Venice falling

Compacting soil means the flood-prone city is sinking

Venice is still sinking and will probably continue to do so for a long time, a new study suggests. That’s bad news for the local government, which had already put a stop to groundwater pumping in an effort to curb the city’s subsidence.

Venice sank an average of 1 to 2 millimeters per year between 2000 and 2010, with some areas (red) subsiding even faster. American Geophysical Union

“It was thought that the sinking had pretty much been stabilized, but now we know it will continue into the future indefinitely,” says Yehuda Bock, a geodesist at the Scripps Institution of Oceanography in La Jolla, Calif.

Today’s sinking has little to do with human activities, Bock and colleagues report online March 28 in Geochemistry, Geophysics, Geosystems. Soil is compacting beneath Venice, bringing the city down with it. And Venice rides atop a slab of Earth’s crust that is slowly diving beneath the Apennine Mountains, giving the city a tilt noticed for the first time in the new study.

Data collected by GPS devices and satellite radar systems showed the city dropping an average of 1 to 2 millimeters per year between 2000 and 2010.

That’s significantly slower than it sank decades ago, says Bock, and slower than other cities such as New Orleans are sinking today. So some researchers don’t think this natural subsidence is worth worrying about.

“We think this is a very small number,” says Tazio Strozzi, a physicist with the GAMMA remote-sensing consulting company in Gümligen, Switzerland. Previous satellite measurements have detected a similar amount of subsidence, he says.

But Bock worries that even a millimeter or two a year could, over time, compound the risks of changing sea levels. During the 20th century, sea levels rose 13 centimeters at Venice, helping to make floods seven times more common. 

“They built barriers to raise during high tide,” says Bock. “They’ll probably have to use them more often than they had anticipated.”

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