The specific objective of this study is to investigate impact of the presence of secondary tidal basins on long-term estuarine morphodynamics. To this end, first, a morphodynamic model using Delft3D software is set up, which is able to reproduce large-scale bottom features similar to those observed in real estuaries. Second, runs are conducted with the latter model in case that secondary tidal basins are present. The Scheldt estuary is selected as the study area, where multiple secondary tidal basins were present in the past (e.g. Sloe and Braakman). To compare model results with observa- tions, secondary tidal basins are constructed at the same positions as those of former basins Sloe and Braakman.
In the case of using present geometric shape of the Scheldt estuary (default case), good resemblance between modeled and measured bathymetry is observed, even though significant differences occur. In particular, the simulated formation of a shallow region in the mouth of the estuary flanked by two distinct southern and northern channels, and of a system of straight and meandering channels inside the estuary, are large-scale features that are comparable to observed bathymetry. Moreover, the simulated connection between the main ebb channel in the estuary and the southern channel in the mouth, with the latter extending seaward over time, is confirmed by historical bathymetric data. A significant difference between simulated and measured bathymetries is that the southern channel in the mouth area forms too far south compared with the observed main channel in this area (Wielingen). Another major difference is that many small-scale secondary channels appear in the mouth, which are not observed in the field. Appearance of these small-scale bathymetric features is attributed to neglecting of waves.
Similar to the case of absence of secondary basins (default case), a system of straight flood and meandering ebb channels occurs also in case of presence of the Sloe basin. Main difference is that in the latter case, the part of the ebb channel that is located near this basin forms ∼ 2 km more to the south with respect to its location in the default case. These results suggest that the presence of this secondary basin causes a locally southward migration of the ebb channel. Similar southward migration of the ebb channel occurs in case that both Sloe and Braakman basins are present. However, in this case, the meandering ebb channel is wider and the flood channels are less pronounced compared with those in the case of only the Sloe basin. Adding only the Braakman basin weakens the connection between the landward and seaward parts of the ebb channel, which results in a decrease of the sinuosity (meander) of this channel. The presence of Braakman basin does not result in a local displacement of the ebb channel, which reveals the importance of basin location. Results further show that the presence of secondary tidal basins leads to changes in the shape of the meandering ebb channel, such that the connection between the latter channel and the southern channel in the mouth area is weakened, particularly in the case of Braakman basin. This means that the presence of a secondary tidal basin inside the estuary not only have local morphodynamic effects, but it can also have significant impact on the morhodynamic evolution of the mouth of the estuary. This is also confirmed by the fact that sand balance of the entire estuarine system (estuary+mouth) changes significantly in case that secondary basins are added to the system.
Based on the model results, it can be stated that observed northward migration of the ebb channel near the location of former secondary basin Sloe is due to closure of this basin. Moreover, these results suggest that the closure of Sloe and Braakman basins probably have affected to morphological evolution of the Scheldt mouth area.