M.Q.T. Groenewegen1,2,3*, S. de Vries1, A. Colina Alonso1,2, R. Abraimi3, P.K. Tonnon2, S.G.J. Aarninkhof1,
1 Delft University of Technology, 2 Deltares, 3 Waterschap Hollandse Delta
The Bollen van de Ooster, a sand bar in the outer delta of the Grevelingen, is separated from the coast of Goeree-Overflakkee by a relatively deep channel, called the Schaar. Since the closure of the Grevelingen in 1971, the Bollen van de Ooster has evolved into an intertidal alongshore parallel shoal of approximately eight kilometers length. Migration of its eastern edge along the coast during the past years led to an erosion trend at the coastline of Goeree-Overflakkee that is measured to be up to 27 m/year. This potentially threatens the local hard sea defence, the Flaauwe Werk. Uncertainty about this behaviour raises questions about the necessity to take measures. In this particular research we aim to provide a better understanding in the changes that have occurred, bring more certainty about the future and therefore contribute to informed decision-making.
Methods & Results
A data analysis of vaklodingen and JarKus, numerical modelling with a 2DH Delft3D model and a case study were applied. The data analysis revealed a periodic arrival of shoals at the coastline since the closure of the Grevelingen until the mid-nineties. Thereafter the Schaar reached a depth of approximately 12 meter. Since 2003, a continuous decrease in depth and flow surface can be observed. The morphological development shows many similarities with two other cases: the recent attachment of the Noorderhaaks at the coast of Texel and the attachment of the Bornrif at Ameland.
Modelling results show that significant flow velocities arise in the channel for a limited time duration during low tide when the Bollen van de Ooster is emerged. This is the result of tide-driven flow along the coast. Simulation of waves showed the small influence of the wave angle on the wave-induced flow velocities near the channel. Waves coming from the north are refracted considerable due to the extension of the Haringvliet outer delta. The result is a large net longshore sediment transport rate at the seaward side of the Bollen van de Ooster. This could explain the pace of the migration in eastern direction. Moreover these sediments are a source for the channel, contributing to the decreasing channel dimensions. Lastly it was found that wind forcing is able to affect the flow velocity in the channel and during more energetic conditions even reverse the flow direction.
Based on these findings we concluded that waves are the dominant forcing mechanism in shaping the morphology. The attachment of the Bollen van de Ooster is expected to occur in the near future. Attachment implies the disappearance of the eroding currents. The duration at which this attachment can be expected could not be deduced from this study. The erosion of the coastline will continue as long as the channel is present. Based on the width of the dune row and the presence of beach groynes the potential threat of the morphological development for the primary flood defences seems minor.
Figure 1 Schematization of morphological evolution of the Bollen van de Ooster near Goeree-Overflakkee and the expected future morphological development.