Anthropic and natural variations effects on basin hydrodynamics and settling time in multi-inlet systems: Insights from Marano and Grado lagoon

Multi-inlet lagoons are complex environments with a generally high socio-economic value, which often require interventions aimed at stabilizing the inlets or improving navigability. The effects that arise from anthropogenic or natural changes localized in a single basin can concern the entire system since the basins are interconnected. This study analyses the changes occurring in the Marano and Grado lagoon over the last 52 years, with a 2DH numerical approach and the correlation between different hydrodynamic variables. In particular, tidal prisms, water levels, residual currents, asymmetries between tidal flood and ebb are analysed and discussed in the light of a new parameter called settling time. It suggests the time required by each basin to drain the volume of water stored during the rising tidal phase. The construction of the jetties to protect the Porto Buso inlet and the narrowing of the Lignano cross-section inlet have increased the settling time of the respective basins, while the loss of salt marshes in the Grado basin has reduced it. The results obtained show that any modification of the settling time of a basin in a tidal multi-inlet system lengthens or shortens the response of the basin itself to the tidal forcing. This also determines level gradients, and transverse flows that modify the hydrodynamic pattern of the neighbouring basins.