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Venice could provide gateway to 21st-century flood control method

A Venetian grocer wears waders while minding the store.
A Venetian grocer wears waders while minding the store.
Photo courtesy / Italy's Ministry of Public Works, Venice Water Authority, Consorzio Venezia Nuova
The proposed underwater floodgates will fill with air and rise  during very high tides; the design allows each gate to move separately, creating  a barrier strong enough to withstand battering waves.
The proposed underwater floodgates will fill with air and rise during very high tides; the design allows each gate to move separately, creating a barrier strong enough to withstand battering waves.

Venice, historic city of canals and gondoliers, is planning to defend itself in the 21st century by building massive, swinging gates. The idea may conjure images of medieval drawbridges, but these high-tech gates won't protect citizens from foreign invaders. Instead, they've been designed to keep the marauding sea at bay.

After a centuries-long war with flooded streets, the Italian government is expected to decide later this year whether to proceed with a public-works project that would build a series of floodgates at three inlets along the lagoon surrounding the city. These gates, which will rest on the sea floor and swing upwards in response to rising tides, will cost between $3 billion and $4 billion and take eight to 10 years to construct.

Three MIT professors have been involved with the project since 1995, when they were hired to oversee a two-year environmental assessment. Since then, as the consultants who "look over the shoulders of the consultants," they have shepherded the project through twists and turns as numerous as the city's canal-streets, said Rafael L. Bras , the Bacardi and Stockholm Water Foundations Professor at MIT, who chairs the advisory committee that reports to the Venice Water Authority. His committee oversees the consultants who are designing and planning the project, called MOSE - the Modulo Sperimentale Elettromeccanico, or Experimental Electromechanical Module - after a prototype of a single gate that was built a decade ago to test some of the engineering design.

"Our job is to keep them honest, to watch for the appropriateness of the action," said Bras, a hydrologist and hydroclimatologist who was head of the Department of Civil and Environmental Engineering from 1991-2001. With him on the committee are Professor Emeritus Donald R.F. Harleman of civil and environmental engineering, and Professor Paola Rizzoli of earth, atmospheric and planetary sciences, who is also the MIT director of the MIT-Woods Hole Joint Program in Oceanography. Professor Chryssostomos Chryssostomidis of ocean engineering participated in earlier activities. Professor Andrea Rinaldo from the University of Padua is also a member of the group. Both Rizzoli and Rinaldo are Venetians.

In his own research program, Bras studies the relationship among atmospheric phenomena, surface hydrologic processes and the landforms of river basins to enhance our understanding of human impact on the global environment and to improve predictions of hydrologic events like floods.

And flooding is a phenomenon Venetians know well.

Venice is actually a collection of many small islands connected by bridges in a lagoon that's separated from the Adriatic Sea by a strip of long, narrow barrier islands. When the tides are higher than normal, water surges through the lagoon inlets and floods the city, forcing people to walk around on raised catwalks set up for these occasions. On Nov. 4, 1966, the city was under a 1.94-meter tide (nearly six and a half feet), almost 1.5 meters over the normal tide, causing the worst flooding in recent history. In 1997, Venice flooded about 100 times.

Because flooding occurs more frequently now, the ground floors of buildings have been abandoned and many people have moved away. During medieval times, Venice was a thriving metropolis of 250,000 people; today the population is a mere 60,000.

"It's hard enough to live in Venice because of difficulties in mobility; there are no cars or trucks to move goods or drive you to the market. If, on top of that, businesses have to deal with an infrastructure under constant attack, it's understandable why it's difficult to maintain a viable, vibrant city," said Bras.

Floods usually occur between October and February, when winter storms bring lower atmospheric pressure and higher winds, leading to increased tide levels. But this year, it also flooded several times in June, when the city was unprepared. "It was a disaster, like snow removal in Boston in the summer," said Bras, who added that flooding tides of 1.4 meters (four and a half feet) were once rare, but now occur almost every year.

The increased flooding is caused by two things: the city is sinking and sea level is rising.

After World War II, water needed by new industry was obtained by pumping groundwater from nearby areas. As the water table decreased, the islands began sinking. The practice of pumping groundwater was stopped shortly after the devastating flood of 1966, and the city is now sinking at the much lower rate of 0.4mm per year.

At the same time, the Adriatic Sea and other ocean waters have been rising at the rate of 1.6mm per year, perhaps due to natural causes. "It could be the ubiquitous sea level rise that occurs around the world and has been going on fairly consistently since the last glacial period," said Bras. "You can debate whether it is accelerating or not, but so far I see no evidence of acceleration."

The end result of this rising and sinking is that Venice floods when high astronomical tides - the tides induced by the moon - combine with high meteorological tides - the surge caused by stormy weather (low atmospheric pressure combined with winds).

But if the government approves and constructs the MOSE project, this combination of high tides will no longer flood the city. Instead, the rising tides will be blocked by new floodgates, which will be placed at the inlets where the Adriatic Sea enters the lagoon. The gates are designed to prevent the Adriatic Sea from flooding the lagoon at high tides in even the worst storms.

The project is a series of 79 gates - each about 30 meters high, 20 meters wide and four to five meters thick - to be installed on the bottom of the sea at inlets at Lido, Malamocco and Chioggia. Forty of the panels will stretch across Lido, the widest of the inlets. When a tide of 1.4 meters or higher is predicted, the hollow gates will fill with air and rise, creating a barrier to the seawater.

Critics fear the gates will prevent water exchange between the lagoon and the Adriatic Sea, which could damage the lagoon's ecosystem. But research on water exchange in the lagoon shows that the lagoon is flushed daily by the tides, indicating that if the gates are up during a storm, the tides will quickly flush the lagoon as soon as the gates come down.

"Pollution prevention in the lagoon is always the best practice," said Bras. "In fact, the water quality of the lagoon has been improving over the last decade in response to pollution prevention practices, so that dependence on natural flushing would be less essential. In addition, during normal periods, the gates can be opened and closed in such a way as to induce additional mitigating flushing."

Other critics claim that the design is based on outdated predictions of rising sea levels. They say the gates will become obsolete in half a century. Bras, Harleman, Rizzoli and Rinaldo addressed that criticism in an article in Eos, a publication of the American Geophysical Union, last spring.

"The bottom line is that the gates work. The barriers, as designed, separate the lagoon from the sea in an effective, efficient and flexible way, considering present and foreseeable scenarios," the authors wrote.

"I advocate that this is the only solution to the problem," Bras said in an interview. "I have no doubt that it will work for 50, 70 years. I have no way of knowing what will happen in 100 or 120 years. If we see massive rises in sea levels, then we'll have big problems in Cape Cod and elsewhere. It won't be just Venice, which will in fact be in a better position by acting now."

A version of this article appeared in MIT Tech Talk on November 6, 2002.

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