Summaries
nvironmental studies as a support for the
ompletion of the Oosterschelde works
uring the final phases of the construction of
t ie storm surge barrier it happened that parts
f the barrier had to be closed for hydraulic
rsasons At the occasion of the closure of the
ompartment dams, the barrier was even
t losed completely for some days. As this
could possibly damage the natural
c nvironment, strict measures were taken to
t nsure that the situation went not out of hand
mainly by measuring quality parameters
such as the tidai movement, oxygen and salt
concentrations.
nd so, for instance, the pattern of the closing
cpart of the barrier gates was changed when
i appeared that mussels in the channel
f ammen were being threatened by silt
c aposits; sometimes an extra low-tide was
e oked to help birds foraging.
I was also for environmental reasons that the
c rmpartment dams were scheduled to be
c osed in spring and autumn, in order to
f otect fishing and procreation interests.
I /aluation of the hydro-meteo service
I uring the planning and the execution of the
C osterschelde works it was of great importance
t at the working vessels should be informed
t eforehand about the weather conditions. For
i certain time the UPS control office on the
i land of Neeltje Jans functioned as an
f dvance post of the forecasting centre at
c erikzee. At the end there were even
f recasters aboard those ships.
C enerally it can be said that the accurateness
o the predictions rose up to expectations.
The decision to implement at Zierikzee a
branch-office of the Meteorological Institute
has no doubt been a right one. This office has
now developed into the main centre of
prediction activities for the North Sea and its
coastal waters.
Closure of the compartment dams
In October, 1986, and April, 1987, the
Tholense Gat and the Krammer were closed
respectively. With that the construction of the
compartment dams came to an end. Both
closures were realised in the last phase with
the help of sand only, which was pumped in
large quantities into the closing gaps, at first
forming layers upon layers horizontally, and at
last vertically, as a cap, growing from both
shores.
Sandfill closures on this scale fell entirely
beyond experience. The maximum permissible
water velocity was fixed at 2 m/s. Under
natural conditions however they could easily
reach 5 m/s. So in order to make these
sandfill closures possible at all, it appeared
necessary that the storm surge barrier in the
mouth of the Oosterschelde should be closed;
partially at first, but in the end totally for a
couple of days. This kept tidal movements out
almost completely.
Both closures were carried out in due time,
and even sooner, because the pumping
capacities of the dredges appeared to be
greater than was expected; also the efficiency
of the working methods surpassed expectancy.
Sandfill closures: developments in theory
and experience
The preparation of the sandfill closures of the
compartment dams led designers far beyond
the limits of experience in this field. A design
therefore had to be set up full of theoretical
uncertainties. In order to gather more
information about the physical processes that
play a part in sandfill closures, intensive
measuring and observation programs were
carried out, during the actual execution.
It appeared that the sand formed very weak
slopes under water, by way of many successive
losses of stability on a small scale. This made
the dam as a whole more robust than could
have been expected.
It has proved possible to upgrade considerably
the formulas for sand losses, by incorporating
new elements, like the delay between transport
capacity and actual transport. Also new
insights have been reached into the
productions that can be reached, and into the
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