JETTIES are dikes at the mouth of a river or across a harbor bar to increase the riverine or tidal current by narrowing the channel and thus scour out a deeper bed, to accommodate navigation. Single jetties are solely at the mouths of rivers with strong currents, to de flect these to one side of its natural channel, but in most rivers and in all harbors they are 'rou ble, forming an entire artificial channel. Briefly, the physical principles are: The power of water to transport solid matter varies as the square of its velocity, so that increasing the strength of current two-fifths will about dou ble its sand-carrying capacity; the velocity in creases with increase of slope and decrease of friction; the slope is increased by narrowing the channel, since it forces flood waters inside or outside to rise higher at the entrance, and the friction decreases as the width of the channel; and lastly, if a channel of a given depth and width passes a given quantity of water, then a narrower channel involves either a permanently greater height of water if the bed were rigid, or the scouring of the bed to a depth which, multiplied by the new width, will produce an equal cross-section with the old. The increased slope and the correspondent velocity vanish as the water cuts a deeper basin; but the velocity due to lessened friction does not, nor do the deepened channel, the greater discharge through it and the greater tidal fluctuation due to the larger basin. The channel is scoured along until the deepening sea establishes an equilibrium of action.
The system is not new. A number of im portant European rivers were jettied even be fore the middle of the last century, and others not much later. The Danube at Sulina had been deepened to 21 feet, from 8 feet before; and in 1874 Eads found seven German rivers, includ ing the Oder and Vistula, improved so that with initial channels of 4 to 7 feet, they then had 13 to 23.5. Several of these are still deeper now, the result increasing somewhat for many years under the same conditions. The Neva m Rus sia has also such works; as also the harbors of Calais and Dunkirk in France. In the United States a very great number have been con structed, bath for rivers and harbors; the great est of all are the jetties at the mouth of the Mississippi, and as the general principles are alike in all, these may be briefly described.
The Mississippi discharges its waters to the gulf not by one channel, but in the main by three, running through (passes° 12 to 17 miles long from the delta land to the sea and widely divergent. The largest is the Southwest Pass;
next the easternmost, Pass I l'Outre, with two branches. In the middle is the smallest, South Pass, 600 to 800 feet wide, and taking not over a 10th the total discharge, with a shoal at its head only 15 feet deep, and a bar at its mouth only 8 feet; so that, with 30 feet of water through the delta, it was unserviceable for deep water navigation. Capt. James B. Eads offered to build jetties to deepen the bar at Southwest Pass from its then 13 to 28 feet; but Congress preferred South Pass as cheaper and simpler, needing work only at the head and foot. Work was begun in June 1875 and within nine months the water was 13 feet on the bar; by 1879 it was 29 feet; it is now over 30.
The west side of the pass had silted up into land 4,000 feet farther out than the east, so that the west side of the two parallel dikes built out to 30 feet depth in the Gulf was about 7,800 feet, while the east was 11,800. First piles were driven in two rows 1,000 feet apart (the piles .12 feet apart) to mark the lines of the projected jetty-walls. Then mattresses were built, of wil low branches, or young willows 15 feet long, cut with the leaves on, laid in four courses, each crosswise to the next and fastened together at top and bottom by pine planking 2A inches thick, doweled with hickory pins; this com pressed the willows to a thickness of about 2 feet and their brush-ends projected 3 or 4 feet. These mattresses were 100 feet long; for the bottom course they were made 50 feet wide, but steadily narrowed for each of the four courses at first needed to bring them to the surface of the water, the top one being 20 feet wide. Wider ones were used in deep water. These were built on shore, on ways as for launching boats; towed by tugs to the places indicated by the line of piles and sunk by loading one or two tons of stone on top. Once down, they speedily filled with sediment and became solid. At the sea end foundations of mattresses 200 to 300 feet broad were laid. For two or three years these stone-laden mattresses gradually sunk in the soft bottom and new ones were added at the top to bring the surface even. The willows not imbedded in sediment were riveted with stone. Where exposed to storms, they were considerably sloped and more thoroughly riveted. The sea ends were afterward capped with concrete blocks. The jetties have under gone considerable repairs since then, but have essentially done their work of making the river navigable for large vessels.