greatest dike breaches in history), Zierikzee, Ouwerkerk (two gaps close
together), Oosterland, Sirjansland, Stevensluis;
in the island of Zuid-BevelandKruiningen (a group of three breaches),
Bath.
Oosterland and Sirjansland were closed by means of dams of stones
and sandbags, the others with concrete caissons, hundreds of them pre
fabricated in the Netherlands and a small number of giant Phoenix caissons
originally intended for the Normandy invasion harbours and now towed
from Britain to Zeeland.
It meant a busy time for the hydraulics laboratory. Models were built
to scales varying from 1 20 to 1 150 and wooden boxes with a length
of a few feet or less were made to represent the 7500-ton Phoenix or the
smaller Dutch caissons. Observations on the spot enabled the laboratory
people to adjust the tidal currents through the models.
Above and below: Small-scale general rehearsals in the
laboratory for the closing of the gaps. The actual project was
often a striking large-scale repetition of the laboratory
experiment.
these materials, the gaps were blocked by large steel or concrete ships or
caissons. This is the new way of closing a breach by means of "blockships".
The first instance in which a breach was closed with a large unit
prepared in advance was at Hellevoetsluis. Two ships were linked up and
provided with screens which were made to fit into the breach.
A serious setback was the failure of "operation despair", a desperate
attempt to close the Zierikzee gap by means of old steel barges. It was not
possible to protect the bed on which they were to rest by brushwood
mattresses and the result was that the clay was scoured away from under
the barges, which eventually slipped down into the deep holes.
Strangely enough the laboratory people were greatly encouraged by
this operation, for the manoeuvring of the ships was done in accordance
with the results of their experiments and it appeared that their predictions
based on the models were completely satisfactory.
This was again illustrated by the next attempt in the same gap. This
time a strip of soil at some distance from the gap, running from the dike
on one side of the gap to that on the other side, was covered by brushwood
mattresses. Strings of caissons were put on top of them and again the
operation, previously studied with the model, was carried out with com
parative ease.
A large part of the enclosing dike round the Schelphoek gap was
constructed in the same way. The enclosing dike, however, had to cross
two deep gullies and there large caissons had to be used, in one instance
a Phoenix. The placing of this giant was also duly studied in a special
model.
By the end of August all gaps had been closed with the exception of
the one at Ouwerkerk. The closing of this exceptionally vicious breach
required the utmost exertion of everyone working in the field, along the
assembly lines (for the supply of materials) and also in the laboratory.
The erosion was very serious and deep holes were formed, continuously
threatening the stability of the brushwood mattresses and of the struct
ure resting on them. Several models were built and operated. The ero
sion was studied, so that the engineers could take additional precautions
at the danger spots by sinking more mattresses.
Next the spectacular final act was prepared. It proved necessary to
sink three huge Phoenix caissons on their beds made of brushwood
and stones in a very short time. Again the laboratory engineers watch
ing from a hundredfeet light mast saw their model manoeuvres turn
ed into reality.
None of those present will forget November 5th and 6th. In turn the
big monsters were floated into their correct positions and sunk. Midnight
was approaching when, in the presence of Her Majesty Queen Juliana
and numerous officials, the last caissons touched the bottom and the sirens
of tugs and dredges and cranes proclaimed that the nightmare was over
and the dikes of the Netherlands where whole again.
It is evident that the manoeuvring of such a large unit is a difficult
job. The ships must be floated into position just before the turning of the
tide. Only a few minutes are available to push, pull and turn the bulky
thing into position and to sink it.
The operation is first rehearsed with a small-scale model. In the labor
atory threads represent cables, tons are reduced to ounces and the man
oeuvre is varied and repeated so many times that the man who is re
sponsible for the operation knows exactly where to fasten his cables, and
how much the strain will be in each of them. He knows too where to locate
his winches on the dike or on the pontoons of dredgers or floating cranes
and how to apply his tugs.
Four weeks after the flood, on 28th February, the gap of Hellevoetsluis
was closed. The laboratory engineer who was present got the impression
that he saw the model experiment over again, only on a larger scale.
This happened several times, for models were made for most gaps
where the clay had been eroded away and the water had cut deep into
the sand beneath. These gaps were:
in the twin island of Schouwen-DuivelandSchelphoek (one of the
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