Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/605

Rh K I V R J E 581 feet. The South Pass, however, was selected by Congress for improvement, partly on account of the smaller cost of the works required, and partly because it was the pass which had been pre- ferred by a previous commission. The depth of the South Pass through the delta, a distance of about 13 miles, was 30 feet ; so that the obstructions to be removed were restricted to the head and mouth of the pass. Dredging had been tried for deepening the outlet of the South- West Pass, and had necessarily failed in pro- ducing any permanent improvement, for sediment soon filled up again'the portion of the channel which had been enlarged beyond its natural limits of maintenance. The object aimed at in the South Pass was to contract the width of the channel at the head and outlet, so that the current might be forced to regain its required section of channel by scouring out in depth what it lost in width. At the mouth of the South Pass this result could be effected by concentrating the current over the bar with parallel jetties, thus prolonging the banks of the pass artificially into deep water, and contracting the current sufficiently to ensure the requisite depth. At the head, however, of the South Pass, the conditions were more complicated ; for, as soon as the entrance channel there was contracted, a portion of the discharge tended to desert the impeded pass for the other more open passes, and the head works by themselves would therefore have deprived the South Pass of a portion of its natural flow, which would have necessarily led to a reduction of the channel below the head. Accordingly the entrances to the other passes had to be correspondingly reduced, so as not to absorb more than their former proportion of the dis- charge and thus leave the discharge through the South Pass un- diminished. The works consist mainly of willow mattresses, which are specially suitable where osiers are abundant, and where settlement on the soft alluvial bottom is inevitable. The funnel-shaped entrance at the head of the South Pass was contracted across the shoal into a uniform channel, 800 feet wide, by means of mattress dykes ; and mattress sills, 30 to 40 feet wide, 60 to 70 feet long, and 2 feet thick, were laid right across the entrances to the other two passes to restrict the volume of their discharge to its normal amount. The parallel jetties on each side of the mouth of the South Pass consist of tiers of willow mattresses, 100 feet long, from 20 to 50 feet wide, and about 2 feet thick, consolidated with rubble stone, and capped at the outer ends with concrete blocks to secure them against waves (fig. 23). They have beeu raised to flood-tide level to within 1000 feet of their extremities. The east jetty is 2 miles long, and the west jetty 1| miles; they terminate at the same distance out, in a depth of thirty feet. They are placed about 1000 feet apart, and are curved slightly towards their extremities, so as to bring the channel at right angles to the westerly littoral current in the gulf. The jetty channel has been contracted to a width of 700 feet by mattress spurs, in order to promote the scour in the central channel, which was lessened at first by leakage through the mattresses owing to their want of consolidation from difficulties in the supply of stone. The works were commenced in 1875, and completed in 1879. The channel was to be made, according to the terms of the con- tract, 26 feet deep and 200 feet wide, with a central depth of 30 feet. According to a survey made in May 1884, the least central depth through the jetty channel is 33 feet, and the width of the 26-foot channel is 290 feet ; and beyond the jetties the least central depth is 31 - 8 feet, and the least width of the 30-foot channel is 70 feet. At the head of the South Pass there now exists a straight channel with a minimum central depth of 35 feet ; and the 30- foot channel has a minimum width of 275 feet. There is now a depth of 30 feet right through the South Pass. The latest surveys indicate only a slight shoaling beyond the outlet, showing that hitherto the accelerated current, being pro- truded into deep water and aided by the littoral current, has not created a fresh bar by the deposit of its sediment. As the littoral current cannot be expected to convey away more than a portion of the sediment brought down, the material must be accumulating beyond the outlet, and after having filled up the deep places in front, will gradually rise beyond the ends of the jetties, where the dispersed river current will be unable to carry it away. This shoaling, however, may be delayed for a considerable time by the distance to which the velocity of the trained current carries the sediment, and the depth in which the sediment is deposited ; and when it becomes prejudicial to navigation, it can be readily removed by an extension of the jetties. Conclud- The method of lowering a bar at the mouths of rivers ing re- by means of jetties has been applied, as indicated above, to both tidal and tideless rivers ; but the systems employed for each type of river are based on different principles which must not be confounded together. A tideless river is maintained solely by its discharge, and therefore the more its channel is contracted the greater is the depth marks. attained. As, however, a tidal river depends largely for its maintenance on its tidal flow, a contraction at its mouth checks the entrance of the flood tide and reduces the tidal flow. The contracted width between the Adour jetties (fig. 16) has reduced the tidal rise at Bayonne in spite of the openings formed in them for the admission of the tide; and the narrow width of the cut at the Maas outlet (figs. 13 and 14), whilst affording an improved depth in the cut; is prejudicial to the depth elsewhere. A parallel channel with high jetties is suitable for tideless river mouths ; but a slightly diverging channel, with the outer ends of the jetties below high-water level, is expe- dient for tidal rivers. Converging jetties, like those of Dublin and Charleston (fig. 17), would be perfectly use- less at tideless outlets ; but these jetties, by not being unduly contracted at their extremities, and by being kept low towards their ends, freely admit the tidal flow, whilst the increased capacity obtained by their enlarged form increases the tidal scour at the outlet. The comparatively small improvement in depth over Charleston bar, as com- pared with the magnitude of the works, may be due to the want of concentration of tidal scour, owing to the small elevation of the inner portions of the jetties, which allows of the dispersion of part of the ebb. The greatest difficulty in training tidal rivers is so to adjust the width of channel that the free admission of the flood tide may be secured whilst affording adequate scouring power for the current. If the influx of the tide is checked by a sufficient reduction of width to ensure improvement in depth by scour, the capacity of the estuary is eventually reduced, and a portion of the scouring power is lost, as in the case of the Seine. It is better, therefore, to regulate the width so as to ensure a free admission of the tide, and to provide for any deficiency in scour and depth by dredging. Deepening by dredging can be easily and economically effected to any desired extent, as shown by the Tyne and Clyde improvements ; whereas tidal capacity in an estuary, when once lost, can never be regained. For further information about the works described, reference may be made to Rivers and Canals, by L. F. Vernon-Harcourt ; River Tyne Improvements, by P. J. Messent ; "The River Clyde," by James Deas, Proc. Inst. 0. E., vol. xxxvi. ; ' ' The Delta of the Danube," by Sir Charles Hartley, Proc. Inst. C. E., vols. xxi. and xxxvi. ; A History of the Jetties at the Mouth of the Mississippi River, by E. L. Corthell. (L. F. V.-H.) RIVIERA OF GENOA. See ITALY, vol. xiii. p. 437. It is customary to speak of the Eastern and Western Riviera (Riviera di Levante and Riviera di Ponente), which meet at Genoa. RIXDORF, a large village to the south-east of Berlin, and practically an outlying suburb of that city, with which it is connected by tramway, is chiefly interesting as a foundation of Moravian Brethren from Bohemia, who settled here in 1737, under the protection of King Frederick William I. German Rixdorf, which is now united with Bohemian Rixdorf, was a much more ancient place, and appears as Richardsdorf. in 1630 and as Riegenstorp in 1435. The inhabitants of the united community (who numbered 18,729 in 1880, though only 3421 in 1852) are engaged chiefly in weaving, in the manufacture of india- rubber goods, and in the various industries of the neigh- bouring capital. RIZZIO, DAVID, a servant of Mary, queen of Scots, was, according to Buchanan, a native of Turin, and came to Scotland in 1561 in the train of the Piedmontese ambassador. He entered the queen's service as a musician in 1564, and was also employed by her as private foreign secretary. He was murdered in 1566, as has been related in the article MARY (vol. xv. p. 596). RJEV. See RZHEFF.