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. 42

Tijdschriftenbank Zeeland

Driemaandelijks bericht Deltawerken | 1985 | | pagina 44