Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/603

RIVER ENGINEERING 579 for the purpose of facilitating tlie removal of the bar and for sheltering vessels entering the river, and were originally designed to terminate iu a depth of 13 feet at low water ; but eventually a larger scheme was adopted for forming a refuge harbour in com- bination with the improvement of the river, and the piers, which are still in progress, are to extend into a depth of 30 feet. In 1861 extensive dredging operations were commenced for improving the depth of the river, and a maximum of 3,515,000 cubic yards was removed in 1866 ; the work has been regularly continued, and the total amount dredged since the commencement, in 1838, reached 49,668,000 cubic yards in 1884, being an average of nearly 2,000,000 cubieyards annually since 1861, when systematic dredging operations were begun. The improvement in depth that was eifected between 1860 and 1884 is shown on the longitudinal section of the river (fig. 9) ; and the deepening of the river between Newcastle and Hedwin Streams, a distance of 8J miles, is in progress, being carried on by six dredgers. The river has also been regulated by making a straight cut across Lemington Point, and widening the channel from 150 feet to 400 feet opposite Blaydon ; the obstruction offered to the Tyne by the old Newcastle Bridge has been removed by rebuilding the bridge with larger openings ; and a sharp bend in the river has been eased by removing the high projecting rock at Bill Point (fig. 8). The tidal capacity of the river has been in- creased by 14,000,000 cubic yards; the bar has been lowered 14 feet since 1860 ; and the least depth at low water up to Newcastle is 20 feet, and 18 feet for 3 miles above. The deepening of the channel has produced a very beneficial lowering of the flood line in the river, thereby preserving the adjacent lands from inundation. 1 The improvement of the river has led to a great increase in the tonnage of the vessels frequenting it, and a large development of its trade. The average tonnage of the vessels, which was 163 tons in 1863, had risen to 396 tons in 1883 ; and the total tonnage of the vessels entering and clearing the Tyue ports rose from 4,382,000 tons in 1863 to 13,043,000 tons in 1883. River Clyde Improvement Works. The Clyde is a small river, with a drainage area of only 945 square miles, and a tidal flow of about 20 miles ; it opens, however, into a deep well-sheltered estuary, or arm of the sea, called the Firth of Clyde, and is free from a bar. The rise of spring tides at its mouth is about 10 feet. The Clyde was by nature an insignificant stream, with numerous hard gravel shoals, and a ford 12 miles below Glasgow which could be crossed on foot. The regulation of the river by cross jetties, and the removal of hard shoals, was commenced in 1773. Early in the present century the jetties were made more uniform, others were added, and their ends were eventually connected by low training walls which were gradually raised as deposits formed behind. The river had to be subsequently widened to accommodate the increasing trade and a larger size of vessels. As scour alone could only produce a very moderate depth, systematic dredging operations were commenced in 1844, and reached a total of 28,648,000 cubic yards by the middle of 1884, the maximum accomplished in a single year (1878-79) reaching 1,502,000 cubic yards. Dredging is -still being continued with six dredgers in order to maintain as well as deepen the river ; for the channel up to Glasgow has been deepened so far beyond its natural limit that any matter in suspension which enters the river is readily deposited. Of the 1,041,000 cubic yards dredged in 1883-84, as much as 703,000 cubic yards consisted of deposit, or more than two-thirds of the whole quantity removed. The river has a depth of 24 feet at high water from Glasgow to Port Glasgow, and from 13 to 15 feet at low water. The tide falls 8 feet lower at Glasgow than it did before any works were begun, which not merely adds to the tidal capacity of the river, but also prevents the fresh-water floods which formerly inundated the low-lying portions of Glasgow. The improved depth has caused the average tonnage of the vessels frequenting the port of Glasgow to rise from 199 tons in 1863 to 315 tons in 1883 ; whilst the total tonnage entered and cleared has increased from 1,757,000 tons in 1863 to 5,544,000 tons in 1883. River Tees Improvement Works. The Tees was formerly a very irregular winding river between Stockton and Middlesborough, and after passing that town it opens out into a wide sandy, estuary about 6 miles long and 3 miles across at its widest part. It is tidal for about 17 miles, and the rise of spring tides at its mouth is 15 feet. The improvement of the river between Stockton and the estuary was commenced in 1810 by making a straight cut near Stockton ; another cut was made in 1830, and the river was also regulated by cross jetties. These works provided a more direct channel, and increased the depth by about from 2 to 5 feet. The channel, however, between the jetties was irregular in depth ; and the navigable channel through the estuary was shallow and variable, with a rocky shoal across it, and a bar at the mouth of the river. Accordingly, in 1853 training walls were commenced, both for connecting the ends of the cross 1 P. J. Messent. C.E., River Tyne Improvement, 1882. jetties, and also for guiding the channel through the estuary. Dredging was commenced in 1854 for removing shoals and deepen- ing the channel ; and the ridge of rock across the estuary channel lias been removed by blasting. The total amount dredged up to Oct. 31, 1884, reached 13,145,000 cubic yards, and a maximum of 1,220,000 cubic yards was dredged in 1883-84. Vessels of 3000 tons, drawing 21 feet of water, can leave Middlesborough fully laden ; the average tonnage of vessels frequenting the Tees has risen from 169 tons in 1873 to 303 tons in 1883; and the total ton- nage entered and cleared has increased from 1, 212,000 tons in 1873 to 2,528,000 tons in 1883. It is proposed at present to deepen the channel by dredging, so as to obtain a depth at low water of 12 feet up to Middlesborough, and 10 feet from thence up to Stock- ton ; and it is hoped that eventually 2 feet additional depth may be attained by the same means. Two breakwaters have been designed, starting from opposite sides of the estuary and con- verging over the bar, in order to protect the entrance, to facilitate dredging, and to keep out drifting sand. The southern break- water has been completed, and the northern breakwater is in pro- gress. The breakwaters and the training walls are constructed of slag obtained free of charge from the neighbouring iron-works. Training Walls on the Tidal Seine. The Seine has a very winding course between Rouen and the sea, Plate V, as well as in its upper portion, but it possesses a good natural figs. 10, depth between Rouen and La Mailleraye, a distance of about 37 11, 12. miles. Below this point, however, the natural condition of the river was very unsatisfactory, for the channel through the estuary was constantly shifting, and high shoals existed at Aizier and Villequier with a depth over them of only 10 feet at spring tides, so that vessels of from 100 to 200 tons found the passage difficult, and even dangerous at times. Training walls, formed of mounds of chalk obtained from the neighbouring clitfs, were commenced in 1848, and were gradually extended along both sides of the channel, as shown in fig. 10> and were terminated at Berville in 1869, a further extension of the northern wall for 1| miles having been refused in 1870 for fear of endangering Havre. These works have effected a remarkable increase in depth in the channel between the walls, as shown by a comparison of the longi- tudinal sections of the river in 1824 and 1875 (fig. 11); for dredging has only been used for deepening the worst shoals. The improvement, however, ceases beyond the termination of the works ; and the channel between Berville and the sea is still changeable and shallow. High walls were for the most part adopted down to Tancarville on the right bank and to La Roque on the left bank. These walls, however, produced such rapid accretion behind them that low walls, raised only from 3 to 5 feet above low water of spring tides, were adopted below these points. This precaution, however, has not arrested the accretion in the estuary, which is still proceeding, though sixteen years have elapsed since the works were stopped. The accretion resulting from the training works had reduced the tidal capacity of the estuary by 274,000,000 cubic yards in 1875 ; and a survey in 1880 showed that a still further loss of over 40,000,000 cubic yards had occurred between 1875 and 1880. More than 28,000 acres of land have been reclaimed in the upper part of the estuary, as shown by cross lines on the plan ; whilst large tracts have been raised to high-water level, as indicated by dotted lines, extending as much as 8| miles below the ends of the training walls. The walls were originally merely rubble chalk mounds ; but the bore and the currents injured the mounds, so that the walls are being strength- ened by pitching or concrete on the river slope, with an apron of concrete, and piling at the toe (fig. 12). The regulation of the river, by bringing deep water about 25 miles nearer the sea, enables vessels of about 2000 tons, and drawing about 20 feet, to pass the shallow estuary between the sea and Berville at high tide and thus reach Rouen. The prolongation of the training walls has been frequently urged ; but the fear of injuring Havre, and the difficulty of devising a suitable channel which would effectually serve both Honfleur and Havre on opposite sides of the estuary, have hitherto prevented the continuation of the work. The Tancarville Canal is in progress for connecting Havre with the Seine at Tancarville (fig. 10), so as to enable river craft to avoid the dangers of the lower estuary ; and the canal is being made so as to be capable of being readily converted into a ship-canal if the growing accretions should impede access between Berville and the sea. Works at the Mouth of the Maas. The Scheur branch of the Maas, which forms the most direct Plate V. channel to Rotterdam, gradually silted up at its outlet, so that figs. 13, vessels had to seek more southern and circuitous channels. The 14, 15. length of the deepest channel was shortened in 1829 by the construction of the Voorne Canal, but even this course became inadequate for the increasing draught of vessels. Accordingly, in 1862 works were commenced for providing a new direct outlet for the Scheur branch of the river by a straight cut across the Hook of Holland, with fascine- work jetties for training and maintaining