Sea-level rise isn't the only thing that has Venice's famous canals rising ever-so-slightly every year: The city is also sinking, a new study shows, in contrast to previous studies that suggested the city's subsidence had stabilized.
The study's findings also showed that the Italian city is slowly tilting slightly to the east, something scientists had never noticed before.
Venice's subsidence was recognized as a major issue decades ago, when scientists realized that pumping groundwater from beneath the city, combined with the ground's compaction from centuries of building, was causing the city to settle. But officials put a stop to the groundwater pumping, and subsequent studies in the 2000s indicated that the subsidence had stopped, said lead author of the new study, Yehuda Bock, a research geodesist with Scripps Institution of Oceanography at the University of California, San Diego, in La Jolla, Calif.
But the new study, detailed in the March 28 issue of the journal Geochemistry, Geophysics, Geosystems, used a combination of measurement techniques that provided data on both the absolute and relative shifts in elevation of the area, along with GPS measurements and space-borne radar (InSAR) data.
The team use data from 2000 to 2010 to track changes in the elevation of Venice and its surrounding lagoons and found that the city of Venice was subsiding on average about 1 to 2 millimeters a year (0.04 to 0.08 inches per year). The patches of land in Venice's lagoon (117 islands in all) are also sinking, with northern sections of the lagoon dropping at a rate of 2 to 3 mm (0.08 to 0.12 inches) per year, and the southern lagoon subsiding at 3 to 4 mm (0.12 to 0.16 inches) per year.
"Our combined GPS and InSAR analysis clearly captured the movements over the last decade that neither GPS nor InSAR could sense alone," said study team member Shimon Wdowinski, associate research professor of marine geology and geophysics at the University of Miami.
The team also found that the area was tilting a bit, about a millimeter or two eastward per year, something never noticed before. That means the western part — where the city of Venice is — is higher than the eastern sections.
What's causing the sinking?
The forces causing the subsidence now are likely natural ones that have been impacting the area for a long time, particularly plate tectonics. The Adriatic plate, on which Venice sits, is subducting beneath the Apennines Mountains and causing the city and its environs to drop slightly in elevation. The compaction of the sediments beneath Venice also remains a factor.
Floods are happening more frequently along Venice's canals now, Bock said, with residents having to walk on wooden planks to stay above the floodwaters in large parts of the city about four or five times a year.
A multibillion-dollar effort to install flood-protection walls that can be raised to block incoming tides is nearing completion, he said. These barriers were designed to protect the city from tides that are coming in higher as overall sea levels are rising in response to climate change. But builders should also take into account the rate of subsidence to make sure the barriers can do their job, Wdowinski said.
Pietro Teatini, a researcher with the University of Padova in Italy who was not involved in the study, says that while it is important to monitor the subsidence, the amount measured by the team is small and much less than compared to what the city experienced when groundwater pumping was going on.
Venice subsided about 120 mm in the 20th century due to natural processes and groundwater extraction, in addition to a sea level rise of about 110 mm at the same time, Teatini said in a statement. Bock and his colleagues calculate that the city and surrounding land could sink by about 80 mm (3.2 inches) relative to the sea in the next 20 years if the current rate holds steady.
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