Venice flood barriers scheme 'will soon be obsolete'

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A £2bn scheme to save Venice with dozens of inflatable flood barriers is doomed to failure because sea levels are rising even faster than predicted, a scientific study to be published tomorrow says.

One critic of the ambitious plan to build 79 massive gates at the three entrances to the Venice lagoon says the scheme will not only become obsolete soon after it is built, but might even have to be dismantled because of the environmental problems it will generate. Paolo Pirazzoli, a marine geophysicist with the French National Centre for Scientific Research in Meudon, says in a devasting critique of the project that the planners have failed to take future increases in sea levels into account.

"According to the best climatic models, there is a clear risk that the [flood] gates will become obsolete within a few decades and that they may have to be demolished shortly after their construction to separate the lagoon from the sea more effectively," Dr Pirazzoli said.

The Italian government, led by Silvio Berlusconi, approved the Experimental Electromechanical Module project – known as MOSE – in December 2001. It is due to take eight years to build, although construction has not yet started.

Each concrete barrier will measure 20 metres (66ft) wide, 20 to 30 metres high, and five metres thick. They will have a water-filled core that can be quickly emptied with pressurised gas to make the gate rise when a predicted storm surge threatens to raise tides by more than about one metre.

Dr Pirazzoli, a member of the American Geophysical Union, says this specification is based on tidal surges above average sea levels for the period from 1884 to 1909. In effect, the barriers are designed to cope with average increases in water levels of 87cm above the present-day average, he says.

Furthermore, Dr Pirazzoli claims that the MOSE project – whose origins date back more than two decades – is based on outdated assumptions about future rises in average sea levels and does not take into account the latest predictions, made in 2001, of the Intergovernmental Panel on Climate Change.

"The planned barriers would protect the lagoon from storm surges at the present time, when the lagoon inlets would be closed for a few hours. Protection seems doubtful, however, in the case of near-future background sea level rise, forcing closure durations to increase."

Dr Pirazzoli's study is to be published tomorrow in Eos, the journal of the American Geophysical Union.

"This weakness in the project can be explained by the fact that the system was officially put forward in 1981 and has not been subsequently adapted to the predictions of greenhouse gas buildup-related sea-level rise which have been foreseen since 1982," he says.

The designers of the MOSE project assumed that the "most realistic" increase in average background sea levels by 2100 would amount to about 22cm, with a "pessimistic" scenario of a 31cm rise.

"Such optimism in near-future sea-level change is not supported by new modelling studies ... the Experimental Electromechanical Module system could hardly cope with a relative sea-level rise much greater than about 0.30 metres," Dr Pirazzoli says.

Heavy rains that swell the rivers draining into the Venice lagoon, strong winds, high tides and storm surges could, on top of increases in average sea levels, cause a catastrophic breach of the barriers, rendering them useless.

Dr Pirazzoli also believes the barriers would need to be raised far more frequently than planned, causing pollutants to build up in the lagoon and destroying its delicate ecosystem, which relies on frequent "flushing" with seawater.

Dr Pirazzoli is one of a number of scientists who believe Italy should opt for less expensive, "soft" defences against flooding, protecting individual buildings and walkways until better assessments of climate change can give more accurate predictions of rising sea levels.

A team of marine engineers and geophysicists led by Rafael Bras of the Massachusetts Institute of Technology, which have studied the project on behalf of the Italian government, dispute Dr Pirazzoli's assumptions. Their calculations suggest that the barriers would protect Venice even with the predicted rise in average sea levels.

"The bottom line is that the gates work ... To argue that the design of the barriers did not consider sea-level rise is just wrong," Dr Bras says. "The barriers, as designed, separate the lagoon from the sea in an effective, efficient and flexible way, considering present and foreseeable scenarios."