Wind waves in shallow microtidal basins and the transition from tidal flats to salt marshes,

Shallow microtidal basins are characterized by extensive areas of tidal flats and salt marshes that lie within specific ranges of elevation. Both 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 transition from tidal flats to salt marshes. The model accounts for sediment deposition, tidal oscillations, and sediment resuspension by wind waves. The case study to which the model is applied is the Venice lagoon, Italy. Model results show that the transition from tidal flats to salt marshes occurs abruptly when the deposition rate reaches a critical value related to the local wind climate. 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 wind and wave climate. 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 variability of tidal flat elevation is finally related to deposition rates, tidal oscillations, and wind and waves characteristics.