Abstract
AbstractCompound flooding in coastal regions, that is, the simultaneous or successive occurrence of high sea levels and high river flows, is expected to increase in a warmer world. To date, however, there is no robust evidence on projected changes in compound flooding for northwestern Europe. We combine projected storm surges and river floods with probabilistic, localized relative sea‐level rise (SLR) scenarios to assess the future compound flood hazard over northwestern coastal Europe in the high (RCP8.5) emission scenario. We use high‐resolution, dynamically downscaled regional climate models (RCM) to drive a storm surge model and a hydrological model, and analyze the joint occurrence of high coastal water levels and associated river peaks in a multivariate copula‐based approach. The RCM‐forced multimodel mean reasonably represents the observed spatial pattern of the dependence strength between annual maxima surge and peak river discharge, although substantial discrepancies exist between observed and simulated dependence strength. All models overestimate the dependence strength, possibly due to limitations in model parameterizations. This bias affects compound flood hazard estimates and requires further investigation. While our results suggest decreasing compound flood hazard over the majority of sites by 2050s (2040–2069) compared to the reference period (1985–2005), an increase in projected compound flood hazard is limited to around 34% of the sites. Further, we show the substantial role of SLR, a driver of compound floods, which has frequently been neglected. Our findings highlight the need to be aware of the limitations of the current generation of Earth system models in simulating coastal compound floods.
Highlights
Compound flooding in coastal regions, that is, the simultaneous or successive occurrence of high sea levels and high river flows, is a significant hazard
We evaluate the adequacy of the copula model in capturing the upper tail dependence using Mean Error to Standard Error (MESE) statistics (Frahm et al, 2005) between empirical upper‐tail‐dependence coefficient (UTDC) estimate, bλemp and simulated UTDC from the parametric family of the copula, bλCθ
This suggests that sea‐level rise (SLR) plays an important role in future compound floods in northwestern Europe and should not be neglected
Summary
Compound flooding in coastal regions, that is, the simultaneous or successive occurrence of high sea levels and high river flows, is a significant hazard. Sea‐level rise (SLR) increases the water levels in the mouth of rivers (Nicholls et al, 2011), increasing the probability of extreme coastal water levels. The main drivers of coastal compound flooding are storm surges, SLR and river floods, which in turn, are driven by a range of processes in the atmosphere and oceans, and on the land surface. Quantifying compound flood hazard under climate change poses a particular challenge at the intersection of climate dynamics and statistical analysis, as the estimation of event probabilities is affected by climate projection uncertainties in relation to the different drivers and in relation to the interdependencies between them (Zscheischler & Seneviratne, 2017)
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