May 9, 2002 WASHINGTON - The Italian government recently decided to move forward with planning for the construction of underwater, mobile floodgates to mitigate flooding in Venice, situated on islands in a lagoon in the Adriatic Sea. The soundness of the plan is discussed by several scientists in the May 14 issue of Eos, published by the American Geophysical Union.
The approved plan to protect Venice, called MOSE (Modulo Sperimentale Elettromeccanico, or Experimental Electromechanical Module), involves the construction of 79 gates at three lagoon inlets. When waters rise 1.1 meters [43 inches] above "normal," air will be injected into the hollow gates, causing them to rise, blocking seawater from entering the lagoon and thereby preventing the flooding of Venice. The floodgates will take approximately eight years and $2.6 billion to construct.
Some critics of MOSE, such as Paolo Antonio Pirazzoli of the French Centre National de la Recherche Scientifique (CNRS), are skeptical as to whether the gates will actually prevent flooding. In his Eos article, Pirazzoli states that the design of the gates is based on outdated predictions of sea-level change, utilizing a scenario that differs by nearly 0.26 meters [10 inches] from recent estimates of rise in sea level over the next century, made by the Intergovernmental Panel on Climate Change (IPCC). Pirazzoli also asserts that the MOSE designers did not consider sea-level rise associated with land subsidence or increased water levels associated with extended rainy or windy periods.
Pirazzoli argues that once sea-level rise exceeds 0.31 meters [1 foot], possibly within the next 100 years, MOSE will become obsolete and will need to be replaced with watertight gates. Therefore, Pirazzoli contends, the Italian government should follow "soft" techniques, such as raising street level elevations, and await further assessment of sea-level rise to find "an updated, wise solution, more able to cope with foreseeable sea-level change."
In the same issue of Eos, MOSE supporters Rafael L. Bras, Donald R.F. Harleman, and Paola Rizzoli of the Massachusetts Institute of Technology, comment on Pirazzoli's view. The writers, who worked on the design and assessment of MOSE, state that the gates will indeed be effective barriers to flooding. They note that the sea-level rise scenario they utilized was based on recent research and that the floodgates are designed to prevent flooding in the event of a 0.3 to 0.5-meter [12 to 20 inch] rise in sea level. Furthermore, they say, it is not necessary to consider further land subsidence, because it was the result of groundwater removal that was ended in the 1970s, and it has not been a problem since then.
Bras and his colleagues note that as flooding occurs with greater frequency, steps will have to be taken to protect the city, and the cost of doing nothing may be greater than the cost of constructing the MOSE gates. They believe that, "the barriers, as designed, separate the lagoon from the sea in an effective, efficient and flexible way, considering present and foreseeable scenarios." With regard to Pirazzoli's contention that the mobile floodgates would eventually have to be replaced with watertight gates, they respond that if water levels continue to rise, the gates would just remain closed more often, in effect serving as "permanent" barriers.
Environmentalists argue, however, that keeping the gates closed for increasingly longer periods of time could be detrimental to the lagoon's ecosystem, which relies on exchange of waters between the lagoon and the Adriatic Sea to flush pollutants from the lagoon. Without this cleansing flow, they say, toxic substances may build up in lagoon waters, damaging its delicate ecosystem.
In order to understand how frequent closing of the gates would impact the lagoon's ecosystem, it is necessary to understand water flow patterns and exchange rates through the lagoon inlets. Miroslav Gacic and colleagues have taken preliminary steps in addressing these issues. Their research, published in the same edition of Eos, is based on a series of ship-borne surveys of water flowing through the inlet over an approximately forty-five day period.
Although the results are preliminary, the authors conclude that flow through the inlets is controlled primarily by tides. They also determine that the lagoon waters have an exchange rate of about one day, meaning that the lagoon is well-ventilated and quickly flushed. The researchers note that better assessments will be made when data representing several seasons become available.
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