Image from the digital archive of the Aquagranda project:

The future of flooding in Venice: projections, predictions and protection

In order to plan the future defence infrastructures for Venice and other coastal cities, it is critical to make assessments of the impact of emissions on sea levels during this century, according to the authors of a new issue published in Natural Hazards and Earth System Sciences.

The study was coordinated by Università del Salento, the Institute of Marine Sciences of the National Research Council of Italy and Ca’ Foscari University of Venice. The researchers analysed historical and present-day data on Venice, a UNESCO World Heritage Site, to explore why the flood risk has increased in the recent past — climate change scenarios show that this risk might increase further and even accelerate during the 21st century.

Relative sea rise is the key factor to consider when examining the threat of flooding in Venice and coastal cities. Relative sea rise is the sea level’s change relative to the local solid Earth’s surface. 

Long-term projections show great uncertainty in the relative sea level of Venice: it is predicted that by 2100 the sea level will rise between 17 and 120 cm.

Davide Zanchettin, lead author of the article and professor at Ca’ Foscari University of Venice, attributes this uncertainty to the fact that projections are based on a wide range of emission scenarios, as well as on an inadequate understanding of the processes, both happening remotely and inside the Mediterranean and Adriatic sea, that contribute to changing sea level.   

“For a projection to be useful, it must be well constrained” Zanchettin says “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.”

“Small increases can have a large impact”

It is important to have accurate projections of the rise in sea level, especially for coastal cities like Venice, where the rise also increases the risk of extreme weather and consequently the risk of serious flooding.

These extreme events in Venice should not only be attributed to storm surges caused by sirocco winds, but also to a greater variety of factors, such as meteotsunamis and massive planetary atmospheric waves, which have caused and will continue to cause extreme events. 

“When you are this close to the upper limit of the tidal range, any meteorological event can be hazardous and cause an extreme flood,” says lead author Piero Lionello, one of the lead authors of the article and professor at Università del Salento.  “Small increases can have a large impact.”

In the short-term, Venice is protected from flooding mostly thanks to the effective and timely deployment of the new MoSE coastal defence system, which has been operated several times since October 2020 to protect the city. 

The MoSE project relies on being able to predict the water level 4-6 hours ahead of the peak. This allows for opening and closing the barriers, in order to protect the vulnerable low-lying city from flooding at some times, and provide access to the port and maintain the environmental balance of the lagoon at others.

“The MoSE system will be operated on the forecast,” says Georg Umgiesser, one of the lead authors of the article. “If the forecast is wrong, the operation of the MoSE becomes wrong—and that is very important both economically and ecologically.” 

Reducing the uncertainty that exists in any forecasting model is imperative. Umgiesser suggests this is particularly effective when combining multiple independent models to account for differences in the numerical models.

Giving governments time to plan

The wide ranging impacts of a high emissions scenario on the operation of the new MoSE system and flooding in Venice are made clear in this special issue, both in the short and longer term. In a plausible but unlikely scenario in which the sea level would rise quickly,  a year-round closure of the lagoon might be needed as early as 2075.

This extreme closure of the coastal defence system would have serious environmental and economic impacts on the city of Venice and the lagoon.

“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.” Lionello notes. “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 adaptation strategies. It’s clear that we need to be prepared to act.”