Assessing the restoration potential of the Orbetello Lagoon with Slim 4 : In collaboration with DEME
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- The Orbetello Lagoon (Tuscany, Italy) is a semi-enclosed Mediterranean coastal lagoon subject to recurrent eutrophication crises driven by nutrient enrichment and restricted tidal exchange through three narrow inlets. The severe anoxic crisis of August 2024, which caused a large-scale fish kill and significant damage to local aquaculture, illustrates the urgency of evidence-based management: two interventions are under active consideration by authorities — artificial pumping at the Fibbia and Nassa inlets, and bathymetric restoration through channel dredging —yet their hydrodynamic effects had not been quantified under a common modelling framework. This thesis simulates eight management scenarios using the 2D finite-element hydrodynamic model SLIM, validated against 2024 ARPAT water-level observations (RMSE = 0.052 m, 𝑟 = 0.851 at the interior Diga station). Water renewal is quantified through Lagrangian Particle Tracking (LPT) diagnostics — the Water Residence Time (WRT) and Water Exposure Time (WET) — computed from dual-season particle seedings in January and June 2024 to resolve both winter background dynamics and the critical summer eutrophication-risk period. A nudging-based pump implementation and a bathymetric sensitivity analysis complement the scenario comparison. The results establish a clear hierarchy. Active pumping (Status Quo) reduces domain-mean WRT by 42% in winter (88.8 to 51.8days) and 73% in summer (95.7 to 25.4days), making it the overwhelmingly dominant intervention. Crucially, the Natural scenario exhibits higher WRT in summer than in winter — the lagoon renews water least efficiently precisely when ecological risk is highest — making active management a structural correction, not merely an enhancement. All eight scenarios display a bimodal summer WRT distribution: pumped scenarios cluster at 25–28 days and unpumped scenarios at 75–96 days, with no intermediate state, exposing a systemic fragility that argues for combining active forcing with passive improvements. Among passive interventions, Wide Dredging (pit removal and channel widening, Scenario 07) is the most effective, achieving 74.9days in summer. The combined scenarios (03, 06, 08) show no domain-mean improvement over pumping alone, yet spatially they reduce WRT in the highest-stagnation zones — the eastern Levante basin in summer and the eastern and northern Ponente in winter — improvements that domain-mean metrics conceal and that are directly relevant for crisis prevention.