How often will the 100-year flood occur in the future?
Heavy rainfall becomes more intense with every degree the Earth warms. This has an impact on flooding. Using hourly data from 384 rivers in the Alpine region, SLF researchers have modeled how flooding will change by the end of the century. The forecasts: significantly bleaker than previous ones.
Spring and summer bring the snowmelt season—which causes mountain streams and rivers to swell, sometimes to the point of overflowing. The Hydrology and Climate Impacts in Mountain Regions group at the WSL Institute for Snow and Avalanche Research (SLF) focuses on this topic.
Previous studies have shown that flooding in the Alpine region could decline or remain stable by the end of the century due to lower amounts of snow melting in the spring. These projections are based on daily Model input data (see box), which, however, do not adequately account for short, intense rainfall events. For this reason, the Federal Office for the Environment (FOEN) commissioned SLF hydrologist Paul Astagneau to develop new forecasts. The researcher had access to data from 384 rivers in Switzerland and Austria, whose hourly The discharge was measured. Using a hydrological model, Astagneau simulated flood events from today through the end of the century—and was astonished by the results: «We didn’t expect such large discrepancies compared to the daily data.»
Daily vs. Hourly Data
Previous studies on flooding and climate change simulated how much water passes through a specific point in a stream over the course of an entire day. These data do not adequately capture short, intense downpours—they are «spread out» over the course of the day and are therefore no longer visible. SLF hydrologist Paul Astagneau therefore calculated the amount of water passing through a specific point per hour. In other words, instead of using one data point, he used 24 data points per day.
Extremes are changing more dramatically
Flooding in the Alps is caused by summer thunderstorms, prolonged precipitation on saturated soil, or a combination of rain and snowmelt. Due to climate change, the intensity of precipitation is increasing—particularly on timescales of less than one day. More specifically: For every additional degree Celsius, hourly heavy precipitation increases by about seven percent. As a result, the rain more than compensates for the decrease in snowmelt, as the analysis of hourly simulations shows: With climate change, flooding is more likely to increase than decrease even in snow-fed Alpine streams. In addition, floods will occur more frequently, and this will already be the case by the middle of the century. The 100-year flood—which occurs on average once every 100 years—will become five to fifteen percent more intense in the Alps. This refers to the peak discharge, i.e., the highest water level expected during this flood event. At the same time, the 100-year flood could occur more frequently in the future—every 45 to 80 years. For moderate floods, such as the biennial flood, a decline is still evident in the mountains, albeit a smaller one than previously assumed. Paul Astagneau summarizes: «Extreme events are changing more significantly than moderate floods.»
