South Pass feity-uvrks.—The Danube in Europe and the Mississippi in America have been made- the subject of elaborate engineering works with the object of improvinp,- the navigability of these rivers at their mouths. The jetty-works of the South Pass of the Mississippi River con structed by the late Captain James B. Eads are shown in perspective in Figures 2 and 3 (fi/. 55), and a brief description of them will be of interest. The object of the works was to secure and maintain a channel sufficiently wide and deep to be navigable by the largest vessels, and this required to be artificially opened through the great deposits of sediment by which the entrance to the river from the Gulf of 'Mexico was obstructed, rendering navigation extremely difficult, and, before the execution of the jetties, impossible, for vessels of considerable draught.
The plan of the work (which was begun in 1874, and practically fin ished about five years later) involved the removal of the point where the sediment of the river was originally deposited—namely, in the shallow water at the entrance of the Pass—farther ont into the deeper water of the Gulf, where the filling- up again of the channel would be indefinitely post poned. In accomplishing, this object, the banks of the Pass were extended by the erection of artificial walls, within which the waters of the river are confined. These walls were designed to be carried sufficiently far out toward the deep waters of the Gulf, and to be so proportioned in width in relation to the quantity of discharging water as to cause an increased veloc ity of current, by whose aid the stream would naturally scour out for itself a channel of the required depth and width. In carrying this design into effect, two extensive lines of jetties were built along the course of the stream at the above-named situation, tbese jetties constituting artificial banks to the river, intended to confine its waters within a comparatively narrow channel, and to prevent them from diffusing themselves over a great area as they enter the sea. (The manner of building these artificial walls will be found described on page 292.) The lines of the jetties are moo feet apart. The east jetty, from the land's end to deep water, in which the jetty-heads are built, is about 12,5oo feet in length. It extends in a nearly straight line for a distance of Soo feet, and then curves to the west, in order to strike most favorably a littoral current from the east which was ascertained to exist in the Gulf. For the greater part of its length the east jetty is built on a lateral shoal having an average depth of about 6 or 7 feet; at the Gulf-end it terminates in 3o feet of water. The west jetty begins about 4000 feet lower dowu the river, and extends parallel to the other throughout its whole length, terminating at the same place. As collateral improvements executed in conjunction with
the jetties, two auxiliary Works were carried into effect. These comprise the closure of the mouth of one of the principal lateral channels (the Grand Bayou) and the building of a dyke at the head of the Pass. These had for their purpose the deflection of a large volume of water into the South Pass, with the object of causing an increased velocity of current, and conse quently a greater scouring action. The effect of these engineering works has been very favorable.
Straightening Winding the stream makes one or several windings or loops, its course may be straightened by cutting the channels through at the points of the curve lying nearest to each other. By this expedient a certain amount of land is gained, the necessity of con stant repairs to the banks is obviated, and the course of the river is short ened. In work of this nature it is not necessary to excavate the new chan nel to the full section of the river-bed, but merely to dig a channel of say one-tenth to one-thirtieth its width, according to the size of the river, and down to low-water level, leaving to the river itself the task of excavating its new bed by erosion.
Correction of the instructive example of the application of the foregoing principles is afforded by the engineering works by which the rigime of the river Rhine has been established on the Bodensee-Bavarian border. The final results accomplished by these works—executed between IS19 and IS6S—which embrace a correlated system of channel and shore improvements in connection with eighteen channel-cuts, will appear from the following brief statement: The river flows either in a straight course or in gentle curves between parallel banks defended by stone embank ments, and with a uniform breadth of 787.2 feet. The former length of the river between Lauterberg and Worms, of ninety-three miles, is now reduced to fifty-four miles. The fall of the river—which in its former ser pentine course was very irregular, so that boats going against the current required in some places but one horse, and in others five horses, for towage— is now rendered quite uniform, tbe surface slope being- o.4 in zoo° at Lauter bergr, decreasing regularly to o.1 in moo at Worms. The numerous short cuts have increased the depth of the river-bed 7.38 feet, and considerable reaches of ground that formerly were always under water are now laid drv and under cultivation. The benefit accruing from these improvement works is further seen in the more rapid draining off of high water, and, as a consequence of the straightening. of the channel, in the diminished fre quency of ice-jams.