Aerial shot of Burano, Venetian Lagoon. Credit: Abstract Aerial Art/ DigitalVision/ Getty Images.
One day, the Venice lagoon may look very different from now. Its canals, shallows and salt marshes – island formations periodically submerged by the tides – could change because of the Mo.S.E. (Experimental Electromechanical Module), a flood-protection system which started operating in late 2020.
Mo.S.E. is a barrier made of 78 hollow steel gates, each 30 metres wide, that lie flat on submerged concrete foundations. If tides are expected to exceed 110cm, the gates are filled with compressed air and rise to block the flood waters. According to a study published in Science Advances, the Mo.S.E. has significant consequences for hydrodynamics and sediment transport within the lagoon1.
The study was done by researchers at the University of Padua and University of Ca 'Foscari of Venice. “In the last 100 years, we have already lost about 75% of the lagoon salt marshes, and the Mo.S.E. will certainly not help to preserve the remaining ones,” says Alvise Finotello, a researcher in the Department of Environmental Sciences at Ca’ Foscari University of Venice and first author on the study.
By integrating modelling tools and field measurements, the researchers analysed the effects of the mobile gate system on the morphological evolution of the Venice lagoon. They compared the model data, which reproduced the height of the tide and subsequent transport of sediments, with data from 15 closures of the Mo.S.E. between October 2020 and January 2021. The model data were also compared with depth data collected in 1970 and 2012, and data on salt marshes collected from 2018 to 2021.
“Although the Mo.S.E. closures have so far been reduced in number, their morphological effects are already significant,” says Davide Tognin, a researcher in the Department of Civil, Environmental and Architectural Engineering at University of Padua.
The reduction in tide levels alters the balance between tidal currents and wind waves, increasing erosion of sediments from the lagoon shallows. The sediment, transported by water, settles in the canals, but not over the marshes as would normally happen. As a result, the ability of the marshes to withstand a rise in sea-level is undermined.
The authors say these morphological changes would have occurred with any other engineering solution that closed the lagoon. “We are by no means advocating against the use of the Mo.S.E.”, says Tognini. “Closing the inlets during storm-surge events is key to avoid flooding the city, but our results emphasize the need to find a trade-off between the needs of flood mitigation and the conservation of the lagoon habitat.” The authors say complementary measures to mitigate the negative effects of barrier closures must be considered soon, as the Mo.S.E. closures are expected to become both more frequent and longer as sea levels rise.
“This is a very good work that brings together field work and simulations. The mathematical result is sophisticated, the conclusions are convincing,” says Andrea Rinaldo, professor of hydrology at the Swiss Federal Institute for Technology (EPFL) in Lausanne, who was not involved in the study.
