IMPROVEMENT OF TIDAL RIVERS FOR NAVIGATION Whereas the size of tideless rivers depends wholly on their fresh water discharge, the condition of tidal rivers is due to the con figuration of their outlet, the rise of tide at their mouth, the dis tance the tide can penetrate inland, and the space available for its reception. Accordingly, tidal rivers sometimes, even when pos sessing a comparatively small fresh-water discharge, have a much better natural navigable channel at high tide than the largest deltaic rivers, as shown by a comparison of the Thames, the Humber and the Elbe with the Danube, the Nile and the Mis sissippi. Tidal water is, indeed, unlimited in volume ; but, un like the drainage waters, which must be discharged into the sea, it only flows up rivers where there is a channel and space available for its reception. Consequently, works which exclude the tide from a river may have injurious effects on the channel, as did the sluices which were erected long ago across the fen rivers of eastern Anglia to secure the low-lying lands from the inroads of the sea. The tidal influx is also liable to be reduced by the accretion which may result in an estuary from the construction of training works. The aims of all tidal river improvement should be to facilitate to the utmost the flow of the flood-tide up a river, to remove all obstructions from the channel so as to increase the scouring efficiency of the flood and ebb tides, and to reduce to a minimum the period of slack tide when deposit takes place.
The progress of the flood-tide up a river and the corresponding ebb are clearly shown by a diagram giving a series of tidal lines obtained from simultaneous observa tions of the height of the river Hugli during a high spring-tide in the dry season. The observations were taken at intervals at sev eral stations along the river and the lines exhibit on a very dis torted scale the actual water-levels of the river at these periods. The steep form assumed by the foremost part of the
tide lines from the entrance to beyond Chinsura, attaining a maximum in the neighbourhood of Konnagar and Chinsura, indi cates the existence of a bore. This is caused by the
in the channel obstructing the advance of the flood-tide, till it has risen sufficiently in height to rush up the river as a steep, break ing wave, overcoming all obstacles and producing a sudden reversal of the flow and abrupt rise of the water-level. Such phenomena
are observed on the Severn, the Seine, the Amazon and other rivers. A bore indicates defects in the tidal channel, which can only be reduced by lowering the obstructions and by the regula tion of the river. No tidal river of even moderate length is ever completely filled by tidal water; for the tide begins to fall at its mouth before the flood-tide has produced high-water at the tidal limit. This is clearly shown in the case of a long tidal. Every im provement of the channel, however, expedites and increases the filling of the river, whilst the volume of water admitted at each tide is further augmented by the additional capacity provided by the greater efflux of the ebb, as indicated by the lowering of the low-water line.
The improvement of tidal rivers mainly by dredging is specially applicable to small rivers which possess a width sufficient for navigation like the Clyde and the Tyne; for such rivers can be considerably deepened by an amount of dredging which would be quite inadequate for producing a similar increase in depth in a large, wide river, with shifting channels. Both the Clyde below Glasgow and the Tyne below Newcastle were originally insignificant rivers, almost dry in places at low-water of spring tides; and the earliest works on both rivers consisted mainly in regulating their flow and increas ing their scour by jetties and training works. They have been improved, since 1840 on the Clyde and 1861 on the Tyne, by continuous systematic dredging. The Clyde has been given a mini mum depth of 27 ft. in the chan nel at low-water of spring tides up to Port Glasgow; 25 ft. up to Rothesay Dock, and 24 ft. up to Glasgow Harbour. A depth of 32 ft. at low-water has been provided alongside some of the quays. The Tyne has been progressively deepened (fig. 6) until in 1929 the channel from the sea to the Northumberland dock (31 m.) has a minimum depth at low-water spring-tides of 3o ft.; from thence to Derwen thaugh, above Newcastle, 25 ft.; and about 12 ft. on to Newburn, the rise of tide at springs increasing these depths by 15 feet.