Regulation of the Non-Tidal Portions of Rivers for Navigation

The deepening of the bed of a non-tidal river along a consider able length by dredging merely lowers the water-level of the river during the low stage ; and though this deepening facilitates the passage of floods in the first instance, it does not constitute a permanent improvement even in this respect, for the deposit of the detritus brought down by the river as the floods abate soon restores the river to its original condition. Nevertheless, where sand-banks obstruct and divert the channel of a river at its low stage, as in parts of the Mississippi below Cairo, it has been found possible before the river has fallen to its lowest level to form a channel through these sand-banks, with a depth of 9 or 10 ft. and 25o ft. wide, by suction dredgers, aided by revolving cutters Or water-jets (see DREDGERS AND DREDGING). Sand is discharged through floating pipe-lines into a part of the river away from the channel; and the navigation can thus be maintained through the low stage at a reasonable cost. Though, however, these channels across the shoals, connecting the deeper parts of the river, can be easily kept open on the Mississippi till the return of the floods, they are obliterated by the currents in flood-time, and have to be dredged out afresh every year on the abatement of the floods.

Low-water Channel.

The concentration and fixation of the low-water channel is effected by closing subsidiary low-water chan nels with dikes across them, and narrowing the channel at the low stage by low-dipping cross-dikes extended from the river banks down the slope, and sometimes pointing slightly up-stream so as to direct the water flowing over them into a central channel. The contraction of the channel can occasionally be more effectually accomplished, though at a greater cost, by low longitudinal dikes placed along either side of the low-water channel, some distance forward from the banks but connected with them generally at intervals by cross dikes at the back to prevent the current from scouring out a channel behind them during floods. By raising these dikes only slightly above the surface of the bed of the river, except where it is expedient to produce accretion for closing an old dis used channel or rectifying the course of the river, the capacity of the channel for discharging floods is not affected ; for the slight obstruction to the flow produced by the dikes at the sides is fully compensated by the deepening of the low-water channel in the central course of the river.

This system of obtaining a moderate increase in depth during the low stage of a river, whilst leaving the river quite open for navigation, has been adopted on the Rhone, the Rhine and the Mississippi. The improvement of the navigation of the river Po by extensive regulation works of this description has re cently been begun. In the case of the RhOne below Lyons regu lation works were preferred to canalization, in spite of the rapid fall of the river amounting in some places to I in 25o, on account of the large quantities of sand and gravel carried down by it. The

comparative regularity of the discharge, owing to the flow being derived from tributaries having their floods at different times of the year, has aided the effects of the works. Owing, however, to the unfavourable natural condition of the river, the depth does not exceed 5 ft. at lowest water level ; and the rapid current forms a serious impediment to up-stream navigation.

The Rhine is much better adapted for improvement by regula tion works than the Rhone, for it has a basin more than double the area of the Rhône basin, and its fall does not exceed 3.1 ft. per mile at Strasbourg and 2.5 ft. per mile through the rocky defile from Bingen to Kaub, and is much less along most of the length below Strasbourg. These works systematically carried out in wide shallow reaches between the Dutch frontier and Mainz, aided by dredging where necessary, have secured a navigable depth at the low stage of the river of 10 ft. from the frontier to Cologne, 81 ft. from Cologne to Kaub, and 61 ft. up to Bingen, sissippi are almost always constructed of fascine-mattresses weighted with stone; but whereas the regulating dikes on the upper river are usually similar in construction, a common form for dikes in the United States consists of two parallel rows of piles filled in between with brushwood or other materials not affected by water, and protected at the sides from scour by an apron of fascines and stone.

Protecting and Easing Bends.

Unless the concave banks of a river winding through wide, alluvial plains are protected from the scour of the current, the increasing curvature presents serious impediments to navigation, sometimes eventually becom ing so intensified that the river at last makes a short cut for itself across the narrow strip of land at the base of the loop it has formed. This, however, produces considerable changes in the channel below, and disturbances in the navigable depth. Pro tection, accordingly, of concave banks is necessary to prevent excessive curvature of the channel and changes in the course of a river. On the Mississippi the very easily eroded banks are pro tected along their upper, steeper part by stone pitching or a layer of concrete, and below low-water level by fascine-mattresses weighted with stone, extended a short distance out on the bed to prevent erosion at the toe. Dikes, also, projecting into the chan nel from the banks reduce the curvature of the navigable channel by pushing the main current into a more central course ; whilst curved longitudinal dikes placed in the channel in front of con cave banks are also effective in keeping the current away from the banks, which is sometimes still further promoted by dipping cross-dikes in front.