Pan-European hydrodynamic models and their ability to identify compound floods

Dominik Paprotny,Luc Feyen,Sebastiaan N Jonkman,Michalis I Vousdoukas,Oswaldo Morales-Nápoles

doi:10.1007/s11069-020-03902-3

Abstract

The interaction between storm surges and inland run-off 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 UK. First projections of compound flood hazard under climate change have been made, but no study has so far analysed whether existing, independent climate and hydrodynamic models are able to reproduce the co-occurrence of storm surges, precipitation, river discharges or waves. Here, we investigate the dependence between the different drivers in different observational and modelled data set, utilizing gauge records and high-resolution outputs of climate reanalyses and hindcasts, hydrodynamic models of European coasts and rivers. The results show considerable regional differences in strength of the dependence in surge–precipitation and surge–discharge pairs. The models reproduce those dependencies, and the time lags between the flood drivers, rather well in north-western Europe, but less successfully in the southern part. Further, we identified several compound flood events in the reanalysis data. We were able to link most of those modelled events with historical reports of flood or storm losses. However, false positives and false negatives were also present in the reanalysis and several large compound floods were missed by the reanalysis. All in all, the study still shows that accurate representation of compound floods by independent models of each driver is possible, even if not yet achievable at every location.

Highlights

Compound floods are a specific type of floods, when two or more drivers of those coincide in space and time: storm surges, waves, tides, precipitation and high river discharges
The number of days between the occurrence of a storm surge event, and the maximum precipitation/river discharges that result in the highest value of the upper tail dependence coefficient, is referred to hereafter as the “time lag”
The performance of models driven by ERA-Interim reanalysis and RCA4 hindcast was rather satisfactory in this region, though with some overestimation of surge–precipitation dependence was found mainly around the English Channel and North Sea

Summary

Introduction

Compound floods are a specific type of floods, when two or more drivers of those coincide in space and time: storm surges, waves, tides, precipitation and high river discharges. Growing consideration is given to possible co-occurrence of hazards previously considered independently (Leonard et al 2014). This attention is drawn primarily by damages caused by both coincidence of surge and excessive rainfall during tropical cyclones in the USA, including the $150-billion deluge in Houston during hurricane Harvey in August 2017 (van Oldenborgh et al 2017). Even in the USA, along coasts outside the paths of hurricanes there is very little dependence between coastal water levels and heavy precipitation (Wahl et al 2015), while correlation between river flows and surges is spatially diverse (Moftakhari et al 2017). European coasts are affected by extra-tropical cyclones, with diverse mechanisms of fluvial and pluvial floods. Several large European cities located in river estuaries are prone to coastal floods, such as Antwerp, Hamburg, London and Rotterdam

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