caissons' across the mouth of the Eastern-
scheldt a lifting ship was designed to trans
port the caissons, the pits and the piers from
the construction-site to the closure gaps,
to dig in the caissons there, to place the
piers on the caissons and to bring back the
detached pit.
Meanwhile the design for the piers has been
changed. As a result the designed lifting ship
will not be built. The lifting ship would have
been able to place 18 caissons and
piers a year. The design consists of one
transverse pontoon and two longgitudinal
pontoons with a 23 meters wide well. A 50
meters high portal-crane that rests with four
legs on the longitudinal pontoons could tow
the load - partly floating - through the water.
The effects of the motions of the ship are
reduced by swell absorbes. For this design
numerous tests and calculations have been
carried out by the Netherlands Shipmodel
Basin in Wageningen.
Future control of the storm-surge barrier
The manner in which the storm-surge barrier
will be put to use forms the basis for the
design. The demands on the operating me
chanisms, for instance, will be entirely dif
ferent if the storm-surge barrier is not closed
at the turn of the tide but during tidal flow
conditions.
In collaboration with the Rand Corporation a
study called 'Barrier Control' has been initi
ated to review all aspects of the manage
ment. This study will be focused in the first
place on the closing procedure of the barrier
when imminent storm-surges are being fore
cast. The security-system that is to come into
action is now being submitted to a policy-
analysis.
Removal of the bottomprotection across the
mouth of the Easternscheldt
The design to close the Easternscheldt with
piers made it mandatory to remove the
bottomprotection that was previously laid in
the axis of the dam-alignment, i.e. concrete
block mattrasses, mats of stone asphalt and
fascines lined with woven polypropylene
sheets. The layers of rubble around the
cableway-towers had to be removed too.
The stone asphalt mats proved to be the most
difficult to remove as they are interlaced with
steel wire. These mats were divided into
strips with a cutting device that was dragged
along the sea-bottom. The bottomprotection
was hauled up by a dredger which, for this
purpose, was equipped with rock-buckets.
The removal of the cableway-towers from
the closure gaps of the Easternscheldt
As the plans for a full closure of the Eastern
scheldt will not be implemented, the support
ing towers of the cableway, already placed
in the closure gaps, had to be removed. The
superstructure was easily dismantled. The
hollow steel piles with which the towers were
founded in the bottom of the Easternscheldt
were pushed up by water-pressure. To main
tain the pressure within the piles a pressure-
lid was installed at the top and a plug - a
combination of bentonite and silversand
at the bottom of the pile.
The progress made so far in raising the dikes
along the Frisian coast. The dike between
Wierum and Paesens
In 1974 the raising of the Frisian sea-dike
between Wierum and Paesens up to the
Delta-level was started. Additional work also
included a part of the adjacent dike reach
between Wierum and the Oost-Holwerder
polder. It proved possible to construct the
dike-body over a length of 6,2 kilometers in
one season. The existing dike was reinforced
on the outside. The new dike has a crest
height varying between N.A.P. 8.30 m. and
8.80 m. Great care has been taken to pre
serve the historically evolved alignment of
the dike.
The wet cross-section in the storm-surge
barrier
An interdepartemental working group has
studied what would be the most desirable
dimensions of the wet cross-section in
the storm-surge barrier with which the
Easternscheldt can be sealed off. The fol
lowing wet cross-sections have been examin
ed: a cross-section with a minimum size of
11,500 m2, an intermediate solution of 14,000
m2 and a maximum size of 20,000 m2. These
cross-sections will cause a vertical tidal
range at Yerseke of 2.30 m., 2,70 m. and
3.10 m. respectively, (the tidal range being at
present 3.50 m.)
As far as security and hydraulics are con
cerned ail three alternatives are equally good.
For the preservation of the environment and
the fishing industry a larger wet cross-section
is to be preferred to a smaller one. Moreover,
the more the tidal range approximates the
present one, the easier it will be to predict
the future situation.
The majority of the committee recommends
a wet cross-section of 14,000 m2.
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