Spatial coherences of flood-generating processes in Europe and their impact on flood statistics

Flood events in Europe are caused by different generating mechanisms that lead to events with different peaks, volumes and hydrographs. Understanding such mechanisms is crucial not only for deterministic or stochastic modelling of floods, but also for practical purposes such as hydrological planning and design estimation. In this study, driving mechanisms of floods are analysed and the associated catchment and atmospheric attributes controlling these flood types are identified through a classification and regression tree approach. In addition, the role of flood types in flood statistics is analysed using type-based flood statistics. It is shown which flood types dominate the more frequent floods and which flood types are most frequently associated with extreme floods. Ordinary and extraordinary floods are identified by a Likelihood-Ratio test and tested for a significant difference in the frequency distribution of flood types. Our results show that the flood types vary regionally in Europe. In the Alpine region, heavy rainfall floods are responsible for the most extreme flood events, while in the northern parts of Europe flood events caused by snowmelt lead to the largest peaks. This is reflected in the flood statistics in the type-specific distributions, which have a different tail heaviness. These findings provide information to identify the most crucial circumstances in which floods become extreme and on the flood event itself.