The frequency of floods in Venice is increasing, Bock said, and now about four or five times a year residents have to walk on wooden planks to stay above the floodwaters in large parts of the city. A multi-billion-dollar effort to install flood-protection walls that can be raised to block incoming tides is nearing completion, he said. The adjustable 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.
To ensure that the gates can hold back sufficient tidal water in the long run, their builders "have to take into account not only the rising of sea level, but also the subsidence," Wdowinski said. The land on which those gates are being built is descending and taking the barriers down with it relative to the rising seas, he said, effectively doubling the amount of elevation change in store for Venice.
The patchy land in the surrounding lagoon could need shoring up as well. Over the next 40 years, the natural barriers that protect the Venice lagoon and city could drop by 150-200 mm, Wdowinski said, so officials may need to reinforce those sinking sediments in some way over the coming years.
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.
"One millimeter is nothing with r
|Contact: Kate Ramsayer|
American Geophysical Union