Dependence between high sea-level and high river discharge increases flood hazard in global deltas and estuaries

Abstract

When river and coastal floods coincide, their impacts are often worse than when they occur in isolation; such floods are examples of 'compound events'. To better understand the impacts of these compound events, we require an improved understanding of the dependence between coastal and river flooding on a global scale. Therefore, in this letter, we: provide the first assessment and mapping of the dependence between observed high sea-levels and high river discharge for deltas and estuaries around the globe; and demonstrate how this dependence may influence the joint probability of floods exceeding both the design discharge and design sea-level. The research was carried out by analysing the statistical dependence between observed sea-levels (and skew surge) from the GESLA-2 dataset, and river discharge using gauged data from the Global Runoff Data Centre, for 187 combinations of stations across the globe. Dependence was assessed using Kendall's rank correlation coefficient ( ) and copula models. We find significant dependence for skew surge conditional on annual maximum discharge at 22% of the stations studied, and for discharge conditional on annual maximum skew surge at 36% of the stations studied. Allowing a time-lag between the two variables up to 5 days, we find significant dependence for skew surge conditional on annual maximum discharge at 56% of stations, and for discharge conditional on annual maximum skew surge at 54% of stations. Using copula models, we show that the joint exceedance probability of events in which both the design discharge and design sea-level are exceeded can be several magnitudes higher when the dependence is considered, compared to when independence is assumed. We discuss several implications, showing that flood risk assessments in these regions should correctly account for these joint exceedance probabilities.

Growth of the Decision Tree: Advances in Bottom‐Up Climate Change Risk Management

“…; Ward et al. ), and methodologies for managing such risks at a large scale (e.g., Hall et al. ; Salman and Li ), especially with sea level rise (e.g., Wu et al.…”

“…Neither has this manuscript addressed risk assessment issues associated with the coastal zone. The reader is referred to literature related to the risks of compound river flood and storm surge events (e.g., Zheng et al 2013;Ward et al 2018), and methodologies for managing such risks at a large scale (e.g., Hall et al 2005;Salman and Li 2018), especially with sea level rise (e.g., Wu et al 2002;Ericson et al 2006;Revi 2008;Jongman et al 2012).…”

Increased Extreme Coastal Water Levels Due to the Combined Action of Storm Surges and Wind Waves

“…When two or more of these mechanisms occur simultaneously, flood severity may be exacerbated leading to increased coastal flood risk. Examples of compound flood events include the combination of river discharge and surges (Moftakhari et al, ; Ward et al, ), rainfall and surges (Wahl et al, , in the U.S. coasts and Wu et al, , in Australia), and rainfall, surge, and waves (Bilskie & Hagen, ; Paprotny et al, ). If these concurrent events display statistical dependencies (e.g., through a common forcing mechanism), the probability of their joint occurrence (i.e., the chance of two or more extreme conditions occurring at the same time) is higher than that expected considering separately the extremes of each variable, with a consequent increase of the likelihood of coastal flooding.…”

Trends in Compound Flooding in Northwestern Europe During 1901–2014

“…Previous researchers have identified a moderate to strong correlation between high coastal water levels (HCWLs) or storm surges and floods (either pluvial or fluvial) globally , and regionally across Europe (Kew et al, 2013;Paprotny et al, 2018;Petroliagkis et al, 2016;Reeve et al, 2008;Svensson & Jones, 2001;Svensson & Jones, 2004), the United States (Moftakhari et al, 2017;Sadegh et al, 2018), Australia (Wu et al, 2018;Zheng et al, 2013Zheng et al, , 2014, and China (Lian et al, 2013;Tu et al, 2018). In addition, they have focused on the joint probability of compound floods Kew et al, 2013;Moftakhari et al, 2019;Moftakhari et al, 2017;Sadegh et al, 2018;Ward et al, 2018) considering individual drivers and the conditional probability (Bevacqua et al, 2017) of occurrence of river floods given moderate to extreme coastal water levels or surges. These metrics are valuable for risk management and infrastructure design in delta areas (Wu et al, 2018).…”