The Thames, moreover, differs entirely from any of the foregoing rivers,
and must be considered on its own merits. The map (Fig. 1) shows that it
is already dammed and provided with locks at thirty-four places between
London and Oxford, the object of these dams being the maintenance of
a uniform level of water for navigation and boating purposes, and to
prevent the river running dry in the dry season and exposing the muddy
foreshores.
But from Teddington Weir to its estuary the Thames is tidal, and there
is no obstruction to the tidal flow except the bridges and the half-tide
weir at Richmond, which merely holds up sufficient water to cover the
foreshores for the advantage of the riparian owners and of boating.
_The Tidal Thames._
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.
_Upland Water._
But there is a daily flow over Teddington weir—excluding the water
abstracted by the London water companies—varying during the year on the
average as follows:—
Cubic yards.
Jan. 11,800,000
Feb. 5,300,000
March 4,100,000
April 3,250,000
May 4,720,000
June 2,900,000
July 1,760,000
Aug. 1,590,000
Sept. 1,160,000
Oct. 1,900,000
Nov. 3,530,000
Dec. 8,230,000
Average daily flow, 4,186,000 cubic yards.
Below Teddington, numerous small affluents add to this volume of upland
water as follows:—
Cubic yards
per day.
The River Lea and Essex streams on the north bank 60,000
Streams in the Kent district 500,000
To this must be added a large quantity of spring
water rising in the bed of the river and land
drainage—quantity uncertain 1,000,000
Sewage effluents discharged at Crossness and Barking 1,176,000
Storm water overflow from London sewers 580,000
---------
Total upland fresh water daily average 7,502,000
This gives an average volume of 7½ million cubic yards of fresh water
descending and mingling with the oscillating tidal water of the river and
estuary, which slowly pushes the latter down into the North Sea. Taking
the high-water volume in the river as above at 180 million cubic yards,
the proportion of fresh water from the upland daily flow is 1/24th, and
therefore it will take 24 days to change entirely the water in the tidal
river.
Mr. W. P. Birch has shown that the combination of fresh water and sewage
which enters the river below Teddington remains in the river, oscillating
up and down with the tides for 45 days before it finally gets pushed out
into the North Sea.
[Illustration: THAMES MUD.]
In this way the discharge of effluents at Crossness and Barking passes
up and down in front of London for more than a month, and it becomes
apparent that the tidal action keeps the river continually saturated with
about 45 days’ soilage. It is no wonder, therefore, that the conditions
of colour, smell and turbidity of the river below Teddington are so vile
as compared with the Upper Thames, especially as to the above sources of
filth must be added the tidal current, which is so rapid that it keeps
the mud continually in suspension, washing it up at one time, depositing
it at another, but never permanently leaving it except in the places
unscoured by the upland water, such as docks, backwaters and places out
of the main current. It has been acknowledged by all writers that if
the upland water should be stopped the Thames would become a stagnant
oscillating ditch, because all filth discharged into it would remain in
it permanently.
The docks trap a very large proportion of this mud, and it costs at least
£60,000 per annum to clean it out. The mud enters with the locking water
and with that pumped to make up the basins.
_Effect of Dockisation on the River._
It is proposed to construct across the river at Gravesend a dam or
barrage similar to that across the Nile, containing numerous adjustable
sluices, and in addition a series of very large locks, the dam to hold up
the river to about Trinity high-water level (see section, Fig. 4).