Mega floods: Can we be better prepared for the next big one?

Study in Nature Geoscience says we can learn from looking back… internationally.

In July 2021, a devastating series of floods swept through Europe, claiming the lives of over 200 individuals. The aftermath of this natural disaster resulted in widespread devastation in Germany and Belgium, and also closer to home in Luxembourg. The impact was particularly severe in western Germany, where the flooding wreaked havoc, resulting in 49 fatalities and causing over €30 billion in damages (source: The Guardian).

Described as mega floods, these natural disasters, despite their apparent regularity over the past few years, catch entire populations off guard, resulting from a lack of preparedness. “In regions highly exposed to flooding, determining the worst-case scenario is thus crucial for designing effective protective measures,” says Laurent Pfister, Head of the Catchment and Eco-hydrology group at the Luxembourg Institute of Science and Technology (LIST). Traditionally, this determination is based on local historical records, using the most severe flood events of past decades or centuries as a benchmark for future expectations. “The conventional methods that rely solely on local data, usually confined within national borders, has until now struggled to forecast these extreme events accurately due to limited information,” he adds.

Now, a comprehensive research initiative, led by the Technical University of Vienna, and to which Pfister has contributed, has unveiled a significant revelation: while mega floods may take citizens and stakeholders by surprise at a local scale, they have not changed much in time relative to their variability at the scale of the entire European continent.

Gaining insights from cross-border flood history for better local assessments

The research, published earlier this month in Nature Geoscience, seeks to understand the triggers and characteristics of these floods, crucial for developing effective flood protection measures. “The conventional challenge in flood prediction also lies in the scarcity of long-term data to rank these events accurately,” adds Pfister. The study distinguishes itself in that in it the researchers collected and examined data from over 8,000 gauging stations, including Luxembourg, spanning the years 1810 to 2021 across Europe.

Pfister explains, “Unlike conventional analyses limited by shorter time spans, this study has encompassed data sets covering centuries, and has also proposed a shift from single-station analysis to a Europe-wide approach, capitalizing on the shared behaviour of rivers in specific categories.”

Going by the study, we can anticipate mega floods in one region by leveraging data from similar river basins elsewhere on the continent. The approach involves learning from areas with comparable climatic and hydrological characteristics, where mega-floods may have already occurred.

“Our goal was to categorize rivers based on similarities in behaviour, climate, land use, and geology. By grouping them, we aim to extrapolate insights from historical flood records of similar-behaving rivers to estimate the frequency and size of mega floods in specific categories,” Pfister added.

The research team also demonstrated in the article that flood disasters, such as the one in the Rhine basin in 2021, could have been anticipated using this method. In fact, it comfortably falls within the anticipated range when continental data is taken into account.

The findings stress the importance of adopting an international perspective for more effective flood prevention and emphasizes the need to go beyond assessing only geographically adjacent areas. “This shift encourages a move beyond national flood-risk assessments and shows the value of information sharing on mega floods across countries and continents. This can ultimately reduce the surprise factor and potentially save lives.”

What lies ahead

Addressing a question about the study's relevance in the context of climate change, Pfister clarifies that more than a predictive model the study is a retrospective analysis, and it does not consider future scenarios or changes in climate variables. Instead, it leverages historical data to assist stakeholders in assessing flood risks for planning flood protection measures.

He also emphasizes that the study is groundbreaking in Europe, with similar investigations conducted in Northern America. As for future plans at LIST, the focus shifts from process-focused research towards making use of the accumulated knowledge of river systems gained over the past 30 years in the Alzette and Sûre River basins in Luxembourg. The aim is to project how our national rivers might respond to future changes, considering factors like climate change and land use alterations. This involves combining climate models and hydrological models to make more accurate projections, aiding stakeholders in making informed decisions regarding infrastructure protection.

Looking even further ahead, Pfister discussed plans to delve into the past using natural archives such as freshwater mussels and tree rings. “By analyzing growth bands in these biological recorders, we hope to gain insights into the hydrological history of rivers over thousands of years, ultimately improving our capabilities of understanding and anticipating potential future changes.”