Bio-physical impacts on fine sediment dynamics in an idealised Wadden Sea basin

This research has focussed on the identification of the relevant processes behind the long-term fine sediment dynamics. Three main influences were considered: the tide, wind waves and biological stabilization/destabilization. The study was performed by means of computational modelling, within the process-based model, Delft3D. By modelling several different scenarios, the separate influences of the tide, wind waves and biology were investigated. The results showed that the tidal asymmetry, entering the basin from the sea, was the main contributor to the import of fine sediments. In general, the tide promoted a large import towards the intertidal areas below the MSL line, and only a very small import towards higher elevations, due to the limited transport capacity of the tidal currents in these areas. Waves were the main contributor to the export-flux of fine sediments by the model, by increasing the bed shear stress in the intertidal areas and causing a relatively larger distribution of sediment towards the deeper areas. Waves were also the main contributor to the high suspended sediment concentrations inside the modelled basin, causing a substantial increase in the basin-average SPM concentrations. In case of a combined influence of waves and biology, particularly the interaction between the seasonal variations appeared to be of importance, which further enhanced the export flux from the modelled basin during late autumn and early winter. With respect to the distribution of sediment, the biological influences caused a landward shift in the deposition and the sediment was distributed much more evenly than if no biological effects were included in the model.Civil Engineering and Geoscience