Summaries
The shifting system of the 'Cardium'
The execution of work in the mouth of the
Oosterschelde is very much determined by the
accuracy with which the mattress laying vessel
'Cardium' can be placed and shifted. The
steering process should run with utmost
precision. The cardium is shifted by means of 8
winches operating simultaneously. It is
improbable that so many orders can be
assimilated without some form of automation.
As a completely automatic system could not be
developed or installed on time a semi-automatic
one was decided on, whereby 3 orders are to
be passed by hand: going forwards and
backwards and rotating round the mid-fore and
the mid-aft of the vessel. The computer works
out which motion is required by each of the
eigth winches and causes these to be carried
out.
The control of hollows caused by scouring
Scouring hollows occur as a result of the
erosion of the original sand bed. These can
eventually contribute to the collapse of hydraulic
constructions. Such scouring is also caused by
the building of the Oosterschelde barrier and is
given a fresh impulse at each building phase.
The length of the protective revetment of the
bottom on both sides of the barrier has a great
influence on the occurence of scouring hollows.
Wheather or not these become dangerous
depends on the mechanical soil composition.
The determining factor is not the depth of the
hollow, but the greatest slope gradient.
Scouring can be kept under control by
lengthening the protective revetment, by
securing the gradient of the hollow, or even by
dredging a hollow and reveting it.
The chance that scouring hollows will occur
has been calculated according to the probability
method, as well as in how far they contribute
to the chance that the Oosterschelde barrier
will be impaired. This has resulted in an
optimal combination of measures to be taken:
Revetment of the bottom up to 650m from the
axis, securing of the hollow slopes and in
certain places compaction of the bottom.
Sandasphalt in hydraulic engineering
Sandasphalt is a mixture of sand reclaimed on
the spot together with 3-5% of bitumen. This is
a permeable material which can withstand a
current velocity of up to 3m/sec. It can be used
for auxilliary dikes or as a construction floor
while at the same time functioning as a filter
layer, at a gradient up to 1:2. It is now possible
to predict its deformation behaviour and failure
characteristics by computer. In general its
life-span is no longer than ten years. A supply
pipe or a dump ship is the best way of bringing
it into position under water.
Fresh/Salt water studies in the Oosterschelde
and in the Zoommeer
This article deals with the consequences of the
hydrological situation in the future salt water
area of the Zijpe and the future fresh water
area of the Volkerak after 1986.
As a result of the storm surge barrier a change
will occur in the pattern of exchange with the
sea water. This means that fresh water will
remain twice as long in the Oosterschelde as it
does at present. Fortunately the fresh water
load on the estuary will diminish with the
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