by regulation works, which have been carried out below St Paul in
shallow and shifting reaches, with the object of obtaining a minimum
navigable depth during the low stage of 6 ft. along the upper
river from St Paul to St Louis just below the confluence of the
Missouri, and 8 ft. thence to Cairo at the mouth of the Ohio.
Various materials are used for the regulation works according to the respective conditions and the materials available in the locality. On the Rhone below Lyons with its rapid current, the dikes have been constructed of rubble-stone, 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-stone or pitching on the face, with a rubble mound forming the toe exposed to the current; but occasionally fascines are employed in conjunction with stone or simple rubble mounds. The dams closing subsidiary channels on the Mississippi 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 iles 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. Other forms of dikes sometimes used are timber cribs filled with stone, single rows of sheet piling, permeable dikes composed of piles supporting thin curtains of brus wood for promoting silting at the sides, and occasionally rubble-stone in places needing special protection.
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 becoming 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. Protection, 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 protected 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 channel 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 concave banks (figs. 4 and 6) are still more effective in keeping the current away from the banks, which is sometimes still further promoted by dipping cross dikes in front (fig. 5).
Regulation of Depth.—The regulation works at bends, besides arresting erosion, also reduce the differences in depth at the bends and the crossings, since they diminish the excessive depth round the concave banks and deepen the channel along the crossings, by giving a straighter course to the current and concentrating it by a reduction in width of the channel between the bends (figs. 4 and 5). Where there are deep pools at intervals in a river, shoals are always found above them, owing to the increased fall which occurs in the water line on approaching the pool, to compensate for the very slight inclination of the water-line in crossing the pool, which serves for the discharge of the river through the ample cross-section of this part of the river-bed. These variable depths can be regulated to some extent by rubble dikes or fascine mattress sills deposited across the bed of the pool, so as to reduce its excessive depth, but not raised high enough to interfere at all with the navigable depth. These obstructions in the pool raise the water-line towards its upper end, in order to provide the additional fall needed to effect the discharge through the pool with its diminished cross section; and this raising of the water-line increases the de th over the shoal above the pool, so that the general depth in these irregular parts of a river is rendered more uniform, with benefit to navigation.
Canalization of Rivers.
Rivers whose discharge is liable to become quite small at their low stage, or which have a somewhat large fall, as is usual in the upper part of rivers, cannot be given an adequate depth for navigation by regulation works alone; and their ordinary summer level has to be raised by impounding the flow with weirs at intervals across the channel (see Weir), while a lock (see Canal and Dock) has to be provided alongside the weir, or in a side channel, to provide for the passage of vessels (fig. 8). A river is thereby converted into a succession of fairly level reaches rising in steps up-stream, providing a comparatively still-water navigation like a canal; but it differs from a canal in the introduction of weirs for keeping up the water-level, in the provision for the regular discharge of the river at the weirs, and in the two sills of the locks being laid at the same level instead of the upper sill being raised above the lower one to the extent of the rise at the lock, as usual on canals. Canalization secures a definite available depth for navigation; and the discharge of the river generally is amply sufficient for maintaining the impounded water-level, as well as providing the necessary water for locking. The navigation, however, is liable to be stopped during the descent of high floods, which in many cases rise above the locks (fig. 7); and it is necessarily arrested in cold climates on all rivers by long, severe frosts, and especially on the break-up of the ice.
Fig. 7.—Canalized River Main.
Instances of Canalized Rivers.-Many small rivers, like the Thames above its tidal limit, have been rendered navigable by canalization, and several fairly large rivers. have thereby provided a good depth for vessels for considerable distances inland. Thus the canalized Seine has secured a navigable depth of 1012 ft. from its tidal limit up to Paris, a distance of 135 m., and a depth of 612 ft. up to Montereau, 62 m. higher up. Regulation works for improving the river Main, from its confluence with the Rhine opposite Main; up to Frankfort, having failed to secure a minimum depth of 3 ft. at the low stage of the river, canalization works were carried out in 1883–86 by means of five weirs in the 22 m. between the Rhine and Frankfort, and provided a minimum depth of 612 ft. (figs. 7 and 8).
Fig. 8.—Locks, Weir and Haven near Frankfort.
This depth was subsequently increased by dredging the shoaler portion towards the upper end of each reach, due to the rise of the river-bed up-stream, so as to attain a minimum depth of 723 ft. just below the lowest lock, and 745 to 813 ft. in the other reaches; whilst a sixth weir was erected at Offenbach above Frankfort (fig. 7). The Great Kanawha, Ohio, and other rivers, furnish instances of canalization works in the United States.
Limits to Canalization.—On ascending a river it becomes increasingly difficult to obtain a good depth by canalization in the upper part, owing to the progressive inclination of the river-bed; thus, even on the Seine, with its moderate fall, whereas a depth of 1012 ft. has been obtained on the Lower Seine by weirs placed on the average 1312 m. apart, on the Upper Seine weirs are required at intervals of only about 423 m. to attain a depth of 612 ft. Accordingly, the higher parts of rivers are only suitable for floating down trunks of trees felled on the hills, or rough rafts of timber, conveying small loads of produce, which are broken up on reaching their destination. Moreover, sometimes an abrupt fall or rocky shoals make it necessary to abandon a section of the river and to continue the navigation by atera cana.
Small River Outlets exposed to Littoral Drift.
Rivers with a small discharge flowing straight into the sea on an exposed coast are more or less obstructed at their outlet
