Regulation of the Non-Tidal Portions of Rivers for Navigation

REGULATION OF THE NON-TIDAL PORTIONS OF RIVERS FOR NAVIGATION As rivers flow onward towards the sea, their current becomes more gentle and their discharge larger in volume and less subject to abrupt variations. Large rivers, therefore, often furnish im portant natural highways for inland navigation in the lower portion of their course, as, for instance, the Rhine, the Danube and the Mississippi. Works are, however, often required in such rivers for preventing changes in the course of the stream, for regulating its depth, and especially for fixing the low-water channel and concentrating the flow in it, so as to increase as far as practicable the navigable depth at the lowest stage of the water-level. Regulation works for increasing the navigable capabilities of rivers can only be advantageously undertaken in large rivers with a moderate fall and a fair discharge at their lowest stage ; for with a large fall the current presents a great impediment to up-stream navigation, and there are gen erally great variations in water-level. Consequently, when the discharge becomes very small in the dry season it is impossible to maintain a sufficient depth of water in the low-water channel.

Removal of Shoals.

The possibility of securing uniformity of depth in a river by the lowering of the shoals obstructing the channel depends upon the nature of the shoals. A soft shoal in the bed of a river is due to deposit from a diminution in velocity of flow, produced by a reduction in fall and by a widening of the channel, or to a loss in concentration of the scour of the main current in passing over from one concave bank to the next on the opposite side. These changes from one side of the river to the other are called crossings. The lowering of such a shoal by dredging merely effects a temporary deepening, for it soon forms again. The removal, moreover, of the rocky obstructions at beyond which the same depth is maintained up to Philippsburg, 221 m. above Mannheim, and is being extended up to Strasbourg. (See also CANAL AND CANALIZED RIVERS.) Soon after reaching the Dutch frontier in its seaward course the Rhine divides into three branches of which the most important is the Waal taking about of the water of the undivided river. The Waal and its westward continuations under various names form the main navigable waterway between Germany and Rotter dam and the sea. Its importance to inland navigation may be

gauged by the volume of river traffic passing Lobith at the frontier which increased from 28 million tons in 1903 to 73 million tons in 1927. The improvement of the Waal was commenced about the middle of the 19th century, but the works carried out in it be tween 1909-16 afford one of the most interesting and successful examples of river regulation effected in the present century (fig. 2). The channel at its low stage has been deepened, throughout its length of 53 m., from 71 ft. to I II ft. almost entirely by training works, regulation of width, and its reformation in curved reaches, so that nowhere is there a straight section left in the river. Low spur-groynes or cross-dikes have been largely employed and very little dredging work was necessary. Since 1916 no dredging has been required to maintain the depths.

The Mississippi also, with its extensive basin and its moderate fall in most parts, is well suited for having its navigable depth increased by regulation works, which have been carried out below St. Paul in shallow and shifting reaches, with the object of ob taining a minimum navigable depth during the low stage along the upper river from St. Paul to St. Louis just above the con fluence of the Missouri, thence 9 ft. to New Orleans.

On the Rhone below Lyons with its rapid current, the dikes are constructed of rubble, consolidated above low water with concrete. The dikes on the Rhine consist for the most part of earthwork mounds protected by a layer of rubble or pitching on the face, with a rubble mound forming the toe exposed to the current ; but occasionally fascines are employed in conjunction with the stone or simple rubble mounds. On the Waal the newer cross-dikes have a core of sand protected by a mattress weighted with stone. The dams closing subsidiary channels on the Mis rapids produces a lowering of the river above the rapids by facilitating the efflux, which may result in the appearance of fresh shoals at the low stage of the river. Where, however, narrow rocky reefs or other hard shoals stretch across the bottom of a river and present obstacles to the erosion by the current of the soft materials forming the bed of the river above and below, their removal may enable the river permanently to deepen its bed by natural scour.