Venice, a city intertwined with water, faces persistent flooding. Built on low-lying islands within a shallow lagoon, its unique geography maintains a delicate balance between land and sea. Regular inundations impact daily life, infrastructure, and the preservation of its historical and cultural heritage.
Geographical and Environmental Factors
Venice’s vulnerability to flooding stems partly from natural geological processes, including subsidence, where the land gradually sinks. The city rests on soft sediment layers that compact over time, contributing to a sinking rate of approximately 1 to 2 millimeters per year. Tectonic movements of the Adriatic Plate also contribute to this descent. Historically, excessive groundwater extraction in the mid-20th century further accelerated subsidence, causing the city to drop by about 12 centimeters in some areas, though this practice has since stopped.
Rising global sea levels, driven by climate change, exacerbate Venice’s situation by increasing baseline water levels in the Venetian Lagoon. Between 1993 and 2019, the average sea level around Venice rose by approximately 2.76 millimeters per year, a rate higher than historical trends. Projections suggest that by 2100, sea levels could be 17 to 120 centimeters higher than current levels. These increases mean even normal high tides can become problematic, pushing water levels closer to the city’s ground.
The natural tidal patterns of the Adriatic Sea also contribute to Venice’s susceptibility to flooding. Astronomical tides, influenced by the moon’s gravitational pull, are amplified during new and full moons and equinoxes, leading to higher water levels. The Adriatic Sea’s semi-enclosed nature and shallow depth enhance these tidal effects, sometimes leading to oscillations similar to water sloshing in a bathtub. These factors predispose the city to high water events.
Human Impact and Historical Changes
Centuries of human intervention within the Venetian Lagoon have altered its natural hydrodynamics, intensifying flooding. From the 13th to the 17th centuries, the Republic of Venice diverted major rivers like the Po, Brenta, Sile, and Piave away from the lagoon. This prevented silting that threatened navigation, but inadvertently reduced the natural sediment supply that once built up the land.
More recently, large-scale dredging operations, particularly for deep industrial canals (1961-1969) to accommodate large container ships, modified the lagoon’s floor and water circulation. These alterations made the lagoon less resilient to high water events by reducing its natural ability to buffer incoming tides. These historical and contemporary changes have made Venice more vulnerable to water level fluctuations.
Beyond physical alterations, human-induced climate change amplifies Venice’s flooding problem. While global sea level rise is a direct consequence, climate change also contributes to more frequent and intense extreme weather events. This includes stronger storm surges and persistent low-atmospheric pressure systems that push more water into the Adriatic Sea and lagoon. These amplified conditions contribute to the frequency and severity of flooding.
Understanding Acqua Alta
“Acqua alta,” meaning “high water,” describes the temporary tidal flooding that regularly affects low-lying areas of Venice. It is a specific event resulting from meteorological and astronomical factors, not just a general high tide. This phenomenon typically manifests when low atmospheric pressure systems combine with strong southerly “Sirocco” winds, pushing water from the Adriatic Sea northward into the Venetian Lagoon. The Adriatic’s unique shape and shallow nature can amplify this effect, creating a “seiche” or standing wave that elevates water levels.
While astronomical high tides provide a base level, meteorological conditions trigger an “acqua alta” event. Low atmospheric pressure creates an “inverse barometer effect,” allowing the sea level to rise, while persistent southerly winds drive water into the lagoon. When these conditions align, water can quickly inundate parts of the city. The most immediate impact is the flooding of St. Mark’s Square, one of Venice’s lowest points.
During an “acqua alta” event, elevated walkways, known as “passerelle,” are deployed for residents and visitors to navigate flooded streets and squares. These events usually last a few hours, corresponding to the natural tidal cycle, before the water recedes. While St. Mark’s Square can flood at relatively low tide levels (around 80 cm), more significant events affect wider city areas.
Current Solutions and Future Outlook
To combat flooding, Venice implemented protective measures, notably the MOSE (Modulo Sperimentale Elettromeccanico) barrier system. This project consists of 78 mobile gates at the three inlets connecting the Venetian Lagoon to the Adriatic Sea. When high tides are forecast to exceed 110 to 130 centimeters, compressed air is pumped into these hollow gates, expelling water and causing them to rise from the seabed. This creates a temporary barrier that seals off the lagoon, preventing incoming high water from flooding the city.
The MOSE system first protected Venice from significant flooding in October 2020. While effective in safeguarding the city from high water, its operation and maintenance are costly, exceeding €300,000 per activation. Concerns persist about its long-term effectiveness with accelerating sea level rise and its potential ecological impact on the lagoon’s ecosystem, as frequent closures can disrupt natural water circulation and sediment flow, potentially affecting salt marshes.
Complementary measures enhance Venice’s resilience. These include raising ground levels in vulnerable areas, such as parts of St. Mark’s Square, and improving the city’s drainage systems. While the MOSE system provides a line of defense against immediate flood threats, ongoing adaptations and careful management of the lagoon environment remain important for the city’s long-term future.