delta in front of an outlet is proportionate to the size of the channel, and the length of the jetties required for lowering the bar by scour in front of any channel is proportionate to the discharge of the channel. Consequently, the conditions are more unfavourable for the improvement of the outlets of the larger delta channels than of the smaller ones; though, on the other hand, the larger channels crossing the delta are generally more suitable for navigation on account of their size, and the natural depth over their bars is greater owing to the larger discharge.
The discharge of the main branch of the Rhone, which formerly flowed into the Mediterranean and the Gulf of Foz through six mouths, was in 1852–57 concentrated in the direct eastern channel by embankments along sides, which closed all the lateral channels. The entire flow of the river, being thus discharged through the eastern outlets, increased for a time the depth over its bar Rhone. from 412 ft. to 934 ft.; but as the great volume of alluvium brought down, including an unusually large proportion of sand rolled along the bed of the river, was also all discharged through the. one outlet, the bar soon formed again farther out, and naturally advanced with the delta in front of the outlet more rapidly than formerly when the deposit was distributed through six divergent mouths. Accordingly, the very moderate deepening produced by the embankments was not long maintained, and the average depth over the bar has not exceeded 612 ft. for many years past; the St Louis Canal was constructed to provide a deeper outlet for the navigation.[1] This want of success was due to the selection of an outlet opening on a sheltered, somewhat shallow bay, instead of a southern outlet discharging into deep water in the Mediterranean and having a deep littoral current flowing across it, and also resulted from the closing of all the-other outlets, whereby the whole of the deposit, as well as all the discharge, was concentrated in front of the badly situated eastern outlet. The southern Roustan branch was reopened in 1893 to prevent the silting-up of the outlet of the St Louis Canal.
The Danube traverses its delta in three branches, the northern one of which, though conveying nearly two-thirds of the discharge of the river, is unsuitable for improvement owing to its splitting up along portions of its course into several channels, and eventually flowing into the sea through twelve mouths of a small independent delta advancing about 250 ft. annually Danube. across a shallow foreshore. The central Sulina branch was selected for improvement in 1858 in preference to the southern St George’s branch, which had a more favourably situated outlet and a better channel through the delta, on account of the much smaller expenditure required for carrying out jetties to the bar in front of the Sulina outlet, which was only half. the distance from the shore of the bar of the St George’s outlet, owing to the much smaller discharge of the Sulina branch.[2] The jetties, begun provisionally in 1858 and subsequently consolidated and somewhat extended, were finally completed in 1877. They increased the depth over the bar from an average of about 9 ft. previously to 1858 up to 2012 ft. in 1873, which was maintained for many years. In 1893, however, the increasing draught of vessels rendered a greater depth necessary; the wide inshore portion of the jetty channel was therefore narrowed by inner parallel jetties, and a powerful dredger was set to work in the jetty channel and outside, whereby the depth was increased to 24 ft. in 1897, and was fairly maintained up to 1907, when a second dredger became necessary to cope with the shoaling. The somewhat small ratio of sediment to discharge in the Danube, the fineness of the greater portion of this sediment, its comparatively moderate amount owing to the small proportion of the discharge flowing through the Sulina branch, and its partial dispersion by the southerly littoral current and wave action, have prevented the rapid formation of a shoal in front of the Sulina outlet. Nevertheless, the lines of soundings are gradually advancing seawards in the line of the outlet channel, and there are signs of the formation of a new bar farther out, whilst the deposit to the south by the current and waves has deflected the deepest channel northwards. Accordingly, a prolongation of the jetties will eventually be necessary, notwithstanding the removal of a portion of the deposit from the outlet channel by dredging.
The selection of the outlet of the south pass of the Mississippi delta for improvement by parallel jetties in 1876–79, in spite of the south-west pass possessing a larger channel and a better depth over its bar, was due, as at the Danube, to motives of economy, as the bar of the south-west pass was twice as far off from the shore as that of the south pass (fig. 13). There fascine Mississippi. mattress jetties, weighted with limestone, and with large concrete blocks at their exposed ends (see Jetty), 214 and 112 m. long, and curved slightly southwards at their outer ends to direct the sediment-bearing current more directly at right angles to the westerly littoral current, increased the depth of 8 ft. over the bar in 1875 up to 31 ft. between the jetties and out to deep water (fig. 14). The prolonged current of the river produced by the jetties has; as at the Sulina outlet, carried the main portion of the heavier sediment into fairly deep water, so that the greatest advance of the foreshore in front of the south pass has occurred in the 70-ft. line of soundings, though the shallower soundings have also advanced.
Fig. 14.—Deltaic Jetty Outlet, South Pass, Mississippi.
The shoaling, however, in the jetty channel necessitated its reduction in width by mattresses and spurs from 1000 ft. to 600 ft., and also dred ing to maintain the stipulated central depth of 30 ft., and 26 ft. depth for a width of 200 ft., out to deep water; whilst the outer channel was deflected to the east and narrowed by the alluvium carried. westwards by the littoral current and also deposited in front of the jetty outlet. Accordingly, dredging has been increasingly needed to straighten the channel outside and maintain its depth and width; and since the United States engineers took in hand its maintenance in 1901, the available depth of the outlet channel has been increased from 26 ft. up to 28 ft. by extensive suction dredging.
In order to provide for the increasing requirements of sea-going vessels, the dredging of a channel 35 ft. deep and 1000 ft. wide, cut from the large south-west pass outlet to deep water in the gulf, was begun at the end of 1903; and jetties of fascine mattresses weighted with stone and concrete blocks have been carried out about 4 and 3 m. respectively from the shore on each side of the outlet for maintaining the dredged channel[3] (fig. 15).
Fig. 15.—Deltaic jetty Outlet, South-West Pass, Mississippi.
These works differ from the prior improvement of the south pass in the adoption mainly of suction dredging for the formation of the channel in place of scour alone, so that it will be unnecessary to restrict the width of the jetty channel to secure the desired depth; whilst as the discharge through the south-west pass is rather more than three times the discharge through the south pass, and the bar is double the distance seawards o the outlet, the slightly converging jetties, in continuation of the south-west pass, are placed about 3400 ft. apart at their outer ends; and have been given about twice the length of the south pass jetties. As soon as the dredging of the channel has been completed (which depends on the appropriations granted by Congress) the south pass will be abandoned, and the south-west pass will form the navigable approach. Dredging will be required for preserving the depth of the outlet of the south-west pass; and when the large volume of sand and other alluvium discharged by the pass accumulates in front sufficiently to begin forming a bar farther out, an extension of the jetties will be necessary to maintain the elongated channel free from drift, and extend the scour, especially in flood-time.
Improvement of Tidal Rivers for Navigation.
Whereas the size of tideless rivers depends wholly on their fresh-water discharge, the condition of tidal rivers is due to the configuration of their outlet, the rise of tide at their mouth, the distance the tide can penetrate inland, and the space available for its reception. Accordingly, tidal rivers sometimes, even when possessing a comparatively small fresh-water discharge, develop. under favourable conditions into large rivers in their lower tidal portion, hav1ng a much better natural navigable channel at high tide than the largest deltaic rivers, as shown by a comparison of the Thames, the Humber and the Elbe with the Danube, the Nile and the Mississippi. Tidal water is, indeed, unlimited, in volume; but, unlike the drainage waters which must be discharged into the sea, it only flows up rivers where there is a channel and space available for its
