N. 6 - 28 mar 2012
International info a cura di Cecilia Migali
The water flowing through Venice's famous canals laps at buildings a little higher every year - and not only because of a rising sea level. Although previous studies had found that Venice has stabilized, new measurements indicate that the historic city continues to slowly sink, and even to tilt slightly to the east. The findings are published in 'Geochemistry, Geophysics, Geosystems', a journal of the American Geophysical Union.
"Venice appears to be continuing to subside, at a rate of about 2 millimeters a year," said Yehuda Bock, a research geodesist with Scripps Institution of Oceanography at the University of California, San Diego, in La Jolla, California, and the lead author of the new article on the city's downward drift. "It's a small effect, but it's important," he added. Given that sea level is rising in the Venetian lagoon, also at 2mm per year, the slight subsidence doubles the rate at which the heights of surrounding waters are increasing relative to the elevation of the city, he noted. In the next 20 years, if Venice and its immediate surroundings subsided steadily at the current rate, researchers would expect the land to sink up to 80 mm (3.2 inches) in that period of time, relative to the sea.
Bock worked with colleagues from the University of Miami in Florida and Italy's Tele-Rilevamento Europa, a company that measures ground deformation, to analyze data collected by Gps and space-borne radar (Insar) instruments regarding Venice and its lagoon. The Gps measurements provide absolute elevations, while the Insar data are used to calculate elevations relative to other points. By combining the two datasets from the decade between 2000 and 2010, Bock and his colleagues found that the city of Venice was subsiding on average of 1 to 2 mm a year (0.04 to 0.08 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 Shimon Wdowinski, associate research professor of Marine Geology and Geophysics at the University of Miami.
The issue of Venetian subsidence and what it means to the city is controversial, Bock said - especially as it relates to the effort to protect the city with gates. Pietro Teatini, a researcher with the University of Padova in Italy, said that it's important to monitor the ground movements, but in his opinion a subsidence of 1 mm per year is not a significant drop. Instead it is relatively stable for the area, he said. Venice subsided about 120 mm in the 20th century due to natural process and groundwater extraction, plus saw a sea level rise of about 110 mm at the same time.
Teatini and his colleagues are also investigating geoengineering solutions for the Venice, including an idea to inject saltwater into the aquifers deep below the city, reversing the subsidence caused by groundwater pumping. A paper by the research team outlining the proposals was published last December in Water Resources Research.