Midget experiments Giant construction
50 - 75
75 -100
75 -100
75 -100
50 - 75
25 - 50
Flooded area according to normal yield
75 -100
50 - 75
25 - 50
1953
General rehearsal in the laboratory precedes the
full-scale project
o - 25
1954
0-25%
1955
0-50%
1956
0-75%
20 40 60 80 100 120
x 1000 hectares of total affected area
I of normal yield
The soil after salt-water flooding
Few inhabitants of the low parts of the Netherlands realize that they
are living under exceptional conditions. The coast is protected along its
whole length by a sea wall, consisting of sand dunes or man-made dikes.
Behind this line of defence the level of the land lies as low as 7 metres
below mean sea level. If the dikes and the dunes were not there, the lowest
parts of the land would be covered by the sea twice a day.
A breach in a dike causes flooding of the land, which is only limited
by secondary, inner dikes, in some cases far inland. At low water the
water rushes from the land through the gap back to the sea. Some hours
later, at high water, the water returns to the flooded land, passing through
the gap with equal velocity.
These strong alternating currents bite deep into the soil, which offers
only a slight resistance to erosion. The gap becomes wider and wider and,
therefore, the closing of a breach is always a race against time; it must be
closed before it has been enlarged so much that it gets out of hand.
The rate at which the gap grows is not the same for all dike breaches.
In new "polders" the land behind the dike is comparatively high, not
much below the level of high water in the sea. After the gale which caused
the breach and during which the sea rose to an extraordinarily high level
has subsided, conditions return to normal and only a small amount of
water covers the land for a few hours; the current passing through the
breach is not strong and consequently the amount of erosion is also limited.
During the periods of low water there is hardly any current, so repairs
can be done quickly and in many instances the breach can be closed before
it has had an opportunity to widen.
When the land is low conditions are quite different. This is the case
where the land consists of old "polders", which have had ample time
gradually to sink lower and lower. In the course of centuries the land may
settle to a depth of several feet. This is the case in the unhappy island of
Schouwen; the surface of the land is only a few inches higher than the
level of the water at low tide. An incessant stream of water passes through
the breaches made in the night of the gale; for six hours the water flows
inland and afterwards the current runs in the reverse direction for the
same period. There is hardly any period of dead water; the current
slows down and immediately after the turning of the tide it gains speed
again.
This has two consequences: there is strong erosion and all the time
repair work is hampered by the current.
In the beginning the erosion is not so serious. The surface of the land
consists of clay which can resist the scouring effect of strong currents
without wearing off too quickly. But this top layer is nowhere very thick
and so it is worn away if erosion continues a long time. Then the under
lying sand is exposed; it crumbles away under the force of the currents
and deep holes are formed in a short time. In a few days depths of more
than a hundred feet may be reached.
"k "k
Many breaches were closed before this stage was reached. In some
instances, for example near Hellevoetsluis in the island of Voorne-Putten,
the bottom was protected by sinking willow mattresses before the clay layer
had disappeared. After this has been achieved, the greatest danger is over
and the closing of the gap can be taken in hand.
Several materials may be used for this last stage. In the first few
months after the floods the sandbag was in great demand. Many breaches
were closed with them. Stones also have been used in great quantities.
Quarries in Belgium, in Germany and in other countries provided thousands
of tons of stones, which were used to weigh down the willow mattresses
and to build dams on top of them.
In places where the currents were strong enough to displace sandbags
or heavy stones, or where time did not permit of the building of dams with
