Scientists question the value of flood prevention in Venice

The Experimental Electromechanical Module (EEM), approved by Silvio Berlusconi’s government in December last year, comprises 79 mobile flood barriers at the three inlets of the lagoon of Venice, and is designed to protect the city from storm surges. However, according to a paper in this week’s Eos journal, published by the American Geophysical Union, the barriers do not sufficiently account for the sea level rises predicted by the Intergovernmental Panel on Climate Change (IPCC) over the next century, or for local subsistence.

The 20-metre-long gates will lie on the floor of the lagoon until the water level threatens to rise to 0.87 metres above current mean sea level. They would then be raised by injecting compressed air, but would leave a narrow passage for the movement of water, and would oscillate with the waves.

According to the IPCC, the most likely sea rise estimate for the period between 1990 and 2100 is 0.48 metres. However, the designers of the EEM project have considered a scenario of a rise of only 0.22 metres by 2100, less than the total rise throughout the 20th century, says Paolo Antonio Pirazzoli of the National Centre for Scientific Research in France, and author of the Eos paper.

Pirazzoli cites the example of a storm surge in October 1976 that reached 0.9 metres, causing the tide to reach the flooding level of 1.24 metres above datum in Venice. This is not an exceptional level, he states.

Even if the system does work, it will cause a severe impact on the ecosystem within the lagoon, says Miroslav Gacic and his colleagues from the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, at Trieste, Italy. The ecosystem relies on the exchange of waters between the lagoon and the Adriatic Sea to flush pollutants from the lagoon.

However, not everyone agrees with these negative analyses. In response to Pirazzoli and Gacic, researchers from the Massachusetts of Technology (MIT) in the US and the Università di Padova in Italy, who worked on the design of the barriers, have defended the flood prevention scheme in a third paper in Eos. They note that the sea-level rise scenario that they used in the design were basedon recent research and that the floodgates are designed to prevent flooding in the event of a 0.3 to 0.5 metre rise in sea level. They add that it is not necessary to consider further land subsidence because it was the result of groundwater removal that was ended in the 1970s, and has not been a problem since.