Photograph by Alvaro Leiva, Age Fotostock/Getty Images
Published January 12, 2012
Want to save sinking Venice from rising seas? Fight water with water, a new plan suggests.
Injecting billions of gallons of seawater could "inflate" porous sediments under the canal-crossed city, causing the Italian city to rise by as much as a foot (about 30 centimeters), scientists say.
(Read "Charlie Chaplin's Venice" from the new issue of National Geographic Traveler.)
Known to Venetians as the acqua alta, or "high water," flooding driven by high tides submerges the lowest 14 percent of the Italian destination four times a year, on average (interactive map of Venice's flood zones).
And it's only getting worse.
Venice dropped about 5 inches (12 centimeters) between 1950 and 1970, when groundwater was pumped out for industrial uses, according to city data. Currently, though, the city is sinking at a rate of less than two inches (five centimeters) a century, according to a 2002 study in the journal Geophysical Research Letters.
But even as Venice's descent slows, the surrounding Adriatic Sea is swelling—with the average local sea level predicted to rise by perhaps a foot by the end of this century, said hydrologist Giuseppe Gambolati, a proponent of the seawater-injection method.
(See National Geographic magazine's "Vanishing Venice.")
Under the plan, a dozen wells surrounding Venice in a six-mile (ten-kilometer) circle would pump water into the ground over a ten-year period—nearly 40 billion gallons in all (150 billion liters).
"When you inject water, you cause an expansion of the injected formations," said Gambolati, a hydrologist at the University of Padua in Italy.
"If land is settling, then you offset the settlement and sinking stops. [Once] land is stable, you induce an uplift."
Parting of the Waters
The modest rise could reduce the need for the still-under-construction MOSE floodgate system, aka Project Moses. Segments of the electromechanical system sit on the seabed at each of the Venetian Lagoon's three inlets. MOSE's giant panels are to be inflated to seal off the marshy lagoon when severe floods threaten.
By 2100 MOSE might be needed about 35 times a year if the average local sea level has risen by about 10 or 11 inches (roughly 28 centimeters), according to a 2010 study in the journal Climate Dynamics.
The injection project could reduce that number to 4 times a year, according to a new study co-authored by Gambolati and published in the December 7 edition of the journal Water Resources Research.
For the study, the researchers combined raw seismic data from the 1980s—when oil and gas companies were still allowed to use explosives to delve under the lagoon for geological observation—with more recent data. The result is a 3-D map that depicts the impermeable layer of clay under Venice in unprecedented detail.
That clay, Gambolati said, is key.
"The seawater would be pumped into a number of sandy layers ... between 650 and 1,000 meters' depth [2,133 and 3,280 feet]," he said. "The presence of a clay cover above ensures that the injected seawater will not flow upward. Water will diffuse laterally within the layers it is pumped into."
(Related: Venice "Ancestor" City Mapped for First Time)
It's Been Done Before
Civil engineer Ron Wong, an expert in the underground injection of fluids for enhanced petroleum recovery, said the concept of "raising" Venice is definitely feasible if the geology below the city is right.
"We have more than 20 years of experience in Alberta injecting steam or water into the ground, and we have observed a very similar kind of heave, around 30 centimeters [1 foot] of permanent deformation," said Wong, who said heads the University of Calgary's department of civil engineering.
"But it has only worked here in dense sand." Wong said the study appears to show that the ground below Venice has similar properties.
In Alberta, Wong added, high-tech monitoring tools precisely track land as it slowly rises, to make sure the uplift is uniform. "If the heave is not uniform, you can cause a lot of damage on the ground," he said.
The University of Padua's Gambolati said his team's modeling study suggests that the entire city of Venice can be raised in unison to avoid any structural damage that might be caused if some sections rise higher than others.
If the injection project is greenlighted, it would take only a year or two to get the decade-long pumping process underway, Gambolati estimates.
The plan is also economical, he stressed, certainly in comparison with the MOSE system.
"We did not make a detailed breakdown of the cost," Gambolati said. "However, offhand, the overall cost could be expected over the range of 200 to 300 million Euros [about U.S. $255 to $383 million].
"Consider that the planned cost of MOSE, as of today, is five billion euros [six billion dollars]," and that's before you factor in maintenance costs, he said.
Let It Flood?
So far, Venice's sometimes submerged streets and squares aren't dimming tourist enthusiasm. In 2011, for example, more than 20 million tourists visited the city, generating some two billion dollars in declared revenue.
The novelty of the acqua alta, in fact, might be one of the city's draws, said journalist Erla Zwingle, an American who's called Venice home for 17 years.
"My husband, who is a native Venetian, said that if the MOSE system were operating, they'd have to open it periodically to let the flood waters flow, because the tourists love it so much," said Zwingle, who writes a blog about everyday life in Venice.
"They take off their shoes and wade into the Piazza San Marco (picture) and take pictures of one another. To them, it's kind of fun."
Even when waters are high, many parts of the city don't flood. Those that do can often be traversed via several miles of passerelle, raised boards laid down by the city. Many hotels keep stores of galoshes or boots on hand.
For Venetians, Floods Are "a Small Nuisance"
The floods, added Zwingle, are relatively short—usually lasting a few hours till the tide recedes—and predictable. "It doesn't happen all the time or even most of the time."
The acqua alta season generally runs from September to April, and if a flood is going to happen, "it's most likely to happen when the moon is full or new, when there's a southeast wind, and/or when there's low atmospheric pressure. It doesn't strike out of the blue," Zwingle said.
"People know in advance, so it's easy to deal with and low impact. It comes up and it goes back down."
To residents, Zwingle adds, flooding is neither fun nor a catastrophe—just a part of life in Venice.
"It's an inconvenience, a small nuisance, but people are used to it all," she said.
"Venetians are the only people who are not excited by the high water. A resident will just pull on his or her boots and pursue a normal schedule," she said.
In fact, Zwingle said, Venetians are more likely to get worked up over more familiar civic concerns, like lack of municipal funding and medical care or failing schools.
"Venice has been here for 1,500 years, and they've had high water for centuries," she said. "If it were such a problem for daily life that it couldn't be faced anymore, people would have moved away long before now."
The Future of Food
How do we feed nine billion people by 2050, and how do we do so sustainably?
We've made our magazine's best stories about the future of food available in a free iPad app.