in October 1986 to provide for the closure of
the Tholense Gat. In April 1987 the tide will be
influenced once more with the help of the
storm surge barrier, this time for the closure of
the Krammer. The influcence exerted by these
manoeuvres upon the water levels and water
movement has been explored as far as possible
beforehand. These explorations have
subsequently been used as the basis for a
so-called description of effects with regard to
environment and fishery. The vertical tide is of
the utmost importance to birds, fishes, see-bed
life and for the mud-flats and shallows.
Prolonged stagnant water levels are
unfavourable. The horizontal tide serves as a
pump for all sorts of alternation processes.
Should this movement be interrupted an
increase of sediment will occur and the salinity
will be reduced. At the same time the chances
of stratification will increase. A. survey is given
of the changes which have occured within the
first weeks after a number of former closures.
A group of experts from differing fields will be
present to provide advice during closure of the
gaps.
Technical designs
The designs presented in this report date from
the end of 1983. They will have to be adjusted
to new data in a number of respects. A
description is given of a sand-fill closure of the
Krammer and a sand - and a stone-fill closure
of the Tholense Gat.
A risk analysis has been set up for each closure
giving the factors which can lead to a delay of
two weeks or more in the completion of the
work, together with a quantification of the
chances of these factors occurring.
Sand-fill closure of the Krammer
The Krammer will be closed in 1987; the
Krammer locks cannot be operational before
then. The closure gap will be filled with sand,
first from the north bank and later from both
banks. The storm surge barrier will be used to
limit sand loss. Should this reach 50% of the
production then the next scenario for the storm
surge barrier will be put into effect in such a
manner that the current velocities in the
closure gap will not exceed 3 m/s. Finally the
Krammer will be closed off while the storm
surge barrier is shut. Sand production will be
raised as much as possible. Extensive
measuring- and observation systems as well as
mathematical models will be in use during
execution.
Sand-fill closure of the Tholense Gat
The Tholense Gat will also only be able to be
closed when the storm surge barrier is in a
position to influence conditions. In this case
however use of the storm surge barrier will be
limited, also because conditions in October,
when this closure is planned, are more
favourable. The Tholense Gat will be sand-filled
from both banks. In this way the current will
remain concentrated in the centre of the gap.
Stone-fill closure of the Tholense Gat
A design has also been made for the closure of
the Tholense Gat with boulders. This will not
be used but is considered of sufficient interest
to warrant description in this report. This work
would be spread over the years 1985 and 1986.
Closure could take place before the storm
surge barrier was operative as the reduction of
the wet cross-section occasioned by the
placing of the sill-beams in the barrier would
be sufficient to allow for an undisturbed
execution. In order to make the closure less
costly a sand-sill of up to N.A.P. - 9,50 m.
would be built up in the closure gap. After this
an embankment closure would be built up
using progessively heavier boulders. Sliding
and liquifaction could not be ruled out with this I
form of closure. For this reason extensive
measures were provided in the design for the
protection of the sea-bed.
Cost estimates and price formation
The customary cost price accounting method
of the Delta Service is applied to the estimate
of the costs involved in the afore-mentioned
closures. The amounts of material to be
processed serve as the basis for the accounting
of the duration the working stock is used. In
this way an approximation of the average
estimate costs is reached.
Besides this the financial consequences of the
technical risks must be assessed. If for any
reason delays should occur in the work, the
costs of hiring working stock continue. In a
case of state contracting not all the risks are
carried by the contractor. In any case the credit
worthiness of the contractor should be capable
of covering possible damages arising.
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