A Scenario-Based Eulerian–Lagrangian Framework for the Management of Semi-Enclosed Basins

Semi-enclosed basins such as lagoons and harbors are often ecologically or economically valuable, yet they are characterised by weak water exchange with the open sea and a high sensitivity to contamination. This thesis proposes an end-to-end methodology to support their management and protection, linking long-term metocean variability, hydrodynamics and pollutant dispersion in a single, scenario-based framework. The approach begins by condensing a multi-decadal hindcast of wind and waves into a limited number of representative metocean scenarios using clustering techniques. These scenarios are then used to drive three-dimensional hydrodynamic simulations, whose velocity fields feed Lagrangian particle tracking models for passive tracers and, in one case, for oil and hazardous and noxious substances. Single- and multiple-particle statistics are applied to the resulting trajectories to quantify water renewal, retention and mixing over a range of spatial and temporal scales. The methodology is applied to two contrasting semi-enclosed systems: a coastal lagoon and a large commercial port. In the lagoon, results reveal extremely limited connectivity with the open sea, with water renewal largely controlled by wind rather than tides. Simulations of lagoon-wide releases and of a sewage plume both indicate a strong tendency to retain tracers, underscoring the system’s vulnerability to nutrient and contaminant accumulation. In the port, spills released at accident-prone locations frequently remain confined within the harbour for days, as basin geometry and local circulation favour trapping over export. Overall, the thesis shows that a library of precomputed metocean–hydrodynamic scenarios, coupled with Lagrangian diagnostics, can provide a practical and transferable tool to assess flushing, retention and contamination risk in semi-enclosed basins, supporting both contingency planning and informed environmental management.