Identifying compound flood hazard potential from riverine and coastal interactions at the global scale

Compound flooding is an extreme impact event resulting from the interaction of multiple physical and anthropogenic drivers. Low-lying coastal areas are particularly vulnerable to this as they are exposed to different sources of flooding driven by the interaction of oceanographic, hydrological and meteorological processes. A few studies have recently investigated the risk from compound flooding at the continental scale based on modelled variables of flood drivers such as storm surge, precipitation and river discharges. At the global scale, this assessment has only been performed based on observations thereby excluding a large extent of the global coastline.

In this study, we derive and analyze indicators to assess the compound floods hazard potential from river discharge and storm surge interactions at the global scale. Comprehensive time-series of mean daily discharge and maximum daily storm surge, obtained from global models using the ERA-Interim (1979-2014) reanalysis dataset as consistent forcing, are paired along the global coastline. First, we calculate the number of annual maxima events co-occurring and find large global patterns. In some regions, such as the East coast of the United States or in East Asia, the probability of extreme river discharge and high storm surge interacting shows a strong seasonality and greatly deviates from independence. An analysis of the corresponding weather systems for these co-occurrences highlights that these conditions stem from synoptic weather patterns. We further analyze different compound flood mechanisms by defining critical areas in probability space using copula theory and characterize the strength of this relationship with the Kendall's rank correlation coefficient. Compound flood hazard hotspots have a consistently large statistical dependence in all cases tested, for which we also derive their synoptic atmospheric conditions.

This analysis provides a better understanding of the spatial characterization of the compound flood hazard potential in delta and estuarine areas at the global scale and identifies regions at high risk from compound flooding. Because the statistical dependence is closely linked to quantification of the joint probability, this also emphasizes the need to refine global flood risk assessments to account for these interactions.