with See also:quay walls, are sometimes carried out into a wide dock, at right angles to the See also:line of quays at the side, 'to enlarge the See also:accommodation; and they also serve, when extended on a large See also:scale from the coast of a tideless sea under shelter of an out-lying See also:breakwater, to form the basins in which vessels lie when discharging and taking in cargoes in such a See also:port as See also: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 See also:beach by parallel jetties, made solid up to a little above low water of See also:neap tides, on which open timber-work is erected, provided with a planked See also:platform at the See also:top raised above the highest tides. The channel between the jetties was originally maintained by tidal scour from low-lying areas See also:close to the coast, and subsequently by the current from sluicing basins; but it is now often considerably deepened by See also:sand-See also: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 See also:action of the waves; whilst the upper open portion serves to indicate the channel, and to guide the vessels if necessary (see See also:HARBOUR).

The bottom See also:

part of the older jetties, in such See also:long-established jetty ports as See also:Calais, See also:Dunkirk and See also:Ostend, was composed of See also:clay or rubble stone, covered on the top by See also: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 See also:air, at a depth of 22 ft. below low water of See also:spring tides; and their solid See also:masonry portion, on a See also:concrete See also:foundation, was raised 51 ft. above low water of neap tides (fig. 2). Jetties at See also: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 See also: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 See also: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 See also: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 See also:lower it by scour. Thus the available depth of the approach channels to See also: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 into deep water on both sides of the channel. Other examples are provided by the long jetties extended into the sea in front of the entrance to See also:Charleston harbour, formerly constructed of fascines, weighted with stone and SEA.shifting outlet of the river Yare to the See also:south of See also:Yarmouth, and has also been successfully employed for fixing the wandering mouth of the Adur near See also:Shoreham, and of the See also:Adour flowing into the See also:Bay of See also:Biscay below See also:Bayonne. When a new channel was cut across the See also:Hook of See also:Holland to provide a straighter and deeper outlet channel for the river See also:Maas, forming the approach channel to See also:Rotterdam, low, broad, parallel jetties, composed of fascine mattresses weighted with stone (fig. 5), were carried across the foreshore into the sea on either side of the new mouth of the river, to protect the jetty channel from littoral See also:drift, and cause the See also:discharge of the river to maintain it out to deep water (see RIVER ENGINEERING). The channel, also, beyond the outlet of the river Nervion into the Bay of Biscay has I WO ST o.1T been regulated by jetties; and by extending the-south-See also:west jetty out for nearly See also:half a mile with a See also:curve See also:concave towards the channel the outlet has not only been protected to some extent from the easterly drift, but the bar in front has been lowered by the scour produced by the discharge of the river following the concave See also:bend of the south-west jetty.

As the See also:

outer portion of this jetty was 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 See also: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 See also: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 See also: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 See also:Sulina See also:branch of the See also:Danube into the See also:Black Sea, and the FIG. 6.—River Nervion Outlet, Western Jetty.

south pass of the See also:

Mississippi into the Gulf of See also:Mexico (fig. 4), formed of rubble stone 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 See also:access to them; and they have also carried the sediment-bearing See also:waters sufficiently far out to come under the See also:influence of littoral currents, which, by conveying away some of the sediment, See also: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 See also:block up its outlet during severe storms. The river is thus forced to seek another exit at a weak spot of the beach, which along a low coast may be at some distance off; and this new outlet in its turn may be blocked up, so that the river from See also:time to time shifts the position of its mouth. This inconvenient See also: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.

End of Article: JETTY

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