The PROPOSED THAMES BARRAGE, 1904
A VIEW OF THE RIVER FROM THE GRAVESEND BANK AS IT WOULD APPEAR IF THE DAM WERE CONSTRUCTED
Drawn By H. C. Brewer from Materials Supplied By Mr. T. W. Barber
click to enlarge
Mr. T. W. Barber, M.Inst.C.E., and Mr. Jas. Casey, M.I.N.A., have suggested that the difficulties of which the shipping interests complain might be met by the construction of a barrage across the river from Gravesend to Tilbury, a comparatively simple engineering feat after the great Nile dam (about 1¼ miles in length), especially as the bed of the stream is here firm chalk. This would, it is claimed, give a navigable depth of water, varying from 65ft. at Gravesend to 32ft. at London Bridge, without dredging, or any interference with the river bottom or banks. Some of the advantages which would, the advocates of the scheme claim, be secured are as follows: - Ships drawing 30ft. could proceed to London Bridge at any hour of the day or night, without waiting for tides; ships of all tonnages and draughts could traverse the river, anchor anywhere, lie alongside any wharf or quay, always remain at one level for loading or unloading, and need not lie out in the river or obstruct the free navigation; dock entrances could be left open, thus saving the cost and time lost in working them - the London and India Docks Company estimates the cost of working their entrances at £50,000 per annum; while greatly increased safety of navigation would result, there being no possibility of grounding, swinging with the tides, or collisions due to tidal drift. In addition to these, London would be provided free with a lake of fresh water forty-five miles long, and from a quarter to a half-mile wide. In short, we should have a vast inland lake from Gravesend to Richmond.
It is not necessary to emphasise in any way the fact that something must be done in the tidal Thames to bring the Port of London up to date, and to maintain it as the great inlet of British commerce. What with numerous newspaper articles, magazine reviews, reports of Royal Commissions and others, and a general murmur of complaint from all persons who use the port for their business or the river for traffic purposes, there have recently been abundant evidences that things are not as they should be. Everyone is agreed on this point, but when it comes to the question of a remedy, there agreement ends and confusion begins.
And, first, to briefly catalogue
the complaints
from all sources. They
are as follows: -
(a) Insufficient depth of water
in the river for the increasing size and tonnage
of steamships.
(b) Tide-waiting at Gravesend
and at the dock entrances, inward and outward.
(c) Excessive dues.
(d) Vexatious restrictions
owing to conflicting and overlapping authorities
in the river.
(e) Excessive cost of barging,
pilotage, and labour in loading and discharging.
(f) Loss of time at the port.
(g) Dangerous
navigation, due to tides, bends in the river,
narrow channel, fogs, and the crowded state of
the river. That these complaints are well
founded is generally admitted.
The Royal Commission on
the Port of London, the
Board of Trade, as representing the Government,
the Thames Conservancy, the dock companies and
others recommend the deepening of the river by
dredging as a remedy for (a), and as a partial
remedy for (b) and (f). As to (c) no remedy seems
to be proposed by either, but rather an increase
of dues, or in lieu thereof a charge upon the rates
of London through the London County Council.
Partly to amend (d) it is proposed by all the
above authorities, except the Thames Conservancy,
that a Port Trust should be created to
control the river, instead of the present conflicting
authorities of the Thames Conservancy,
Trinity House, the City Corporation and the
Watermen's Company.
But as to (e) there is no suggestion of amendment,
nor is it expected that the proposed
deepening of the river will materially improve
the dangerous navigation (g).
The Government has
sought to give effect
to the Report of the
Royal Commission on the Port of London in
this Bill, which reached the stage of Committee
of the whole House, and was then suspended till
next Session (1904).
But as there were seventy petitions presented
against the Bill, and a large number of amendments
stand on the notices for Committee of the
whole House, it may justly be concluded that
the Bill satisfies no one, and that the attempt of
the Government to force it through the House by
stifling discussion of most of its vital points in
Committee was a flagrant violation of public
rights, and will have a disastrous effect on the
future settlement of the question.
In 1755 Smeaton proposed the
dockisation of the River Clyde
as a means of providing a
sufficient depth of water for
the increasing trade of the Port of Glasgow.
His plan was rejected, and the Clyde Trustees
have since expended £7,430,000 in dredging and
improving the river to a low-water depth of
20 ft., and now spend annually a large sum in
maintaining this depth.
Thos. Howard proposed the dockisation of the
Avon at Avonmouth in 1877 to provide a sufficient
depth of water for vessels passing to the Bristol
Docks up and down the Avon, there being a rise
and fall of tide in the Severn of nearly 40 ft.
His proposal was not adopted because the
extraordinary range of tide would have left the
entrance unapproachable at low water, causing
delay in the Severn Channel.
Messrs. L. Murray and W. C. Mylne recommended
the dockisation of the River Wear in
1846, but this was not carried out.
The Czar of Russia has recently approved a
great dockisation project, consisting of a dam
with locks and sluices across the Straits of Kertch,
in the Black Sea, to raise the level of the Sea of
Azov for the purpose of facilitating navigation
to the port of Taganrog and the River Don. The
Sea of Azov will then become a fresh-water lake,
with an increased depth of water (14½ ft.) and
an area of 10,000 square miles. The dam will
be nine miles long, and is estimated to cost
£5,000,000.
There is, however, no actual instance of the
dockisation of a tidal river from which any data
can be obtained.
The Upper Thames
Shewing Existing Dams and Locks Between London and Oxford
The Thames, moreover, differs entirely from
any of the foregoing rivers, and must be considered
on its own merits. The map 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.
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.
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.
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.
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.
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: -
Average flow over Teddington Weir, by month | |
---|---|
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: -
Affluents added to the water volume | |
---|---|
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.
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.
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 Fig. 4).
The immediate effects will be these: -
(a) The tides, Neaps and Springs, will be
stopped at the dam.
(b) The river will be converted into a long
lake having numerous affluents, the principal
of which will be its natural flow
over Teddington Weir.
(c) It will have a slow downward current,
never reversed, so that all that enters it
will pass downwards to the dam.
(d) Its level (normally at Trinity high water)
can be regulated to any level above low
water by the sluices.
(e) Within from 25 to 45 days of the closing
of the dam the upland water will have
pushed over the dam all the oscillating foul
water of the tidal river, and thenceforward
the water of the lake will be the same as
that of the upper river, and any soilage in
it must enter it by sewage or land drainage.
(f) There will thus be obtained by one work
a navigable depth of water varying from
65 ft. at Gravesend to 32 ft. at London
Bridge, without dredging or any interference
with the river bottom or banks.
The River Thames Below Blackwall,
As it will appear when dockised.
But the consequent effects upon the business
and usage of the river will be tremendous: -
(g) Ships drawing 30 ft. can then proceed to
London Bridge at any hour of the day or
night, without waiting for tides.
(h) Ships of all tonnages and draughts can
traverse the river, anchor anywhere, lay
alongside any wharf or quay, always remain
at one level for loading or unloading (an
immense boon to shipowners and wharf wharfingers)
and need not lie out in the river or
obstruct the free navigation.
(i) Dock entrances can be left open, thus saving
the cost and time lost in working them.
(The London and India Docks Co. estimates
the cost of working their entrances at
£50,000 per annum.)
(j) There will be no mud entering the docks
and backwaters, the water in which will
freely circulate with the clean river water.
(k) Exceptional tides, being stopped at the dam,
will not overflow the river banks as now
sometimes happens.
(l) Reduced cost of towage, barging, repairing
river banks, camp-shedding, quays, dredging,
management, control and policing of the
river.
(m) Greatly increased safety of navigation:
no grounding, swinging with the tides,
collisions due to tidal drift. The tides are
responsible for most of these accidents and
for many lives lost - casualties which would
not occur in a lake.
In addition to these there is a most valuable
asset created in the advantage the new conditions
open up for -
(n) Pleasure traffic, boating and sailing, fishing
and the provision of efficient steamboat
services, with fixed piers. London will be
provided free with a lake of fresh water
45 miles long and from a quarter to half-a-mile
wide. It is certain that this will give
rise to extensive pleasure boating of all
kinds, which will have ample room owing to
the removal of all vessels from mid-stream
anchorages to the shores.
The illustrations show the present crowded condition of some of the reaches of the river and the clearance that will be effected by a barrage.
Blackwall Reach.
Perhaps the most
important advantage
created by the barrage
will be the permanent supply of water for
the increasing demands of the London area.
By the Act of 1903 has been created a Water
Board which is empowered to purchase the
water companies' properties and to administer
them in the public interest. These companies
claim £47,000,000 for their properties. The
ratepayers pay them £3,000,000 annually for
their water, and the companies pay £30,000
annually for the greater part of the water which
they draw from the Thames.
The figures are as follows: -
Water volumes drawn by water companies | |
---|---|
Gallons per day. | |
From the River Lea | 52,500,000 |
From the wells in the Lea Valley | 40,000,000 |
From the wells in the Kent Co.'s district | 27,500,000 |
From the River Thames | 185,000,000 |
Total | 305,000,000 |
So that two-thirds of London's water supply
comes from the Thames; and as the other sources
named above cannot be expanded for future
requirements, it is evident that for the increasing
demands of London either the Thames or some
more distant source must be looked to.
The Royal Commission on the water supply
of London estimated that in 1941 these requirements
will reach 423 million gallons per day, so
that at that date 303 million gallons must be
obtained from the Thames or elsewhere.
Now if the Thames is dockised, and the tides
kept out of the river, it is evident that much
less upland water than is now considered necessary
will suffice to keep the river lake fresh and
clean, because all sewage and effluents entering
the river will be carried directly down to
Gravesend; there will be no muddy foreshores
and no stirring up of the river mud by the
tidal scour.
The river will be, in fact, in exactly the same
circumstances as most large lakes - that is, a
large body of fresh water, having a main inlet
of fresh water at one end, many small inlets
along its banks, and one main outlet at its lower
end at Gravesend. Such lakes abound all over
the world: they are the purest of all waters and
never become stagnant.
It is proposed, therefore, that the Thames lake
should be regarded as a storage reservoir, so far as
water supply is concerned. It will contain sufficient
for 320 days' supply, even at the estimated
requirements of 1941; for to whatever extent its
waters may become contaminated at and below
London, these pollutions cannot work back up
the river towards Teddington. It follows,
therefore, that between Teddington and London
water may safely be drawn off for town supplies,
or the supply may be taken as now from above
Teddington.
An inspection of the table of flow over Teddington
Weir above will show that in the
winter and spring enormous quantities of water,
above the quantity considered necessary for
scouring the river, flow down and are lost.
A minimum flow of 200 million gallons is
fixed by law as the amount needed in summer
to keep some sort of cleanliness in the lower
river; but in January ten times this amount
flows away. It is only for a short time in the
months of August or September that the natural
flow over Teddington Weir - including the water
drawn by the water companies - is a little below
423 million gallons daily, and in those months
the surplus might be taken from below the weir
without affecting the river materially.
If this be objected to, however, there is another
remedy available. The Upper Thames may be
used as an aqueduct to convey a larger supply,
to be derived from neighbouring watersheds or
from wells, the water so obtained to be regulated
to meet the requirements, enabling a sufficient
amount to be run over the weir to keep the lower
river in motion at its upper end. Further down,
the small but numerous affluents and springs
will keep the river in motion, as they are not
affected by the Teddington flow, but give a
continuous supply to the river. Mr. Topley, the
eminent geologist, in his evidence before the
London Water Commission, 1892, stated that
there are outside the Thames basin large areas
from which water could be obtained, such as
East Kent, West Suffolk, Norfolk, Hampshire
and Wilts.
It is evident that in this way an enormous
prospective outlay for a supplementary water
supply for London in the near future may
be obviated, and that without adding to the
existing plant of the water companies the
new Water Board may inherit free of cost a
future source of supply which will make their
purchase of the London Water Companies' stocks
a good investment and a cheap one for the
ratepayers.
The possibilities of this scheme are not exhausted, as there remains to be mentioned the opening of railway communication across the river by a tunnel under the dam and of road communication by a roadway over the dam. These are clearly shown in the accompanying Figs. 4, 5 and 6.
Fig. 4. Section Of The Thames From Teddington To Gravesend
Showing Proposed Permanent Mean Water Level And Tidal Sections.
click for larger version
Fig. 2. The Thames From London To Gravesend.
n.b. Figures 2 and 3 are each shown in two halves
Fig. 3: The Thames Estuary.
The tunnel will be constructed in the foundation
of the dam, and the road formed on the top
of the dam, and provided with opening bridges
across the locks.
A glance at a railway map will at once show
the strategic value of the railway route thus
opened up between the Midlands and the North,
and Dover and the South Coast, avoiding the
conjested London lines; also for national and
military direct traffic between the Government
arsenals and the Colchester and northern routes
and depots. All the northern lines will thus
have access by the Tilbury line to the continental
routes.
The Port of London above the barrage will be the finest and safest harbour we possess for the fleet, having an immense deep-water protected area. The barrage can be fortified, and will constitute the most effective prevention against any foreign invasion by way of the Thames estuary. The tunnel and roadway will be of great service in this connection also.
This, which has been increasing for many years, is becoming a serious matter, and has attracted much comment. One of the advantages that will be obtained from the barrage will be the raising of the underground water-levels in the chalk and other strata of the Thames basin. In this way a permanent improvement in the water supply by wells throughout this large area will result.
Among these may be mentioned: - No further scouring of bridge or other foundations. No backing up of the foul waters of the small tributaries, such as the Lea, Barking Creek and others. Improved living conditions and reduction of disease, especially in the neighbourhood of the river, resulting from the cessation of ebb and flow, of smells and exposure of mud banks. Increased value of properties bordering the river. Fixed piers for passenger steamers.
Fig. 7 is a general plan showing the barrage in relation to Tilbury and Gravesend shores.
Fig. 5 is a cross section of the river showing the vertical dimensions and contours.
Fig. 6 shows a section and details of construction.
Generally it is proposed to form the barrage of mass concrete, faced with granite on all exposed faces. The tunnel will be formed in the solid monolith as the work proceeds, and afterwards connected north and south with the existing railways. The foundation is in the chalk. The method of construction will be by cofferdam, to enclose an area sufficient for the walls and locks, which, when completed, can be opened for the up and down traffic of the river while the construction of the weirs and sluices is proceeded with. The sluices will be left open for the free passage of the tides until the closing of the barrage, which will take place at high water of a Spring tide.
The locks will be worked electrically from a
power-house built upon the central pier of the
locks; the power to be obtained from dynamos
operated by the fall of part of the water flowing
over the dam. A pilot tower will be fixed from
which the river traffic will be signalled and
regulated, and the locks, movable bridges, etc.,
controlled.
The locks as shown are four in number, each
provided with internal gates in addition to the
outer ones, in order that these locks may be
worked in long or short lengths to suit the traffic.
The lengths provided in this way will be 300 ft.
500 ft., 700 ft. and 1000 ft., and the widths 80 ft.
and 100 ft. It is not likely that these dimensions
will ever be exceeded by steamships.
The number of vessels passing up and down
the river per day averages 220, but few of these
exceed 300 ft. in length. It will be easy to lock
this number up and down, or three times the
number with this series of locks, one important
advantage to the shipping being that, instead of
waiting tides at Gravesend, each vessel as she
arrives, at any hour, can be locked in a few
minutes, up or down, without waiting.
Special provision will be made for rapidly and
safely passing into and out of the locks with the
use of power capstans and gear. The sluices
will be of steel, sliding in roller guides, balanced
and operated each by its own motor.
At or near low water a large volume of water
will be sluiced into the lower river to scour the
approach to the locks as often as found necessary.
A system of signalling from the Upper Thames
to the barrage will be employed to notify any
heavy rainfall or freshet coming down the river,
so that by lowering the sluices water may be rapidly discharged to maintain the required level
in the river, and at certain fixed dates it may be
desirable to let down the water-level for a fixed
time to allow of the repairing of dock entrances,
walls, and other river-side works.
The estimated cost of the barrage complete is £3,658,000, including compensations and other contingencies. A toll of ¾d. per ton on the shipping passing up and down will pay the interest on this sum. This ¾d. per ton additional toll will, it is estimated, be many times compensated for by reductions in the river and dock dues and other expenses, as below: -
Estimated savings effected by dockisation | |
---|---|
Savings Effected by Dockisation. | Per Annum. £ |
Dredging in the river | 200,000 |
Repairing banks, campsheds and groynes | 10,100 |
Mudding in all docks | 50,000 |
Cost of operating dock entrances and pumping | 70,000 |
Saving in time of vessels ascending and descending the river | 225,000 |
Saving in towage | 20,000 |
Saving in barging | 185,000 |
Saving in warping, buoying, lying off, etc. | 20,000 |
Saving in management of river | 70,000 |
Total annual saving | £850,100 |
This is equal to a reduction of 6.8d per ton on
the tonnage of shipping (30,000,000) entering and
leaving the Port, or equal to 7½ times the interest
on the cost of the barrage.
To the credit of the barrage must also be set
the removal from the prospective future of
enormous outlays contemplated for: -
£ | |
---|---|
Purchasing docks, estimated at | 30,000,000 |
Improving docks and dredging river | 7,000,000 |
Cost of a water supply from Wales or other source | 24,000,000 |
Total | £61,000,000 |
This measure is the Government's attempt to put into law the recommendations of the Royal Commission on the Port of London, 1902, but with amendments. It is proposed to purchase the entire docks and warehouses, leaving the wharves to run on their own resources; to create a Port Trust to control the entire river and docks; to charge the loan for purchase, etc., upon the London County Council - i.e., about £35,000,000: and to dredge the river to about 30 ft. at low water up to the principal dock entrances.
Apart from its cost and the
grossly unfair policy of
financing and running the
docks against the wharfingers, it is evident that
this scheme is based upon the possibility of
dredging the river to the depth required. Fig. 8 is an actual section of the river, showing the
proposed dredged channel as compared with a
dockised river.
It seems incomprehensible that any expert
authorities should have advised the Government
that the river can be effectually dredged. The
fact is that it is quite impossible to dredge it to
the required depth of about 15 ft. below the
present bottom, because experience has shown
that with such a river and scouring current the
channel will fill up again nearly as fast as it is
dredged, the material coming from the foreshores
and the estuary. This will give rise to dangerous
slipping in of river banks and walls. The
estimates of the cost of this dredging (£2,500,000)
are therefore entirely misleading.
The present bottom is formed and stands at
the natural angle of repose for its present volume,
width and currents, and any great interference
with this contour such as is proposed - with
slopes of 7 to 1 - will not stand, the general
slope of its bottom now being from 20 to 50 to 1.
The Port Trust that undertakes this will find
itself spending enormous sums annually in continuous
dredging and repairing banks and in
compensating owners; all, of course, added to
the annual cost of maintenance and to the dues,
or charged to the ratepayers.
Glasgow and the Clyde have been instanced as
examples of what can be done by dredging. But
the Clyde below Glasgow is not a river comparable
with the Thames below Gravesend, but an estuary
with a very moderate current and tidal range of
from about 4 ft. to 10 ft., and the dredging has
merely made and kept open a channel in this
estuary. The Thames, on the other hand, is a
narrow river with a strong scouring current and a
range of tide of from 16 ft. to 21 ft. Further
than this, Glasgow has spent seven millions in
this work, and has to pay large sums to keep
the channel open, dredging nearly a million
cubic yards every year.
But there are other difficulties. When the river
has been deepened as proposed, the tidal volume
will be increased about one-third, and therefore
its current strengthened and increased, probably
two knots per hour. What is worse, the tidal
range will be increased proportionately, which
means that the high tides will be higher - probably
3 ft. or more - and the low tides lower, by a similar
amount, than now. Spring tides may be expected
to run the river nearly dry at low water above
London Bridge. Results - frequent inundations
of waterside districts, more grounding at low
water, and more dangerous navigation. Such
results have always followed increased tidal
volume.
Fig. 8. Section of the Thames below Blackwall
shewing Proposed Dredged Channel compared with a Dockised Channel.
click to enlarge
But a dredged channel is necessarily a narrow one (see Fig. 8), and ships will have to negotiate the sharp bends in a narrow channel and against a stronger tide, and also to swing at anchor, for which a wide area is necessary.
Although this proposal has
been mooted for some time
past, scarcely any valid
objection has been brought forward, but such as have been mentioned are
mostly based on misconceptions.
One writer thought the river would become
stagnant. As a matter of fact the sources of
stagnation would be carried down the river by
the fresh-water flow continuously, and there is
no more reason to anticipate stagnation in the
lower river than the upper river, where it has
for ages been held up in the same way by
numerous dams.
Another writer talks of the "cleansing power
of the tides", and it is a pity to see greater
authorities, who ought to know better, speaking
also in this way. It has been abundantly proved
that the tides - as far as a clean river is concerned - are
wholly detrimental. They back up
twice daily the natural drainage of the river for
five hours, and keep it in solution and circulation
for forty-five days before removing it, the effect
being exactly similar to backing up in a sewer.
The Pool Below Tower Bridge.
It has also been suggested that the sewage
effluents discharged into the river at Crossness
and Barking may cause the river below to
become foul. Here again is misconception.
The effluents - after precipitation of the solids,
which is chemically effected, and the carrying
out to sea of the resulting sludge to the amount
of two million tons annually - contain very little
impurity (only seven grains per gallon), and it
has been proved by Dr. Dupré that 9/10ths of
this becomes oxidised and absorbed in the large
volume of water between the discharge and
Gravesend. It is well known that in the case of
"sewage effluents poured into a sufficiently large
volume of otherwise comparatively pure water,
the dissolved organic matter contained in it
disappears with remarkable rapidity" (Sir Alex.
Binnie).
Another critic suggests that the lower river will
soon silt up under the new conditions. Most
persons - seeing the filthy state of the water - naturally
think there must be a large deposit
from it. But it has been shown that this suspended
matter is the result of tidal currents keeping the
mud stirred up everlastingly. An examination
of the affluents of the Thames shows that they
contain very little suspended matter, and therefore
when the locked Thames has deposited its
charge of suspended matter any future soilage
must come from its affluents - that is, from the
upland waters and the sewage effluents, which
latter will only affect it below the point of their
discharge.
A calculation from official data of the quantities
actually now passing into the Thames,
from all sources, gives less than 1/10th of an
inch annually over the river bottom; so that
in ten years the deposit will not exceed 1 in.,
even without any improvement in the prevention
of pollution. It has been estimated by Dibdin
that the sewage outfalls could be removed to
Gravesend, below the barrage, for the sum of
£4,000,000.
But the condition of these effluents is commonly
much exaggerated. The total annual discharge
of suspended matter at 7 grains per gallon (as
given by Dibdin) amounts to 32,000 tons per
annum, but much of this becomes chemically
combined with the river water and some remains
in suspension till it passes Gravesend, leaving
only a small quantity to deposit in the river. A
single dredger can remove 600 tons per hour;
therefore a few hours' work will remove the
whole quantity.
A more valid objection at first sight is that
ships and barges will lose the motive power of
the tides up and down. This would appear,
however, to be a very beneficial loss, because at
the same time they will avoid the tide-waiting
and waste of time which add considerably to
the cost of transit. But against this loss must
be set the fact that most ships now have steam
power and can make their own destination,
while tugs will be able to handle much larger
fleets of barges than is now possible in the
tide-way, and at all hours of the day. Sailing
vessels will be able to sail up and down,
which they can only do now with the aid of
the tide.
Another suggestion is that when the barrage
has closed the river the tides below it may
accumulate to a higher level and overflow the
low-lying lands below Gravesend. This is, however,
a mistake, the fact being that with a reduced
tidal volume and momentum in the estuary the
tidal range will be reduced, there being no river
to fill up, the high tides will be lower and the
low tides higher than formerly.
Finally, a word or two as to the vague idea
that seems to be in the minds of most people
accustomed to tidal rivers - that in some
mysterious way the tides by their continual
movements are beneficial, keeping the air in
motion, etc. All this is pure imagination and
arises probably from living on the banks of a
tidal river, for most rivers are non-tidal. There
happen to be round our coasts some phenomenal
ranges of tide; hence the resort to docks, which
are almost unknown in other countries. The
ranges of their tides being small, docks are not
needed, and scarcely any tides occur in their
rivers, which, however, are far cleaner than the
Thames.
There are of course some low-lying lands
bordering the river the drainage from which
will have to be pumped into the river. This is,
in fact, partially done now, but the matter is
a small one.
Prof. Flinders Petrie, in a letter to the Times, is strongly in favour of this proposal, and looks to it to relieve the squalor of the East End, with its crowded and unhealthy living, by extending the manufacturing districts down the river banks, providing a belt of factories along each bank and a belt of garden villages behind them, with fast lines of railway to Town between.
To carry out the proposals of this article, a
committee has been formed to bring the subject
before the notice of Parliament and of the public,
and it is suggested that a Board of Harbour
Commissioners should be formed, somewhat on
the lines of the Port of London Bill of last
Session. The new Board would be constituted
under the usual Commissioners' Acts to control
the entire Lower Thames, taking over the powers
of the existing authorities, but without any
interference with the docks, the warehouses or
the wharves, the business of which, if the river is
rendered properly navigable, could be carried on
without making any demands upon the rates of
London.
A new era of prosperity would then open up
for the trade of London, and its Port would
become the finest in the world, with the largest
business attached to it.
The committee will include many influential
gentlemen connected with and interested in the
improvement of the Port of London. The scheme
originated with Mr. Jas. Casey, M.I.N.A., and the
author is responsible for the engineering details,
as also for the information set forth in the foregoing
article.