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Earlier this month, Venice suffered some of its worst floods for almost a century. And as global warming becomes a reality of 21st-century life, the problems can only get worse. The Italian government is considering a controversial scheme to protect the city during high tides using a system of mobile flood gates. Some critics claim that the plan risks disrupting the ecology of the lagoon, others say that it underestimates the rising sea levels. Camillo Dejak of the University of Venice is a leading expert on the city’s ecosystems. He tells Susan Biggin that these flood gates are Venice’s only realistic hope.

The first serious flood in recent memory was in 1966. Why has it taken so long to come up with a solution?

The 1966 flood was a surprise. It was a freak. There had been no floods as exceptional before then. Just think, a hundred years ago, a tide of that dimension was expected every thousand years. Today, subsidence means that we would expect it to happen every 140 years, while a sea-level rise of 20 centimetres due to global warming would reduce this to 40 years. Even in the early 1970s there were very few high tides, so the government probably didn’t want to spend large sums of money on the problem. But after a second flood in 1979 there was almost a revolution in Venice. A public meeting was held which the entire city attended to protest about why nothing had been done since 1966. The scientists and politicians had no defence, and it almost came to a lynching. Imagine all the shopkeepers-all their goods on the ground floor went under water that year.

What did the government do then?

It had already held an international competition in 1975, inviting proposals for dealing with the flooding problem. Five projects were submitted, but none was considered adequate and the government abandoned the competition without a winner. In 1980, it set up a committee of the best hydrodynamics engineers in Italy to amalgamate the five entries from the competition into a single project. This project, from which the Modulo Sperimentale Elettromeccanico project (MOSE) derives, was submitted in 1982 to the Ministry of Public Works. In 1984, after some revision, it was approved within the framework of a special law for Venice, which also established that the work should be carried out by a single concessionaire. The project was then entrusted to the Venice Water Magistracy, which chose as concessionaire the Consorzio Venezia Nuova. The cost will work out at about 4000 billion lire (£1.3 billion).

Has Venice always suffered from flooding?

Archaeological evidence from 50 BC shows that there were frequent high tides up to 75 centimetres during the Middle Ages. After the diversion of the main river in the 18th century, the phenomenon didn’t become serious again until the 1966 flood, when Venice went under a freak tide of 194 centimetres. Piazza San Marco, the heart of Venice, floods at about 80 centimetres of tide. Even last month, we had a high tide of 144 centimetres-more than half a metre above the level of the Piazza.

What causes a high tide?

There are three factors. First, the regular tides controlled by astronomy, but these never cause problems by themselves, otherwise we’d be under water every couple of weeks. Then there’s the difference in atmospheric pressure between the north and south Adriatic, which results in an oscillation of the sea with a period slightly different from the astronomical one. If this oscillation, known as “sieces”, is in phase with the astronomical effect on the tides, we get very high water. The third factor is the wind, which is much more difficult to forecast. There are two main winds in Venice. The wind from the south, the sirocco, pushes water into the lagoon, increasing the water level. The bora, which blows in from the north-east, presents no risk since it pushes the water out of the lagoon through the southernmost of the three inlets. The flood we had last month was almost entirely caused by the sirocco wind.

Is it possible to predict these floods?

It is like predicting an earthquake: it is very difficult if the wind is the main cause-as it was in 1966 and earlier this month. In fact, I did predict the big flood of 1979. For a period, the astronomical tides and the sieces were out of phase. This put Venice under water for two days with high tides in rapid succession. I reasoned that if it carried on, in a few days these two effects would act together rather than counterbalancing each other. And if this occurred at a full or new Moon, we would go well under water. And we did, we got 166 centimetres two days before Christmas.

How would MOSE work?

There would be 79 flap-gates across the three inlets. Normally, they would rest on the sea floor. But during a high tide compressed air would be pumped into the gates, making them buoyant enough to float up and close off the inlets. The gates are designed to withstand a sea-level rise of up to 2 metres, and would close whenever the waters reach at least 1 metre. The system is therefore effective for defence against all high waters.

Some ecologists claim repeated use of the barriers will restrict circulation of water in the lagoon, increasing pollution and damaging the ecosystem. Do they have a point?

Everyone is talking pessimistically about it without any scientific basis. It isn’t the currents, but the resulting eddy-diffusion that determines how the nutrients and pollutants disperse. You can work out the growth of plankton and other organisms from the relative concentrations of nutrients and pollutants in the lagoon water. We developed a general model for this at the university. This model can be used for studying barrier closure and the consequent effects on pollution and nutrients. The alternating closure of the gates could even have a cleansing effect on the lagoon water!

Apart from its misgivings about MOSE’s environmental effects, the environment ministry is unhappy about entrusting the future of Venice to a single consortium. Is it an unhealthy monopoly?

We have already gone from a competition with five proposals to a single concession. Do we want to go back, start again and take another 25 years, which was the time needed in Italy to work through these technical and, in particular, bureaucratic procedures?

Do you have any criticisms of MOSE?

Not for the hydraulic system itself. I’m not a specialist in hydraulic systems, and I have no issue to raise at all with the full-size prototype of the basic MOSE element that I’ve examined. Today the project is perhaps the most refined of its kind, as it has been continuously brought up to date.

Are you happy with the way the rise of the sea level is predicted in MOSE?

This is where I am critical. It’s crazy to promise that the mobile gates will be closed any time the level exceeds a fixed threshold, determined now, such as 1 metre. The project uses only average sea levels for predicting high tides. But the major increase in the number of high tides above 110 centimetres is due to the increasingly marked oscillations around the average. An increase in average sea level of about 20 centimetres between the 1950s and 1990s does not explain an increase in floods from a handful per decade around the middle of the last century to 44 in the past decade alone. We have to find out why they are increasing.

Shouldn’t that happen before the barriers are built?

It is no good at all claiming, as the environment ministry and others have suggested, that we have 20 years before we have to decide about the barriers. On the basis of the new findings released at the Hague climate conference, we have to accept that the project is urgent. If the greenhouse effect worsens and the temperature increases by an average of 6 °C, we are going to have to spend increasing sums to deal with the consequences, far beyond what has been earmarked for MOSE. This is true for every coastal zone. We are now paying the price for what hasn’t been done in the past.

How serious a problem is global warming for Venice?

Twenty years ago, when I began studying it, I was taken to task by the press for frightening the Venetians with outrageous tales. But it is a reality. There are two effects. When the temperature increases, seawater expands, raising the level of the oceans. But more energy in the atmosphere also means more turbulence, so oscillations of the sea level become more pronounced. Both these effects are difficult to predict with mathematical certainty. I’m afraid that by the time we manage to measure them convincingly, it’ll be too late to do anything about it.

Some time ago, a group from the University of Padua proposed raising Venice by pumping waste CO₂ under the city. The idea has been revived by Carlo Rubbia, head of Italy’s energy and environment agency ENEA. Would it work?

Every time I hear about a new idea, I shudder. Actually, this could work, but the problem is how to pump in the CO₂. There are endless studies. Twenty years ago, a small part of the Isola di Poveglia in the Venice lagoon was raised artificially, by pumping in a dense liquid. So it has been done before, but only on a small scale and there are immense problems because the ground below Venice is highly complex. The buildings are basically propped up on wooden stilts that are rotting and shifting.

Are there no alternatives to MOSE?

The alternative is that we do nothing and Venice goes under water. There are no alternatives-but alongside MOSE there must be complementary measures. One example is the “macroinsulae”, a system of local defences, put forward by engineers in Venice, in which the city is divided into blocks completely isolated from the exterior by deep walls. With this system, the barriers would be raised when the water reaches 140 centimetres above the sea level rather than 1 metre.

Did you look outside Venice for ideas?

I did, but my opinion, as well as that of several experts, is that solutions used in other countries, such as London’s Thames Barrier or the infrastructure used in the Netherlands, cannot be transferred to Venice. For us, one criterion was to find an aesthetic solution that respected the landscape and was almost invisible on the surface.

So what happens now?

We are waiting for the governmental committee to decide how to go forward. But we’re having elections in about six months’ time and no one is likely to do anything now. I hope that, at least after the elections, the government will decide to go ahead with the project, partly in view of the fact that the Mayor of Venice is patently in favour.

The stumbling block is political, then?

The politicians are the deciding factor. It’s all about time periods. The high tides in the lagoon go through cycles. I have found a combination of two superimposed waves of 11 years and about 16 years. Unfortunately, politicians here work in cycles of five years, which is too short by comparison.

Could there be an international solution?

It has already been done. We had a positive evaluation of the project by an international committee of experts, appointed in 1996 by the Italian government. It’s time to resolve the issue here, in Venice and in Italy.

Will this project ever be launched?

I hope so. The recent floods may help convince the sceptics. I have lived through the entire history, from the 1970s to 2000. You can’t understand the lagoon without its history. Without it, the delays seem like madness.