Floods and landslides in 2021: wet summer resulted in highest damage costs since 2007

The wet and stormy summer of 2021 had a clear impact on the costs of damage caused by floods and landslides. For the first time since 2007, these costs rose significantly above CHF 300 million, the inflation-adjusted average for the years 1972 to 2020, ranking ninth highest in the 50-year observation series. By far the largest share of damage costs (97%) resulted from flooding  due to overflowing bodies of water and overland flow in June and July. Flooding also caused one death, in the canton of Basel-Landschaft.

Summer 2021 saw an unusually high number of storms occurring in rapid succession. Between Lake Geneva and Lake Constance, only a few places escaped being hit by one of the many thunderstorms that swept through in June and July. As such, the large number of widespread damage events was the primary reason for the above-average costs, not individual and localised severe events (which was the case in other years that saw similarly high costs).

In 2021, the greatest damage caused by thunderstorm-related flooding occurred in Cressier (canton of Neuchâtel) on 22 June, in the Bernese Jura and the districts of Waldenburg and Sissach in Basel-Landschaft on 23 June, in Vordemwald in Aargau on 24 June and in the district of March in Schwyz on 25 July. From mid-July onwards, constant rainfall caused many lakes and rivers to rise above the high-water marks, resulting in considerable damage, especially in the communities bordering the lakes at the feet of the Jura. In Ticino, thunderstorms and persistent rain led to flooding as streams burst their banks, especially in late July and early August. The mountain cantons of Valais and Grisons mostly escaped such damage in 2021. WSL analyses do not take into account hail and storm damage, which was substantial in some regions in 2021.