Sediment dynamics and budget in a microtidal lagoon — A numerical investigation

The water circulation and sediment transport in a shallow microtidal basin (the Lagoon of Marano-Grado, Italy) were investigated by means of a 3D finite element modelling system. The framework of the numerical model consists of a 3D finite element hydrodynamic model, a third generation unstructured spectral wave model and a sediment transport and morphodynamic model for both cohesive and non-cohesive sediments. In order to describe the exchange of water and sediment through the lagoon's inlets, the model was applied to a domain comprising the lagoon itself and the whole Adriatic Sea. The developed model reproduces well the tidal motion and the wind induced circulation in the lagoon and the adjacent open sea. The transport of the fine sediments, resuspended by wind waves in the shallow tidal flats, resulted to be strongly related to the residual circulation induced by wind stress and nonlinear interaction between the tidal flow and bottom topography. The westward residual flow influences the exchange with the open sea through the six inlets, which therefore contribute with either a positive or negative net supply of water and sediment to the lagoon. The model provides estimates of the lagoon sediment budget revealing a net loss of about 40⋅ 103 m3 of sediment in 2007. The results demonstrate the potential application of numerical models to simulate the recent morphological evolution and to improve the understanding of sediment dynamics in coastal embayments.