Study: Century Floods in the Alpine Region Will Occur Every 45 to 80 Years in the Future

Data Basis: Hourly Values Instead of Daily Averages

The study identifies a warmer atmosphere as the main driver. When the atmosphere warms, it can absorb more moisture, leading to more extreme heavy rainfall. According to the calculations, the intensity of hourly heavy rainfall increases by around seven percent for each additional degree Celsius.

At the same time, this effect overcompensates for the decreasing snowmelt at higher elevations. "The researchers cited climate change-related, more intense heavy precipitation as the reason for the increase in extreme events. These overcompensated for the decreasing snowmelt."

HQ100 Loses Its Significance as a Planning Metric

The practical implications are substantial. According to the study, the peak discharge of a hundred-year Alpine flood could be around five to 15 percent higher in the future. Instead of occurring on average once per century, such an event should statistically be expected every 45 to 80 years, based on the Austrian data.

In Switzerland, the HQ100 serves as the central reference value for flood protection and spatial planning. "Based on historical data, terrain models, and precipitation amounts, the cantons, in collaboration with the Federal Office for the Environment (FOEN), produce so-called hazard maps." These statistics also form the basis for building codes and protective structures along waterways.

Current Events Confirm the Trend

The research casts new light on a region long considered a special case in the scientific community: in the Alps, climate change had seemed to tend to dampen flood risk, since glaciers and snowfields act as natural buffers. This picture must now be revised, the authors emphasize.

Concrete evidence is provided by current events. At the end of June 2024, rivers in Valais and Ticino swelled within a short time to levels that statistically occur only every 100 to 300 years. In September 2024, days of rainfall caused severe damage in parts of Central Europe, including Austria.

Internationally, the World Weather Attribution initiative classifies the triggering heavy rainfall of the Central European flood of September 2024 as more intense and more likely due to climate change. In their annual review, the researchers found a corresponding climate change fingerprint in 15 of 16 flood events examined worldwide – the exception being the floods in Emilia-Romagna in 2023.

Consequences for Hazard Maps and Spatial Planning

The trend is expected to become apparent from around the middle of the century. The term "century flood" is currently losing its reliability due to climate change. Moderate floods, such as the two-year event, are still slightly declining in the mountains, albeit more weakly than previously assumed.

The Swiss Federal Institute for Forest, Snow and Landscape Research WSL announced the results on Thursday. The Federal Office for the Environment had commissioned the new projections from Astagneau. "Therefore, authorities must regularly revise these statistics and hazard maps upward."

The study carries the digital object identifier (DOI) 10.1126/sciadv.aed6012 and is accessible via https://dx.doi.org/10.1126/sciadv.aed6012. It paints a picture in which the Alps as a water world are coming under increasing pressure – with direct consequences for settlement planning, insurance, and emergency protection throughout the region.

The researchers emphasize that the results concern not only the frequency but also the peak discharges themselves. This intensifies the burden on protective infrastructure that was previously designed for rare exceptional events. Municipalities and cantons face the task of reviewing their planning foundations at shorter intervals than before.