Self-organization of shallow basins in tidal flats and salt marshes

Shallow tidal basins such as 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 and 2.0 meters above the mean sea level (m a.m.s.l.), whereas salt marshes lie at an average elevation higher than mean sea level (i.e., between +0.1 and +0.5 m a.m.s.l.). Only a small fraction of the tidal basin area has elevations between 0.6 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 intermediate range of elevations. A conceptual model describing this evolutionary trend has recently been proposed. The model assumes that the 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 wave-tidal model applied to the Lagoon of Venice, Italy. Both the present and the 1901 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 that explains the long-term evolution of shallow tidal basins based on the results presented herein is finally proposed and discussed.