Wind waves in shallow microtidal basins and the dynamic equilibrium of tidal flats

Shallow microtidal basins are characterized by extensive areas of tidal flats that lie within specific ranges of elevation. These landforms are inherently flat and their evolution strongly depends on the balance between sedimentary and erosive processes. Here we present a stochastic point model for tidal flat evolution to study the influence of tidal currents and wind waves on tidal flat equilibrium. The model accounts for sediment deposition and sediment resuspension by wind waves and is applied to the Venice lagoon, Italy. Model results show that the equilibrium elevation of tidal flats depends on the relationship between shear stress caused by wind waves and depth. It is found that wind wave shear stresses peak for a specific water depth which is a function of the local wave climate and fetch distance. Above this critical depth, tidal flats are unstable, since an increase in elevation reduces wave height and therefore erosion, preventing the system from recovering equilibrium conditions. The critical depth for equilibrium depends on fetch distance but not on substrate characteristics and, for the Venice lagoon, varies from 1.5 m for unlimited fetch (>3000 m) to 0.6 m for a fetch of 1000 m. The sediment characteristics determine instead the sediment input necessary to maintain the tidal flat in equilibrium at a specific elevation. Sediment inputs for tidal flats composed of fine sand need to be much higher than those required for tidal flats composed of cohesive material. Finally, we show that the spring-neap modulation of the tide is critical for tidal flat equilibrium, with erosive events occurring mostly during spring conditions that equilibrate the sediment deposition during neap tide.