Historical and future development of the tidally averaged transport of sandy sediments in the Scheldt estuary: a 2D exploratory model

To investigate the historical development of the tidally averaged transport of sandy sediments in the main branch of the Scheldt estuary over the last decades (1950–2013), a 2D exploratory model has been developed. This model comprises the depth-averaged (2DH) shallow water equations, driven by an asymmetric tidal forcing at the seaward side, as well as an advection-diffusion equation to describe the depth-integrated dynamics of the suspended sediment concentration. The tidally averaged sand transport results from a subtle balance between the various contributions; advective contributions due to internally generated and externally prescribed overtides and the diffusive and topographically induced contributions. A seaward tidally averaged sand transport is found near the open boundary, whose magnitude has increased since ca. 1950. Moving upstream, the magnitude of the seaward transport decreases and changes into a smaller landward transport with a local maximum near the landward boundary. This maximum has increased over the years. Varying parameters that capture changes in the environment, e.g. historical changes in the bathymetry, future mean sea-level rise or changes in tidal forcing, results in changes in the tidally averaged sand transport that are systematically analysed and related to changes in the various contributions. Our model shows that there is a competition, in terms of determining the magnitude and the direction of the tidally averaged sand transport, between the effects of historical bathymetric changes, changes in tidal forcing and (projected) SLR. Even small changes in the tidal forcing at the seaward boundary can have a large impact on the magnitude and the direction of the tidally averaged sand transport. This hampers accurate predictions of sediment transport and morphodynamic changes in tidal systems, due to the uncertainty in the response of the tidal dynamics to the projected sea-level rise.