Forty wholly removable, regulating weirs were constructed in the Seine several years ago. When wholly closed up in summer, they maintain the required depth of water for steamboat navigation. When wholly open in floods, they cause no stoppage in the river surface. A remarkable barrage mobile has been in action for several years at a place called Port à l’Anglais, above Paris, and above the junction of the Seine and the Marne. When all is open there is not a ripple on the river flowing by. M. Gambuzat, the chief engineer of the river Seine, informed Mr. Lynam that all those wholly removable regulating weirs in the Seine were remarkably effective, and suitable for regulating that great commercial river. Mr. Lynam declared that, if in July 1861, a month previously to the great flood, the Killaloe weir-mound had been wholly removed, and a wholly removable weir, like that in the Seine at Port à l’Anglais, had been constructed, and subsequently been properly manœuvred, during the month of August, none of the crops in the level of the Shannon, above the Killaloe weir-mound, would have been materially injured.[284]
The cost of high weirs on large rivers is considerable. For instance, the most recent weir on the Seine at Poses, retaining a depth of 16½ feet of water, cost 151l. 5s. per lineal foot; and the Mulatière weir, on the Saône at Lyons, retaining a depth of water of 10 feet, cost 118l. 11s. 7d. per lineal foot.
On all navigations and canalised rivers liable to floods, the great difficulty is to be able to pass away the water without impeding the traffic, and without flooding the surrounding country. This has been accomplished on the Weaver to a very great extent by means of what are, practically, movable weirs, at Dutton, Saltersford, Hunts, and Valeroyal. They are flood-gates, or sluices, capable of being lifted clear of the water, and thus allowing an uninterrupted passage, and consist of doors 15 feet by 14 feet deep, built of rolled iron beams with timber sheathing. These are supported by masonry piers, and are lifted by means of overhead gearing, so that the attendants are entirely above water and on a permanent bridge. Friction is practically dispensed with, owing to their working on rollers. The rollers hold the doors from their seating, and would thus allow the passage of the water. To prevent this, “stopwaters” have been introduced, consisting of pieces of hard wood weighted at one end, until the specific gravity is about the same as that of water; they then float vertically, and are held in such a position that the pressure of the water forces them into the angle formed between the door and the masonry.
This plan of sluice has practically reduced by one-half the flood level at Northwich, and instead of having floods of 8 to 12 feet, the highest that has occurred since their erection is one of 6 feet.
On the Aire and Calder Canal a form of sluice has been invented and applied by Mr. Bartholomew which appears to have merits and originality. A large culvert is made alongside the whole length of the lock, with a very large sluice at the upper end, measuring 7 feet by 5 feet, the ordinary sluice being 2 or 3 feet square. Another sluice is provided at the other end, and when this is closed and the lock is empty, the upper sluice, which is self-balanced, like a throttle-valve, is raised. Three orifices are made into the elongated lock, which are arranged in such a way that the vessels are divided, and do not knock against each other while in the lock. In emptying the lock, the upper sluice is let down and the lower sluice is drawn, the water entering the culvert through the orifices and discharging at the lower end. In working the sluices, a man only requires to turn the handle and it raises itself, while with three turns in the other direction it is lowered. The locks on this system are 215 feet long, 22 feet wide, and have 9 feet of water on the sills.
Dams.
The proposed dam on the Nicaraguan Canal is to be of concrete, faced with timber, and will be 1225 feet long on the crest, and 52 feet high. The embankment will be 6500 feet long and 51 feet high in the centre. There are, however, much larger dams than this. Of masonry dams, Verviers, a small city of Belgium, near the frontier of Prussia, with a population of about 38,000, has one—that of Gilleppe—154 feet high and 771 feet long. The water supply of the town of St. Chaumonde, in France, has a dam about 140 feet high, and the water supply of St. Etienne is held by the Furens dam, 170 feet high.
The Villar dam, 162 feet high, holds the water supply of Madrid and other dams in Spain, some of them dating back to Moorish days—Puentes, Alicante, Val de Infierno, Nijar, Elche, and Almanza range from 164 feet to 68 feet in height. In England the Vyrnwy dam, at the Liverpool waterworks is 136 feet high and 1255 feet long. The San Francisco waterworks dam, 170 feet high and 700 feet long, and the Quaker Bridge dam, 278 feet high and 1300 feet long, will, when built, be still larger.
Of earthen dams or embankments, some of the most notable are the Montaubry dam, on the Canal du Centre, 54 feet high; the dam, 66 feet high, by which the water supply of Dublin is impounded; the reservoir dam of the Bolton waterworks, England, over 120 feet high; the Oued Muerad dam in Algeria, 95 feet high. In India and Ceylon such examples are very numerous; the embankment of the Ashti reservoir is 58 feet high and 12,709 feet long; the Karakvasla dam is over 70 feet high; the Tansa reservoir dam (water supply of Bombay) is to be 8500 feet long and 118 feet high; the embankment of the Cummum tank in the Madras Presidency is 102 feet high, and although it ranks among the earliest works of Hindoo history, it is still in such condition as to fulfil its original intention. In Ceylon there are old tanks with embankments from 3 to 12 miles long and 50 feet to 70 feet high.
The materials used for the construction of a weir or dam across a river are principally earth, timber, fascines, stone, &c. The most simple form of dam is that made of gravel protected by fascines kept in place by piles. Such dams are principally used for temporary works. Dams are often made of timber, stones, and earth combined, and covered with planking laid parallel to the current, and the bottom of the channel at the foot on the downstream side should be protected by an apron formed of a platform of planks resting on piles, or by a stone pitching. Dams of this kind built of dry stone and timber often do not become weirs except during floods; that is to say, the water does not pass over their crests except at such times, and at other seasons of the year any surplus finds its way through the interstices between the stones.