Investigating Wind-Waves Impact and Sea Level Rise on the Long-term Morphological Development of Estuarine Shoals

Estuarine shoals are valuable areas with functions of nature, safety and navigation. It is of utmost importance to understand their underlying physical processes and long-term evolution to achieve protection. The first aim of this report is to determine the impact of wind-waves on long-term morphological development of estuarine shoals. The rapid sea level rise requires a valid model prediction of esturine shoal evolution in the future. Whether waves should be included for forecast needs to be evaluated. This rises the second aim to investigate wind-waves impact on estuarine shoals evolution under the sea level rise. A 2-D, process-based numerical model (Delft3D) is applied. This study sets up a large scale realistic model covering the whole Western Scheldt geometry and focuses on shoal Van Ossenisse by constructing high resolution grids and imposing wind-waves. Simulations are compared on a timescale of 50 years, with and without wave effects. Results revels that waves tend to slightly migrate shoals along their propagation direction. Waves erode sediment in the intertidal area, resulting in a lowering elevation of the shoal and high suspended sediment concentrations in water column. On the one hand, high SSC combined with wave asymmetry and wind-driven flow enhance sediment transport rate along wave propagation direction over top of the shoal, causing more sediment appearing in the lee side of the shoal. On the other hand, wave-induced suspended sediment follows tidal currents and transport to shoal edges where low bed shear stress exists. These cause shoal widening at lower intertidal area and upper subtidal area. Channel velocity hence increases with response to channel area reduction. This leads to erosion in the channel and channel deepening. In a longer timescale, waves impact do not lead to fundamental difference on estuarine autonomous behaviors. It reveals that waves impact may be sensitive to some parameters (eg. sediment grain size, tidal range, type of boundary conditions), especially in the inner channel. It is recommended to do more investigations. Sea level rise scenarios are carried out by imposing gradually sea level rise at the seaward boundary. Its timescale is 100 years. Both in wave and no wave case, sea level rise leads to elevation of shoal height and area lose in shoal edges, resulting in steeper slope in the intertidal area. Larger channel area and volume are presented with sea level rise. Waves maintain their function under the sea level rise. It lowers and widens the shoal, resulting in the increase of intertidal area and volume. Sea level rise does not change the tendency of waves impact.