Abstract
Current global riverine flood risk studies assume a constant mean sea level boundary. In reality high sea levels can propagate up a river, impede high river discharge, thus leading to elevated water levels. Riverine flood risk in deltas may therefore be underestimated. This paper presents the first global scale assessment of the joint influence of riverine and coastal drivers of flooding in deltas. We show that if storm surge is ignored, flood depths are significantly underestimated for 9.3% of the expected annual population exposed to riverine flooding. The assessment is based on extreme water levels at 3433 river mouth locations as modeled by a state-of-the-art global river routing model, forced with a multi-model runoff ensemble and bounded by dynamic sea level conditions derived from a global tide and surge reanalysis. We first classified the drivers of riverine flooding at each location into four classes: surge-dominant, discharge-dominant, compound-dominant or insignificant. We then developed a model experiment to quantify the effect of surge on flood hazard and impacts. Drivers of riverine flooding are compound-dominant at 19.7% of the locations analyzed, discharge-dominant at 69.2%, and surge-dominant at 7.8%. Compared to locations with either surge- or discharge-dominant flood drivers, locations with compound-dominant flood drivers generally have larger surge extremes and are located in basins with faster discharge response and/or flat topography. Globally, surge exacerbates 1-in-10 years flood levels at 64.0% of the locations analyzed, with a mean increase of 11 cm. While this increase is generally larger at locations with compound- or surge-dominant flood drivers, flood levels also increase at locations with discharge-dominant flood drivers. This study underlines the importance of including dynamic downstream sea level boundaries in (global) riverine flood risk studies.
Highlights
IntroductionCurrently, global flood risk studies either examine riverine or coastal floods (Dottori et al 2018, Hallegatte et al 2013, Hinkel et al 2014, Hirabayashi et al 2013, Jongman et al 2012, Ward et al 2013, 2017, Vitousek et al 2017, Vousdoukas et al 2018, Winsemius et al 2016)
Surge exacerbates 1-in-10 years flood levels at 64.0% of the locations analyzed, with a mean increase of 11 cm. While this increase is generally larger at locations with compound- or surge-dominant flood drivers, flood levels increase at locations with discharge-dominant flood drivers
Compound flood events can occur from the interplay between riverine and coastal flood drivers, for instance when: high sea levels propagate up a river leading to elevated water levels; and/or the drainage of high river discharge is impeded by elevated sea levels
Summary
Global flood risk studies either examine riverine or coastal floods (Dottori et al 2018, Hallegatte et al 2013, Hinkel et al 2014, Hirabayashi et al 2013, Jongman et al 2012, Ward et al 2013, 2017, Vitousek et al 2017, Vousdoukas et al 2018, Winsemius et al 2016) As such, these studies have not accounted for compound events, in which the combination of multiple drivers and/or hazards can interact to modulate risk (Zscheischler et al 2018). By comparing simulated water levels from original andshuffled’ time-series, the effect of surge-precipitation dependence on extreme inland water levels was examined This approach eliminates the need for a-priori event selection but requires models that realistically simulate interactions between multiple drivers. No global analysis of surge-discharge interactions based on simulated water levels exists To fill this gap, we developed a global compound flood model framework with the aim to identify dominant flood drivers in deltas globally and assess the effect of surge on riverine flood hazard and impact.
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