Mixing characteristics of the Dutch Wadden Sea: Elucidation based on 11 years of simulation results

The Dutch Wadden Sea (DWS) is an event-driven aquatic system between the Wadden Islands and the Dutch mainland. The dynamics of the system is mainly modulated by the collective influence of seasonally varying wind, the meso-tidal forcing and the freshwater release from two sluices. A three-dimensional hydrodynamic model (GETM- General Estuarine Transport Model) was applied to this region to study the hydrodynamics of the system for over a period of 11 years. In the present study, we have extracted the depth-averaged current from each inlet that connects the DWS with the North Sea. Furthermore, Simpson-Hunter parameter, which is the ratio between the depth and the cubic power of the current speed, was estimated at each inlet. This empirical formula is adopted to determine the mixing capacity of the DWS inlets on a seasonal and inter-annual scale. The initial results manifested that the Texel and the Vlie inlet undergo well-mixed nature in general; however, sporadic stratification events occur in the low wind condition during slack tide. On a contrary, stratification events occur quite often in the other three inlets (i.e., Eierlandse Gat, Borndiep and Friesche+Pinkegat). On a seasonal scale, the frequency of stratification events in the minimum during autumn and maximum during spring. The present results bear a resemblance to the earlier ones’ where low to intermediate magnitudes of Simpson number (quantification of the density-driven stratification) were obtained in the Texel inlet which is indicative of low stratification. Also, the density-driven stratification was mainly controlled by the wind climatology. On a nutshell, the present research emphasizes the influence of wind on the mixing capacity of the DWS. In addition, further simulations are continued considering the scale of 40 years.