MOSE is a set of 78 hollow steel gates lying flat on the seabed across the three inlets that connect the Venetian Lagoon to the Adriatic. When a tide above 110 cm is forecast, air is pumped in, the gates swing up, and the lagoon is closed off. Closing takes about 30 minutes (Ferrarin et al., 2024).
The name is an acronym, Modulo Sperimentale Elettromeccanico, or experimental electromechanical module (Michielotto et al., 2026, p. 845). Construction started in 2003, the gates were first raised in October 2020, and the system was declared fully operational at the end of 2023 (Cibic et al., 2025, p. 2). Four barriers cover three inlets, because the Lido inlet is wide enough to need two sections, at San Nicolo and Treporti (Michielotto et al., 2026, p. 848). Since October 2020 the gates have gone up more than 150 times, for an average of 8.6 hours each and about 146 hours a year, which is 1 to 2 per cent of the year (Michielotto et al., 2026, pp. 848, 850, 852).
For a visitor, the practical effect is that the flooding which used to close shops and reroute walking trips several times a winter is now mostly headed off. The first closure under real conditions, on 3 October 2020, held the lagoon and Piazza San Marco at 70 cm while the Adriatic outside reached 130 cm (Lionello et al., 2021, p. 2635). The harder question is what the closures cost the lagoon, and the research of the last two years has started to answer it.
How does the MOSE flood barrier work?
Each of the four barriers is a row of hollow flap gates, 78 in total, hinged along a single horizontal axis on the bed of the inlet. Under ordinary conditions they are full of water and lie flush with the seabed, and their design allows water, sediment and regular ship traffic to pass straight over them (Michielotto et al., 2026, p. 848). That is why there is nothing to see from a boat on a normal day.
When water levels are predicted to pass 1.10 m above the Punta della Salute datum, compressed air is pumped into each gate. Once buoyant, the gates rise on their hinges and temporarily disconnect the lagoon from the open sea (Michielotto et al., 2026, p. 848). All three inlets are normally closed at the same moment, and the operation takes about 30 minutes (Ferrarin et al., 2024). When the surge has passed, water is let back in and the gates settle to the bottom.
One detail is worth knowing before reading any confident account of the rules: no formal operational protocol for MOSE has been published (Michielotto et al., 2026, p. 848). The operating aim, as described by people working on the system in late 2024, is to keep the water inside the lagoon always below 110 cm (Turner, 2025, p. 306). The 110 cm trigger is a management decision driven by the high cost of each closure rather than a limit of the hardware (Cibic et al., 2025, p. 2).
When does MOSE close, and how often?
The threshold is 1.10 m above the Zero Mareografico di Punta della Salute, the datum Venetian tide bulletins use, which Italians call acqua alta molto sostenuta. That datum now sits about 32 cm below present mean sea level, which is why a 110 cm reading does not mean 110 cm of water in the street (Michielotto et al., 2026, p. 848). Piazza San Marco, the lowest ground in the city, starts to flood at around 80 cm; at 110 cm roughly 12 per cent of the city area goes under, an extent the authorities treat as manageable with footbridges and raised pavements (Turner, 2025, p. 306; Michielotto et al., 2026, p. 848).
Closures cluster between October and February, matching the storm surge season, though the gates have also been raised in May and late August (Michielotto et al., 2026, p. 848). Counts have risen steeply over time: 52 events above 110 cm between 2000 and 2009, and 95 between 2010 and 2019 (Michielotto et al., 2026, p. 848). November 2019 was the worst month in the tide gauge record, with 15 events over 110 cm and four above 140 cm (Lionello et al., 2021, p. 2635).
Since the gates entered service the system has closed more than 150 times, averaging 8.6 hours per closure and 146 hours a year (Michielotto et al., 2026, pp. 848, 850). If you are in Venice during a closure you will notice almost nothing: the water simply stops rising.
Who decides to raise the gates
MOSE is run by three companies. Thetis and Comar monitor and operate the gates, while Consorzio Venezia Nuova has held exclusive rights to coordinate construction since 1984, and the three workforces sit under three national unions (Turner, 2025, p. 306). The forecasting teams work from a control room at Arsenale Nord, building lagoon water level models and cross-checking them against other regional forecasting centres (Turner, 2025, p. 309).
The error bar on a forecast is about 10 cm either way, largely because of wind, which can tilt the surface of the lagoon towards the historic centre when it blows from the south (Turner, 2025, p. 309). A 10 cm uncertainty on a 110 cm trigger explains a great deal. That uncertainty, combined with a precautionary approach rather than any change in the weather, accounts for how often the gates go up: wind speed, wind direction and tidal levels have stayed within the range of the last three decades (Michielotto et al., 2026, p. 848). All available forecast systems underestimated the peak of 12 November 2019 by as much as 45 cm (Lionello et al., 2021, p. 2637).
Before a closure the Arsenale Nord team notifies the port authority, fishing associations and medical response teams (Turner, 2025, p. 307). The project has not been free of scandal: a 2014 police operation at Consorzio Venezia Nuova uncovered an embezzlement scheme running to hundreds of millions of euros (Turner, 2025, p. 304).
What a closure does to the water behind the gates
Until recently this was modelled rather than measured. In 2025 a research team published the first field experiment: 18 floating enclosures of about 0.8 cubic metres in the shallow Palude di Cona in the northern lagoon, sealed off from sideways water exchange for up to 48 hours in July 2019 and again in October 2020, the second run coinciding with a real closure for a 135 cm tide (Cibic et al., 2025, pp. 1, 4).
The headline result is reassuring. One or two days of closure does not suffocate the lagoon. No hypoxia developed inside the enclosures even during the second heatwave of 2019, and temperature and salinity stayed effectively unchanged (Cibic et al., 2025, pp. 8, 14). Crabs, worms and shellfish were unaffected (Cibic et al., 2025, p. 16).
What does change is the lagoon's metabolism. Suspended clay settles out within about four hours, so the water clears. Ammonium rises 2.4 times after 24 hours, bacterial carbon production in the water column climbs 86 per cent, seabed diatoms double, and the plankton shifts towards larger cells (Cibic et al., 2025, pp. 16, 18). The authors are careful about scope: these results describe the most isolated and shallow parts of the lagoon, not the whole basin (Cibic et al., 2025, p. 18). Their warning is about the future rather than the present, since closures happen mostly in autumn, when cold water slows oxygen consumption. Longer closures at higher temperatures would be a different test (Cibic et al., 2025, p. 15).
The salt marshes are paying for the dry streets
The lagoon's barene, the salt marshes that make up much of its surface, survive by catching sediment. Because the rivers were diverted away centuries ago, that sediment now comes from fine silt stirred up off the tidal flats, and it arrives above all during storm surges, when the water is deeper, muddier and stays longer. Those are the conditions that trigger a MOSE closure (Michielotto et al., 2026, pp. 846, 849).
A 2026 study modelled the whole lagoon for 2020 to 2023 under three scenarios: the barrier as operated, no barrier, and an optimised closure rule. Under current operation the flooded marsh area falls by 27.5 per cent on average and by more than 75 per cent in some events, sediment accumulation drops by about 32 per cent, and the marshes lose roughly 2.6 mm a year of vertical growth (Michielotto et al., 2026, pp. 849, 850). Relative sea level is rising about 4.4 mm a year, so the marshes lose more than half the growth they need to keep up (Michielotto et al., 2026, p. 848).
The same study found the problem may be one of timing rather than hardware. Closures ran on average 107.2 per cent longer than flood protection required, and an adaptive rule could have cut their number by about 30 per cent while keeping the city below its safety thresholds (Michielotto et al., 2026, pp. 849, 851).
Will MOSE still protect Venice in 2100?
Relative mean sea level at Venice rose 4.9 mm a year over 1991 to 2020, against a long-run average of 2.5 mm a year since 1870, and about 1.5 mm a year of that recent figure is the ground sinking rather than the sea rising (Ferrarin et al., 2024). Every extra centimetre means more closures, because the trigger stays where it is.
Modelled on a 2050 horizon, the numbers move fast. Against 16 acqua alta events and 357 hours of closure in the 2020 control run, a moderate trend scenario gives 34 events and 917 hours, and a high emission scenario gives 54 events and 1,324 hours, or 11 per cent of the year (Ferrarin et al., 2024). A 0.22 m rise alone would triple the number of flood events and bring nine exceptional floods, the kind that put about 60 per cent of the city under water.
Further out, closing the inlets for three weeks a year is virtually certain before the end of this century even under a low emission scenario. A six-month closure, which the authors use as the criterion for the present defence strategy being inadequate, is likely before 2100 under a high emission scenario (Lionello et al., 2021, p. 2638). That is where the argument stops being about the gates and becomes an argument about what supplements them.
