[Illustration: THE THAMES FROM LONDON TO GRAVESEND.]
[Illustration: 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.
[Illustration: Fig. 5.]
_National and Military Aspect of the Scheme._
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.
_The Depletion of the Thames Basin._
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.
_Minor Advantages._
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.
_Works and Construction._
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.
[Illustration: Fig. 6.