Understanding and forecasting floods

An unusually large number of thunderstorms in quick succession in June and July 2021 led to floods with damage costs of over 435 million Swiss francs – the costliest storm since 2007. Back then, property damage amounted to around 700 million Swiss francs. Overall, the highest costs recorded in the Swiss flood and landslide damage database - around three billion Swiss francs - was also caused by a flood (in August 2005): floods are the natural hazard that causes the highest costs in Switzerland.

Such damages are reduced by avoiding construction in at-risk areas and providing more open space around waterways. If these measures are not sufficient, new protective structures are needed. All this requires hazard maps, for which our research provides the data and basic methodology.

For example, we have been studying steep mountain streams for more than fifty years. Using measuring systems which we developed, we can determine water flows, bedload volumes and snowpack in Alptal (Schwyz). No other mountain torrent in the world has been surveyed as thoroughly as this one.

Additionally, we develop computer models and alarm systems for flood warning. For this, we analyse current events and use our long time series of measurements. Since September 2008, we have been operating a flood early warning system for the city of Zurich and the Sihl valley. It improves safety for the city and Zurich's main railway station.

Snowmelt can play a significant role in causing flooding. At the WSL Institute for Snow and Avalanche Research SLF, we run an operational snow-hydrological service (OSHD). This analyses where and how much snow there is in Switzerland, and how much of it will melt. This enables lowering water levels in reservoirs in good time to prevent flooding.

How will extreme floods alter with climate change? This question is being investigated by a new joint professorship of WSL and ETHZ in Davos.