Saving Venice from flooding may destroy the ecosystem that sustains it

A system of moveable walls, called Moses, protects Venice from colossal high tides that are worsening with climate change. But they’re also destroying the marshes that keep the lagoon alive.

By Frank Viviano
Published 26 Jul 2022, 10:34 BST
The main gate of Palazzo Ducale in Venice on November 13, 2019. Over the night before, ...
The main gate of Palazzo Ducale in Venice on November 13, 2019. Over the night before, the second highest tide ever recorded submerged parts of the city in six feet of water.
Photograph by Marco Zorzanello

The cataclysmic flood of November 12, 2019, washed unremarkably into Venice, Italy’s Piazza San Marco around 6 a.m. Two hours later, the rising waters began to tail off at about three feet above normal sea level, leaving 90 percent of the city untouched. Venetians breathed a collective sigh of relief. It was just another mildly unpleasant acqua alta, high tide, in the lagoon.

The calm lasted until 4 p.m., just before night fell. Sirens began to sound, and within an hour the ancient squares and narrow walkways along the city’s 26 miles of canals had vanished under ferocious torrents of seawater. “This wasn’t simply a tide,” says Marco Malafonte, who co-owns a property management firm with his wife, Caroline Gucchierato. “It was a colossal wave, something we’d never seen before. A tsunami."

The couple split off in separate directions, joining ad hoc rescue teams. In the San Marco quarter, Venice’s lowest-lying district, Gucchierato came to the aid of an elderly French tourist standing up to her neck in the raging flood, pinned against a stone wall. She had perched her infant grandson on a ledge, holding onto him until help arrived. Vaporettos, the iconic 80-foot-long Venetian water buses, “were thrown up onto walkways and bridges like children’s toys,” recalls Malafonte.          

St. Mark's Square is one of the lowest areas in Venice, making it especially vulnerable to high tides.
Photograph by Marco Zorzanello

At 8 p.m., the surge finally crested at just over six feet, with 85 percent of the city submerged. It was the second highest flood tide ever registered in Venice, nearly eight inches above the average height of its residents.

Before the world climate crisis began building in the 1970s, that terrible November night might have been written off as a freakish disaster. But in 2022, Venice—La Serenissima—is a poster child for climate change. The occasional destructive acqua altawhich occurred just a few times per century before 2000, has become the new normal as sea levels rise around the globe. Of the 25 worst acque alte registered in Venice in the past 100 years, each topping 4.5 feet, more than half have happened since December 2009.

That has prompted Venetian officials to spend billions on a series of moveable walls to block high waters from the city. Called Mose in Italian, Moses in English, the system is in use and seems to be working. In effect, Moses temporarily divorces La Serenissima from the sea, closing off the entire Venetian Lagoon from menacing tides in the turbulent northern Adriatic Sea. The audacious engineering experiment is a last-ditch effort to stave off a final disaster for one of the world’s most beautiful and fragile cities. But it poses fatal environmental risks to the lagoon’s equally fragile and disappearing salt marshes, whose protective structure and biodiversity have been giving life to Venice for 1,800 years. 

Photograph by Marco Zorzanello

How Moses works

Moses began taking shape in 1987. It is a system of mobile tide barriers formally called the Experimental Electromechanical Model, but its acronym invokes the biblical prophet who parted the Red Sea, allowing the Israelites to escape captivity in Egypt.

The project has engaged some of Europe’s largest construction firms, under the direction of an Italian government-sponsored consortium called Venezia Nuova (New Venice). Its cost to date has ballooned to more than £5 billion from an initial euro budget projection equivalent to £3.75 billion. The price tag is highly controversial in Italy, as is a long string of work stoppages due to unpaid debts and political scandals.

In 2014, then Venice mayor Giorgio Orsoni was arrested on charges of accepting over half a million dollars in illegal campaign contributions from Venezia Nuova in return for alleged influence in contract allocations. Although the judicial verdicts were inconclusive, Orsoni and 24 city council members subsequently resigned from their posts.            

This vessel, called a Jack-up, was created to install and maintain the gates of Moses.
Photograph by Marco Zorzanello

The Italian Ministry of Infrastructure and Transport, in conjunction with the consortium, began manufacturing the system’s components in 2003. On-site installation opened in 2008. The long-term goal is to protect Venice until at least the end of the century, when sea levels are expected to have risen another two feet. Although Moses will not be fully operational until December 2023, by late 2020 its partially completed barriers were ready for live testing. 

The functional heart of the project is a battery of four gigantic surge barriers that span three inlets to the lagoon from the Adriatic Sea. The two largest barriers, comprised respectively of 21 and 20 steel gates, are separated by a control centre on an artificial island and stretch a combined half mile across the Lido inlet, just east of the main islands of urban Venice. Each is outfitted with a lock that allows small vessels to leave or enter the lagoon when the barrier is raised.

A third set of 19 gates was installed 7.5 miles south at the 46-foot-deep Malamocco inlet, where a much larger adjacent lock provides access to the lagoon during high tides for cargo freighters and industrial ships. The fourth set, with 18 gates, lies near the port of Chioggia on the lagoon’s southern limits, with a double lock serving fishing boats, pleasure craft, and emergency vessels.

One of the buildings that support the Moses system of tidal flood walls that protect the city. The mobile gates, installed at three inlets off the city's shores, rise to protect Venice from increasingly higher tides from the Adriatic Sea.
Photograph by Marco Zorzanello
A researcher from the National Center for Research (CNR) calibrates a testing machine for monitoring sea water quality. As tides rise and flood Venice more often, the public organization is using the city as a model for Mediterranean coastal flooding.
Photograph by Marco Zorzanello
The Acqua Alta research tower was installed on January 1970 off the Gulf of Venice by the CNR, to conduct scientific experiments and research. The tide forecast for the Venetian authorities comes from this station so that they can decide when Moses's walls should be raised.
Photograph by Marco Zorzanello

The most experimental aspect of Moses is that all of its 10-ton gates are discreetly invisible, hidden in seabed cradles, until there’s a tidal alarm. Whenever the sea level rises to 3.5 feet above normal, they are currently programmed to be lifted automatically by 156 electronic hinges, two per gate. The hinges pump compressed air into the gates and elevate them to their maximum 10-foot height from the water’s surface in 30 minutes. When the danger passes, they are electronically refilled with water and lowered back into their underwater casings.

Moses was activated for the first time on October 3, 2020 to meet a tide higher than four feet. The results were dramatic, and largely stilled the most common criticism of the project: that it wouldn’t work. In fact, for the first time in its long struggle with high waters, Venice remained astonishingly dry. Over the following 20 months, its gates were lifted 33 more times, in tests ranging from 30 to 92 minutes, with the same results. “The barriers are ready. They absolutely protect Venice,” Moses’ High Commissioner Elisabetta Spitz declared in a National Geographic interview on May 16, 2022. 

The cost of keeping Venice dry

The vaporetto that ferries passengers from Piazza San Marco sails east along the two largest spans of Moses and the high-security command centre on the artificial island between them. The gates themselves are hidden in their cradles. But their land-based operative infrastructure, clusters of giant compressors, warehouses, and office buildings, fills the skyline. 

The Moses barriers were tested on the Lido inlet gate in 2019. Under normal tidal conditions, the gates are full of water and rest in their housing structures. When a high tide is forecast, compressed air is introduced into the gates to empty them, and they rise, stopping the tide from entering the lagoon.
Photograph by Marco Zorzanello
This natural salt marsh around the Venetian Lagoon has irregularly shaped banks and serpentine internal channels, essential for absorbing high tides.
Photograph by Marco Zorzanello, National Geographic

A team of four University of Padova marine research scientists meets me at the Punta Sabbioni wharf, near the tip of the 12-mile-long—and barely three-feet-high— peninsula separating the northern end of the lagoon from the Adriatic. Just to our west are the deep-green fields of Sant’ Erasmo, the fertile garden island that provides Venice with its best artichokes, zucchini, and tomatoes. We board a small, rented launch, piloted by hydraulic engineer Alvise Finotello, which carries us to the San Felice salt marsh. We don mud boots and step ashore.

At low tide on a sun-struck morning in May, San Felice is an airy field of salicorn, cord grass, sea lavender, and other estuarial plants waving in the morning breeze. The lagoonal waters have retreated into a web of narrow channels teeming with small fish, crabs, and seagrass. By early evening, the entire marsh will be fully submerged by the tide, its flora and fauna busily capturing nutrients necessary to their health—as well as the lagoon’s.

This is the universe that Moses threatens, one of “halophytic” vegetation, salt-tolerant plants that spend part of each day on land and part underwater, where they are nourished by clouds of incoming and outgoing tidal sediment that the flood gates block when in place. The sediment enables the plants to grow, and in the process, reinforce the sand banks and structure of the lagoon: its very existence. The salt marshes are “hotspots for biodiversity,” D’Alpoas says; without them and the ecosystems they foster, the lagoon would die. They also account for one of nature’s most critical roles in the struggle to confront climate change.

“They are extraordinarily efficient at sequestering carbon dioxide and storing it in the soil as organic carbon,” says geologist Massimiliano Ghinassi. “One square kilometre of the Venetian marshes annually removes 370 tons of carbon dioxide from the earth’s atmosphere, at a rate 50 times greater than that of tropical forests,” such as those in the Amazon.          

Finotello, fellow hydraulic engineer Davide Tognin, and team leader Andrea D’Alpoas, an environmental engineer and leading authority on the lagoon, provide me with background, while Ghinassi inserts a long, T-handled cylindrical tool into the spongy ground. The cross-disciplinary mixture of experts is deliberate, D’Alpoas explains. “Eliminating the walls between pure science and engineering brings diverse perspectives into our work—and better results.”

This invasive grass, called Spartina, was found for the first time in the Venetian Lagoon in the early 2000s. Scientists are still deciding whether the grass is beneficial to the ecosystem.
Photograph by Marco Zorzanello, National Geographic
Seahorses like this museum species have been reintroduced in the grasses of the lagoon after being decimated by the farming of an invasive clam.
Photograph by Marco Zorzanello, National Geographic
On the left, environmental engineer Alberto Barausse of the University of Padua is inspecting the surrounding area. Biologist Davide Tagliapietra, center, and station Director Laura Airoldi compare notes on a freshly collected sample of invasive Spartina.
Photograph by Marco Zorzanello, National Geographic

Ghinassi bends his weight into the tool with a twisting motion, then carefully pulls it back up, loaded with a core sample of the marsh’s soil. “This is the biography of San Felice roughly 500 years ago,” he says, pointing at the deeper end of the sample, which is tightly banded with striations in shades of gray, brown, and red. “It records the evolution of the marsh, and the flora and fauna it has supported, in great detail. The upper, more recent, end is rich in plant debris covered with a drape of mud laid down by the high tide, a window on the process of extracting carbon.”  

An expert on marine sedimentology—the sea’s interaction with the sand, silt, and clay that lies around and underneath it—Ghinassi can read much of that history on sight, examining it more closely back at his university laboratory with advanced soil-analysis instruments. He has done fieldwork in Norway, Turkey, Sudan, Ethiopia, Eritrea, Greece, Scotland, England, Spain, and the United States over the last 20 years—a resume exemplifying the worldwide character of problems now facing the marine environment.      

The Padova team has been evaluating the marshes’ health intensely for more than three years. Their conclusions are not encouraging. While the world justifiably worries about the fate of La Serenissima and its treasures, environmentalists warn that her mythic partner may already be approaching its death throes. Thanks mostly to heedless human interventions with the lagoon’s sediment flows, the vital salt marshes of the Venetian lagoon have shrunk to an expanse of just 16 square miles, barely one-sixth of their 100 square miles in the 17th century.

How to degrade a salt marsh

In the 1920s, Italian economic planners undertook a “modernising” program that transformed the continental shore of the Venetian Lagoon into one of Italy’s most heavily industrialised regions. Scores of manufacturing plants and refineries were built. Deep channels were gouged through the lagoon bed to accommodate heavy freighters bound for the port of Marghera, just two miles west of La Serenissima’s main island, and more recently, huge 5,000-passenger cruise ships. 

The industrial facilities extracted immense amounts of groundwater from compressed sediment under the lagoon, causing Venice to subside 4.5 inches in the 20th century, at the same time Adriatic sea levels rose by four inches. Although the Italian government closed most of the wells in 1970, the subsidence is irreversible and slowly increasing.

Researchers collect soil samples from an artificial sandbar in the Venetian Lagoon. The aim of this activity is to study insects and microorganisms present in these soils, to understand the differences between a natural and an artificial sandbar.
Photograph by Marco Zorzanello, National Geographic
This artificial sandbar is being built with sediment dredged from the deepest navigable waterway in Venice Lagoon.
Photograph by Marco Zorzanello, National Geographic

Put bluntly, environmentalists now fear that Moses’ efforts to save urban Venice from crumbling into the sea may complete the destruction of the very ecosystem that gave birth to the city and sustained it for 15 centuries.

The chief threat, they say, endangers the crucial interaction of the marshes with tidal currents. The Padova team accumulated extensive data on the first 15 tests of the flood gates, focusing on San Felice and two other marshes between October 3, 2020, and the following winter. Their analysis suggests that the barriers could reduce the annual supply of sediment to the marsh plants by 25 percent, with potentially fatal effects on the future of the lagoon, since the sediment distributed by the tidal ebb and flow—the “sedimentary budget” —shores up the existing land and lagoonal banks.  

What’s more, notes Davide Tognin in a related study, 70 percent of the lagoon’s necessary sedimentation occurs during episodes of high winds—precisely when the barrier tends to be raised.

"It is clear that the defence of the city of Venice and of the inhabited centres from high waters is an indispensable issue, and not under discussion,” says D’Alpaos. “We don’t question this need.” Instead, researchers urge that the barriers be raised when the tide reaches a slightly higher level—4.3 feet rather than 3.6 feet, which they say would reduce the loss of sediment to an environmentally sustainable 10 percent.

The lagoon near the island of Mazzorbo.
Photograph by Marco Zorzanello, National Geographic
These boxes are used to fish crabs in the lagoon.
Photograph by Marco Zorzanello, National Geographic
An aerial view of the island of Burano in the Venetian Lagoon.
Photograph by Marco Zorzanello, National Geographic

A final decision about activation levels has not yet been established, says commissioner Spitz. The purpose of its current testing regimen, she emphasises, is arriving at the “optimal point” for raising the gates. The bottom line, however, is that “Venice is a very delicate city,” Spitz adds, a treasure trove of historic monuments, art, and culture. “It cannot risk another event like November 2019.”

“Yes, there would still be flooding” at 4.3 feet, perhaps affecting up to half the city, D’Alpaos concedes. “But much of it could be controlled by complementary measures, such as a permanent storm drainage system at Piazza San Marco and raising pedestrian sidewalks in lower-lying areas.”

After billions in public expenditures and decades of work on Moses, any compromise on its protection of Venice is certain to generate a harrowing political acqua alta. But compromising the fragile health of the marshlands invites yet another sort of catastrophe. “If Moses is used too often and raised for too many hours,” D’Alpaos flatly predicts, “the marshes will die.”

La Serenissima surrounded by a stagnant, dead lagoon is unthinkable.

Venice in the 21st century is an enchanted 1,500-year-old dream immersed in an apocalyptic contemporary nightmare. She is not alone in facing the dire challenge of rising sea levels and climate change, a recent Oceanography Society report observed. But thanks to her sheer fame, the authors added, the city is a maritime equivalent to “the canary in the coal mine,” whose determined efforts to meet existential crisis “may be the example that hastens the world to act.”

Fog descends on the lagoon.
Photograph by Marco Zorzanello

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