Samenvatting

The realization of the storm surge barrier and the two secondary dams changed not only the hydrodynamics but also the geomorphological characteristics of the Oosterschelde estuary. These engineering works have created a disequilibrium between erosion and sedimentation. At this moment there is a sand deficit in the Oosterschelde that leads to the erosion of the tidal flats as a response to this new state.
In order to give an adequate answer to this large scale erosion in the Oosterschelde new approaches that are more cost-efficient and sustainable should be explored. In the framework of the programme Building with Nature, several solutions are being investigated in order to conserve or restore the intertidal area. One possibility is the use of ecosystem engineering.
Pacific Oysters (Crassostrea gigas) are ecosystem engineers that can influence flow, wave action and sedimentation and erosion patterns within the reef, and in the surrounding of the reef. To use these organisms - by constructing artificial reefs that develop into living oyster reefs - for coastal protection, better insight is needed of the effect these organisms have on the morphological development of tidal flats.
The purpose of this research is to assess the suitability of oyster reefs as a sustainable coastal protection method for the intertidal areas of the Oosterschelde. The research focus on the erosion and sedimentation patterns observed in and around naturally developed oyster reefs in the Oosterschelde. A long term analysis of existing height data (transects crossing tidal flats) and also a short term analysis of height measurements around a few reefs using a DGPS and mini sand nourishments techniques were used in order to reach the aimed goals.
From the long term analysis it is concluded that the areas without oyster reefs are more prompt to erosion than areas with oyster reefs. Where oysters are present along the transects, they preferentially settled low in the intertidal, often near intertidal gullies or after the settlement, the oyster reefs promoted gully formation, especially at the edges of the reefs. This fact is very interesting and important when constructing artificial reefs in the Oosterschelde. It demonstrates that artificial reefs should be constructed preferentially low in the intertidal, as here the chance of developing a living oyster reef will be the best.


The reefs in the short term analysis show lower heights due to erosion on the exposed side of the reef and higher heights on the sheltered area. The existence of higher heights behind the reef is not necessarily a synonym of sedimentation promoted by the reef but can be seen as a decrease of the erosion rates promoted by the oyster reef.
The results are encouraging from the point of view of using these ecosystem engineers as a short term solution to prevent the fast erosion of the tidal flats in the Oosterschelde.