The minimum low-water depth in the river Thames below Thames Haven in its natural state was about 27 ft. (see HAR BOURS). A channel at least 3o ft. deep at low-water spring-tides now extends from the sea to the King George V. dock, 37 m. above the Nore and Shoeburyness (fig. 7). Shallow portions of the chan nel between Shoeburyness and Purfleet have been dredged and above Purfleet the river has been considerably deepened in recent years throughout its course to secure this depth. Above the King George V. dock the river has been dredged to lesser depths at low water, decreasing to 14 ft. at London Bridge.
Regulation and Dredging of Tidal Rivers.—Considerable improvements in the navigable condition of tidal rivers above their outlet or estuary can often be effected by regulation works aided by dredging, which ease sharp bends, straighten their course and render their channel depth and flow more uniform. Examples are the Nervion between Bilbao and its mouth, the Weser from Bremen to Bremerhaven at the head of its estuary, and the Whangpoo from Shanghai to Woosung where it enters the Yangtse estuary. These works resemble in principle the regula tion works on large rivers with only a fresh-water discharge, pre viously described; but on tidal rivers the main low-water channel should alone be trained with an enlarging width seawards to facili tate the tidal influx, and the tidal capacity of the river above low water should be maintained unimpaired.
To secure a good and fairly uniform depth on a tidal river, it is essential that the flood and ebb tides should follow the same course in order to combine their scouring efficiency and form a single, continuous deep channel. In wide, winding reaches, however the flood tide in ascending a river follows as direct a course as prac ticable; and on reaching a bend, the main flood-tide current, in being deflected from its straight course, hugs the concave bank, and, keeping close alongside the same bank beyond the bend, cuts into the shoal projecting from the convex bend of the bank higher up, forming a blind shoaling channel as clearly indicated near Brul point in fig. 8. This effect is due to the flood-tide los ing its guidance and consequently its concentration, at the change of curvature beyond the termination of the concave bank, where it spreads out and passes gradually over, in its direct course, to the next concave bend above along the opposite bank. The ebb
tide, on the contrary, descending the river, follows the general course of the fresh-water discharge in all rivers, its main current keeping close along the concave bank and crossing over opposite Brul point to the next concave bank below. The main currents, ac cordingly, of the flood and ebb tides in such reaches act quite inde pendently between the bends, forming channels on opposite sides of the river and leaving a central intervening shoal or bar crossing.
In tidal rivers the main ebb-current, being reinforced by 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 together would in certain cases unduly restrict the tidal flow; and, in a river like the Hugli dependent on the tidal flow for the maintenance of its depth for two-thirds of the year, with channels changing with the wet and dry seasons and even shifting their position from day to day, deepening by dredging can never be permanent. Hitherto frequent re-dredging has been relied on for maintaining the requisite depths at the bar crossings in the Hugh.
The training and dredging works carried out in the Whangpoo since 1906' have been successful in deepening the alluvial channel of the river to about 3o ft. at low-water up to Shanghai including von Heidenstam, Int. Nay. Congress Papers, 1919, and 1923.
the bar crossing in a wide reach between Gough island and Woo sung where formerly the low-water depth was no more than 13 to 15 ft. The problem which had to be faced differed from that of the Hugli in that the volume of the land-water run-off is rela tively small. Moreover the concave bank of the river above the crossing was being rapidly eroded, accentuating a sharp bend be low it. The condition of the lower part of the river before and after the improvement works were effected is shown in fig. 9.
The erosion of the Taylor's bank, forming the concave side of the main sea-channel of the river Mersey, which threatened to intensify the bend to a dangerous degree, has been countered by the revetting with stone of the concave face of the bank below low-water (see HARBOURS).