Images like this one from the Matte district of Bern during the floods in July 2021 are once again topical in Switzerland: after heavy rainfall, water levels rise and sometimes cause major damage, such as in Valais or Misox. But what happens when previous limits are broken and records are shattered? As climate change progresses, these limits are shifting. Warmer air can absorb more water - and heavy precipitation increases in intensity as a result. What will happen to the Aare in Bern? What will happen to the Limmat in Zurich or the Rhone in Martigny? How much damage would be caused, how many people or workplaces would be threatened? 

Watercourses react very differently 

So far, no one has been able to say for sure, because “many factors play a role in the effects at specific locations. For example, the condition of the shore, the proximity to buildings or existing protective structures,” explains Andreas Zischg, Co-Head of the Mobiliar Lab. “These factors vary from location to location and each site must be considered individually”. 

The expected damage is therefore very different along a river and from watercourse to watercourse. This means that the same increase in discharge has completely different effects in the Berner Matte than on the Hinterrhein in Thusis. “At some locations, floods that only slightly exceed the previous maximum values can lead to much greater damage. At other locations, however, even significantly larger floods only cause very minor additional damage,” says Zischg. This information is essential for efficient prevention, “because only in this way can suitable measures be taken at the weak points at an early stage and the available resources be used optimally to protect the population,” explains Zischg. 

Tool opens up new possibilities 

Zischg and his colleagues close this information gap with their newly developed tool. “Our method provides a differentiated overview of the consequences of record floods in Switzerland,” says Zischg. 

The most important findings: Based on the largest flood observed to date, damage to buildings increases by more than 40 percent on average with an additional discharge of 10 percent. With an additional discharge of 20 percent, the damage rises sharply by 80 percent. 

Setting priorities in flood management 

The tool is primarily intended to better manage the consequences of climate change. “Priorities can now be set in flood risk management,” explains Andreas Zischg, ”using a uniform methodology throughout Switzerland.” Flood protection measures should now be implemented on the river sections that are classified as particularly at risk using the new instrument, says the researcher.