Numerical modelling of tidal circulation and flushing processes in the Venice Lagoon under different MoSE mobile barriers operation scenarios

As climate change and rising sea levels increase the frequency of MOSE barrier closures, the prolonged isolation of the lagoon water poses significant concerns about the deterioration of key ecosystem parameters over time. While the primary function of the MOSE system is to protect the historic city of Venice from flooding, its increased operation has profound implications for the lagoon's hydrodynamic and ecological health. Reduced exchange with the Adriatic Sea can impair tidal flushing, alter natural circulation patterns, degrade water quality, and elevate the risk of hypoxia - particularly during summer months when higher temperatures and increased nutrient loads can trigger severe oxygen depletion. In a future scenario with higher sea levels, summer closures could become more frequent, exacerbating these environmental hazards and potentially leading to habitat loss and a decline in biodiversity. Previous scientific research has mainly focused on operational logistics, flood-prevention efficacy, and the impacts of full closures on maritime traffic. In contrast, the hydrodynamic consequences of alternative barrier operation strategies remain underexplored. Specifically, the potential of partial inlet closures - closing-opening combinations designed to actively enhance internal circulation - has not been systematically investigated. This represents a critical gap in knowledge. Furthermore, understanding how these physically altered circulation patterns translate into changes in ecological connectivity and habitat suitability is essential. To this purpose, a particular transport-scale metric, such as confinement, is fundamental for defining habitat properties and shaping biological communities in transitional environments like the Venice Lagoon. As for implemented combinations, in addition to the reference case “ALL-OPEN”, where there are no closures and the mobile barriers are not operating or do not exist, and the other reference case “CLOSURE”, with full closures as in present-day use, the main two alternative partial closing-opening combinations are “COMBO”, which tests basic water replacement by allowing water to enter only from one inlet (Lido) and exit from another (Chioggia), so that stagnant, higher-concentration water is selected over newly admitted water; and “BIGCOMBO”, which refines the same principle as “COMBO”, with a larger redistribution of the exchange. This doctoral thesis directly addresses these interconnected gaps through a structured, three-part numerical modelling study. Each part is designed as an independent, publishable scientific contribution, together forming a comprehensive multi-scale analysis from fundamental physical mechanisms to applicable ecological indicators.