The co-incidence of storm surges and extreme discharges within the Rhine-Meuse Delta

The Netherlands is a low-lying coastal area and therefore threatened by both extreme river discharges from the Meuse and Rhine rivers and storm surges along the North Sea coastline. To date, in most flood risk analyses these two hazardous phenomena are considered independent. However, if there were a dependence between high sea water levels and extreme discharges this might result in higher design water levels, which might consequently have implications for flood protection policy in the Netherlands. In this study we explore the relation between high sea water levels at Hoek van Holland and high river discharges at Lobith. Different from previous studies, we use physical models forced by the same atmospheric forcing leading to concomitant and consistent time series of storm surge conditions and river discharge. These time series were generated for present day conditions as well as future climate projections and analysed for dependence within the upper tails of their distribution. In this study, dependence between the discharge at Lobith and storm surge at Hoek van Holland was found, and the dependence was highest for a lag of six days between the two processes. As no significant dependence of the threats was found for cases without time lag, there is no need for considering dependence in flood protection and policy making. Although future climate change is expected to lead to more extreme conditions in river discharges, we cannot conclude from this study that it will change the magnitude of the dependence for extreme conditions.