the Fourth Week of Advent
free while helping to build churches and support pastors in Uganda.
Click here to learn more!
Bible Encyclopedias
Jetty
1911 Encyclopedia Britannica
or 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, which are 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, sometimes of low solid projections, and occasionally only differ from breakwaters in their object.
Jetties for regulating Rivers. - Formerly jetties of timber-work were very commonly extended out, opposite one another, from each bank of a river, at intervals, to 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 from each bank, served the same purpose. As, however, this system occasioned a greater scour between the ends of the jetties than in the intervening channels, and consequently produced an irregular depth, it has to a great extent been superseded by longitudinal training works, or by dipping cross dikes pointing somewhat up stream (see River Engineering).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 lie in deep water (fig. I); or more solid structures FIG. I. - Timber Jetty across Dock Slope.
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 j etties, 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 outlying 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, a.nd 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 224 ft. below low water of spring tides; and their solid masonry portion, on a concrete foundation, was raised 5R ft. above low water of neap tides (fig. 2).
