Battling that sinking feeling in the City of Canals
The fortunes of Venice have always ridden on water. The sea afforded refuge from the Huns decades before the fall of Rome. It carried the ships that built a mercantile empire 900 years ago. It brought gondolas and innumerable tourists. Now it threatens to undo 1,400 years of culture, architecture, and civilization.
Built on 117 small islands in a lagoon, the City of Canals has long been plagued by floods. But their frequency and severity has steadily increased over the past four decades, driven by rising worldwide sea levels and overconsumption of ground water.
The city's world-famous St. Mark's Square now stands about 2 inches above normal high tides, but is 4 inches underwater an average of 100 times a year. The situation is getting worse: This year, Venice flooded in July, in the middle of what should have been the dry season.
"Picture if, from November to March, you had to walk in water in Faneuil Hall and the first floor of every building in the area was useless," said Rafael L. Bras, head of civil and environmental engineering at the Massachusetts Institute of Technology. "What kind of city can have a normal life under these conditions?"
So, Bras and others at MIT are now working with an Italian consortium on an innovative system designed to keep the Adriatic Sea at bay. Construction began in May on a breakwater at the Malamocco inlet to Venice Lagoon, the first phase of a $4 billion, eight-year project to protect the city.
The project's centerpiece will be mobile walls embedded in the sea floor at the lagoon's three inlets. When storms approach, 79 giant hollow steel barriers will be pumped full of air and bob in the water, deflecting flood waters.
Traditionally, storm barriers have relied on rigid, armored walls strong enough to stop incoming waves, but the structures designed for Venice will undulate two or three degrees as they are buffeted by storm waves. Even as storms push the flexible walls toward Venice, the lagoon's waters will push them back, providing the main support for the structure.
"By using a system that is based on buoyancy, the structure itself doesn't have to be rigid," said Bras, consultant to the Venetian water authority and member of the committee that oversaw development of the project's environmental impact statement. "You're letting the water itself take some of the force."
The modern attempt to protect Venice from the sea grew out of the Italian government's response to disastrous floods in 1966. That November, flood waters whipped by sirocco winds reached more than 6 feet above the zero point on the Punta della Salute flood gauge near St. Mark's Square.
"For three days, it was total paralysis of the city, waiting for the water to go down," said MIT oceanographer Paola M. Rizzoli, a Venetian high school student at the time. "At the end of three days, we were eating eggs because that's all there was left to eat."
The Italian government made protecting Venice a national priority and began training oceanographers, including Rizzoli, to tackle the problem.
The idea of mobile sea barriers was first advanced about 30 years ago, but Italy's and Venice's politics delayed the process. Rizzoli said the only effective alternative to the sea barrier would be to lift the entire city by nearly 2 feet.
The new barriers' mobility is expected to decrease storm-related erosion that can undermine traditional sea walls over time. Rigid barriers deflect storm waves, creating powerful currents that scour away the sea floor supporting the walls' foundations.
Computer models show the walls also will improve water circulation in the 212-square-mile lagoon -- though project opponents dispute that claim, arguing its frequent use will instead impair critical water exchange with the sea.
A healthy lagoon environment depends on regularly mixing fresh and salt water, on balancing sedimentation and erosion, and on periodic flushing to bring in nutrients and carry away pollutants. But centuries of Venetian public works projects diverted major rivers flowing into the lagoon, and the jetties that deflect inlet-choking sediment also make tidal mixing less effective.
Opening and closing inlets in synchronism with rising and falling tides would move Adriatic Sea water through the lagoon more effectively, according to Walter Munk, professor emeritus of geophysics at the Scripps Institution of Oceanography in La Jolla, Calif., and onetime consultant on the project.
"If used properly, the gates would not be a problem with regard to pollution," Munk said. "They would actually be a benefit. . . . It's a vastly more efficient way of cleaning the lagoon."
Computer models also indicate the sea walls will protect Venice from rising sea levels over the next century. Global-warming-induced thermal expansion of the oceans and melting polar ice caps are projected to raise sea levels by as much as 12 inches by 2050 in some areas, according to the UN's International Panel on Climate Change. Opponents have said that the project's design was based on earlier, more modest predictions of sea level rise over the next century, and that the barriers won't be large enough to accommodate more recent forecasts.
Bras argued that the seawalls will work for a long time, regardless of which predictions are used. Rising sea levels will eventually force inlet closures, he said, which could affect the lagoon's tidal exchange with the sea.
But, he said, "in the first 75 years, we have nothing to worry about."
Sean McNaughton can be reached at rmcnaughton@globe.com.
