Venice is continuing to sink, albeit at a relatively slow rate of about
2mm a year, new satellite measurements show. The famous City of Water in north-east Italy experienced major
subsidence in the last century due to the constant extraction of water from
below ground. That was stopped and subsequent studies in the 2000s suggested
the decline had been arrested. But work by a US and Italian team indicates
Venice is still descending, even tilting to the east slightly. With waters
rising in the Venetian lagoon also by about 2mm (0.08in) a year, the combined
effect is a 4mm-a-year increase in sea level with respect to the land. The city
is already subjected to regular floods, which require citizens sometimes to
walk on raised boards. These floods, however, should be better constrained by a
new system of barriers set for completion in 2014. The value of the latest data
is in how it helps local authorities plan defences much further into the
future, says team-member Yehuda Bock from the Scripps Institution of
Oceanography in California. "It's critical information that they need to
take into account," he told BBC News.
Powerful duo
Dr Bock worked on the
Venice project with colleagues from the University of Miami in Florida and
Italy's Tele-Rilevamento Europa, a company that specialises in the measurement
of ground movement from space-borne sensors. The team used a combination of GPS
and satellite radar to map how Venice and its lagoon were shifting over time. Scientific
GPS receivers will provide very precise point measurements. These can then be
used to "anchor" the relative changes in height discernable across
the whole region from repeat radar snapshots. Dr Bock explained: "GPS
gives the absolute movement with respect to the Earth, which is important for
subsidence. "And the radar gives basically how points in an area move
relative to each other. Put the two together and you get thousands of points on
the ground that are tied into an absolute reference frame." The analysis
indicated that the city through the 2000s was subsiding on average by 1-2mm a
year, with some other lagoon locations dropping by up to 3-4mm per year. No-one
can really state how the trend will behave in the future, but if the current
rates of subsidence and sea level rise are maintained, the city can expect to
drop up to 80mm (3.15in) with respect to the average height of the water in the
lagoon over the next 20 years. The new inflatable gates should be able to
handle this, though.
strange research
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