To understand clearly the conditions to be dealt with, it is necessary to consider the daily movements of tide, the affluents, the dock and wharf business and the traffic of the river.
The maps ([Figs. 2 and 3]) show the tidal river and estuary from Teddington to the North Foreland. [Transcriber’s Note: It seems ‘Teddington’ here is an error for either ‘London’ or ‘Southwark’; that’s what the maps show, anyway.]
The river proper—that is, from Teddington to Gravesend—is forty-six miles long, and averages one-third of a mile wide. Its depth at low water varies from 6 ft. at Teddington to 10 ft. at London Bridge and 40 ft. at Gravesend, and the rise of tide at London varies from 17 ft. to 21 ft. and at Gravesend from 15 ft. to 19 ft., the current usually averaging four knots per hour. At London Bridge the Spring tides flow 5 hours and ebb 7½ hours; while at Gravesend they flow 6 hours and ebb 6½ hours.
The river winds about considerably. The straight line distance from Teddington to Gravesend being thirty-three miles, shows that thirteen miles are added to the river in its bends, some of which—as those at Grays, Erith, Blackwall and Limehouse—are short and tortuous.
The longitudinal section ([Fig. 4]) of the river from Teddington to Gravesend gives graphically all the data necessary for our purpose. Ordnance Datum (O.D.) is the common datum line of the Government maps. Trinity High Water (T.H.W.) is the water datum usually adopted in the river. High and low water, ordinary and Spring tides (H.W.O.T.—L.W.O.T.—H.W.S.T.—L.W.S.T.) are the levels of the respective states of tide in the river at various points. The highest and lowest known tides are also given, as well as the level of the river bottom and the levels of the principal dock entrance sills and of the crowns of the Thames tunnels, showing their depths below the river bottom.
Tidal Wave.
The curved lines (in various forms of dotting) represent the levels of the surface of water at various states of Spring tides and clearly show the tidal wave which ascends the river and by its momentum and volume raises the high-water level at the upper end several feet above that at Gravesend.
The Thames Estuary.
From Gravesend to the Nore is an immense triangular area with sandy bottom, muddy foreshores and several deep channels running in the general direction of the Essex coast line, that is, N.E. to the North Sea. The area may be roughly estimated at 120 square miles, and the navigable depth of the principal channels at from 60 ft. to 26 ft. at low water Spring tides.
The volume of the estuary at high water Spring tides may be taken at 2600 million cubic yards, and at low water Spring tides at 1500 million cubic yards, the volumes of the river from Gravesend to Teddington being respectively 180 million and 80 million cubic yards, so that the volume of tidal water entering the river each tide is about 100 million cubic yards.