Floods with compounding hazards increased almost threefold in 30 years

From the Emilia-Romagna floods of May 2023 - which struck after a prolonged drought - to storms that bring extreme winds and inundation, the reality of flood risk is rarely simple. Yet most risk models, insurance frameworks, and early warning systems still treat floods as standalone events.

A new international study led by JRC scientists, in collaboration with the Potsdam Institute for Climate Impact Research, Vrije Universiteit Amsterdam, and the University of Louvain, now found that more than 70% flood events in Europe are in fact compound events, that is they combine multiple hazards that contribute to societal or environmental risk. They involve at least one compounding hazard - such as drought, heatwave, or windstorm occurring before or alongside the flooding.

On average, losses from floods combined with other hazards were 2.8 times higher than those from floods alone over the study period. Every event in the top 1% of economic losses belonged to one of these compound event types.

The data reveals a worrying trend: floods are increasingly occurring alongside two or more other hazards. Between the 1980s and the 2010s, these compound events rose by 186% - compared to just 16% for floods occurring alone.

Compound event patterns across Europe at NUTS-3 level

Number of distinct hazard types recorded per region (0-6)

Most frequent hazard type from 1981 - 2020

Complexity as a risk factor

To build the first systematic, pan-European picture of its kind across Europe, researchers combined a database of historical flood impacts with meteorological and hydrological records, covering 1 349 flood events between 1981 and 2020. They examined five types of combined flood events: flood preceded by wet conditions, drought-to-flood transitions, heatwave-flood sequences, cold spell-floods, and windstorm-floods.

Using machine learning with explainable AI and a causal inference approach, the authors found that regions where floods occur alongside other hazards consistently suffer greater average losses - even when accounting for the size of the flood and local vulnerability. In already fragile regions, the effect is even stronger, suggesting that physical and social risks are reinforcing each other.

What this means for policy

Europe's disaster risk management frameworks are increasingly required to adopt multi-hazard approaches. The EU's Anticipate disaster resilience goal calls for better data and analytical tools to support risk-informed decision-making, while the upcoming second European Climate Risk Assessment (due 2028) has explicitly identified floods occurring alongside other hazards as a priority area for deeper analysis - a gap that studies such as this one aim to help address.

These findings have clear implications across several policy areas. For regional risk assessments and civil protection planning, incorporating compound hazard complexity would help identify the regions facing greater risk, enabling better-targeted preparedness and response.

For multi-hazard early warning systems, which the EU is actively developing, integrating information about combined hazards - not just flood severity - could significantly improve the ability to anticipate the most damaging events.

For the insurance industry and financial risk modelling, the systematic underestimation of compound flood risk points to the need to revisit loss models and pricing frameworks that treat hazards in isolation.

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