The morphology of tide-dominated systems is progressively influenced by human activities and climate change. Quantitative approaches aiming at understanding or forecasting the effects of interventions and climate change are often aggregated, thereby simplifying or schematizing the investigated area. In this work, we advance on the knowledge of sediment transport processes shaping tidal systems and on methodologies translating schematized model output into physically realistic variables. In terms of improved physics, we systematically evaluate the influence of sand-mud interaction processes. Most tidal systems are shaped by a mixture of sand and mud. Morphological models typically compute transport of sand and mud independently, despite studies clearly demonstrating that their physical behavior is mutually dependent. We investigate the effects of two interaction mechanisms (erosion interaction and roughness interaction, applied with varying mud erodibility) with a schematized process-based morphodynamic model. We convert model output into metrics that describe the meso-scale configuration of the modeled systems, allowing a quantitative comparison of scenarios. Modeled patterns and intertidal flat shape, size and composition widely vary with mud erodibility settings, but equally depend on the evaluated sand-mud interaction mechanisms (with erosion interaction having a larger effect than roughness interaction). Sand-mud interaction thus needs to be accounted for from a physical point of view, but also to improve predictions of tidal basin evolution models, particularly the (bimodally distributed) sediment composition of intertidal flats.

The sediment composition of the seabed governs its mobility, hence determining sediment transport and morphological evolution of estuaries and tidal basins. Bed sediments often consist of mixtures of sand and mud, with spatial gradients in the sand/mud content. This study aims at increasing the understanding of processes driving the sediment composition in tidal basins, focusing on depositional processes. We show that bed sediments in the Wadden Sea tend to be either mud-dominated or sand-dominated, resulting in a bimodal distribution of the mud content where the two modes represent equilibrium conditions. The equilibria depend primarily on the sediment deposition fluxes, with bimodality originating from the dependence of suspended sand/mud concentrations on the local bed composition. Our analysis shows that bimodality is a phenomenon that is not only specific for the Wadden Sea; it can be expected for a wide range of suspended sediment concentrations and thus also in other systems worldwide.

Multiple tidal inlet systems like the Wadden Sea have long been considered as separated basins, bordered by so-called tidal divides. Recently, it was however shown that fluxes of water and sediment occur over the borders of these basins, especially during wind events. In this paper, the wind-driven fluxes over these borders and the residual flow of water through the main inlet are studied. The study is based on flow measurements at the tidal divides and in the main inlet of the Ameland Inlet system in the Dutch Wadden Sea and on numerical modelling. The measurements were carried out during 40 days in the fall of 2017, including both calm conditions and storm events. Numerical simulations of a full year have been used for upscaling results from the measurements to system scale exchange flows, and to unravel the effects of several mechanisms. The wind-driven variability in exchange flows between back-barrier basins at tidal divides was measured in the field and reproduced by the numerical model. Water level set up increases the water depth and thus the conveyance capacity at tidal divides, such that the exchange flows increase in magnitude. The flow conditions due to wind forcing are similar for both tidal divides of the Ameland Basin. The conveyance capacity and therefore the total volume exchange are however different. This leads to a residual compensation flow through the main inlet, which is directed outward (i.e., in the ebb direction) during winds from the prevailing southwestern wind direction. The net discharge through the main inlet is therefore a consequence of the residual flows over the tidal divides.