The national commission came out against the barriers; the International College found in favor. Zitelli is still smarting at the fact that her group's objections were not taken seriously. She argues that the environmental impact of the construction will be very high, because deep, transverse channels must be excavated for the gates' foundations. She enumerates several technical objections, such as that 20,000 pilings must be sunk 30 meters into the mud, "and must stay in a straight line to resist the weight that will be put on them. But the bottom is not flat. There are small, discontinuous lumps of limestone at different levels. How do you get these pilings straight? And if the substrata are not stable, the gates could move apart."

She gestures to some gondolas, bobbing about in front of us on the water. "You see those? They don't move in the same way, at the same rate; they don't reach the same height. This is called disharmonic wave resonance, and this disharmony could be so strong in windy conditions that the gates could pull apart." She is worried too about biological encrustation under water, which she estimates will be "around 30 kg per square meter per year." I ask if this could stop the gates from working; she shrugs.

She also points to what she believes are uncertainties in forecasting when to raise the gates. "Computer models predicting high tides currently have a margin of error of plus or minus 20 cm. Only last week the sirens failed to predict how high the tide would go." (Brotto disputes these figures, citing a 10 cm margin of error.) "That means that if you decide to close for tides of 110 cm, the actual tide could be 90 cm or 130 cm. During winter, this includes all medium-high tides in Venice. Today, high tide is 90 cm, and there is no flooding. So you can't accurately predict the tide until an hour and a half beforehand, but they will have to decide before that, in order to warn shipping."

Brotto does not accept these figures: She says that the first information comes in 48 hours before a high tide, and that "it is possible three hours before the high tide to have a forecast that is within 10 cm of error, 97 percent of the time." She adds that mathematical modeling is improving all the time.

Zitelli concludes that even the International College of Experts said the project would need to be reviewed in 2050. "Yet this is being sold to us as a project with a 100-year technical life."

If the Venice barriers are completed on schedule, they will be finished in 2011, nearly half a century after the 1966 flood. For Dominic Standish, a British academic and journalist who lives near Venice (and who has connections with an international think tank that has been linked to the Bush administration's position on the environment), this represents an inexcusable delay. "I feel a little bit jealous of London for having got its barrier before these things became such a political issue," he says. "It's a sign of our times that we feel so hesitant about protecting something that is so important in the history of the world. Governments have now adopted the precautionary principle: There is a desire to prove that any action is risk-free before proceeding. It's a way for them to seem to be interested in emotive issues: a way of connecting with people."

No doubt the sums of money involved have troubled successive Italian governments. But Italy's often sclerotic politics has not helped: In the '70s, there were doubts that Italy would even survive as a state; in the '80s, shifting coalitions were preoccupied by their internal state, as Christian democracy and communism faded and declined. In 1998, a Green environment minister halted the project; the Amato government subsequently revived it with conditions that took account of some of the environmentalists' concerns. It is finally happening now because Berlusconi has based his plan for economic recovery on the implementation of a number of grand projects.

Throughout this, as Green M.P. Giorgio Sarto admits, the center-left has been hopelessly divided, not only on the wisdom of having the barrier at all (which, given the technical arguments raging around it, is understandable) but also on whether to oppose the role of CVN. As water seeps into the city, life is draining out of it. Today, CVN rents a palace belonging to General Insurance. There are no insurance companies working in the city anymore -- few major employers at all, in fact. Even the public-private partnership responsible for urban maintenance is moving out to the mainland. Somers Cocks, of Venice in Peril, believes all this is directly related to the flooding, and that if the waters could be controlled, businesses might start investing once more. "How acceptable is it for Venice to flood? My view is not at all. It is damaging not just to the buildings but to the way the city is lived in. The only way you can deal with Venice in the short term is to close the gates relatively frequently. Then do you get a stinking, rotting lagoon? The data differs."

For some environmental groups, that's too big a risk, certainly at this stage. Some would rather see pavements raised first, although pavements have already been raised as far as most architects and art historians are comfortable with, and any further lifting would spoil the proportions of the architecture, making doorways impossibly low.

Until recently, until postwar industrialization and port development, environment and architecture have lived in symbiosis in Venice. But that point of equilibrium appears now to have passed, and what needs to give depends on one's priorities. The trouble, though, is that tampering with one end of things affects everything else: Venice is only Venice thanks to the vigorous natural dynamics of the lagoon system. Most scientists, it is probably fair to say, think that the gates are a solution, certainly for the time being. Brotto argues that they will work "up to a 60 cm rise in sea levels. There is considerable dispute about how far global warming will cause sea levels to rise, but if it is as much as 60 cm, we will also lose Ferrara and Ravenna. Venice will be the only dry place in the northern Adriatic." For Somers Cocks, too, the argument that global warming may make the gates obsolete by the middle of the century is a red herring, "because what happens between now and then? Flooding 80 times a year and the constant worry of another extreme event?"

A three-year collaboration between Cambridge University and Corila, the Venice-based lagoon research consortium, resulted this autumn in the publication of a book, "The Science of Saving Venice" by Caroline Fletcher and Jane da Mosto, summarizing everything currently known about the ecology of the Venice lagoon. A wonderfully clear statement of what scientists currently understand, this eschews the political posturing of so much of the debate about the barrier. The authors do, however, note rather tartly: "It is remarkable that an enormous amount of data collected by numerous institutions (but mainly by the Venice Water Authority) has not been circulated nor made readily accessible to the scientific community, nor the public at large. This has obstructed the comparison of research results and findings from different modeling approaches and the development of a non-ideological, constructive, science-based debate."

So much money has been funneled through CVN and the Venice Water Authority into research that it is difficult to find scientists locally who are genuinely independent. And with the fruits of all this research not widely disseminated, there remain engineering and ecological uncertainties. For Somers Cocks, though, much of the opposition -- whether it takes the form of environmental or engineering objections -- is rooted in ideology: "an objection to private companies, a sense that there is a capitalist plot." She thinks that antipathy to the barriers is symptomatic of Venetians' failure to face up to the reality of their future. Since the lagoon would, left to its own devices, merge with the sea eventually, Venice is faced with an unpalatable choice: Abandon itself to the tides, or wall itself off and turn the lagoon into a lake.

"The greens say if you fix the lagoon, you fix the problem," says da Mosto. "But it's not an either/or, not least because one day we're almost certainly going to have to turn the lagoon into a lake. We need to know everything we can about its ecology by then. The more we know, the more we realize we need to know, especially given the decreasing role of natural dynamics in the lagoon."

The barriers will almost certainly be built, even though no one yet knows who will operate them, nor how the government will manage the funding (currently being allocated only on a year-by-year basis). Romano Prodi, who will oppose Berlusconi in the next election, recently came out in their favor. Venice will hold back the tides for a little longer -- and after that, who knows? (Engineers in London are already working on the successor to the Thames Barrier; in Rotterdam, they are planning for the next barrier but one.)

The tides of economic globalization are another matter. At present, Venice is a city close to being swamped by the international tourist trade, and finding no other identity it can assert. If it is to avoid being turned into merely the museum quarter of the Venice metropolitan area, it has to engage with globalization, not avoid it. In theory, it ought to be easy enough for Venice (rather as Dubai has done) to offer incentives for appropriate businesses, those based mainly on computers and the exchange of ideas, to set up in the city. But it remains to be seen whether there is the political will to engage with modernity in this way. For Professor Ortalli, the city is already becoming like Babylon. "Venice cannot survive," he says, standing on the palazzo steps. "I think it has died already. I am spending a lot of time fighting this barrier not to save Venice, but to save my soul."

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