Simulation studies for storm winds, flow fields and wave climate in the Wadden Sea

The motivation for this study is to assess the possibility of using wind, current and water level fields as simply as possible for the determination of the Hydraulic Boundary Conditions (HBC) in the Wadden Sea. Since storms are intrinsically unsteady processes, the purpose of this study is to investigate physically-based methodologies to simplify the wind fields that generate severe surges and use then as realistic forcing on the water in the Wadden Sea. An additional purpose is to perform a sensitivity analysis to various model settings for the determination of the wave conditions in the eastern Wadden Sea

Firstly, this was realized by performing a detailed analysis of the characteristics of historical storms generating severe surges in the Wadden Sea, with emphasis on the eastern part. The results show that high water levels were generated by rising wind speeds with directions approximately shifting from near-south to near-north. The historical time- and space-varying wind fields were tentatively schematised by synthetic steady-state values to carry out numerical experiments and testing realistic HBC in the Wadden Sea. Simulations with the WAQUA model applied to the CSM-ZUNO-Kuststrook nested domains showed that the wind field cannot be simplified as a stationary wind field with constant speed and/or direction if physically-based speeds have to be maintained. From this result it is concluded that no simple way exists to specify wind, current and water level fields for the determination of the HBC, at least according to the present methodology

Secondly, a sensitivity analysis was performed for the eastern Wadden Sea to assess the effect of various model settings on the wave conditions along the dikes of the Wadden Sea. These simulations were performed with the SWAN model on a dedicated curvilinear grid for the eastern part of the Wadden Sea. The model simulations include alternatives such as omitting currents, replacing a spatially varying water level with a horizontal water level, developments in bathymetry and sensitivity to offshore boundary conditions. The results showed that the waves in the eastern Wadden Sea are primarily locally generated, that currents cannot be neglected, and that the local depth strongly dominates the local wave conditions

Several recommendations stem from this study: among those, it seems most appropriate to investigate the viability of using parametric unsteady storm profiles to generate realistic (tide free) surges. Regarding the wave modelling, also, it is recommended to include a two-way coupling between the flow and wave models and to further investigate the propagation of low-frequency storm waves into the Wadden Sea