Severe flood and landslide damage and eight deaths in 2017

Flooding, landslides, debris flows, rockfall and rock avalanches in Switzerland in 2017 caused damage totalling around CHF 170 million. This is the highest amount since 2007. Overland flow caused particularly serious damage to buildings and a rock avalanche claimed eight lives. These were the findings of the annual evaluation of the Swiss flood and landslide damage database kept by the Federal Institute for Forest, Snow and Landscape Research (WSL).  

Natural hazard events occurring in 2017 claimed eight lives and caused damage to property totalling around CHF 170 million. This is the highest amount since 2007 (CHF 700 million), but still significantly less than the inflation-adjusted average of CHF 307 million for the years 1972-2016. Roughly 94% of the estimated damage was caused by flooding or debris flows, while landslides accounted for 2% and rockfall for nearly 4%. Most damage (about 66%) was caused by thunderstorms and other heavy precipitation, and more than 5% was caused by persistent rain. In 29% of cases, damage was caused by other or unknown factors.

The greatest damage in 2017 was caused by two serious events: flooding in the Zofingen area (Canton Aargau) in July and the rock avalanche in Val Bondasca, followed by debris flows in Bondo (Canton Grisons) in the Val Bregaglia in August.

Flooding in the Zofingen area

On 8 July, mainly overland flow (not channelized), but also overflowing streams and landslides in the Canton of Aargau caused serious devastation. The urban part of Zofingen flooded very rapidly indeed. The rainstorm left hundreds of businesses, basements, underground car parks and underpasses under water, and the station car park containing more than 100 vehicles was also affected. In Uerkheim the Uerke overflowed its banks and flooded the valley floor, causing damage to numerous residential buildings, shops and businesses. In nearby Bottenwil and Oftringen, extensive damage was caused to buildings.

High death toll after a rock avalanche

On 23 August 2017, around 3.1 million m3 of rock crashed down from Pizzo Cengalo into Val Bondasca (Canton Grisons). This rock avalanche buried eight hikers on their way from the Sciora Hut into the valley. Despite intensive search efforts, their bodies could not be recovered. The rock avalanches following the mudslides in Val Bondasca filled the retention basin and damaged numerous buildings and much of the infrastructure in Bondo in the municipality of Bregaglia (Canton Grisons). The total volume of debris flowing down to Bondo totalled around 500,000 cubic metres. Thankfully, there were no deaths or injuries in the residential area.

Further damage mainly caused by thunderstorms

After heavy rainfall on the evening of 29 May, bedload volumes blocked the passage of a stream in St. Moritz (Canton Grisons), which subsequently overflowed its banks and flooded offices and storage facilities on an industrial estate.

In eastern Switzerland, especially in the St. Gallen Rhine Valley, flooding that occurred on 2 September caused extensive damage to many buildings in the municipalities of Widnau, Au, Balgach, Altstätten and St. Margrethen.