A conceptual model for the long term evolution of tidal flats in the Venice lagoon

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, whereas salt marshes lie at an average elevation higher than mean sea level. Only a small fraction of the tidal basin is characterized by intermediate elevation. Recently a conceptual model has been proposed demonstrating that the bimodal distribution of bottom elevations stems from the characteristics of wave induced sediment resuspension. The model shows 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 which comes out to be inherently unstable. The conceptual model has been validated through comparison with numerical results obtained with a two-dimensional windwaves – tidal model applied to the lagoon ofVenice. In thisworkwe propose a new framework to explain the long-termevolution of tidal flats in shallow basins. This framework is verified by the application of the conceptual model to different bathymetries of theVenice lagoon reproducing configurations of the lagoon during the last century. The variation displayed by the bottom elevation PDF curve of the Venice lagoon in the last century shows that the lagoon experienced an initial salt marsh degradation phase followed by tidal flat deepening.This result enable us to infer the likely future trend of theVenice lagoon and to suggest the possible morphological consequence of an increased sediment supply induced to face the actual lagoon degradation.