Golf-stroominteractie in ondiepe waters. Vortex force versus gradiënt radiation stress benadering

This study focuses on investigating wave-current interaction in the Belgian coastal area of the North Sea. The examination of this interaction is crucial for better understanding of both wave and hydrodynamic processes which is essential for improving coastal management, navigation safety and the accuracy of forecasting models. The reciprocal interaction between wind-induced short gravity waves (referred to as waves) and the barotropic mean flow (referred to as currents) occurs throughout the water column. These interactions can be divided into two categories: the effects of currents on waves and the effects of waves on currents. The latter group includes processes such as wave-induced currents and wave set-up. To study this phenomenon, the wave field is simulated using the SWAN (Simulating Waves Nearshore) model, which accounts for the wave-induced force and Stokes drift. These wave-related parameters are integrated into the COHERENS (Coastal Hydrodynamics and Sediment Transport) circulation model in a depth-averaged mode. Subsequently, the wave model (SWAN) and the current model (COHERENS) are coupled online, enabling a comprehensive analysis of the wave-current interaction. To validate the coupled-set, two well-known academic test cases are employed: the planar beach case and the single-barred beach case. Following successful validation, the coupled-set is applied to a two-dimensional test case based on the Belgian coastal waters. This test case involves the calculation of wave-surge and wave-induced currents using both existing methods for wave force calculation. The obtained results for the storms occurring in January 2007 and March 2008 are then compared against observational data. The study concludes that in the depth averaged mode, the two methods perform very similar in the academic test cases. The only difference is shown to be the ability of the Vortex Force (VF) method to simulate the undertow. Undertow is in fact the return flow of the mass brought up to the shore by waves to maintain the mass balance in the surf zone. The undertow plays a crucial role in rip currents and sediment redistribution. It is observed that the two methods for calculating the wave-induced force yield almost identical outcomes in the real test case. The simulation of wave-induced circulation based on the Vortex Force (VF) approach is found to provide results that are slightly more in agreement with the measurements compared to the Radiation Stress Gradient (RSG) approach.