Urban Flood Prevention and Ecosystem Trade-offs in Venice, Italy

Developed estuarine and lagoonal systems have progressively evolved toward increasing human control, as their morphology and hydrodynamics have been modified to support settlements, infrastructure, and economic activities. As climate change drives sea-level rise and alters hydrological and meteorological extremes, this human-dominated trajectory is accelerating, with hard-engineering flood-defence systems—including levees, river diversions, and storm-surge barriers—becoming central to coastal flood-risk management in urbanized coastal areas. While the hydro-, morpho-, and ecological effects of individual flood-protection measures are relatively well documented, far less is known about how multiple defenses interact when they operate simultaneously within the same estuarine system.

Here, we investigate the Venice Lagoon (Italy), a paradigmatic anthropogenic tidal basin shaped by centuries of human interventions. In the northern lagoon, a river-levee spillway is used to divert excess river discharge toward the lagoon to protect mainland urban areas from fluvial flooding, while a storm-surge barrier system at the lagoon inlets is operated to limit marine flooding in Venice and other lagoonal settlements. Using numerical modeling supported by field data, we analysed lagoon hydrodynamics during November 2019, a period characterized by exceptional rainfall and storm-surge conditions that triggered repeated spillway operations and severe flooding in Venice. We compared scenarios with and without storm-surge barrier closures and evaluated the effects of projected sea-level rise over a 40-year horizon.

Our results show that storm-surge barrier closures, although effective at reducing tidal water levels within the lagoon, intensify the freshening caused by riverine flood inputs by restricting tidal exchange and increasing hydraulic heads at (and thus flow discharge through) the spillway. Under sea-level-rise scenarios, barrier closures are projected to become both more frequent and longer-lasting, leading to greater volumes of freshwater entering the lagoon and to an expansion of areas affected by altered salinity. Over longer timescales, these coupled processes are expected to drive the lagoon away from its present hydrodynamic and ecological regime, increasing its reliance on active human regulation to maintain stability. This highlights the need for integrated adaptation strategies that explicitly account for interactions among flood-defence infrastructures, so that coastal cities can be protected without undermining the resilience of estuarine ecosystems under ongoing climate change and intensifying anthropogenic pressures.