Despite a ban on artesian wells in the 1960s, Venice is still sinking – though at a far lower rate than before.
According to the American Geophysical Union, it’s not only still sliding slowly under the waves, but is also tilting to the east.
The finding will disappoint conservationists, as previous studies in the early years of this century had indicated that the city had stabilized.
“Venice appears to be continuing to subside, at a rate of about two millimeters a year,” says Yehuda Bock, a research geodesist with Scripps Institution of Oceanography at the University of California, San Diego. “It’s a small effect, but it’s important.”
Given that the local sea level is also rising at 2mm per year, the new findings show the problem to be twice as serious as believed. If things carry on as they are, Venice will subside by up to 80mm – 3.2 inches – over the next 20 years.
The new findings are based on data from GPS and space-borne radar (InSAR) instruments, which provide absolute and relative elevations respectively.
“Our combined GPS and InSAR analysis clearly captured the movements over the last decade that neither GPS nor InSAR could sense alone,” says professor Shimon Wdowinski of the University of Miami.
The measurements also show that the are is tilting a millimeter or two eastward per year. Prior satellite analyses didn’t pick up on the tilt, says Bock, possibly because the InSAR measurements only provided the change in elevation relative to other sites.
After groundwater pumping was banned, studies in the 2000s indicated that the subsidence had stopped. “It’s possible that it was stable in that decade, and started subsiding since then, but this is unlikely,” says Bock.
The frequency of floods in Venice is increasing, and a multi-billion-dollar effort to install flood-protection walls that can be raised to block incoming tides is nearing completion. However, the new findings indicate that this may not be enough.
Pietro Teatini, a researcher with the University of Padova in Italy, is investigating geoengineering solutions, including the injection of saltwater into the aquifers below the city, to reverse the subsidence.