MOSE: the sea gates that keep Venice from flooding

Venice floods several times a year, and the image of tourists walking on raised wooden boards in St Mark’s Square is almost a postcard. Since October 2020 the city has had a second defence: 78 steel gates lying on the seabed at the entrances to the lagoon, which rise to shut out the Adriatic when a high tide is forecast. The system is called MOSE.

The short answer to the title: when the forecast tide is high enough, compressed air empties the gates of water, they swing up on hinges until they break the surface, and the lagoon is sealed off from the sea for a few hours. It works, it cost far more and took far longer than planned, and it was not designed for every kind of flooding. Here is how it runs and what it means for a trip.

Why Venice floods

The flooding has a name, acqua alta, “high water”. According to Wikipedia, it occurs mainly between autumn and spring, when the astronomical tides are reinforced by seasonal winds that hold back the normal ebb. Two winds matter most: the sirocco, which blows north along the Adriatic, and the bora, which has a particular effect because of the shape and position of the lagoon. When they blow towards the inlets, they slow or even block the water leaving the lagoon, so the next high tide arrives on top of the last one. Rain and river inflow add to it.

The city is also lower relative to the water than it used to be. The same source says that towns in the lagoon sit on average 23 centimetres lower with respect to the water level than at the start of the 1900s, because of rising seas and sinking land.

The benchmark disaster is 4 November 1966, when a tide of 194 centimetres submerged Venice, Chioggia and other settlements in the lagoon. The first Special Law for Venice followed, declaring the protection of the city a matter of national priority. More recently, the highest tide in more than five decades, on 13 November 2019, left 85% of the city flooded, according to Wikipedia.

What MOSE is

The name is an acronym, Modulo Sperimentale Elettromeccanico, “experimental electromechanical module”. It originally referred to a full-scale prototype gate tested between 1988 and 1992 at the Lido inlet, and it also echoes Moses, Mosè in Italian, who parted the Red Sea.

The system has four rows of gates across three inlets between the lagoon and the sea:

• Lido: two rows, of 21 and 20 gates, linked by an artificial island that houses the technical buildings.
• Malamocco: one row of 19 gates, with a large lock so big ships can still reach the port.
• Chioggia: one row of 18 gates.

Each gate is a steel box 20 metres wide, between 18.5 and 29 metres long and 3.6 to 5 metres thick, attached to a concrete housing on the seabed by two hinges. That makes 156 hinges, which the source calls the technological heart of the system.

How the gates rise

Under normal conditions the gates are full of water and lie flat in their housings, so ships and tides pass over them. When a high tide is forecast, compressed air is pumped in to push the water out. The gates become buoyant and rotate on their hinges until they stand above the surface and block the tide. When the sea recedes, they are filled again and lower back down.

Raising takes about 30 minutes and lowering about 15. In total the inlets stay closed for around four to five hours. Small locks at Lido and Chioggia let emergency vessels and fishing boats through while the gates are up.

The control room is in the Venice Arsenal, where decisions are made from tide gauges at the inlets and the forecasts. Because the gates are independent, the operators can close all three inlets, close one at a time depending on wind and pressure, or close an inlet partially.

The long road: 2003 to 2020

The timeline shows why the project became a byword for delay:

• 1975: a first call for designs ends without a winner.
• 1984: a second Special Law puts design and construction in the hands of one consortium, Consorzio Venezia Nuova.
• 3 April 2003: the government committee gives the go-ahead and work starts at all three inlets.
• 2014: the seabed housings for the gates are in place at all three inlets (by October); 35 people, including the mayor of Venice, Giorgio Orsoni, are arrested in a corruption investigation tied to the project. According to Wikipedia, there were allegations that 20 million euros of public money was sent to foreign accounts and used to finance political parties. In December 2014 the national anti-corruption authority proposed extraordinary management of the consortium.
• 30 January 2019: the last of the 78 gates is installed.
• 10 July 2020: the first full test raises all 78 gates.
• 3 October 2020: MOSE is used for the first time against a real high tide, keeping parts of the city, including St Mark’s Square, dry.

The completion date slipped from 2011 to 2018 to 2021, and later to 2025. The cost estimate is €5.496 billion, about €1.3 billion above the initial projection.

How well it works, and what it misses

The experts who designed the gates estimated about five closures a year. According to Wikipedia, within two years of the first raising, MOSE had been activated 49 times. Opponents say maintenance and operating costs are high compared with systems in the Netherlands and England, and that the design may not cope with future sea-level rise. The system was meant to hold even if the sea rose at least 60 centimetres, but the 2019 IPCC estimate cited by the source projects a rise of 60 to 110 centimetres by 2100 if emissions keep growing.

There is a second limit. The operating procedure raises the gates for tides above 110 centimetres. Lower tides are handled by raising quaysides and repaving parts of the city, and they still flood the lowest areas. Wikipedia’s article on acqua alta says MOSE should greatly reduce exceptional high waters but not lesser ones. Some news reports and later versions of the procedure mention a higher trigger of 130 centimetres; check the official project and city pages for the current rule.

In practice

• Plan for autumn and winter. That is when acqua alta is most likely, from autumn to spring.
• Check the city forecast. The Comune di Venezia runs a tide forecast and alert centre; use its page the evening before.
• Pack waterproof footwear if you travel between October and March, and leave the rest of the day flexible.
• Know the sirens. The city sounds a dedicated siren system for very intense events, with a number of whistles that signals the expected level.
• Do not count on a perfect day. Closing the gates depends on the forecast, and on days with a moderate tide the lowest streets can still get wet.
• Read the access rules before you go. If you visit for a day, see our guide to Venice’s access fee, which has its own dates and exceptions.

Venice has handled water for more than a thousand years; the first documented acqua alta dates from 782. MOSE is the first time the city has tried to close its own door on the sea.

Frequently asked questions

What does MOSE stand for?

Modulo Sperimentale Elettromeccanico, “experimental electromechanical module”. The name also alludes to Moses.

When did MOSE first operate?

On 3 October 2020, during a real high tide. A full test with all 78 gates had taken place on 10 July 2020.

How many gates are there?

There are 78, in four rows across the Lido, Malamocco and Chioggia inlets.

Does MOSE stop all flooding in Venice?

No. It deals with exceptional tides. Lower high waters are meant to be handled by raised quaysides and paving, and they can still flood the lowest areas.

Sources

• MOSE Project, Wikipedia: name, gate numbers and dimensions, operating principle, chronology, cost, the 2014 arrests and the first activations.
• Acqua alta, Wikipedia: causes, winds, the 1966 record of 194 centimetres, the siren system and the first documented event in 782.
• MOSE Venezia, the official project site: current information on the system.
• Centro Previsioni e Segnalazioni Maree, Comune di Venezia: official tide forecasts and alerts. Figures and the activation threshold change, so confirm them on the official pages.