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

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.

Plastic in global rivers: are floods making it worse?

“…Note that only plastic within the oodplain is accounted for here. As oods are not always singular events, but also occur as complex, compound events [26][27][28][29] plastic mobilisation potential might be considerably higher when for example including transport with surface runoff (especially in urban areas) and strong (gust) winds during such compound events. Additionally, storm surges or tsunamis can transport massive amounts of plastic, such as observed after the tsunami in Japan in 2011, when an estimated 5 Mt of plastic was transported into the ocean 30 .…”

Combined Modeling of US Fluvial, Pluvial, and Coastal Flood Hazard Under Current and Future Climates

“…(2017), and Eilander et al. (2020) have looked at more than one causal mechanism, focusing on either rainfall and river flow or storm surge and river flow. There is thus a pressing need to develop a large‐scale multiperil flood risk analysis that includes pluvial, fluvial, and coastal flooding.…”

“…Floods can arise from a number of sources including extreme river flows, rainfall, coastal water levels and waves, and more rarely groundwater or infrastructure failure (e.g., dam collapses). However, at large scales, only Sampson et al (2015), Wing et al (2017), and Eilander et al (2020) have looked at more than one causal mechanism, focusing on either rainfall and river flow or storm surge and river flow. There is thus a pressing need to develop a large-scale multiperil flood risk analysis that includes pluvial, fluvial, and coastal flooding.…”

Climate Change and Changes in Compound Coastal‐Riverine Flooding Hazard Along the U.S. Coasts

“…Despite several recent studies on interaction between extreme SLs and river flows at local (Fang et al, 2020;Khanal et al, 2019;Moftakhari et al, 2017;P. M. Orton et al, 2020;Jane et al, 2020), continental (Bevacqua et al, 2019), and global (Couasnon et al 2020;Eilander et al, 2020;Ward et al 2018) scale, a comprehensive assessment of the impact of climate change on compound coastal-riverine flooding hazard has not been explored along the coastal U.S. This research gap is important due to several reasons.…”