Jetty

JETTY. The term jetty, derived from Fr. jetee, and there fore signifying something "thrown out," is applied to a variety of structures connected with river, dock and maritime works. Their forms and construction are as varied as their uses and the word jetty is, moreover, often applied to structures which are better described as breakwaters or piers. They are sometimes high open work structures of timber, reinforced concrete, or steel and iron, braced together ; sometimes they are low solid projections of rubble stone, concrete or masonry, and occasionally only differ from breakwaters in their object. The most common uses to which the term jetty should be applied are :—(i) The regulation of river channels where jetties are projected from the banks towards deep water. (2) Structures in continuation of river chan nels at their outlets into deep water, and at the entrances to harbours of lagoon type. (3) Projections from the sides of docks, or in tidal basins, harbours and rivers, alongside which ships may lie for discharging and taking in cargo. These are sometimes described as piers, particularly when of solid construction, and are commonly so called in North American ports. (4) Structures out side the entrances to docks forming the sides of and protecting a convenient approach channel, and (5) An outwork of timber or reinforced concrete piles framed together and protecting a pier, including bridge piers in navigable waters.

Jetties for Regulating Rivers.

Jetties intended to act as groynes are often extended at intervals from one or both banks of a river to contract a wide channel and, by concentration of the current, to produce a deepening of the central channel. Simi larly jetties are sometimes projected from the concave bank of a river to check the erosion on that side. They are variously termed spurs, spur dikes or jetties, cross dikes and groynes, and are formed of timber or of mounds of rubble stone, or of com binations of these materials. Fascines and mattress work weighted and covered by rubble are also frequently employed. This sys

tem of river regulation occasions a greater scour abreast the ends of the groynes than in the intervening channels and con sequently sometimes produces an irregular depth. Longitudinal training works are therefore preferred for the regulation of many rivers. The jetty system does, however, possess the advantage that the length of the groynes may be easily reduced or increased as experience of their effect on the channel shows to be advisable. Spur dikes have been employed in recent years in this way at the south-west pass of the Mississippi outlet. (See RIVER AND RIVER ENGINEERING.) Jetties at Harbour Entrances.—Parallel or nearly parallel jetties are frequently constructed at the entrances to ports on sandy coasts, particularly those formed at the mouths of rivers and at the outlets of lagoons and land-locked bays obstructed by bars. (See HARBOURS.) The older jetties at such ports as Calais, Dunkirk and Ostend were usually formed of clay or rubble heart ing covered on the top by fascine-work and stone pitching and held together by timber piles and bracing. The timber-work was carried high enough to form a platform above the level of the highest tides. The newer jetties at Dunkirk were founded on the sandy beach by sinking caissons by the aid of compressed air to a depth of 23ft. below low water spring-tides. A solid masonry structure was raised above the concrete foundations to about half-tide level and above that again an open timber-work super structure was carried up to well above high water. Compressed air sinking has been employed in forming the foundations of entrance jetties at other French ports as, for instance, at Boulogne, where a new jetty I 74oft. long on the north side of the channel to the inner harbour was built between 1913-27. The channel depth is about I 7ft. at low-water, but the jetty is designed for a future depth of at least 2oft. at low-water spring tides. In this case the open superstructure of the jetty above the solid masonry work is of reinforced concrete.