Venice continues 'downward dip'

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. Tectonic certainty Longer-term, Venice is always likely to have a problem, says Dr Bock's group. Large-scale geological processes are pushing the ground on which the city sits down and under Italy's Apennine Mountains. And although subsidence from groundwater pumping is no longer an issue for Venice, compaction of sediments under the built environment will remain a factor for some time, the scientists say. "Venice is always on the move," Dr Bock told BBC News." "It's been diminished as far as the annual rate is concerned, but there is a natural component of the subsidence that has to do with the tectonics, so no matter what they did by stopping the groundwater pumping, they're still going to have a natural subsidence on the order of about up to 2mm a year. "The figure is a little larger than many were expecting, so I think they may end up using these new barriers more often than they thought." The GPS/radar study is to be published next week in Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union (AGU).