Do Changing Flood Generation Processes Promote the Occurrence of Flood Anomalies in Europe?

Abstract

River floods are generated by numerous processes that may change their relevance and intensity under a changing climate. Causative classification of river flood events can facilitate tracking temporal changes in the frequency of these processes across different regions. In this study we investigate decadal changes in the relevance of flood generation processes of maximum annual floods across 1415 European catchments for the period 1960-2010 and examine their relation to the occurrence of flood anomalies (i.e., flood-rich and flood-poor periods). We classified floods according to the nature of inducing precipitation events and their interaction with the antecedent catchment conditions. Flood anomalies were identified as coherent periods in time with unusually many (or few) flood events corresponding to threshold exceedances compared to the reference condition of independent and identically distributed random variables. Our analysis showed that flood generation processes are changing considerably in Europe: the portion of rainfall-driven floods occurring during dry conditions has increased in the Mediterranean and Central-Alpine regions, while rainfall-driven floods during wet conditions have become more frequent in the Atlantic and Northern regions. In all regions the number of snow-impacted events is decreasing. At the same time regional aggregation of detected flood anomalies indicates a clear switch between regionally prevailing anomalies: in the Northern and Mediterranean regions flood-rich periods in the beginning of time series switched to a flood-poor mode in later years, while in the Atlantic the switch occurred in the opposite direction. In this study we show that the compositions of flood generation processes during opposing flood anomalies have clear and contrasting differences in the four regions. Investigation of regional probability distributions of flood magnitudes of each flood group provides further insights into the relation between shifts in flood generation processes and flood anomalies indicating that future changes in flood generation processes due to changing climate might further enhance the persistence of regional flood anomalies in Europe.