Compound flood potential in Europe

Abstract. The interaction between storm surges and inland runoff has been gaining increasing attention recently, as they have the potential to result in compound floods. In Europe, several flood events of this type have been recorded in the past century in Belgium, France, Ireland, Italy and United Kingdom. Here, we investigate the probability of joint occurrence of storm surges, precipitation, river discharges and waves. A coincidence of those factors have a potential to cause compound floods. We use several datasets covering most of Europe, including observations and data from the European Flood Awareness System (EFAS), ERA-Interim climate reanalysis and a regional climate model within the CORDEX framework, and carry out a statistical analysis based on copulas to assess the likelihood of joint occurrence. Further, we synthesize the joint probability of occurrence of extreme compound events, and their intensity, in the form of a composite index, thus identifying areas where compound floods could be of most concern. The results show considerable regional differences in dependency structure and the resulting joint probability of extreme surge, precipitation and river discharge events. In southern Europe the probability of joint occurrence of storm surge and precipitation is relatively high due to significant flash flood hazard. In northern Europe, along the main corridor of winter storms, dependency between surges and river discharges is higher than elsewhere, with large differences between west-facing and east-facing coasts. The occurrence of compound floods in most of the Nordic countries and along the Black Sea is very unlikely. The results allow the identification of areas at risk from compound flooding. Future studies that utilize statistical and physical methods are recommended to assess interactions between surges and inland runoff at a local scale.