July 20: Climate change is expected to make extreme floods in the Alps more frequent and intense by the end of the century, with floods currently considered “once in 100 year” events potentially occurring every 45 to 80 years according to a new study by the WSL Institute for Snow and Avalanche Research (SLF).
The study commissioned by the Swiss Federal Office for the Environment (FOEN), used hourly discharge data from 384 rivers in Switzerland and Austria to model future flood risks. Researchers found that previous projections which relied on daily data, underestimated the impact of short duration heavy rainfall.
Unlike earlier studies that suggested flooding could decline due to reduced snowmelt, the new analysis found that increasingly intense rainfall driven by global warming outweighs the decline in snowmelt, leading to greater flood risks across the Alpine region.
Lead researcher Paul Astagneau said the team was surprised by the findings noting that hourly data revealed much larger changes than earlier daily simulations.
The study found that for every 1°C rise in global temperature hourly heavy rainfall intensifies by about 7 per cent, significantly increasing the likelihood of flash floods and extreme river discharge.
Researchers projected that future 100 year floods could become 5 to 15 per cent more intense with higher peak river flows than those experienced today. These extreme events are also expected to occur much more frequently, potentially once every 45 to 80 years instead of once a century.
The study explained that flooding in the Alps is driven by a combination of intense summer thunderstorms, prolonged rainfall over saturated ground, and rainfall combined with snowmelt. As climate change increases the intensity of short duration rainfall, these factors are expected to amplify flood risks.
While moderate floods, such as those expected once every two years, may still decline in some mountainous regions because of reduced snowmelt, the decrease is projected to be smaller than previously estimated.
The researchers concluded that extreme flood events will experience the greatest changes underscoring the need for updated flood risk assessments, infrastructure planning and climate adaptation strategies in Alpine regions.