On 8 December 2020, a massive storm surge slammed Venice, Italy. New flood control barriers, which were still in the testing phase, had protected the city against similar events in the preceding 6 weeks. This time, though, forecasters underestimated the magnitude of the surge by about 5 centimeters, so the floodgates remained open, with severe consequences. The surge reached 144 centimeters above the average tide level, flooding most of the city, covering iconic Saint Mark’s Square with a meter of water, and inundating its historic basilica.
The flooding clearly demonstrated that Venice lives on a knife’s edge, with sea level rise threatening its livelihood and perhaps its existence. The flood control system, MOSE (Modulo Sperimentale Elettromeccanico, or Experimental Electromechanical Module), is scheduled to reach full operational status by the end of 2021 and is designed to protect the city from most big events. MOSE provides a series of 78 gates across three inlets where tides enter to block especially high tides. Most of the gates are located on the Lido inlet, which is the closest to Venice and consists of two channels. When MOSE is activated, the floodgates fill with compressed air to bring them to the surface.
But it takes hours to fully engage the system, so its operators need accurate forecasts well in advance to decide how to proceed—especially since each decision can have serious economic consequences as well.
“The port wants to minimize closures to avoid interruption of ship passage and the maritime traffic,” explained a recent review in a special issue of Natural Hazards and Earth System Sciences. “In contrast, shop keepers want to make absolutely sure that high water will not flood their shops and restaurants and want to avoid missed closures because a forecast was too low.” Therefore, only a 10-centimeter forecast error matters—a 105-centimeter water level forecast, which will not activate MOSE, compared to an actual 115-centimeter measured water level could lead to flooding in Venice. But reliable forecasting is essential because MOSE “is the only intervention that will be able to effectively defend Venice from flooding due to storm surges in the near and medium future.”
Tides, Sirocco Winds, and a Sloshing Adriatic
Flooding isn’t new to Venice. The city is built on 118 islands, which have an average elevation of about 1 meter, so it doesn’t take much to create floods, known locally as acqua alta (high water).
Driven by rising sea level and local subsidence, though, flooding has become much more frequent and severe in recent decades. In the past 150 years, fewer than 30 floods have exceeded 140 centimeters above the normal tide level; five of them occurred in November and December of 2019 alone.
“The floods of Venice are compound events, [involving] a variety of combinations of astronomical, meteorological, and oceanic factors,” said Davide Zanchettin, an associate professor at Università Ca’ Foscari in Venice and lead author of a related study in the special issue.
Venice sits on the northwestern elbow of the Adriatic Sea, which separates Italy from Greece. Lunar and solar tides not only ebb and flow through the sea; they also create standing waves that slosh back and forth through the Adriatic’s relatively shallow waters, amplifying the tidal range. Low-pressure systems create strong winds, especially the sirocco, pushing the water toward Venice. Storm systems can bring heavy rains as well. All of those factors combined made Venice a poster child for the consequences of global climate change on 12 November 2019 when the water level reached 189 centimeters above normal—just 5 centimeters below the record high, set in 1966. “The sequence of floods in November 2019 is emblematic in that it shows that such diversity challenges the prediction of extreme sea levels,” said Zanchettin.
It’s Been a Long Time Coming for Accurate Forecasting Systems
Scientists have developed several storm surge forecast systems for Venice, some of which—including ones based on SHYFEM (Shallow water Hydrodynamic Finite Element Model)—have been in use and development for more than 30 years. SHYFEM forecasts are updated as frequently as hourly, and they trigger sirens when major tide events are imminent. As the 2019 and 2020 events show, though, they’re still not perfect.
“You need to incorporate a whole bunch of different disciplines into these kinds of forecasts,” said Stefan A. Talke, a researcher at California Polytechnic State University who studies storm surges and similar events but was not involved in any of the studies within the issue. “You need good meteorological data, good oceanographic data. You need a good idea of what the tides are doing, what the circulation patterns are, what the water temperature and salinity are like. And the bathymetry—the underwater topography of the system—can be tricky in shallow water. These are tough things to model.”
For Venice, however, many of those required measurements are tricky. It’s difficult to incorporate the outlines of the local coastline, for example—in both the lagoon and the Adriatic and beyond. The contours of the lagoon floor change as a result of tide events, dredging, and other human activities. Plus, many forecast models have relatively low resolution, which makes it difficult to predict small-scale changes in and around Venice.
With the local mean sea level in Venice forecast to climb by anywhere from 17 to 120 centimeters by 2100, however, forecasts have less margin for error.
“Storm forecasts are now more important than ever because of the MOSE barrier,” said Ivan Haigh, a coauthor of one of the review articles and an associate professor of coastal oceanography at the University of Southampton. “Also, with mean sea level rising and land subsiding, more frequent lower storm surge events now have the same impact as higher storm surge events in the past.”
Bulwarks Against Future Flooding
As conditions worsen, though, MOSE might have to deploy so frequently that commerce in the historic city is stifled. “With a rise of 50 centimeters, MOSE will be closed about 300–400 times [per year], basically, once a day,” said author Georg Umgiesser, an oceanographer at the Institute of Marine Science in Venice. “This will not be sustainable.”
In the meantime, to ensure that the system is deployed properly, one of the special issue’s papers recommends several actions to improve storm surge forecasts, including improving weather forecasting. This is “probably the most important action to improve water level forecasts,” the study said. Additionally, better incorporation of water level measurements across the region into forecasts and developing better ways to deal with uncertainty are also necessary.
“Only time will show if the scientific community is able to provide such a [forecast] system that allows Venice to continue to remain one of the most beautiful and visited cities in the world,” the paper says.
—Damond Benningfield, Science Writer
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