Flood type classification and assessment of their past changes across Europe

Abstract. This study was carried out to establish the characteristics of observed flood events across Europe in the past in terms of their spatial extent and the processes leading up to the events. Daily discharge data from more than 745 stations of the Global Runoff Data Centre were used to identify peak flows at each station for the period 1961-2010. The identified events at the different stations were further analysed to determine whether they form the same flood event, thereby delineating the spatial extent of the flood events. A pan-European hydrological model was employed to estimate a set of catchment hydrological and hydro-meteorological state variables that are relevant in the flood generation process for each of the identified spatially delineated flood events. A subsequent clustering of the events based on the simulated state variables was used to identify the flood generation mechanism of each flood event. Four general flood generation mechanisms were identified: long-rain flood, short-rain flood, snowmelt flood, and rain-on-snow flood. A trend analysis was performed to investigate how the frequency of each of the flood types has changed in time over the investigation period. In order to investigate whether there is a regional and seasonal pattern in the dominant flood generating mechanisms, this analysis was performed separately for winter and summer seasons and five different regions of Europe: Northern, Western, Eastern, Southern Europe, and the Alps. Continentally, the total number of flood events didn’t show a significant change. However, the frequency of winter long rain events increased significantly while that of summer rain-on-snow events decreased significantly over the investigation period. Regional differences were detected in the dominant flood generating mechanism and the corresponding trends. In Western Europe, the frequency of both winter and summer rainfall events increased significantly. In Northern and Eastern Europe, the frequency of summer rain-on-snow events decreased significantly. In addition, winter short rainfall events increased significantly in Eastern Europe. In the Alps, the frequency of summer short rain events increased significantly.