Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/604

r>so RIVER ENGINEERING the channel across the sandy beach into deep water (fig. 13)'. The cut, three miles long, was only partially excavated, its completion being effected by damming up the old channel and directing the Fresh-water discharge and tidal current through the new cut. The scour soon deepened the narrow cut ; but some of the sand washed from the cut settled in the wider channel formed by the jetties. The cut also, being scoured deeper than the adjacent channel above and below, was not adequately widened, and consequently impedes the entry of the flood tide up the river (figs. 13 and 14). The desired depth of 23 feet at high water not having been attained as anticipated from the works, dredging has been resorted to for deepening "the outlet ; and the widening out of the cut to the proper full width would improve the tidal influx. The jetties consist of fascine mattresses secured by piles and stakes and weighted with stone ( fig. 1 5) ; and their outer portions have been kept down to half-tide level to promote the freer admission of the flood tide, whilst serving equally to concentrate the latter part of the ebb. Charleston Jetties. late V. The largest jetty works in the world for lowering a bar in front gs. 17, of a tidal estuary are being constructed at the entrance to Charles- 3. ton harbour (fig. 17). The jetties are being formed after the type of the Haas jetties, with log and fascine mattresses weighted with rubble stone (fig. 18). They start from the shore about 2 miles apart and converge to a width of about 2000 feet over the bar, which stretches across the entrance to the harbour at If miles from the shore. The object of the works is to concentrate the tidal and fresh-water current from the land-locked estuary, having an area of 15 square miles, into which the Ashley and Cooper rivers flow, and whose mouth forms the entrance to Charleston harbour. The northern jetty, which was commenced in 1878, attained the present length of 14,860 feet in 1881 ; and the southern jetty had been carried out 14,130 feet towards the end of 1883. The outer portions, lying in the direction of the flood cur- rent, are to be raised ; but the inner portions are to be kept low, in order to interfere as little as possible with the littoral drift and thus avoid an advance of the foreshore, and also in order to admit freely the flood tide. Though scour has already commenced, it will be necessary to aid it by dredging in order to attain a depth of 21 feet in place of the former depth of 11^ feet. Improvement of Tideless Rivers. (eltas. Tideless rivers on entering the sea have their velocity checked, and consequently deposit the silt which they previously carried in suspension. In process of time this accumulated deposit forms a tract of low-lying land protruding into the sea, through which the river flows in several shallow channels to the sea owing to the impedi- ments offered to its flow by the sediment which it deposits. The form which these diverging channels assume has led to the term delta being applied to the mouths of tideless rivers and the tract of land which they create (figs. 19 and 22). These deltas are always advancing, and con- sequently reducing the very small fall of the channels through them by prolonging their course. The rate of advance varies with the amount of sediment brought down, the depth of the sea in front, and the extent to which the delta spreads out. The Rhone delta has at present a yearly average progression of 1 40 feet ; the Kilia mouths of the Danube delta have been estimated to advance 200 feet annually ; whilst the Mississippi delta, extending 220 miles into the Gulf of Mexico, is supposed to have taken four thousand four hundred years in forming, which would be equivalent to an average annual advance of 264 feet, the present advance being about 207 feet in a year. etties. The only method of improving the outlet of a tideless river is to concentrate the current flowing through one of the channels, and to prolong the banks of the regulated channel into deep water by means of parallel jetties. ateral The only other way of remedying the impediments to mal. navigation at the mouths of tideless rivers is by avoiding the delta channels altogether, and constructing a canal connecting the deep river above the delta with the sea at some suitable place beyond the influence of the river alluvium. This expedient has been resorted to for the trade of the Rhone ; for, though the discharge of the river was concentrated into a single outlet by forming embank- ments on each side, between 1852 and 1857, which shut off the other three outlets and extended into the sea at its mouth, the increased discharge brought down the whole sediment of the river, and a bar formed again farther out. Accordingly the St Louis Canal was formed, between 1863 and 1873, going from the head of the delta into the Bay of Foz beyond the limits of the delta. A similar plan was proposed for the Mississippi, but was abandoned in favour of the jetty system. The canal constructed by the emperor Claudius for connecting the Tiber with the harbour of Ostia proved a failure, as its outlet at Ostia was not sufficiently removed from the delta, so that it gradually silted up and is now 2 miles from the coast. The success of the jetty system depends upon the exist- Require ence of a littoral current to carry away the sediment in the ments o stream conveyed by aid of the jetties into deep water, or J etptv upon the gradual prolongation of the jetties to keep pace sy with the progression of the delta. The outlets of the Danube and the Mississippi have both been improved by training one of their minor delta channels into deep water ; and hitherto the depth over the bars has been maintained. In the case of the Danube, the southerly current sweeping across the outlet carries away a portion of the issuing sediment. The Mississippi jetties direct the discharge into such deep water, and with so much velocity, that it may be premature to decide to what extent the maintenance of the depth may be due to a westerly current in the gulf ; but hitherto from one cause or the other, or probably from both combined, the bar has not formed again in front of the jetties. Sulina Piers of the Danube Delta. The delta of the Danube commences about 45 miles from the Pla Black Sea, and has an area of 1000 square miles. The river divides figs. 1! into three main branches; the northern or Kilia branch conveys more 20, 2 than three-fifths of the discharge, but it forms an independent delta near its outlet and is consequently unsuitable for improvement (fig. 19). The southern or St George branch is the next largest, and possesses the best channel, but it divides near its outlet into two channels, which are both barred. The central or Sulina branch, though narrower and less good than the St George branch, and conveying less than one-thirteenth of the total discharge, possessed the only navigable outlet in 1858, and was therefore selected for the provisional improvement works begun in that year. The works designed by Sir Charles Hartley consist of piers, starting on each side of the Sulina outlet, which converge till the width between them is 600 feet, and are then carried parallel across the bar (fig. 20). The piers were at first constructed of rubble mounds with piles carrying a platform strengthened at intervals by timber cribs ; but subsequently they were consolidated with concrete blocks (fig. 21). These piers serve to concentrate the discharge across the bar, and increased the least depth at the outlet from 9 feet in 1857 to 20 feet in 1872, a year after the final completion of the works ; and this depth has been since maintained. The sediment-bearing current, moreover, is carried within the influence of the southern littoral current, which, diverting a portion of the deposit, has reduced the rate of advance of the Sulina delta from 94 feet to 44 feet in a year. This progression, however, will event- ually necessitate the extension of the piers. The Mississippi Jetties. Though the Gulf of Mexico is in communication with the Plat Atlantic Ocean, it is almost tideless, for the average rise of tide in figs, front of the Mississippi delta is only 14 inches, and there is only 23. one tide in a day. The Mississippi, accordingly, is a tideless river, and forms a delta which has an area of 12,300 square miles, and has three main channels, or passes, leading the discharge of the river into the gulf (fig. 22). The general features of the delta have been already described, and the various schemes for improving the outlet referred to (see MISSISSIPPI). In flood time the river brings down in suspension 2800 cubic feet of solid matter per second ; and before the jetties were commenced the annual advance of the bars at the mouths of the passes was 300 feet at the South- West Pass, 260 feet at Pass a 1'Outre, and 110 feet at the South Pass. In 1874 Mr Eads offered to make and maintain a channel 28 feet deep across the bar of the South-West Pass, which had only 13 feet of water over it. The South Pass is narrower, having a width of from 600 to 800 feet ; it conveys only about a tenth of the total dis- charge of the river ; it was impeded by a shoal at the head of the pass with a depth of only 15 feet over it ; and the depth on the bar at its mouth, where it expanded to 2 miles in width, was only 8