The effect of surge on riverine flood hazard and impact in deltas globally

Abstract

<p>Current global riverine flood risk studies assume a constant mean sea level boundary. In reality, high sea levels can propagate up a river leading to elevated water levels, and/or the drainage of high river discharge can be impeded by elevated sea levels. Riverine flood risk in deltas might therefore be underestimated if dynamic sea levels are ignored. This contribution presents the first global scale assessment of drivers of riverine flooding in deltas and underlines the importance of including dynamic downstream sea level boundaries in global riverine flood risk studies.</p><p>The assessment is based on extreme water levels at 3433 river mouth locations as modeled by the state-of-the-art global river routing model CaMa-Flood, forced with a multi-model runoff ensemble from the EartH2Observe project and bounded by dynamic sea level conditions from the global tide and surge model GTSM. Using this framework, we classified the drivers of riverine flooding at each location into four classes: surge dominant, discharge dominant, compound or insignificant. The classification is based on rank correlations between annual maximum riverine water levels and surge levels, and annual maximum riverine water levels and discharge. We developed a model experiment to quantify the effect of surge on flood levels and impacts.</p><p>We find that drivers of riverine flooding are compound 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 flood drivers generally have larger surge extremes, 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 13.5 cm. While this increase is the largest at locations with compound or surge dominant flood drivers, surge also affects flood levels at locations with discharge dominant flood drivers. A small decrease in 1-in-10 years flood levels is observed at 12.2 % of locations analyzed due to negative seasonal component of surge associated with dominant seasonal gyre circulations. Finally, we show that if surge is ignored, flood depths are underestimated for 38.2 million out of a total of 332.0 million (11.6 %) expected annual mean people exposed to riverine flooding.</p>

More meteorological events that drive compound coastal flooding are projected under climate change

“…They did not account for the local pluvial effects considered in the present study, which could further influence the water levels 28 . A direct comparison with the results of Eilander et al 28 is not straightforward because of different criteria employed for the selection of compound events 28 , different thresholds used to define the extremes, and different datasets employed. Overall, the large-scale spatial pattern of compound flooding that we find is similar to that identified by Eilander et al 28 and previous analyses of the compound flood potential based on river discharge data 24,26,27 .…”

“…Our findings on the present-day dependence between meteorological tides and precipitation agree in general with those in previous studies 8,10,21,22,24,26,27,40 (Supplementary Discussion). Recently, Eilander et al (2020) 28 quantified the influence of storm surge on extreme water levels in estuaries for present climate. They did not account for the local pluvial effects considered in the present study, which could further influence the water levels 28 .…”

“…Recently, Eilander et al (2020) 28 quantified the influence of storm surge on extreme water levels in estuaries for present climate. They did not account for the local pluvial effects considered in the present study, which could further influence the water levels 28 . A direct comparison with the results of Eilander et al 28 is not straightforward because of different criteria employed for the selection of compound events 28 , different thresholds used to define the extremes, and different datasets employed.…”

“…A direct comparison with the results of Eilander et al 28 is not straightforward because of different criteria employed for the selection of compound events 28 , different thresholds used to define the extremes, and different datasets employed. Overall, the large-scale spatial pattern of compound flooding that we find is similar to that identified by Eilander et al 28 and previous analyses of the compound flood potential based on river discharge data 24,26,27 . In southern Europe and western South Africa, the compound flood hazard in estuaries is relatively low 28 , while we detect a relatively high potential for flooding from concurring precipitation and storm surge extremes (Fig.…”

“…Recent advances in ocean modelling have resulted in the generation of sub-daily continuous time-series of sea level with global coverage 1,25 , which together with estimates of either precipitation or river discharge enable comprehensive global assessments of present-day compound flood hazard 26,27 . For example, Eilander et al 28 , using hydrodynamical modelling, focused on present-day compound flooding in river deltas highlighting that storm surge exacerbates 1-in-10 year flood levels in 64.0% of the analysed deltas worldwide.…”