original dike
new dike
The closing of a tidal gap by constructing a horse-shoe dike
round the gap along the shallow flats outside the
original dike method No. 3, see the accompanying article
building sites in all were organized. In a few months9 time 696 units were
built, involving the use of 23000 cu.m of concrete and 4900 tons of steel.
The caissons were towed from the building sites to the various gaps
without any difficulty.
Spectacular closing of gullies
One of the most spectacular projects with respect to the closing of
gullies was that of the tidal gap in Kruiningen harbour.
The flooding capacity of this gap was about 45 million cu. m, which
means that with each flood tide this quantity of water was forced through
the gap, each time inundating the 1450 ha Kruiningen polder.
Prior to the closing of the actual breach two smaller gaps in the water
defence were closed by means of enclosing dikes.
In two successive days one floating door of unit caissons and the
enormous 7500-ton A-X caisson were placed in position between the
pier heads composed of unit caissons at the turn of the tide. The man
oeuvring into position of the A-X caisson constituted a masterpiece of
seamanship.
The closing of the gap near Schelphoek, which inundated the 8500 ha
Schouwen polder, was a milestone in the history of the struggle against
the water, since one of the largest flooded areas was thereby closed off
from the sea, while moreover the usefulness of a new constructional
method for the conventional enclosing dike had been established.
At the time of the closing the gap in the water defence had a width
of 525 in and a depth of 40 m. The system of gullies which had developed
inland after the dike was breached, but particularly the large flooding
capacity of the gap (125 million cubic metres), made it impossible to
effect the closing of the gullies by means of large caissons, as was done
in the case of the gap in Kruiningen harbour.
For this reason the old approved method of our forefathers was em
ployed, namely closing by means of a horse-shoe dike, but this time the
dike was constructed with modern means, namely floating concrete
caissons.
In strings of 55 and 66 metres the concrete dike sections were
floated into position at high tide above the mattresses of the new enclosing
dike with the aid of special pontoons held in place by means of "spuds".
When sunk and filled with sand they constituted an excellent provisional
water defence.
It was sometimes possible to push on the horse-shoe dike for a distance
of 469 metres at a single day, an achievement which demands admir
ation, particularly if it is considered that the concrete caissons could
only be manoeuvred into position and sunk during the brief period at the
turn of the tide, when there was little or no current running in the gap.
Construction of an entirely new dike at a rate of
200 metres per day
A problem of an entirely different nature than the closing of tidal
gaps presented itself at the island of Overflakkee. With the exception of
the dunes in the west and a polder of 2000 ha in the centre, the entire
island was flooded.
The dikes which formerly protected this prosperous island against the
sea had been so severely damaged over stretches of many kilometres that
it was necessary to build a new dike of about 4s1/2 km on the north side
and a new dike of 181/2 km on the south side of the island.
This extensive project had to be completed in about eight months
before the arrival of the autumn gales.
To produce the necessary quantities of sand the greatest suction
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