Brief communication: The role of using precipitation or river discharge data when assessing global coastal compound flooding

Emanuele Bevacqua,Michalis I Vousdoukas,Theodore G Shepherd,Mathieu Vrac

doi:10.5194/nhess-20-1765-2020

Abstract

Abstract. Interacting storm surges and high water runoff can cause compound flooding (CF) in low-lying coasts and river estuaries. The large-scale CF hazard has been typically studied using proxies such as the concurrence of storm surge extremes either with precipitation or with river discharge extremes. Here the impact of the choice of such proxies is addressed employing state-of-the-art global datasets. Although they are proxies of diverse physical mechanisms, we find that the two approaches show similar CF spatial patterns. On average, deviations are smaller in regions where assessing the actual CF is more relevant, i.e. where the CF potential is high. Differences between the two assessments increase with the catchment size, and our findings indicate that CF in long rivers (catchment ≳5–10×103 km2) should be analysed using river discharge data. The precipitation-based assessment allows for considering local-rainfall-driven CF and CF in small rivers not resolved by large-scale datasets.

Highlights

Compound flooding (CF) happens in low-lying coastal areas due to the interaction of high precipitation runoff and high sea level
The spatial patterns of the potential CF return periods based on either precipitation (Tprec) or river discharge (Triver) are very similar (Fig. 1; Fig. A2 is identical but shows results based on extremes defined considering 2-year return levels)
We find that there is a tendency towards higher differences in the two assessments at locations where either or both those based on precipitation (Tprec) and Triver are high. (This appears consistent with the high uncertainty associated with large CF return periods.) Such a finding has relevant implications, as it indicates that the two assessments tend to be similar, on average, where assessing the actual CF is more important, i.e. where there is a relatively high CF potential (Fig. A6)

Summary

Introduction

Compound flooding (CF) happens in low-lying coastal areas due to the interaction of high precipitation runoff and high sea level. According to Wahl et al (2015), CF is possible to occur when (1) the joint occurrence of high river discharge and storm surge in estuarine regions may elevate water levels to a point where flooding is initiated or its impacts are exacerbated; (2) a destructive storm surge, which already caused widespread flooding, is followed by rainfall, as the latter can drive additional flooding, even if it is not an extreme event on its own; and (3) a moderate storm surge occurs which does not directly cause flooding but is high enough to fully block or slow down gravity-fed storm water drainage, and as a result precipitation causes flooding. CF may occur if (4) precipitation falls on wet soil that is saturated by a preceding storm surge (Bevacqua et al, 2019)

Methods

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Conclusion