A two-point dynamic model for the coupled evolution of channels and tidal flats

A dynamic model for the morphological evolution of channels and unvegetated tidal flats is proposed. Channels and tidal flats are schematized as two reservoirs that exchange sediment through the tidal dispersion mechanism, which stems from the presence of a tidal exchange flow and spatial gradients in sediment concentration. The reference concentration in each reservoir is determined by the shear stress associated to tidal currents and surface wind waves, which are a function of the geometry of the system. A simplified procedure to compute flow partition between channels and tidal flats is developed and compared to the numerical solution of the shallow water equations, showing good agreement. In the absence of wind waves, tidal flats reach a stable dynamic vertical equilibrium close to mean high water level, resembling a creek-marsh morphology. For intermediate wind conditions, an additional stable dynamic vertical equilibrium, characterized by a channel flanked by tidal flats close to mean low water, arises. Such equilibrium stems from a sediment exchange dynamic balance between current-dominated channels and wave-dominated tidal flats, and it likely represents the morphological configuration of most tidal flats. Waves associated with intense winds suppress the channelization process. The model suggests that tidal flat elevation is primarily controlled by waves and can be decoupled from channels. Channel depth is also indirectly controlled by waves, through the influence of tidal flat elevation on channel hydrodynamics. Finally, the model predicts that variations in environmental parameters, such as sea level, storminess, and sediment availability, can induce catastrophic morphological shifts.