The average rate of enlargement (of width) adopted for the trained channel of the Nervion, in proportion to its length, is i in 75 between Bilbao and its mouth; 1 in 71 for the Weser from Bremen to Bremerhaven; and about 1 in 73 for the Whangpoo from Shanghai to its outlet ; and these ratios correspond very nearly to the enlargement of the regulated channel of the Clyde from Glasgow to Dumbarton of i in 83, and of the Tyne from Newcastle to its mouth of 1 in 75. Accordingly a divergence com prised between 1 in 7o and 1 in 8o for the regulated or trained channel of the lower portion of a tidal river with a fairly level bed may be expected to give satisfactory results. The divergence as originally laid down for the Seine training works, i in 200, was found to be too small. (See fig. 1 o.) In rivers in which the chan nels are more or less in natural condition, such as the Thames, the Humber and the Scheldt the divergence is nearly i in 5o.
The general effect of deepening and regulating the tidal compartment of a river by dredging and the removal of obstructions is to facilitate the propagation of the flood-tide up the channel and the efflux of the ebb. Consequently such works often result in the lowering of the low-water lines and the raising of the high-water levels in the upper part of the improved channel and sometimes above it. Thus the deepening of the river Tyne raised the level of mean high-water spring tides at Newcastle by about i ft. between 186o and 1904, and lowered the low-water level by 2 ft. at Newcastle quay. Low-water level at Glasgow has fallen nearly 2 ft. since 1873 and the rise of spring tides has been increased by 1' 8".
The average high-water level in the Thames at Chelsea increased 8 in. between 1890 and 1927, a period which corresponds with the dredging carried out on a large scale in the lower reaches of the river. The maximum flood-levels have also steadily increased since 1874 when a tide rose 16' io" above O.D. at Westminster, consid erably higher than any previous record. This height has been exceeded on several occasions since, the highest recorded at West minster (up to the end of 1927) being 17' 6" in 1881, the same level being reached again in 1882. These high levels were, how ever, far surpassed by the disastrous tidal-flood of Jan. 6-7, 1928 (see Report: Floods from the River Thames, Cmd. 3045 1928) when 18' 3" above O.D. was reached at London Bridge (equivalent to about 18' 5" at Westminster). It is difficult to estimate how far the increase in average and maximum tidal levels in the upper part of the tidal Thames is accounted for by the deepening of the river, for the problem is complicated by the extensive building of embankments and the rebuilding (with en larged waterways) of several of the bridges which span the river. There is little doubt that the deepening of the channel has re sulted in raising the high-water levels in the Thames as in other rivers where similar improvements have been effected.
Tidal rivers flowing straight into the sea, without expanding into an estuary, are sub ject to the obstruction of a bar formed by the heaping-up action of the waves and drift along the coast, especially when the fresh water discharge is small. The scour of the currents is of ten in such cases concentrated and extended across the beach by parallel jetties for lowering the bar, as at the outlets of the Maas and the Nervion rivers. Except in the case of large rivers, the jetties have to be placed too close together, if the scour is to be adequate, to form an easily accessible entrance on an exposed coast. Accordingly, in the small bay into which the Nervion flows, a harbour has been formed by two converging breakwaters, which provide a sheltered approach to the river and protect the outlet from drift. Similar provision has been made at Sunderland for the mouth of the Wear; whilst the Tynemouth piers formed part of the original design for the improvement of the Tyne, under shelter of which the bar has been removed by dredging (See HAR BOURS and JETTY.) Training Works Through Estuaries.—Many tidal rivers flow through bays, estuaries or arms of the sea before reaching the open sea, as, for instance, the Mersey through Liverpool bay, the Tees through its enclosed bay, the Liffey through Dublin bay, the Thames, the Ribble, the Dee, the Shan non, the Seine, the Scheldt, the Weser, the Elbe and the Yangtse, through their respective estuaries, the Yorkshire Ouse and Trent through the Humber estuary, the Garonne and Dordogne through the Gironde estuary, and the Clyde, the Tay, the Severn and the St. Lawrence through friths or arms of the sea. These estuaries vary greatly in their tidal range, the distance inland of the ports to which they give access, and the facilities they offer for navi gation. Some possess a very ample depth in their outer portion, though they generally become shallow towards their upper end; but dredging often suffices to remedy their deficiencies and to extend their deep-water channel. Thus the St. Lawrence, which possesses an ample depth from the Atlantic up to Quebec, has been rendered accessible for large sea-going vessels up to Mon treal by a moderate amount of dredging ; whilst dredging has been resorted to in parts of the Thames and Humber estuaries, and on the Elbe below Hamburg, to provide for the increasing draught of vessels; and the Mersey bar in Liverpool bay, about II m. seawards of the actual mouth of the river, has been lowered by suction dredging from a depth of about 9 ft. down to about 26 ft. below low-water of equinoctial spring-tides.