Venice flooding is caused by a combination of Adriatic tidal cycles, southerly sirocco winds, and low atmospheric pressure pushing seawater into the lagoon. The MOSE barrier system, operational since 2020, now prevents the most severe events, though moderate flooding still occurs during the October-to-March season. The historic low point is Piazza San Marco, which begins to flood at just 80 cm above the standard tidal datum. At 110 cm, roughly 12% of the city goes underwater. At 130 cm, nearly 70% is submerged. The catastrophic flood of November 2019 reached 187 cm, the worst event since 1966 and a stark reminder of the city's vulnerability. Italy's answer to this ongoing threat is MOSE (Modulo Sperimentale Elettromeccanico), a massive flood barrier system spanning the three inlets that connect the Adriatic to the Venetian lagoon: Lido, Malamocco, and Chioggia. Construction began in 2003 and the system became operational in 2020 after years of delays and cost overruns. When tides are forecast to exceed 110 cm, 78 hinged steel gates rise from the seabed to temporarily seal off the lagoon. MOSE has already prevented multiple flooding events that would have otherwise inundated the city. The Centro Previsioni e Segnalazioni Maree (the city's tide forecasting centre) monitors conditions continuously and issues alerts via its website, a Telegram channel, and a siren system throughout the city. Visitors can check real-time tide data and forecasts before planning their day. The flooding season runs primarily from October through March, with November and December historically seeing the highest tides.
Venice Flooding: The MOSE Barrier and the City's Battle with Water
Updated 8 Mar 2026·Facts verified Mar 2026·2 cited sources·How we research
Frequently Asked Questions
5 QuestionsWhat causes flooding in Venice?
Venice floods due to a combination of astronomical tides, strong sirocco winds from the southeast, and low atmospheric pressure over the northern Adriatic. These forces push seawater through three inlets into the shallow lagoon. The city's gradual subsidence over centuries has made it even more vulnerable, with the lowest areas like Piazza San Marco flooding first.
What is the MOSE flood barrier system?
MOSE is a system of 78 hinged steel gates installed at the three inlets connecting the Adriatic Sea to the Venetian lagoon (Lido, Malamocco, and Chioggia). When tides are forecast to exceed 110 cm, the gates rise from the seabed to seal off the lagoon. The system became operational in 2020 after nearly two decades of construction.
When is Venice flooding season?
The flooding season runs from October through March, with the highest risk in November and December. Summer flooding is extremely rare. During peak season, the Centro Maree issues daily tide forecasts so visitors can plan accordingly.
How much of Venice floods during acqua alta?
It depends on the tide height. At 100 cm above datum, about 3.5% of the city floods (mainly Piazza San Marco). At 110 cm, roughly 12% is affected. At 120 cm it jumps to 35%, and at 130 cm nearly 70% of the historic centre is underwater. Tides above 140 cm, which MOSE now prevents, would flood over 90%.
Has MOSE solved Venice's flooding problem?
MOSE has been highly effective at preventing extreme floods since becoming operational in 2020. It successfully blocked multiple events that would have caused significant damage. However, it does not activate for moderate tides below 110 cm, so minor flooding in low-lying areas like San Marco still occurs. Long-term questions about maintenance costs and rising sea levels remain.
