Morphological development of the Bollen van de Ooster

Abstract

The Bollen van de Ooster, in this report referred to as the Ooster, is a sand bar in the outer delta of the Grevelingen. The Ooster is separated from the coast by a relatively deep channel, called the Schaar. Closure of the Grevelingen in 1971, with the construction of the Brouwersdam, initiated vast changes in morphology. Damming of the estuary mainly affected the tide-induced flow patterns and therefore the relative influence of the waves at the outer delta. In this study the main focus is on the morphological development near the coastline of Goeree-Overflakkee. During the past years an erosion trend of locally up to 27 m/year, just south of the Flaauwe Werk, led to concerns with Rijkswaterstaat. This erosion is caused by the migration of the Ooster along the coast. Out of precaution a beach nourishment has been planned in this area due to the potential danger for the Flaauwe Werk. However, uncertainty about the future morphological development and therefore the necessity to take measures remains. The aim of this study is to provide a better understanding in the changes that have occurred, bring more certainty about the future and therefore contribute to informed decision-making. The coast of Goeree-Overflakkee has had a long history of coastline erosion. Closure of the Grevelingen estuary led to an increase of this erosion trend at the Westhoofd due to a combination of increased tide-induced flow velocities and morphological development of the Ooster. The latter was characterised by an eastward migration which forced the Schaar into the coastline. This process initiated multiple nourishments in the period 1969-1985. The attachment of one of the shoals of the Ooster resolved this problem, resulting in a large accumulation of sediments on the beach. In the years thereafter these sediments have been eroded as the Ooster migrated along the coast and transported further north. The latter resulted in beach widening along the coast of Goeree-Overflakkee. Due to the rapid elongation of the Ooster the erosion problems of the 80’s returned. The Schaar shows a continuous decrease in depth and flow surface since 2003, according to data analysis. Model results show that the channel mainly plays a role during low tide when the Ooster is emerged. Significant tide-induced flow velocities occur in the channel. In general, the decreasing channel dimensions lead to an increase of the magnitude of the flow in the channel. However, this occurs for a limited time duration during the full tidal cycle. A striking observation is the small influence of the wave angle on the wave-induced flow velocities near the channel. Waves coming from the north are refracted considerably 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 Ooster. This could explain the pace of the migration in eastern direction. Moreover these sediments are a source for the channel, contributing to decreasing the channel dimensions. In general the presence of wind decrease the magnitude of the flow in the channel. Based on these findings it can be concluded that the waves are the dominant forcing mechanism in shaping the morphology and attachment of the Ooster is expected to occur in the near future. Attachment of the Ooster implies the disappearing of the eroding currents. The duration at which this attachment can be expected cannot 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 treat of the morphological development for the primary flood defences seems minor.