Morphodynamic Feedback Loops Control Stable Fringing Flats

We apply a 2-D horizontal process-based model (Delft3D) to study the feedback mechanisms that control the long-term evolution of a fringing intertidal flat in the Western Scheldt Estuary. The hydrodynamic model is validated using a comparison with measurements on the intertidal flat and the sediment transport module is calibrated against long-term morphology data. First, the processes that lead to net sediment exchange between channel and flat are studied. Then, long-term simulations are performed and the dependency of sediment fluxes on the tidal flat bathymetry, and the corresponding morphodynamic feedback mechanisms are explained. In the long run, relatively stable states can be approached, which are shown to be typical for wave-dominated fringing mudflats. The system behavior can be explained by the typical feedback mechanisms between the intertidal bathymetry and the hydrodynamic forces on the flat. In the subtidal domain, the impact of small (5-10cm) wind waves increases with a rising elevation due to decreasing water depths. In the intertidal domain, the wave impact increases with increasing cross-sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide-induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves. Plain Language Summary The difficulty in managing the long-term morphodynamics lies in the interdependencies of the underlying processes; the morphology is shaped by the hydrodynamic forces, while it influences these forces at the same time. The resulting internal feedback loops make the long-term morphodynamics difficult to model and to predict. Following a top-down approach, where we learn from the observed steady states, is a useful strategy to get toward an understanding of complex dynamical systems. This starts with realizing that an equilibrium state is not self-evident. Hence, the observation of conserved properties should naturally lead to the important question: why are they conserved? We apply a 2-D Delft3D model to study the feedback mechanisms that underlie the long-term evolution and stabilization of intertidal flats that fringe coastal embayments. We found that small wind waves (5-10cm) have an important influence. Their maximum impact in shallow waters implies a strong stabilizing feedback loop in the subtidal domain, where the minimum water depth strongly increases (decreases) with a decreasing (increasing) bed level. The rest of the intertidal flat is stabilized by another feedback mechanism, induced by the process of wave shoaling and its typical dependency on the cross-sectional slope.