Megafloods, or flood events more severe than any previously recorded in a given place, can be predicted by analyzing past flooding in other similar regions of the same continent, according to a study published this week in Nature Geoscience.
This type of extreme flooding is not uncommon – the study identified 510 such floods from 1999 to 2021 in Europe alone – and towns and cities are often not prepared for the catastrophic impacts. In July 2021, a megaflood among rivers in Germany, Netherlands, Belgium and Luxembourg caused 200 fatalities and more than $40 billion in damage.
Researchers tried to determine if certain megafloods in Europe could have been predicted in advance by studying other previous flood events on the continent in regions with similar hydroclimates (geographic climate and water patterns). By analyzing river discharge data, they found that 95.5% of the observed megafloods fell within the expected peak flood boundaries of similar regions.
“In almost all the cases, it was possible to anticipate the order of magnitude of these megafloods using data from other similar places on the continent,” said Miriam Bertola, a hydrological researcher at the Technical University of Vienna and lead author of the study.
The results reveal the importance of communication between countries when it comes to preparing for and forecasting flooding. According to Bertola, it’s rare for national data and studies to travel across national borders in Europe due to regulations.
“All of these megafloods, based on local data, they look really surprising. This study really shows that we need to start sharing data and knowledge with our neighbors, from other countries,” she said.
As the planet warms, the need to anticipate flooding is more acute than ever. Hotter temperatures cause greater evaporation from lakes and oceans, which falls down as heavy precipitation. Depending on where it falls, river floods can become larger and more frequent in some places and smaller and less frequent in others.
Because Europe falls under the westerlies – prevailing winds that blow from west to east – it is more heavily influenced by the moistening atmospheric conditions of the northern Atlantic, according to Fred Hattermann, vice head of the research department of climate resilience at the Potsdam Institute for Climate Impact Research. Hattermann was not involved in the study.
Due to these conditions, “one can assume floods and megafloods in Europe should increase in number and intensity,” he said.
More needs to be done to understand the reality of floods on a warmer planet, according to Hattermann, as looking at past flooding events has limitations.
“In a changing climate, I don’t think it is enough to only look at historical data,” he said, adding that it could be useful to feed in future climate forecasts to more accurately predict megaflood scenarios.
Still, the forecasting tools used in the study hold promise for preparing communities for these massive floods, since the element of surprise can be the most dangerous aspect of a disaster.
Officials could use the methodology to help implement a warning system, which could save lives and critical infrastructure, Hattermann said. “If you are in Poland or in Slovakia or in Scandinavia, you could have a look at the estimated worst case in your area – you could at least expect it could happen.”