JETTY, an embankment or pier extending into the sea, and built of earth, stone, fascines, timber, or other suitable material, either singly or combined. Jetties are applied to rivers and tidal harbors, to increase the depth over bars by narrowing the channel, and thus concentrating the current. Jetties have been constructed at the mouths of many European rivers, as the Oder and Danube, and the entrances of many harbors, as Boulogne, Dunkirk, and Calais. The Danube jetties increased the depth from 9 to 20 ft. and transformed it into one of the best harbors on the Black sea. The great erosive power of water causes rivers to transport vast quantifies of sediment, which are deposited at the mouths, forming the delta. Capt. Ends says that the suspended matter carried by streams depends upon the rapidity of the current, modified by the depth; and that the relation between the amount of sediment transported and the velocity is very sensitive, a decrease in velocity causing a deposition of suspended matter. Bearing in mind that, other things equal, the velocity increases as the area of the river-section diminishes, the problem is to construct barriers which shall decrease the area. The hydraulic engineer bases his plans upon a careful survey of the delta, and upon a knowl. edge of the amount of water discharged by the river in a unit of time, and the variations of water-line at different seasons, the areas of sections, the location of bars, the direction of prevailing winds, the effect of storms, etc. The bars existing at the mouth of the Mississippi river have been a serious impediment to commerce; various plans have been suggested for the maintenance of a channel. Dredging proved ineffectual. This river and its affluents drain an area of nearly 1,250,000 sq.m.; the yearly rainfall of the basin is 30.4 in. ; and the average discharge, 21,800,000,000 cubic ft. of water per year, or 675,000 cubic ft. per second. Capt. James B. Ends first proposed the application of jetties to the Mississippi river, presented the scheme to congress, and was author.
ized to undertake the work at the risk of himself and associates. In the face of much opposition he brought the energy of the river to bear upon the great bar of sand and silt separating South pass from the deep water of the gulf of Mexico, thus increasing the dapth from 7+ to 30 ft. and achieving a complete success. The merchants of New Orleans have arranged for a visit to their port by the Great Eastern, and direct exporta tion from the river-basin has been stimulated. Acts of congress awarded, Capt. Ends for the expense of the work, *4,250,000—payable in installments as different depths and of channel should be obtained; $1,000,000 for his services, to be paid when it is known that the jetty works duly maintain the channel; and $100,000 yearly for 20 years, to repair the works and preserve the depth. The depth between jetties required by contract is 30 ft.; width of channel, 350 ft.
Description of the Mississippi Jetties.—The river divides into three principal mouths or passes; the jetties are at the entrance to the middle or South pass. The east jetty extends from Eastside Landsend, at or near East point signal, along the edge of the old bar and into the gulf, a distance of 11,800 ft.; its course is a broken and a curved line
deflecting at the gulf end 1700 or 1800 ft. to the right of the first alignment on the shore end prolonged. The west jetty is 1000 ft. w. of the east jetty, parallel to it, and, starting opposite a point 4.000 ft. from the head of the east jetty, extends 7,800 feet. The Kipp darn, GOO ft. long and perpendicular to the west jetty at its head, joins it .with the west shore; its construction is like that of the jetties. The jetty lines were estab lished by driving piles; permanent cross sections were made 500 ft. apart by locating sighting points on and behind each jetty in the sections and in diagonal sections; and peri odical soundings were made which furnished data for the construction of profiles showing the changes in the channel. The chief constructive materials used in the jetties are willow mattresses, stone, palmetto cribs, and pocks of concrete. The boughs arc brought from a crevasse 23 m. above the jetties, where there is a heavy growth of willows; they arc obtained with difficulty, as the mud is covered, in the flood season, with a foot of water. The mattresses are constructed upon inclined planes, having a rise of 1 in 10; the lower end rests in the water, while the upper is 6 ft. above. The mattresses vary in width from 20 1040 ft. or over; they are generally 100 x 40 feet. Longitudinal -strips,. 21 x 6 in., are first laid on the ways, 4+ ft. apart; across these a layer of willow boughs, G in. thick, is placed, with the switch ends extending 2 or 3 ft. beyond the outside strips; a second layer, at right angles to the first, is placed next above; and so on, till the required thickness, generally 2 ft., is attained. Finally, transverse strips are fastened to the bottom strips with hickory pins. The mattress is then launched, and towed to its destination, where it is tied to the piles, loaded with stone, and sunk to its position on the river bed or upon other mattresses. The bottom row of mattresses was sunk throughout the entire length of the jetties before beginning the second layer. The placing of a single mattress was always accompanied by a deepening of the channel somewhere, and, what was still more remarkable, a deposit of sand abutting against the mattress began at once on the sea side. In this manner the jetties have been greatly strengthened, while w. of the west jetty hard fine sand has been deposited, extending from the jetty to outlying reefs and shoals, and reducing the depth at high water from 9 ft. to a few inches. Copt: Ends and assist ants, guided by experience, have modified their plans. A section of the finished jetty, as originally built, discloses a pile at one side; a number of mattresses, separated by lay ers of stone, and diminishing in width from the bottom to tide level, abut against the pile, while, upon the river side, the steps formed, by the mattresses are covered with stone forming a slope. After the action of the water had produced a slope in the river bed conforming to tire new cross-section, another mattress was sunk on the slope adjoin ing the foundation-mattress and also covered with stone. The surface of the jettyabove sea-level was covered with stone, and crowned with dimension-stone laid dry.