Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/597

573 deposit their sediment in the bed of the river, instead of spreading it over the adjacent land, and thus gradually raise the river bed and consequently the height of the floods. The remedies for these defects are strong high banks made of the best materials, and a periodical cleans- ing of the river bed. Neglect of these precautions has led to serious disasters. Large tracts in the Fens have been occasionally flooded by the bursting or overtopping of badly constructed banks. Numerous breaches have occurred in the embankments of the Po, resulting in the devastation of its valley; and the flood level of the Po has been so much raised that it has been decided not to heighten the embankments for fear of occasioning still greater disasters. The gradual silting up of the river Theiss, near Szegedin, produced a rise in its flood level which led to the overtopping of the protecting embank- ment in 1879, and the formation of a breach; and the water thus set free destroyed a portion of Szegedin, and inundated a tract of 200 square miles. The rise of the bed of some rivers in Japan, from the deposit of silt, has been followed up by the gradual raising of the embank- ments ; and this system has been carried out to such a degree, and the accumulated deposit is so great, that some of these embanked rivers have their beds as much as forty feet above the level of the plains over which they flow. These high embankments necessarily require constant attention ; and any failure is attended with serious in- undations. They serve as a warning against the extensive raising of embankments to counteract the silting up of a river. (3) Pumping. When lands are very flat and low, lying sometimes actually below the general drainage level of the district and the waterlevel of the streams, it is impossible for rivers to perform their ordinary function of draining the land by gravitation. It is necessary in such cases to create an artificial fall by pumping the drainage waters up so as to be discharged into the adjacent streams. This method has the advantage of ensuring the effectual drain- age of the lands, provided adequate pumping power is supplied ; but it forms an additional tax on the land, as steam has to be applied to do what is under ordinary con- ditions effected by nature, and the land has also to be surrounded by banks. This system has been adopted for the drainage of the Haarlem Meer reclamation, and also for the lands re- claimed from Lake Y in the construction of the Amster- dam ship canal. The drainage of the Fens is, in several instances, supplemented by pumping ; and a portion of the Witham basin has been secured against floods by this means. lood The formation of large reservoirs in river valleys has ora ge been proposed for storing the surplus waters till a flood has subsided. A reservoir has indeed been formed, by constructing a high masonry dam across a narrow gorge of the Furens valley, which both supplies the town of St Etienne with water and preserves it from inundation. It is also proposed to prevent the floods of the river Chagres from interfering with the Panama Canal, by impounding its flood waters in an extensive reservoir to be formed by building a high dam across a suitable point of its valley. In these cases, however, the deep valleys with their narrow gorges are peculiarly well adapted for the forma- tion of reservoirs having a considerable capacity, whereas most river valleys are unsuitable for reservoirs, and the construction of the long lengths of the banks that would be required would entail a very large expenditure, so that this system could only have a very restricted application, 'analiza- Improvements of the Upper Portions of Rivers. Most iver f * rivers . are not suitable by nature for navigation in their non-tidal portion, or, in the case of tideless rivers, at a considerable distance from their mouths, as the fall of their bed increases towards their source, and they gene- rally present irregularities in depth and flow, with occa- sional sharp bends. Even where the depth is adequate, an irregular or rapid flow offers a great obstacle to up-stream traffic. Moreover, the fall of the waterlevel in dry seasons would often make a river too shallow for navigation. Accordingly, whilst improving the worst bends and remov- ing shoals, it is frequently necessary to retain the water, when the flow is small, so as to maintain a sufficient depth. This is accomplished by dividing the river into a series of sections or reaches, and pounding up the water at the end of each reach by means of a dam or weir. Formerly rivers used to be penned in by a series of Stanc stanches near shoal places, which held up the water, and, when several boats were collected in the pool above a stanch, it was suddenly opened, and the sudden rush of water floated the boats over the shallows below. This primitive method of navigation, termed flashing, was formerly practised on the Thames and the Severn, and also on some of the rapidly flowing rivers in France, such as the Yonne. The stanches on the Severn were removed in 1842; but a few still exist across the Thames above Oxford, where the barge navigation follows a lateral canal. These stanches, consisting of beams swung across the river and supporting a series of spars and paddles, were easily removed in flood time or for the passage of boats. The stanches on the Yonne, which were more recently erected, were of a more elaborate description, known as needle weirs, and are still retained as weirs for holding up the water, though the process of flashing has been discon- tinued. As the demands of navigation increased, these primitive Introi methods proved inadequate, and, moreover, they were - 10 quite unsuited for up-stream traffic. Accordingly weirs were substituted for stanches, to hold up the water in each reach ; and locks were constructed, in suitable side channels, for enabling vessels to be passed from one reach to the next with little loss of water, and with equal facility either up or down. Rivers have been thus converted into still-water navigations, with level reaches forming a series of steps, having a fall at each lock, in place of the natural inclination of the river bed ; so that the up-river traffic is in a great measure relieved from the serious hindrance of an opposing current. In order that the water held back by the weir may be retained within the channel, it is necessary to raise the banks on each side for some distance above the weir ; and, as the gradual rise in the river bed towards the upper end of the reach reduces the depth, it is necessary to deepen the river along the upper portion of the reach to secure a uniform draught of water. A river is thus practically converted into a canal, with this sole difference that it has still to discharge the drainage waters of its basin. This primary object of rivers, to which indeed they owe their existence, was in many instances somewhat overlooked when rivers were utilized for naviga- tion ; and weirs appear to have been often regarded merely as dams for retaining the water, rather than as regulators of its flow. Weirs. Locks have been already considered in the article on CANALS (q.v.); so that it will suffice here to describe briefly the different forms of weirs, which are essentially river works. Weirs have been divided into three classes, namely, overfall weirs, draw-door weirs, and movable weirs. 1 Overfall Weirs. An overfall weir is a solid barrier placed across Overf a stream for the purpose of raising the waterlevel (Plate V. fig. I 2 ), weirs, and only affords an outlet for the discharge of the river when the water rises above its crest. The waterlevel of the river is thus per- manently raised, not merely in dry seasons, but also in flood time, 1 L. F. Vernon-Harcourt, Rivers and Canals, p. 112. 2 The fig. references in the present article all indicate Plate V.