Rising sea levels pose uncertain challenges for Venetian flood defences

The assessment was published in a special issue of Natural Hazards and Earth Systems Sciences exploring why the flood risk in Venice has recently increased. Risk is expected to continue growing, perhaps even accelerating, through the 21st century.

Among the key factors when considering flood risk in coastal cities such as Venice is projected relative sea level rise; the change in sea level relative to the local solid surface of Earth. This accounts for ground sinking and mean sea level rises. Long-term future projections indicate significant uncertainty in the relative sea level of Venice: between 17 and 120cm by 2100. Study author Professor Davide Zanchettin attributes this to the challenges of projections based on a wide range of possible emissions scenarios and to uncertainty about the relevant physical processes.

“For a projection to be useful, it must be well constrained,” he said. “There are important feedbacks in the climate system, for instance related to polar ice sheet dynamics, that we need to understand and better simulate to make more reliable projections.”

Making accurate projections is more important than ever, given the combination of sea level rises and climate change-driven extreme weather events, which present a serious threat for coastal cities. While the extreme weather events that have caused flooding in Venice have thus far been attributed to storm surges caused by sirocco winds, a greater variety of factors will be attributed in future extreme weather, such as meteotsunamis (tsunami-like waves caused by weather conditions) and huge planetary atmospheric waves.

“When you are this close to the upper limit of the tidal range, any meteorological event can be hazardous and cause an extreme flood,” explained lead author Professor Piero Lionello, “Small increases can have a large impact.”

In the short-term, Venice’s flood defences rely on the timely deployment of the new Modulo Sperimentale Elettromeccanico (MOSE) coastal defence system; this has been used several times to protect the city since its first deployment in October 2020. The MOSE, which consists of mobile gates closed at inlets to isolate the entire Venetian Lagoon from the Adriatic Sea, relies on being able to predict water levels four to six hours ahead of their highest point.

The authors emphasise the importance of reducing uncertainty in flooding models – such as by combining independent models – to make effective use of MOSE and protect the city. Dr Georg Umgiesser said: “The MOSE system will be operated on the forecast. If the forecast is wrong, the operation of the MOSE becomes wrong, and that is very important both economically and ecologically.”

The study found that there is a plausible possibility that a year-round closure of the lagoon may be required as early as 2075 to keep up with rising relative sea levels. This would have grave environmental and economic impacts for Venice.

“Sea level is a nasty beast. We could stop global warming completely by stopping the use of fossil fuels and the sea level would continue to rise in spite of this, though at a much-reduced pace,” said Lionello. “But we have the information with studies like these to identify the future risk to coastal cities like Venice.

“Although we don’t know exactly when, the present evidence is that we will need to change our adaption strategies. It’s clear that we need to be prepared to act.”