Venice’s flood barriers are working overtime. How will they change the lagoon?
Now in its fourth year of operation, the Venice system of mobile barriers has proven very effective in safeguarding Venice from acqua alta, the high tides in the lagoon that used to periodically flood the city. MOSE (Modulo Sperimentale Elettromeccanico) barriers, that are lifted from the seabed to stop water from the Adriatic Sea from entering the lagoon, have been raised 31 times since October. In the three previous autumn-to-summer cycles, they were used fewer than 20 times. With predicted a steady rise of the Adriatic Sea sea level, some experts are concerned that MOSE closures will become increasingly frequent, and that this may have an impact on the delicate lagoon ecosystem.
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Another potential problem, which emerged in a University of Padua study, is that the MOSE prevents the salt marshes from being flooded during high waters. This limits the amount of sediment that accumulates on marshes, compromising their ability to compensates for sea-level rise by growing vertically. “The loss of the salt marshes, that are an important habitat for many species, could have an impact on biodiversity” said Zaggia.
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Another problem, says Zaggia, is that the MOSE could change the circulation of fine sediments from the sea that are essential for the morphology of the lagoon. A local research project called Venezia2021 analysed satellite data and found that, in addition to sand made of larger grains, fine sea sediments are transported by the flood tide currents towards the lagoon during storms, especially with northeasterly winds. He notes that structures built more than a century ago at the port entrances are already preventing the entry of such sediments, and MOSE closures could make the problem worse. Zaggia suggests building a sediment bypass, essentially an undersea tunnel equipped with pumps that would drive sediments towards the lagoon, similar to the one built at the Tweed River in Queensland, Australia.
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A more frequent closure of the lagoon would bring about profound changes in its structure and function, transforming it into either a coastal lake or a network of smaller lakes, according to Georg Umgiesser, a CNR ISMAR researcher who has published an article on the topic. Umgiesser recommends studying other coastal lakes, intermittently closed-and-open lakes and lagoons along the warmer coastlines of the Mediterranean, South America, and Oceania. “Understanding the nuances of these environments will provide crucial insights into how a lagoon might function under altered conditions,” he says.