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Study shows extreme floods should come as no surprise

by Trevor Loveday

A new European study has found that 95.5% of extreme flooding events across Europe could be anticipated to avoid surprises and save lives by looking at previous catastrophic floods in catchments with similar flood generation processes elsewhere on the continent.


According to the study report, in most instances of so-called megafloods – where flooding vastly exceeds any previously experienced in sites where they appear – there is a lack of data on them at most of those sites. That means flood defences and emergency response plans are often insufficient leading to significant damage and deaths when they occur.


A team of European scientists, including experts at the UK Centre for Ecology & Hydrology (UKCEH), has looked at the volume of water flowing per second through rivers at 8,000 gauging stations across Europe from 1810–2021 to identify historic megafloods.


The researchers found 95.5% of megafloods could have been anticipated based on previous events at locations elsewhere in the continent that are similar in their climates and variability in terms of how much water their rivers discharge in response to factors such as rainfall and temperature.


Co-author of the study, UKCEH principal hydrologist, Jamie Hannaford, said its findings –published in Nature Geoscience – “demonstrate that while the most extreme floods shock local communities, they are not usually surprising if we take a continental-scale viewpoint.”


He went on: “In the UK we already look beyond local catchments and consider events from other locations when assessing flood risk, but this is still limited to within our borders. A continental scale approach could give us additional information on our susceptibility to extreme floods. This would provide ‘worst case scenarios’ to help ensure appropriate flood defence measures and preparations can be implemented, thereby limiting damage when extreme floods do happen.”


He offers, as an example, the catastrophic flooding in July 2021 at the Rhine tributaries in Germany, and rivers in the Netherlands, Belgium and Luxembourg. In that event, flooding was up to four times greater than in any other event on record in the region. It took residents and authorities by surprise, and caused more than 200 deaths and damage worth $40bn. However, the new study showed that the discharge rate was similar to floods in northern Austria in 2002.


Examples in the UK include the 2009 floods in the Derwent catchment in Cumbria, where the discharge was 58% greater than the second largest event on record there, but not as extreme as flooding in similar catchments in Norway.


Hannaford and fellow UKCEH hydrologist Steve Turner were among the team of 56 researchers representing institutes in 29 countries, led by the Vienna University of Technology.


A study in 2019 by the same research team found that flood events are becoming increasingly severe in north-western Europe, with northern England and southern Scotland among the areas most affected, but decreasing in severity in southern and Eastern Europe.

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