The sinking city: Venice's $3 billion plan to stop a rising sea gets mixed reviews. (Currents).

It's a clear, calm night--no storms, no wind, no unusual weather of any kind--but Venice's Piazza San Marco is flooding. As the tide crests, seawater begins pouring from storm drains, forming a growing pond in the middle of the city's most important square. The vestibule of the sumptuous San Marco Basilica begins filling with water, amusing tourists, some of whom wade laughing in the ankle-deep pond.

But Venice's flooding is no laughing matter. The sea is rising, the city is sinking, and the damage to its historic buildings, bridges and artworks is becoming increasingly apparent. Green algae now grows on the porous brickwork of many of the 14th and 15th century palaces along the Grand Canal because the flooding sea frequently tops the building's waterproof stone foundations.

After decades of debate and study, the Italian government finally has a solution to save Venice from the sea. The centerpiece, approved in December, is a $3 billion project to build 79 enormous hinged gates to separate Venice and its lagoon from the Adriatic Sea. But the project has drawn criticism from environmentalists and a few prominent scientists who warn it will turn into a financial and environmental disaster.

Venice, built on a series of swampy islands in the midst of a large lagoon, has been slowly sinking under its own weight throughout its 1,000-year history. In the 20th century, relative sea-level in Venice increased by more than nine inches due to a combination of sinking land and rising seas.

Global warming will exacerbate the problem by melting glaciers and heating the oceans, causing them to rise by somewhere between three inches and three feet by the end of the century, according to the dozens of scientists serving on the International Panel on Climate Change. Venice's squares, streets and low-clearance bridges are particularly at risk.

Venetians already experience more frequent incidents of "acqua alta"--especially high tidal surges that flood the city in certain wind conditions. The ground floors of most of the city's buildings were abandoned after 1966 tidal surge put much of the city under several feet of water for more than 15 hours. In 1996, San Marco was flooded by high water on more than 80 occasions.

"If you want to preserve the city as a museum for tourists, you don't need to build the gates," says Andrea Rinaldo, a University of Padua engineer who has watched his native Venice lose nearly half its population since the 1966 flood. "But if you want a living city with real residents, decent jobs and everyday shops and stores then you must build these gates and protect the city."

A consortium of Italian engineering firms is already busy putting the final plans together for the construction of the gates, which will lie flat on the seafloor inside the three entrances to the lagoon. When an acqua alta occurs, the gates will swing up to form a temporary wall stopping the rising sea from entering the lagoon and the city's streets and squares.

The gates are designed to protect the city from flood surges of up to six feet in height, sufficient protection to keep up with sea-level rise for at least 70 years, according to Giovanni Cecconi, an engineer with the New Venice Consortium, an alliance of major Italian engineering firms charged with designing, building and operating the massive project. "It's not the final solution, it's just a way to protect the city during this century until another solution can come into place," he says.

Supporters point to similar multibillion dollar flood control projects already constructed by Britain and the Netherlands to protect the London and Rotterdam waterfronts from wind-driven flooding events.

"Italy should do what the Dutch learned to do a long time ago: Act now and think preventively," says the Massachusetts Institute of Technology's Rafael Bras, who led a team that audited Venice's gates project. "Virtually everyone agrees that the only ultimate solution is to separate the lagoon from the Adriatic. So don't take a wait-and-see attitude."

But environmentalists are generally skeptical of the New Venice Consortium, a private for-profit enterprise that effectively serves as the Italian government's Venice flood protection agency. "The Consortium is a little too focused on the gates project because that's what they live off of," says Paolo Lombardi of the Rome office of the World Wide Fund for Nature. He fears the project may consume resources better spent restoring the natural flood-protection functions of the lagoon system.

The gate's most prominent critic, Colgate University archeologist Albert Ammerman, has literally uncovered evidence suggesting that the designs are based on false assumptions about the scale of the problem. "There are fundamental flaws in the scientific studies that the Consortium's gates are based on," he says. "They've really screwed this up."

While studying the archeological origins of Venice, Ammerman and his colleagues dug up evidence showing that the city has been sinking much faster than anyone previously thought. Throughout the city they found layer upon layer of pavements and foundations laid over the centuries in a constant effort to keep the city above water.

Using new carbon dating techniques, his team was able to calculate the rate of relative sea-level rise in the city going back 1,600 years. The data, Ammerman says, shows that the Consortium's plan seriously underestimates the likely rate of sea-level rise during the coming century. By adding Venice's long-term subsidence rate with the best available estimates of future sea-level rise, Ammerman reckons the sea will gain between 12 and 39 inches relative to Venice's streets.

If true the sea still wouldn't be nearly high enough to breach the massive gates when they're closed. The problem, Ammerman says, is that the gates will have to be dosed far more often than their planners say they will. Since virtually all of historic Venice's sewers empty straight into the city's canals and lagoon with little or no treatment, frequent lagoon closures could have serious environmental consequences.

"In a bad year [in the mid-21st century] you could have the gates closed for 100 or 120 days a year, with two-thirds of them in the three-month flood season," Ammerman says. He predicts the resulting pollution crisis would shorten the project's lifespan to 30 or 40 years. The Consortium, he insists, should go back to the drawing board and revamp its designs.

Consortium spokesperson Monica Ambrosini disagrees. "The problem of sea-level rise is very real, and the designs take it into account," she says. The company expects to start construction this year. CONTACT: World Wide Fund for Nature Climate Change Program, [phone], www.panda.org.