Morphological analysis of the beach at the Brouwersdam

Abstract

This thesis focusses on the north-eastward shifting beach at Brouwersdam, which poses a problem for users and stakeholders of the beach. This trend is likely to continue in the foreseeable future and could result in a beach that can no longer fulfil its current functions, like recreation and tourism. Analysis of bathymetric data from the last decades shows that the systems of the Oosterschelde, Grevelingen and Haringvliet are not in equilibrium, the Delta Works have changed hydrodynamic forcing of the system drastically. As a result, the system is transforming to a new equilibrium, a process that isn’t completed yet, nor will it be soon. The time scales of processes like these are in the order of centuries. For the area around the Brouwersdam the result is that the former ebb-tidal deltas of the Grevelingen and the Oosterschelde are pushed shoreward due to the reduced tidal forcing, therefore wave forcing is becoming more dominant. The former shoals Middelplaat and Kabbelaarsbank were pushed against the Brouwersdam and now form the beach there. Over the last decade, the amount of sediment that is lost in the south-western part of the beach is estimated at around 75,000 m3 per year and the amount of dry beach is getting smaller. A morphological model of the area was made to replicate the developments over the last and for the next decade using the SWAN and UNIBEST models. The model set-up consists of a wind- and wave analysis over the last three decades that has been schematized into different wind and wave classes and different tidal conditions, for two different bathymetries (2000 and 2010). The models are able to reproduce the trend, with dominant southwesterly winds and a beach that shifts to the northeast, but the quantities are different from observations. These differences could be the result of the complex flow patterns in the area. The incoming flood current flows from southwest to northeast and stays close to the Brouwersdam and the beach, while the ebb current stays on the outside of the area and flows in opposite direction. The result is a flood dominated flow pattern along the beach, which has its influence on the dominating transport direction and the models aren’t able to replicate this process correctly. In the near future the characteristics of the system are about to change again. There are plans under development to construct a tidal power plant or an inlet sluice in north-eastern part of the Brouwersdam, which means reintroduction of tidal motion in the Grevelingen. A decision on what solution will be chosen is expected before the end of 2014 and should be up and running in 2020. There are a lot of different possibilities for the beach; beach nourishments, the construction of a tidal power plant and eventual compensatory measurements will have a profound effect. How the beach will look like in a decade from now is largely up to the responsible (governmental) parties involved, all possible scenario’s should be carefully evaluated and it is stressed that cooperation between these parties is vital to find a satisfying solution.