Increased Extreme Coastal Water Levels Due to the Combined Action of Storm Surges and Wind Waves

The dependence between extreme storm surges and wind waves is assessed statistically along the global coasts using the outputs of two numerical models consistently forced with the same atmospheric fields. We show that 55% of the world coastlines face compound storm surge wave extremes. Hence, for a given level of probability, neglecting these dependencies leads to underestimating extreme coastal water levels. Dependencies are dominant in midlatitudes and are likely underestimated in the tropics due to limited representation of tropical cyclones. Furthermore, we show that in half of the areas with dependence, the estimated probability of occurrence of coastal extreme water levels increases significantly when it is accounted for. Translated in terms of return periods, this means that along 30% of global coastlines, extreme water levels expected at most once in a century without considering dependence between storm surges and waves become a 1 in 50‐year event. Coastal flooding is caused by a combination of factors, among which storm surges and wind waves are of major relevance due to their potentially large contributions to coastal extreme sea levels and their widespread effects. Based on global scale numerical simulations of these two components, we have investigated the relationship between extreme storm surges and waves along the world coastlines. We find that in more than half of the coastal regions, storm surges tend to be accompanied by large wind waves, thus increasing the potential coastal flooding. Measures for coastal protection often rely on the probability of occurrence of exceedance events (return periods for prescribed water heights), which in turn is determined by the dependence between the contributors to extreme sea levels. The dependency between surges and waves implies that the likelihood of co‐occurrence of extremes is higher than assuming these two variables as unrelated. More specifically, the probability of facing a 1 in 100‐year event is more than doubled in 30% of the global coastlines when accounting for the dependence between storm surges and waves. Considering these dependencies has a strong impact on return period estimates of extreme high waters and is therefore relevant for the design of coastal defenses. Extreme storm surges and wind waves tend to occur in concurrence along 55% of the world coastlinesReturn periods of extreme sea levels are underestimated (by a factor of 2 or higher) in 30% of the coasts, if dependence is neglected