Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/601

RIVER ENGINEERING Jties. Jetties. A bar being caused by the littoral drift, and by the impotence of the expanded current to scour the channel over it to the same depth as elsewhere, it is neces- sary either to arrest the drift or to concentrate the current across the bar. The drift, which comes from the direction of the prevalent winds, might be temporarily arrested by projecting a groyne, or jetty, from the shore on the wind- ward side of the outlet. The material, however, carried along the coast would accumulate against the jetty, and eventually form a bar beyond, or, sweeping round the end of the jetty,, deposit in the channel under its shelter. Accordingly a second jetty is added, on the opposite side of the outlet, to direct and concentrate the current over the bar, and thus increase the depth of the channel, and also to drive into deep water any material that may be carried round the windward jetty, or convey it within the influence of any littoral current farther out. The jetties are either made parallel, or slightly diverging, so as to form a sort of continuation of the banks of the river across the beach, or they are commenced far apart and made to converge towards their extremities, so as to admit a larger volume of tidal water and concentrate the flow into a narrow channel over the bar. The parallel jetty system has been adopted for the new outlet of the Maas (fig. 13), the mouth of the Adour near Bayonne (fig. 16), and the mouths of the Wear at Sunderland, the Yare at Yarmouth, and the Ouse at Newhaven ; whilst the con- verging jetty system has been carried out at Charleston (figs. 17 and 18) and Aberdeen, and at the mouths of the Liffey, the Tyne (fig. 8), and the Tees. The ordinary form of jetty is a timber pier resting upon a k ase o f ru bble stone, like the jetties of the North Sea jetty harbours of Calais, Dunkirk, and Ostend, so that the solid lower portion may concentrate the ebb, whilst the open upper portion permits the passage of the littoral currents in order that a rapid advance of the foreshore may be pre- vented. Such structures, however, simply delay, and do not stop, the advance of the foreshore, as manifested at Newhaven and Dunkirk, where the accumulation of shingle in the one case and of sand in the other brought low- water mark out to the extremities of their western jetties. The solid northern jetties of the Wear and the Yare have naturally produced a similar advance of their northern beaches, both being exposed to a north-easterly drift. A special form of jetty has been constructed at the mouth of the Adour to combine the advantages of open and closed jetties. These jetties consist of a row of cylindrical columns placed at intervals and carrying iron girders on the top. Grooves are formed at the sides of the columns, down which panels can be lowered from the roadway above to confine the issuing current, whilst when the panels are open the spaces between the columns admit the flood tide and the passage of the currents. Nevertheless there are indications of an advance of the foreshore ; the sand passing through the spaces in the northern jetty has encroached upon the channel, and the depths are reduced beyond the end of the jetties (fig. 16). The most important examples of training jetties, includ- ing converging jetties, have been made solid, though sometimes they have not been raised to high-water level at their outer ends, in order to provide for the freer admis- sion of the flood tide. The converging jetties, or walls, at the mouth of the Liffey consist of mounds of rubble stone, and the outer portion of the northern jetty is only raised to half -tide level. The jetties at the mouth of the Maas (figs. 13 and 15) are formed of fascine mattresses; and the Charleston jetties (figs. 17 and 18) are similar in construction ; in both these cases the outer portions have not been raised above half-tide level. The converging jetties or piers at Aberdeen, the Tyne (fig. 8), and the Tees are in reality breakwaters, 1 though they serve the same purpose of protecting the entrance channels from the littoral drift, and promoting scour over the bar, as the less solid structures at Dublin and Charleston. The Tees breakwaters are random mounds of slag ; the Aber- deen breakwaters at the mouth of the Dee are upright walls of concrete ; and the Tynemouth piers are masonry and concrete-block walls upon a rubble foundation. The breakwaters afford a much better shelter for vessels and for dredging operations, but the lower fascine-work jetties at the mouth of the Maas are equally effective in directing the current. Training Walls. The wandering shallow channel of a Objec river through a wide sandy estuary may be improved by traini: training the channel, in a suitable direction, by means of waUs< longitudinal mounds of rubble stone, commonly termed training walls. These walls fix the channel and prevent the current eroding the sandbanks and thus changing its course. Moreover, by guiding the channel into a more direct line, making the ebb and flow follow the same course, and concentrating the current, the scouring capa- city of the stream is increased and the channel is deepened. The flood tide ascends the trained channel more readily, and therefore is able to extend its influence farther up ; whilst the ebb tide flows out of the improved and deepened channel earlier, and thus lowers the low-water line and increases the tidal capacity in the channel. The trained channel must be gradually widened out and Form carried into deep water, otherwise the abrupt expansion traini: which occurs beyond the ends of the training walls would s ' so enfeeble the ebbing current that a shallow shifting channel would be formed only a short distance below. Training walls which stop in the middle of a wide sandy estuary, like the walls carried out on the Seine (fig. 10) and the Kibble, can be only regarded as incomplete works, which sooner or later will have to be extended if the full benefit of a trained channel is to be realized. When the channel is to follow close along one shore of the estuary, a single training wall on the outer side is suffi- cient ; and a single wall is sometimes adequate for main- taining a channel in the middle of an estuary, when placed along the concave side of a bend. The proper width between the training walls depends upon the fall, the tidal range, and the fresh-water dis- charge, and should gradually increase down stream so as to admit as much tidal water as possible with a steady flow. As the scour of the fresh-water discharge is greater with a contracted channel, the tendency is to place the training walls too close together, which, though improving the depth in the channel between the walls, reduces the volume of tidal water that can get up the channel and thus compromises the maintenance of the outlet beyond the walls. The training walls on the Seine and on the Kibble, whilst improving the trained channels, have been preju- dicial to the channels beyond ; and an extension of the works has been authorized on the Kibble. The widths also adopted between these walls are not compatible with an adequate widening out towards their outlet for the free admission of the flood tide, so that these estuaries will eventually be deficient in tidal capacity. The training of a wandering channel is always beneficial Effect to the maintenance and depth of the channel. The traini] wanderings, however, of the channel, which are thus walls - arrested, though very prejudicial to navigation, "are advan- tageous in preventing the silting up of an estuary by the constant erosion and stirring up of the sandbanks which they effect in shifting their position, and which they carry by gradual stages throughout the whole of the 1 L. F. Veraon Harcourt, Harbours and Docks, pp. 200, 317, 341. XX. - 73