Changes in Venice Lagoon dynamics due to construction of mobile barriers.

The MoSE project (construction of mobile barrier to safeguard the Lagoon of Venice) entails changes to the structure of the lagoon's inlets. This could have consequences for the areas near the inlets and for the dynamics of the lagoon ecosystem as a whole. In order to predict the effects of the proposed alterations on the hydrodynamics of the lagoon, a well-tested hydrodynamic-dispersion model was applied. Simulations were carried out considering both idealised and realistic tide and wind scenarios.The results show that with the new structures the Lido sub-basin tends to increase its extension due the southward movement of the watershed, at the expense of the Chioggia sub-basin, whereas the Malamocco sub-basin changes its relative position, but not its extension.The residence time shows variations in agreement with this trend, decreasing in the southern part of the Lido sub-basin and increasing in the inner part of the Chioggia sub-basin.The variations in residence time and return flow factor indicate that they are caused by changes in both instantaneous current velocities and sea-lagoon interaction. In fact the new breakwaters in front of the Malamocco and Chioggia inlets modify the length and direction of the outflow jet (up to 1msd1) and the patterns of the currents around the inlets and the nearby coast. The new artificial island in the Lido inlet changes the current pattern and increases the current velocity on the southern side of the channel propagating this effect up to the Venice city.The risks and benefits individuated from our conclusion are that the Lido sub-basin can improve its renewal time, but the more intense current speeds can be a risk for the conservation of habitats and infrastructures. Finally the micro-circulation between the breakwater and the coast in Chioggia and Malamocco inlets can be a trap for pollutants or suspended sediment.