Megafloods shouldn’t be that surprising

In the German-speaking region, the devastating flood in the German Ahr valley in the summer of 2021, with around 200 deaths, recently made headlines. Dealing with such events and the options for preparing for them – also in light of the increasingly virulent climate change with the trend increase in extreme weather events – have subsequently become increasingly a point of attention.

A ‘megaflood’ is a flood that is more than twice as large as all other floods in a region in the last 150 years, explains Günter, head of the Institute for Hydraulic Engineering and Technical Hydrology at the Vienna University of Technology (TU). out. BLöschl. Ultimately, these are events that locals generally consider impossible – until they happen. The Camp Flood of 2002 in Lower Austria is a typical example of this.

The team examined 510 massive floods
The research team led by BLöschl and lead study author Miriam Bertola has now systematically tackled the topic. The scientists from across Europe collected data from around 8,000 measuring stations across the continent from the 1810s to 2021 and incorporated it into their new analyses. The researchers identified at least 510 such massive floods.

Situation in Camp similar to that in the area in Romania
The basic message of the new work is: “Megafloods are not that surprising if you look beyond your own horizon and also look at other regions,” BLöschl emphasizes. However, it is important here not to “compare apples and oranges” – that is, to ensure that one draws conclusions from areas where similar conditions actually exist. There are many parallels with the situation in Kamp, for example in an area in Romania.

Sandy soils cause the water level to rise quickly
In the case of De Kamp, a flood can be five times as high if the amount of precipitation doubles. This is due to the sandy soils in the Waldviertel area, which can absorb a lot of water, but once saturated the water level rises very quickly. Blöschl: “That is not the case everywhere. In Vorarlberg, where twice as much rain falls, floods are only 50 percent greater.”

Historical floods useful for longer term planning
Based on the new dataset, you can now search for cases across Europe that you can learn from for similar regions. This makes it possible to estimate “what nature can do when things get extreme,” the hydrologist explains. In their work, the researchers talk about more than 95 percent of the mega-floods involving comparable, historic floods elsewhere in Europe. This information is less interesting for real-time flood forecasting than for longer-term planning.

A supra-regional vision would help hydrologists
The Ahr Valley, which is quite similar to the Camp area, could perhaps have learned something from the events of 2002 in Austria or similar incidents in Romania. But this requires a supra-regional view of things. But in many places this is not at all planned in terms of flood protection by the authorities and the administration, according to the hydrologist: “Thinking about floods in Romania or elsewhere abroad does not really come to mind.”

In the future we must therefore ‘orient ourselves on a larger scale’, which is becoming increasingly easier in times of ‘big data’. In addition, there is the important point of raising awareness – also in the minds of people, who must think about the possibility of such and even larger extreme events in order to adapt if necessary, BLöschl emphasized. On the technical side, it is about building a good database, maintaining it and maintaining it to improve structural measures, evacuation plans and the interaction between society and flood risks as a whole.