The specific aims of this study are 1) to set up a morphodynamic model for the Scheldt mouth that accounts for waves, 2) to examine sensitivity of model results to different wave conditions, and finally, 3) to present an example case of how this model can be applied to study the impact a construction of a new shipping channel might have on the morphodynamic evolution of the Scheldt estuary and its mouth area. To this end, the coupled Delft3D and SWAN models were used, which the latter modeling wave action.
The methodology employed is that, first, the model parameters (coupling time, morphological amplification factor) are optimized to reduce the simulation time as much as possible. Next, runs are conducted with and without waves, starting from an initially flat bed until a bottom pattern is obtained that is characterized by relatively small bed level changes compared with the initial changes (nearly morphodynamic equilibrium). In the experiments with waves, a highly simplified wave forcing (with dominant directions south- and northwest) as a well as a more sophisicated forcing (with four different wave conditions) are considered, using the mormerge approach.
Result reveal that when accounting for waves, an ebb-tidal delta forms in the mouth area, which is flanked by two distinct southern and northern channels (”Wielingen” and ”Oostgat”), which are large-scale features that are comparable to observed bathymetry in the Scheldt mouth. The fact that the model simulates two distinct channels (”Oostgat” and ”Wielingen”) in all cases without and with waves, means that tidal motion is the primary forcing that causes the formation of these channels. Furthermore, results from sensitivity experiments to different wave conditions show that in the case that waves are coming from the northwest, the obtained bedlevel does not fundamentally differ from the simulated bedlevel in case of southwesterly waves. In the case of applying four different wave conditions, however, the southern channel (”Wielingen”) is less pronounced than that in the other cases using constant wave conditions. Model results suggest that the observed deepening of Oostgat is part of the natural solution of the system. A major problem in the model is that it simulates too deep channels compared with observations. Nevertheless, by means of an example case, it is shown that the model can be applied to study long-term morphodynamic effects of the presence of a new shipping channel in this area, under the condition that the depth of this channel should be rescaled in proportion to depths of other channels. Finally, due to the idealized approach applied in this model, the precise numbers obtained from the simulations should be considered as indications of orders of magnitudes.