Venice’s Tide Barrier Has Already Cost 6 Billion Euros—Will it Work?

Venice, famous the world over for its picturesque canals and iconic gondolas, has an existential problem: it is slowly sinking. It has dropped 11 inches in the last century. The most visited part of Venice—also one of its lowest points—Piazza San Marco, is so often flooded that elevated ramps have been built on it to keep tourists dry. Native Venetians have learned to take acqua alta in stride with knee-high rubber boots often put to use. But more and more often, the flooding has become so severe that residents have had to use boats, not boots, to get around.

Venice solved the problem most ingeniously—with a tidal floodgate system called MOSE. Its steel gates float up as they are filled with air, sealing off the city from a rising tide. No one, not even the Dutch—legendary for fighting back the sea—has a system like MOSE. Leonardo da Vinci would have been proud.

But before MOSE could be put into use came a deadly reminder of why the system was so urgently needed. On November 12, 2019, a chain of events began that would cause flooding worse than most of Venice’s inhabitants had ever seen in their entire lives. Counterclockwise winds around a strong low-pressure storm system off the southwestern coast of Italy produced 35 mph winds that whipped up wind and forced a storm surge into the Venice Lagoon—already dangerously high from weeks of rain and one of the highest tides of the year. On that November day, 85 percent of the city would be under up to 6 feet of water.

Indeed, Venetians have coped with the city sinking for over a millennium. The city was, after all, built on wooden posts sunk into a muddy delta. It seemed like a good idea at the time. The Roman Empire had collapsed and hordes mercilessly harassed the people of the mainland, driving them onto islands in what would one day be called the Venetian Lagoon.

Being surrounded by a swamp had many small disadvantages (mosquitos that caused malaria, though that was blamed on swamp gas at the time), but one big advantage: a natural moat to keep the hordes at bay.

The settlers went on to create the city-state of Venice, which at its height was a seafaring empire, the financial center of medieval Europe (as told by Shakespeare in The Merchant of Venice), financier of the Spice Trade (Marco Polo hailed from Venice), with a Navy that ruled the Mediterranean.

But new trading patterns and shifts in geopolitical power were to make Venice sink in importance on the world stage.

To Byron, Venice “sinks, like a seaweed, into whence she rose” and to Thomas Mann, the city is depicted as a place of decay, disease and death to which its protagonist succumbs (Death in Venice).

 These days Venice exists mostly for tourists. Most of the city’s residents have fled. 

Literally sinking since its inception made Venetians learn to adjust. When the city sank to where the ground floor was too often underwater, residents simply added another floor on the top of their buildings. But even such heroic measures could not keep up with modern pressures. Pumping groundwater beneath the city for the sake of mainland industry (a practice that started after World War II, now forbidden) made the city sink even faster. On top of that, global warming has been causing sea levels to rise.

It was a great flood on November 6, 1966, that propelled Italy into action. Storm winds raised sea levels, flooding Venice to a depth of 6 feet, and kept blowing, not allowing floodwaters to recede. Sea walls that had stood since the 18th century were demolished, battered by waves. When the storm was finally over, Venice found its canals choked with mud and debris. It seemed as if everything that was once inside had been swept into the canals.

This great flood of 1966 resulted in the first Special Law for Venice, making the saving of Venice a national priority. In 1975 the city started taking bids for an engineering solution to holding back the tides—and placed equal importance on preserving the Venetian Lagoon.

After years of pondering various designs for a tide barrier, the city selected one by Riva Calzoni, a firm from Milan. The winning design featured massive steel sea walls that were pumped with air to float up with the incoming tide, then pumped up with water to lower them into even more massive concrete troughs. There they would lie, out of sight until called upon.

Construction was authorized on April 3, 2003, when then Prime Minister Silvio Berlusconi laid the first stone.

Deadlines for completion came and went and MOSE became enmeshed in scandal from 2014 to 2018. And 35 people, including a former Mayor of Venice, were arrested in a 2018 national investigation.

Every euro spent on MOSE thereafter was scrutinized. Public financing almost dried up. Several companies contracted for the project went out of business.

The nation seemed to lose interest in MOSE. Even the “father of MOSE,” chief engineer Alberto Scotti, became so frustrated by delays that he “rejected his paternity.”

But the sea was unrelenting. Great floods, such as the one in November 2019, reminded Italy of the need for MOSE.

On 10 July 2020, MOSE was deployed amid fanfare with the Italian prime minister, Giuseppe Conte, activating the 78 mobile barriers. But its real test came in October 2020, when a storm lashed the barrier islands of the Venetian Lagoon with 20-foot waves. The sea level was predicted to rise 5 and a half feet. The order to activate MOSE was given. MOSE’s bright yellow barriers bobbed up to the surface, each looking like capsized supersized cargo containers. And for the first time in the history of Venice, the tide was held back.

“Today everything was dry,” said Venice mayor Luigi Brugnaro, who was quick to declare victory. “We stopped the sea.”  

But only a few months later, bridge operators were caught with their gates down. This time, the barriers were not raised because the forecast was for the tide to be too low when they started the 48-hour procedure that preceded barrier activation. By the time the forecast was changed, it was too late. Venice was, once again, flooded.

In November 2021, two of the inlets were closed as a storm threatened a 140-centimeter high tide, enough to flood 60 percent of the city. One lagoon inlet was closed, but two other inlets were left open to let water drain out. MOSE was declared a success (by NASA) that day even though the storm left Venice flooded under 83 centimeters of water.

Most of the criticism of MOSE comes from those who see it as a less than absolute and permanent solution. MOSE still allows some flooding—just not the really deep floods. Critics also say that even if MOSE can hold back some of the highest tides today, it will be hard-pressed to do so against rising global sea levels, making it a stopgap measure only. Sea water rise was predicted to be as much as 60 centimeters (24 inches) not long ago, but experts have been raising the level to as much as 110 centimeters (43 inches) recently.

MOSE, an acronym for MOdulo Sperimentale Elettromeccanico. Mosé is Italian for Moses, the Biblical figure who also had a way with water. A thousand years later, King Canute ordered the tide to recede but was unsuccessful, which is why he has zero floor barriers named in his honor.

MOSE is three systems of gates, each of which closes a gap between barrier islands of the Venetian Lagoon when activated.

Each of the 78 gates is 66 feet wide. The gates lie about 100 feet below the surface, each in a concrete trough. The biggest of the gates weighs 350 tons. Each gate can be operated independently.

When a high tide (130 centimeters or more) is imminent, the gates are activated. They can rise up to an angle of about 45 degrees when fully deployed.

There is a gap of a “few centimeters” between the gates to prevent interference.

MOSE is simply Italy’s biggest engineering project ever, and among the biggest in the world, according to How StuffWorks.com. The cost, initially estimated at 1.3 billion euros has risen to 6 billion euros.

That the barrier is hidden when not in use is “very beautiful but very expensive,” says Luigi Cavaleri, a researcher at Venice’s Institute of Marine Sciences, who frets that MOSE is also too expensive to maintain.

“Those who want to keep milking this cash cow have already lined up to secure the lucrative maintenance contracts,” writes Salvatore Settis in his 2016 book If Venice Dies. Settis sees the project’s ostensible interest in the environment to be a scam. “The MOSE affair demonstrates that Venice’s problems have been used as a pretext to invoke empty rhetorical formulas of preservation, while actually allowing private interests to rob the city blind.”

Construction of MOSE started 20 years ago and was to have been completed in 2011. The latest estimate is 2025.

Most structures that hold water back are immovable structures such as dams and levees. But the chosen design is underwater except when needed. If MOSE is primarily down, it is most likely to preserve the lagoon ecosystem in its present state.

That’s a big “if,” according to Luis D’Alpaos, professor emeritus of hydraulics who wrote, “MOSE is obsolete and philosophically wrong, conceptually wrong.” According to D’Alpaos, the gates will have to work more and more often as sea water rises, effectively sealing off the Venetian Lagoon and turning it into a “stagnant pool of algae and waste.”

A study published in the Journal for Nature Conservation found that a sea level rise of 50 centimeters (about 20 inches) would have the barriers closed an “average of once a day,” effectively sealing off the lagoon.

Man’s meddling with the Venetian Lagoon is nothing new. Four rivers that once flushed the lagoon with fresh water were diverted around the lagoon 600 years ago so that rising deposits of sediment would not provide overland access to enemies. This changed the ecosystem, cut off the fresh water supply, and changed the ecosystem, leaving the cleansing of the lagoon up to the salty Adriatic Sea on the other side of a ring of barrier islands.

It takes energy and time to deploy a moving barrier. MOSE operators estimate that it costs 200,000 euros each time the barrier is deployed—the cost of energy, the air being pumped and bridge staff plus 63 million euro a year.

Operators in the MOSE control center, located in historic Arsenal, monitor tidal forecasts and start raising the barriers when the tide is predicted to rise above 130 centimeters (4.3 feet).

The barrier is deployed for 4 to 5 hours, including the 30 minutes to raise the barriers and 15 minutes to lower them.

London and New York are both studying solutions. New York has a much bigger project to consider as sealing off New York City would take 6 miles of barriers and cost anywhere from $62 billion to $200 billion.

Building a sea wall to protect New York could very well make it worse for Long Island, New Jersey and Connecticut.

“Water that hits an artificial wall doesn’t just go away; it caroms off to somewhere else,” says Justin Davidson, writing for Curbed.com.

How Venice’s Plan to Protect Itself From Flooding became a Disaster in Itself, Washington Post, November 19, 2019.

Venice Holds Back the Adriatic Sea, NASA Earth Observatory

Venice’s $6 Billion MOSE Floodgates May Not Be Enough, Justin Davidson, Curbed.com, December 16, 2020.

Venice Is Saved! Woe Is Venice, Jason Horowitz and Emma Bubola, New York Times, April 1, 2023.

Venice Has Its Worst Flood in 53 Years, Jeff Masters, Scientific American, November 14, 2019.