Deadly floods of 2021—Researchers identify gaps in hazard mapping

The catastrophic flooding in July 2021 claimed the lives of more than 200 people in Germany and Belgium. The high death toll makes it clear that hazard mapping and risk communication must be improved urgently. Researchers at the University of Potsdam, the Helmholtz Centre for Geosciences, the University of Louvain and Johns Hopkins University analyzed the locations and specific circumstances of 224 deaths. Their findings show that many occurred outside the officially mapped flood hazard zones and that older people were particularly affected. The work is published in the journal Earth’s Future.

Heavy precipitation caused by the low-pressure system “Bernd” led to devastating floods in July 2021 across large parts of the German states of North Rhine-Westphalia and Rhineland-Palatinate, as well as the Walloon Region of Belgium. In these three regions, 224 people died in the flash floods, more than half of them in Ahrweiler county in Rhineland-Palatinate. To better understand the underlying causes, the researchers analyzed the individual circumstances of these documented deaths using official records, media reports and interviews with family members.

The results reveal that the areas flooded in 2021 differed significantly from the officially mapped flood hazard zones. Of the 224 locations where victims died or were found, about 50% in North Rhine-Westphalia and 75% in Rhineland-Palatinate were outside the mapped hazard zone for an extreme flood scenario.

“Consequently, the available flood hazard maps were not sufficient to adequately illustrate the risks to the public,” said lead author Annegret Thieken of the Institute of Environmental Science and Geography at the University of Potsdam. To improve their accuracy, it is imperative to take historical floods into account or estimate the worst-case flood scenario and display it on the maps.

In North Rhine-Westphalia, 14 people died while trying to reduce damage, check pumps or begin cleanup in basements. “This points toward deficiencies in communicating safe behavior during flooding,” Thieken said.

In the three regions studied, a total of 81 people died on the ground and upper floors of their homes. Many circumstances suggest they were caught by surprise by the floodwaters. These buildings should have been evacuated in time because staying outdoors in a flooded zone is even more dangerous. Deaths resulting from flooded or collapsed bridges, as well as vehicles that were swept away or stranded, provide evidence of this.

The authors recommend that risk and crisis communication clearly distinguish between damage-reducing and life-saving behaviors. “Warning messages must clearly and timely communicate when evacuation from flood-prone areas is still possible and when it is no longer advisable,” Thieken said. In the latter case, vertical self-evacuation, i.e., going upstairs, should be recommended.

The findings also reveal a lack of strategies to care for vulnerable groups. In all three areas, people over age 60 were significantly overrepresented among flood victims, accounting for 72%. In Rhineland-Palatinate, 12 people died in a flooded residential home for adults with mental disabilities.

“In general, the circumstances of many fatalities in July 2021 demonstrate that evacuation strategies need more attention, especially to better protect older people, people with mobility impairments or people with underlying health conditions,” Thieken said. “Therefore, we call for a revision of the European Floods Directive to better address worst-case scenarios in hazard mapping, risk management and communication.”

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