Tidal Flat – Salt Marshes transition in the Venice Lagoon, Italy

Shallow tidal basins like the Venice lagoon, Italy, are often characterized by extensive tidal flats and salt marshes that lie within specific ranges of elevation. Tidal flats lie just below the mean sea level, approximately between -0.6 m and -2.0 m a.m.s.l., whereas salt marshes lie at an average elevation higher than the mean sea level (i.e. between +0.1 m and +0.5 m a.m.s.l.). Only a small fraction of the tidal basin area has elevations in the intermediate range (i.e. between -0.6 m and +0.1 m a.m.s.l.). This occurrence suggests that the morphodynamic processes responsible for sediment deposition and erosion produce either tidal flats or salt marshes but no landforms located in the above critical range of elevations. A conceptual model describing this evolutionary trend has recently been proposed. The model assumes that this bimodal distribution of bottom elevations stems from the characteristics of wave induced sediment resuspension and demonstrates that areas at intermediate elevations are inherently unstable and tend to become either tidal flats or salt marshes. In this work, the conceptual model is validated through comparison with numerical results obtained with a two dimensional wind waves – tidal model applied to the lagoon of Venice. Both the present and the 1910 bathymetries of the Venice lagoon are used in the simulations and the obtained numerical results confirm the validity of the conceptual model. A new framework to acknowledge long term evolution of shallow tidal basins, which uses the conceptual model, is finally proposed and discussed.