Jetty

an embankment constructed in shallow water at the mouth of a river and parallel with the current, for the purpose of narrowing the stream and causing it to scour out a deeper outlet. Many noble rivers spread out so wide and are so shallow at their mouths that deep draught ships cannot enter them. By confining the current within artificial banks its scouring capacity is increased and a deeper channel is secured. Sometimes one jetty is sufficient, but ordinarily double jetties are built, one on each side of the outlet. Jetties may also be used to set up a scouring action in any part of a river where the soil is easily scoured away. Double jetties have been found useful in harbors where the incoming and outgoing tide may be made to scour a deeper entrance. The harbor of Calais, France, has been improved by the building of jetties. The mouths of a number of rivers in Europe and America have been deepened in this way. The Neva at St. Petersburg has been improved by jetties. The Oder has been deepened from four to thirteen feet; the Vistula, from seven to twenty-three, and the Danube, from eight to twenty-one feet.

In 1875 Congress authorized Captain Eads to construct a system of jetties at the mouth of the Mississippi. The south pass, the central of the three great channels by which the delta is traversed, was chosen. He decided to build two parallel dikes, running a distance of about two miles to deep water. Piles were driven in long rows to mark the lines of the proposed dikes. Willow brush in full leaf, cut fifteen feet in length, was made into huge cribs or mattresses 100 feet long by fifty feet wide, and two feet in thickness. They were built on shore, towed to place, and then sunk by loading them down with rocks. A second, a third, and a fourth tier, each narrower than the preceding one were sunk in like manner. The silt of the river, which is a yellow flood in its lower course, soon settled in the mattresses and made two solid water-tight walls between which the current, now comparatively narrow and correspondingly swifter, scoured out the bottom of the channel. When work was begun the outlet was but eight feet deep at the outside bar. Now ocean-going ships, requiring thirty feet of water, can ascend the river to the wharves of New Orleans.

The longest jetty in the world is that of the Columbia River. It is a single jetty and is over forty-three miles in length. An eight foot bar at the mouth has been scoured out to a depth of thirty feet. A double jetty in the harbor of Galveston, built by the government at a cost of $8,000,000, has proved successful. The tide now scours out and maintains a channel twenty-seven feet deep. The mouth of the St. Johns, Florida, the Brazos, Texas, and the harbor of Charleston, South Carolina, have been deepened by similar means.

See Eads