Figs. 21 and 22.—Training Tidal River at Estuary: River Weser.
the fresh-water discharge, generally forms the navigable channel, which is scoured out during floods. Narrowing the river between the bends to bring the two channels to ether would unduly restrict the tidal flow; and in a river like the Hugli dependent on the tidal influx for the maintenance of its depth for two-thirds of the year, and with channels changing with the wet and dry seasons, »so that deepening by dredging in tie turbid river could not be permanent, training works below low water to bring the ebb-tide current into the flood-tide channel, which latter must not be obstructed aided by dredging, the best prospects of improve at all, offer,
Fig. 23.—Moyapur Reach, River Hugli, Jan. 1896.
- Fig. 24.—James and Mary Reach, River Hugli, April 1890.
The average rate of enlargement adopted for the trained channel of the Nervion, in proportion to its length, is 1 in 75 between Bilbao and its mouth, and) 1 in 71 for the Weser from Bremen to Bremerhaven; and these ratios correspond very nearly to the enlargement cf the regulated channel of the Clyde from Glasgow to Dumbarton of 1 in 83, and of the Tyne from Newcastle to its mouth of 1 in 75. Accordingly, a rate of enlargement comprised between 1 in 70 and 1 in 80 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.
Works at the Outlet of Tidal Rivers.—Tidal rivers flowing straight into the sea, without expanding into an estuary, are subject 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; and the scour of the' currents is generally concentrated and extended across the beach by parallel jetties for lowering the bar, as at the outlets of the Maas (figs, II and 12) and of the Nervion (figs. 19 and 20). In the latter case, however, the trained outlet was still liable to be obstructed by drift during north-westerly storms in the Bay of Biscay; and, 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, a harbour has been formed in the small bay into which the Nervion flows by two converging breakwaters, which provides a sheltered approach to the river and protects the outlet from drift (fig. 19), and a 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 (fig. 17).
Training Works through Sandy 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 Shannon, the Seine, the Scheldt, the Weser and the Elbe 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 giye access, and the facilities they offer for navigation. 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 seagoing vessels up to Montreal by a moderate amount of dredging; whilst dredging has been resorted to in parts of the Thames and Humber estuaries, and on the Elbe a little below Hamburg, to provide for the increasing draught of vessels; and the Mersey bar in Liverpool Bay, about 11 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 27 below low water of equinoctial spring tides, to admit Atlantic liners at any state of the tide. Some estuaries, however, are so encumbered by sand banks that their rivers can only form shallow, shifting channels through them to the sea; and these channels require to be guided or fixed by longitudinal training walls, consisting of mounds of rubble stone, chalk, slag or fascines, in order to form sufficiently deep stable channels to be available for navigation. The difficulty in such works is to fix the wandering channel adequately, and to deepen it
