Acqua alta in Venice and the MOSE barriers explained

The short answer

Acqua alta ("high water") is the high tide that floods parts of Venice. ISPRA, the national environmental research institute that runs the lagoon's tide-gauge network, uses the term for floods that mainly affect Venice, Burano and Chioggia, and explains every tide as the sum of two parts: the astronomical tide set by the Moon and Sun, and a weather-driven rise caused mainly by low pressure and wind, above all the south-easterly Scirocco.[1]

MOSE is the system built to keep those tides out of the lagoon. It is four rows of hinged steel gates, 78 in all, across the three inlets that connect the lagoon to the Adriatic, at the Lido, Malamocco and Chioggia.[3] When a dangerous tide comes in, the gates rise and cut the lagoon off from the sea until the tide drops. They are not in Venice's historic centre: they stand in the three gaps in the coastal strip through which the tide enters the lagoon.[3]

What makes a Venetian high tide

ISPRA describes three things stacking up. The astronomical tide in the northern Adriatic has a range of about a metre at most, larger at full and new moon. On top of that comes the meteorological surge: low pressure raises the sea, and the Scirocco piles water into the closed northern end of the Adriatic. The Adriatic also sloshes back and forth after a sudden change in wind, in oscillations called seiches whose main period, ISPRA says, is about 21 hours.[1]

All Venetian tide heights are measured against one fixed datum, the Zero Mareografico di Punta della Salute (ZMPS), which is the mean sea level of 1897.[1] A tide "of 110 cm" means 110 cm above that 1897 level, not 110 cm of water in the street. ISPRA adds that the sea itself has risen against the datum: mean sea level in recent years has stayed above +30 cm ZMPS, the result of the ground sinking (subsidence) and the sea rising (eustatism) together.[1]

The flood that shaped everything since came on 4 November 1966, when, according to the Consorzio Venezia Nuova's chronology, the Punta della Salute gauge recorded a storm tide of 194 cm and Venice and the other lagoon centres were under a metre of water.[6] Special law no. 171 of 1973 then declared the safeguarding of Venice a problem of "pre-eminent national interest". The two official pages disagree on its date: the Consorzio's chronology gives 16 April 1973, the Lagoon Authority's MOSE page 13 April 1973.[8][6]

How the MOSE gates work

The Consorzio Venezia Nuova, the Ministry of Infrastructure and Transport's concessionaire for MOSE, sets out the layout on its project page.[3]

At rest, each gate is a hollow steel box full of water, lying out of sight in a concrete housing on the channel bed. To close an inlet, compressed air is pumped in and the water pushed out; the gate pivots on its hinges and floats up until it blocks the tide. When the tide has fallen and the lagoon and sea are level, the gates are flooded again and sink back into their housings. The Consorzio puts the average closure at about four to five hours, manoeuvres included.[3]

Construction began in 2003, and the Consorzio's chronology dates the formal start of works to 14 May 2003.[6] The Consorzio says MOSE can protect Venice and the lagoon from tides of up to 3 metres and from a sea-level rise of up to 60 cm over the next hundred years.[4]

Who decides when the barriers go up

Two bodies appear on the official pages, and they have different roles. The Consorzio Venezia Nuova built MOSE as concessionaire.[3] The Autorità per la Laguna di Venezia (Venice Lagoon Authority), a national public body created by a decree-law of August 2020, lists the ordinary and extraordinary management and maintenance of MOSE among its functions.[7]

The Consorzio's description of the control room sets out the decision rule: the barriers are raised on the water level actually measured by tide gauges in front of the inlets, not on the forecast, which serves to raise the alert. That text is written partly in the future tense, describing how the control room at the Arsenale "will" operate, so it reads as the design rule rather than an account of current practice.[4] XTOUR has not read an official statement of the tide level at which a closure is ordered, and does not give one.

What the Authority's closure log shows

The Lagoon Authority publishes a table of closures on its MOSE operations page.[9]

The 2025/2026 log runs from a closure on the morning of 10 September 2025 to one on the evening of 15 May 2026. Not every entry closes all four barriers: several record only the Lido barriers closed. The highest sea-side peak the log records at the Lido during that season is 139 cm, on the evening of 4 February 2026.[9] The Authority's own news page says an anomalous run of high tides led to 30 activations between 28 January and 20 February 2026, and that it has re-analysed the data to improve how the barriers are managed.[10]

One detail in the log matters for reading it. Its footnote says the listed duration runs from the start of raising to the end of re-ballasting, and that the barriers' hydraulic effect lasts about 25 minutes less than the figure shown.[9]

XTOUR's reading of these pages is that closures cluster between autumn and spring and can run on consecutive days, but the log is a record of past seasons, not a forecast of the next one.

St Mark's Square and the local defences

St Mark's Square is the lowest part of the city, and the official pages give different figures for when it starts to flood. ISPRA says the square floods above 80 cm ZMPS, and that this is not counted as acqua alta precisely because the square is the lowest point.[1] The Consorzio's San Marco page gives a lower figure for the basilica: at 60 cm, it says, water starts to enter the narthex of St Mark's Basilica and the paving in front of the entrance, more than 250 times a year; at 90 cm almost two thirds of the square is under water, and at 100 cm, on average seven times a year, it is almost completely submerged.[5] The two pages measure different things, the basilica's narthex and the open square, and neither gives a date for its counts.

The official pages divide the defences in two. The Lagoon Authority's MOSE page lists local defences of the lagoon's settlements against medium-high tides, and the MOSE barriers against exceptional ones.[8] The Consorzio's own MOSE page goes further, saying the barriers defend Venice from exceptional tides and from more frequent ones.[4] Because XTOUR has not read the level at which a closure is ordered, it cannot say at which tide levels the square still floods while MOSE is in service.

The Consorzio's San Marco project is aimed at the frequent floods: it seeks to keep the square passable up to +110 cm by raising the quay and paving, restoring the old drainage tunnels beneath it and laying a clay membrane under the paving.[5] Chioggia has its own small-scale version, which the Consorzio calls "Baby MOSE": two gates on the Canal Vena that cut the canal off from the lagoon at high water.[4] For the sights around the square itself, see the Doge's Palace, St Mark's and the civic museums.

Before you travel

ISPRA's tide forecasts are referred to the Punta della Salute datum, the same datum the Authority's closure log uses, so a forecast number reads the same way as the log above.[9] ISPRA issues a tide bulletin for the northern Adriatic on its Venice lagoon site, and lists the City of Venice's tide forecasting centre (Centro Previsioni e Segnalazioni Maree) among its institutional partners.[2] XTOUR has not reviewed that centre's own service, so it does not describe it.

XTOUR does not publish tide forecasts, flood warnings, which streets flood at which level, footwear or walkway arrangements, or how a high tide affects water-bus services or museum openings. Those change from tide to tide; the city's and ISPRA's own channels are where to check them.