The morphology of the Scheldt estuary and its mouth varies as a result of natural evolution and human impacts. Particularly sea level rise has become an important driver for policy makers dealing with the long-term management of this area. The overall aim of this report is to increase fundamental knowledge of effects of sea level rise on the morphodynamic evolution of the Scheldt estuarine system, using the coupled Delft3D-SWAN morphodynamic model of Nnafie et al., 2017b. The impacts of different rates of sea level rise, as well as the impact of a time-varying tidal forcing on this evolution are also addressed.
Model results indicate that a rising sea level at a rate of 2 mm/yr results in an slight landward retreat of the ebb-tidal delta of the mouth area, whereby channels and shoals undergo increased sedimenta tion over time (0.25 mm/yr on average). This sedimentation is the result of increased sand import from the offshore areas. The latter changes become stronger with increasing rate of sea level rise. In the Western Scheldt estuary, a rising sea level rather causes the redistribution of sand in this area, such that sand is deposited on the shoals and tidal flats. The overall sedimentation rate in the latter area is 0.15 mm/yr. Higher rates of SLR increase sedimentation on the shoals, which is mostly provided through deepening of channels and the reduction of the sand loss to the mouth area.
In the case that the amplitude of tidal currents remains fixed in time, sedimentation rates in the mouth and the Western Scheldt are too small for these areas to keep up with the rising sea level, which eventually might lead to the drowing of the estuarine system. However, the inclusion of stronger tidal currents increases sedimentation rates on the shoals, and it increases the volume of the deep channels, thereby counteracting the drowning of the estuarine system caused by SLR. These results suggest that the observed increasing tidal amplitudes in this area, most likely resulting from the rising sea level, is crucial for the estuarine system to keep pace with the rising sea level.