Flood changes in Europe: from detection to attribution

Abstract

Floods are the most frequent natural disaster in Europe, and there is evidence of changes in their magnitude and frequency during the last decades. Previous studies typically analysed trends in mean annual flood discharges. However, changes in larger and less frequent floods (e.g., the 100-year flood) as well as their causes have not yet been assessed in a quantitative and consistent way. This contribution describes the journey from the detection to the attribution of flood changes in Europe though a collection of data-based studies. A unique pan-European database of annual maximum discharges is used for the analyses. This presentation will answer the following three questions: (i) are changes in small and big floods different? (ii) what are the main drivers of those changes? and (iii) do small and large floods have the same drivers of change? In the first part of this research, regional trends in flood quantiles are assessed across Europe as a function of their return period, using a non-stationary regional flood frequency approach. Results show dependency of flood trends on their return period in all regions except north-eastern Europe. In the second part, a data-based approach for the attribution of flood changes to atmospheric, catchment and river drivers at the catchment scale is developed and applied to a case study in Upper Austria, where flooding has become more intense during the last 50 years. Flood trends here are attributed to long-term changes in extreme precipitation. Finally, the attribution approach is extended to the regional scale and used to assess the contribution of climatic drivers to the observed trends in flood quantiles at the European scale. Findings show that extreme precipitation caused changes in both small and big floods in north-western Europe. Antecedent soil moisture is the main contributor to changes in the median flood in southern Europe, while the contribution of the two drivers to changes in larger floods are comparable. In eastern Europe, snowmelt drives changes in both the median and the 100-year flood. These results provide an improved understanding of decadal changes in flood magnitudes at the regional scale and are useful for informing flood management strategies.