2016

Venice Flood System

Venice is a city where floods more than 5 feet (1.4 meters) high are not unusual. And it is going to get worse with rising sea levels. Eventually the city could be lost to the flooding. Many coastal cities may soon be experiencing similar problems. For example, in New York City, the chance of flooding is now more than 10 times greater than a century ago.

The entire city of Venice is fascinating from an engineering perspective. The city sits on a series of muddy islands in the center of a lagoon. To provide stability, millions of tree trunks were harvested and pounded into the mud hundreds of years ago.

The problem is that the level of the lagoon rises and falls freely with the tides. When storms occur, the lagoon can rise even higher. Even a low-pressure weather system reduces air pressure on the water and the lagoon rises.

To stop the flooding, Venice is installing a system called MOSE to close off the lagoon. A sea wall already separates the lagoon from the Adriatic Sea, with only three inlets to provide access for boats. A set of movable gates has been placed on the seafloor in the inlets. When higher than normal tides are predicted, the gates fill with compressed air and rise up to separate the lagoon from the sea. The gates can maintain up to a 10-foot (3 meter) level difference, keeping the lagoon below its flood stage.

Getting the gates positioned on the seafloor has been a special engineering challenge. The gates, their hinges, and compressed air supply rest in huge, modular concrete trays lowered to the seafloor. Engineers decided to cast them nearby in a purpose-made drydock. Then they float them into precise position and sink them into place.

There are 78 gates total, each 20 meters wide. Once the whole system is operational, expected in 2016, engineers should have complete control over floods and will have saved Venice.

SEE ALSO Concrete (1400 AAA), Leaning Tower of Pisa (1372), Zuiderzee Works (1891), Three Gorges Dam (2008).

Flood in Venice, Italy; acqua alta on Piazza San Marco.