Nonstationary SWAN simulation in the Wadden Sea

Abstract

The spectral wind wave model SWAN plays a key role in the estimation of the Hydraulic Boundary Conditions (HBC) for the primary sea defences of the Netherlands. Since some uncertainty remains with respect to the reliability of SWAN for application to the geographically complex area of the Wadden Sea, a number of activities have been initiated under project H4918 \u91Uitvoering Plan van Aanpak SBW-RVW Waddenzee\u92 (Plan of Action on the Boundary Conditions for the Wadden Sea) to devise a strategy for the improvement of the model. In this context, a number of hindcast studies have been carried out with SWAN for the Amelander Zeegat in the Wadden Sea. In all of these studies, the stationary simulation mode of SWAN was applied, since this is the approach currently followed in the computation of the HBC. Although the Wadden Sea interior is relatively small \u96 typically making stationary simulation appropriate - the wave boundary conditions and the local wind and current forcings vary relatively rapidly, so that stationary simulation may become inaccurate. Furthermore, if the model domain would be extended offshore to compute the HBC for the entire Wadden Sea, nonstationary simulation could become necessary. The aim of the present study is to compare the results of nonstationary and stationary hindcasts with SWAN in the Wadden Sea, for typical W to NW storm conditions. In addition, the effect of including morphological changes in the computations is considered. This study has shown that, when simulating over a large spatial domain that includes the entire Dutch coast and a portion of the North Sea, a notable difference between the results of these two simulation modes is the absence of a phase lag between offshore and the nearshore wave conditions in the case of stationary simulation. Over these relatively large spatial scales, both the nonstationary and stationary modes of SWAN strongly underestimate energy at the spectral peak nearshore of the barrier islands. When simulating on the scale of the Wadden Sea, notable differences between the results of nonstationary and stationary simulations are found in the Amelander Zeegat tidal inlet. However, along the Frisian coast, differences between the two simulation modes are relatively small. In nonstationary simulations on the scale of the Amelander Zeegat, the inclusion of morphological changes has little effect on the wave conditions at the Frisian coast. Based on the results of this study, it is recommended to continue the use of the stationary mode of SWAN for simulations on the scale of the Amelander Zeegat.SB