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Venice flood barrier risks questions raised

A leading oceanographer has raised concerns that Venice’s €4.6bn (£3.8bn) flood barrier project could put public health and the environment at risk.

National Oceanography Centre coastal morpho-dynamics expert Carl Amos said there have not been sufficient studies into the Mobile Barrier System (MOSE) risks in terms of sewage pollution, fish populations and sand flows.

“It is quite horrifying that the level of knowledge with this structure is still rudimentary,” he told NCE.

The barrier gates are designed to be closed for five hours at a time. But Amos said in future high tide events are likely to increase in frequency meaning longer gate closures. Longer closures could cause sewage pollution and water deoxygenation, he said.

“It is quite horrifying that the level of knowledge with this structure is still rudimentary”

Carl Amos

“There is strong evidence that they would have to keep the gates closed semi-permanently in 30 years,” said Amos. “[Sewage is] going to produce a nasty cocktail, which could produce health risks and increase the risk of water-borne diseases.”

Amos said diminishing fish populations due to water deoxygenation would be disastrous for the aquaculture and fishing industries.

However, client Consorzio Venezia Nuova designer Alberto Scotti said the risks were deemed minimal.

He said ecological modelling had found that there would be some sewage pollution and deoxygenation, but any increase would be “very limited”. The models showed that after one or two tides had passed the lagoon would return to its normal state, said Scotti.

Another concern voiced by Amos was that sediment flows could cause sand to settle on top of the gates when they not in use, or underneath the gates when deployed. As a result the buoyant gates may be unable to rise up, or to return to their resting position flat against the seabed.

“It could be catastrophic. Neither one of those scenarios has been looked into,” said Amos, adding that this was what he had been told recently by the Consorzio Venezia Nuova.

But again Scotti argued that the risk had been studied and mitigated. “We have analysed deeply this phenomenon,” he said.The barrier gates are located at the lagoon’s three inlets where, Scotti said, the sand grain size is only 50 micrometres and the tidal flows are fast enough at 1.2m/s. This means sand is unlikely to settle too much on top of the gates, he said. “This material can certainly be removed. But he admitted that the equipment for this was still under development.

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