Flood events cause major damage due to inundation. They can often endanger life and damage or destroy buildings. Foundations can be undermined by bank erosion and degradation. Debris (stones and sludge) and driftwood conveyed by the flood can damage agricultural land and buildings. On the other hand, floods also perform ecological functions, for example by regularly inundating floodplains and modifying channel morphology. Both the frequency and magnitude of flood events are closely tied to climatic changes (e.g. natural fluctuations in atmospheric circulation or warming caused by human activity).
An evaluation of the annual peak discharge data from 1930 to 2017 shows that the frequency of flooding has been rising since the mid-1970s, with an above-average number of flood events observed during the past 30 years. The three most severe flood events since 1930 all occurred between 1999 and 2017. By contrast, there were only very few flood events between 1960 and 1975. Over the past 500 years, climatic changes (such as fluctuations in atmospheric circulation) have caused repeated alternation between phases with many and few flood events. It is not yet possible to conclusively determine whether the increase in the number and magnitude of flood events observed since the 1970s is already a consequence of human-induced climate warming, and in view of this the indicator is not evaluated.
There is no absolute protection against extreme events. The most efficient way to avoid flood damage is to adapt land use in order to prevent development in areas that are susceptible to flooding. Where this is not possible, structural or organisational measures have to be taken in order to lessen the risk or reduce the potential for damage.
The “CLIM17 – River floods” indicator of the European Environment Agency presents the sum of flood events over the period from 1998 to 2008 for various European river basins. While the Swiss indicator shows the development of flood frequency over time, the European indicator shows the spatial differences.
The peak discharges (highest discharge in a given year) are measured at each of the 64 permanent Federal Office for the Environment discharge measurement stations for each year and each station. An annual peak discharge is defined as a flood event if its flow rate is greater than the HQ10 flow calculated for the station (HQ10 flow = discharge that is exceeded statistically every 10 years). The indicator shows the total of all flood events per annum at the 64 stations.