Page:The New International Encyclopædia 1st ed. v. 11.djvu/226

JETTY. On the extreme left to the east lies Pass à l'Outre. about 14 miles in length to land's end. In the middle lies South Pass, which, before its improvement, was about 12 miles long. On the right, to the west, lies Southwest Pass, about 17 miles to land's end. In 1875 Congress authorized Capt. James B. Eads to improve the South Pass by the construction of two parallel jetties. The depth betwetai tlic jetties was required by the contract to be .'iO feet, and the width of the channel was required to be 350 feet. The east jetty was made 11,800 feet long from land's end to 30 feet of water in the Gulf; the west jetty was 7800 feet long, and was built 1000 feet from the east jetty and generally parallel with it. The first work was to drive a row of piles spaced 12 feet apart to mark the inner lines of the two jetties. These piles served to guide the operations of sinking the mattresses and were not intended to give strength to the work.

The jetty structures proper consisted of several layers of willow mattresses, loaded down with stone. The first layer was composed of mat- tresses 100 feet long and 50 feet wide, and the succeeding layers of mattres.ses of the same length, hut decreasing in width to the top mat- tress, which was 20 feet wide. Generally four courses or layers of mattresses were sufficient to bring the mattress-work to the water surface. Each mattress was composed of four layers of willow brush, which, when compressed, gave it a thickness of about two feet. The mattresses were

built on launching ways on shore, towed into position behind the guide-piles, and sunk by load- ing them with stone. After the mattress-work was thoroughly settled, the sea end of each jetty was surmounted by a capping of concrete blocks. Since the original construction the jetties have been considerably added to and repaired. Jn 1899 the United States Army Engineers sub- mitted plans for the construction of jetties at the mouth of the Southwest Pass of the Missis- sippi River, but no actual work had been begun at the end of 1900. A rCsumfi of this proposed improvement and of the previous work on the South Pass by William Starling. United States Engineer Corps, was publislied in Enqineerinf) yews (New York) of August 23 and October 4, 1900. and from this the accompanying cuts are abstracted. See

. The Columbia River, ever since its discovery in 1792, has been the chief harbor of the Pacific Northwest, but the shifting

character and variable depth of the bar channels has always caused its entrance to be held in terror by mariners and shipowners. To improve these conditions a jetty was completed in 1893- 94. Its total length is 4% miles, which makes it the longest jetty in the world. It was con- structed by siidiing mattresses of brush 3 feet thick and 40 feet wide, and surmounting them by a mound or ridge of rubble-stone. From a low-water depth, generally of 19 feet to 22 feet, in shifting and uncertain channels across the bar, the depth has increased to 29 feet in a single, and, so far, permanent channel.

. Yacquina Bay is a narrow estuary some 20 miles long, situated on the Oregon coast 115 miles south of the Columbia River. In its natural condition the harbor throat lay between a rocky lieadhuul on the north and a low sandy point on the soutli. The channel dischargMl into the ocean over a low sandy bar, and was narrow, uncertain of alignment and depth, and bordered by sands upon which there were constant breakers. The plan of improve- ment finally adopted consisted of two jetties starting at the harbor throat, about 2300 feet apart, and converging to a distance apart of iOOO feet: the north jetty being 2300 feet long and the south ,|etty 2000 feet long. Both jettii>s are rubble-stone mounds. In the case of the south jetty, which rests on sand, the rubble mound is supported on a brush mattress about 4 feet thick, but the mound of the north jetty

Water ■Jft. deep at M.U.Tide. Maltreis, 2.5X150' pnliMryMllage,in'}B'0'iy^l^" '<!> VJafer 9ft deep at M.L,"nde. k 9 >i _jr M^H-mtrr__ jM7l7m^' Water 18f|- deep at M.L,Tide.

Fig, 2, typical cross-skctions op east jetty fob soctheastekv pass of Mississippi nivER,

rests on bed rock. The result of the work was to secure a permanent channel 14 feet to 15 feet deep in place of the original variable channel about 7 feet deep.

. Among the harbors on the .Atlantic and Gulf coasts of the United States, where jetties have been com- pleted or are in process of construction, are the following: Charleston. S. C. two converging jetties consisting of a brush mattress foundation and a rubble-stone mound, about 15.000 feet long each; Saint .John's River, Florida, two jetties beginning on the opposite sides' of the river rnouth and converging to a width apart of about 1600 feet, at the ridge of the fan-shaped bar which obstructs the entrance, and thence run- ning parallel as far as it may be necessary to extend them: Sabine P.iss, Tex,, two approxi- mately parallel jetties about 17,000 feet and 15,- 000 feet long, respectively; Galveston, Tex,, two