Salinity Variations in the Venice Lagoon (Italy) Induced by Safeguard Structures: A Challenging Trade-Off Between Urban and Ecosystem Protection in the Face of Climate Change

Estuaries provide vital ecosystem services, but the communities and ecosystems they support are increasingly threatened by flooding driven by climate change and sea level rise. Hard-engineering solutions like levees, seawalls, river diversions, and storm-surge barriers help mitigate flooding risk, but their combined operation within the same estuary can result in complex interactions, leading to unintended and difficult-to-predict ecological and environmental consequences. Here, we investigated the northern Venice Lagoon, where a spillway in a river levee bordering the lagoon and a floodgate system at the lagoon inlets operate as flood defenses. Using numerical modeling informed by field data, we evaluated their combined impacts on lagoon hydrodynamics during November 2019 - a month marked by extreme rainfall and storm surges that triggered multiple spillway activations and severe flooding in Venice City. We compared scenarios with and without floodgate activation and assessed the effects of projected sea level rise over a 40-year timespan. Our results show that floodgate closures reduce salinity by limiting tidal propagation and increasing hydraulic heads at the spillway, which enhances freshwater inflow by up to 40%. Future sea-level rise scenarios predict more frequent and longer floodgate closures (up to +174 hr monthly), boosting freshwater inflow through the spillway and increasing lagoonal water levels (up to +3.6 cm). This might necessitate earlier floodgate activations, further widening areas affected by salinity changes. Our findings highlight the need to carefully evaluate interactions between flood-defense measures to protect coastal cities while safeguarding estuarine ecosystem resilience under climate change and rising anthropogenic pressures.