Compound Flooding: Dependence at Sub-daily Scales Between Extreme Storm Surge and Fluvial Flow

Abstract

Estuaries are potentially exposed to compound flooding where weather-driven extreme sea levels can occur synchronously with extreme fluvial discharge to amplify the hazard. The likelihood of compound flooding is difficult to determine due to multiple interacting physical processes operating at sub-daily scales, and poor observation records within estuaries with which to determine potential future probabilistic scenarios. We hypothesize that fluvial extremes can occur within the peak of the surge in small/steep catchments because of rapid runoff times, whilst the length-scale in larger/flatter catchments will result in fluvial and marine extremes being out-of-phase. Data (15 min river flow and hourly sea level) spanning 40 years were analyzed to assesses the behaviour and timings of fluvial and sea level extremes in two contrasting estuaries: Humber and Dyfi (United Kingdom). Compound events were common in the Dyfi, a small/steep catchment on Britain’s west coast with fast fluvial response times. Almost half of the 937 skew-surge events (95th-percentile) occurred within a few hours of an extreme fluvial peak, suggesting that flood risk is sensitive to the storm timing relative to high tide—especially since flows persisted above the 95th-percentile typically for less than 12 h. Compound events were more frequent during autumn/winter than spring/summer. For the Humber, a larger/flatter catchment on the east coast with slower fluvial response times, extreme fluvial and skew-surge peaks were less frequent (half as many as the Dyfi) and compound events were less common (15% of events co-occurred). Although flows in the Humber persisted above the 95th-percentile for typically between one and 4 days, hence overlapping several high tides and possibly other surges. Analysis of 56 flooding events across both estuaries revealed: 1) flooding is more common in the Dyfi than Humber; 2) Dyfi flooding is driven by 99th-percentile flows lasting hours and co-occurring with a 95th percentile skew-surge; 3) Humber flooding was driven by 95th-percentile flows lasting days, or surge-driven—but rarely co-occurring. Our results suggest that compound flooding studies require at least hourly data (previous analyses have often used daily-means), especially for smaller systems and considering the potential intensification of rainfall patterns into the future.

Multivariate Analysis of Compound Flood Hazard Across Canada's Atlantic, Pacific and Great Lakes Coastal Areas

“…Further, Paprotny et al (2020) found strong dependencies in surge-precipitation and surge-discharge pairs along the northwestern coasts of Europe. Similar analyses have been conducted on the joint occurrence of storm surge and precipitation across coastal zones of China (Fang et al, 2020), Australia (Wu et al, 2018), storm surge, and river discharge in Britain (Robins et al, 2021), storm surge/sea levels and river discharge over the US coasts (Welch, 2020), among others.…”

“…Previous studies have analyzed compound flood events at global (Eilander et al., 2020; Ward et al., 2018), continental (Ganguli & Merz, 2019; Paprotny et al., 2020), national (Ghanbari et al., 2021; Jalili Pirani & Najafi, 2020), and regional scales (Valle‐Levinson et al., 2020; van Berchum et al., 2020) using statistical and process‐based approaches (Hao et al., 2018). These analyses include characterizing the statistical interrelationships between drivers of flooding based on Bayesian networks (Couasnon et al., 2018; Sebastian et al., 2017), copula theory (Bevacqua et al., 2017; Gori et al., 2020; Moftakhari et al., 2017; Paprotny et al., 2018; Xu et al., 2014), bivariate extreme value distributions (Zheng et al., 2014), correlation and linear regression (Robins et al., 2021), bivariate logistic threshold‐excess model (Zheng et al., 2013) among others. Besides, recent studies have assessed the compound flood impacts and risks through process‐based modeling and hybrid statistical‐dynamical framework (Ganguli et al., 2020; Ganguli & Merz, 2019; Najafi et al., 2021; Wang et al., 2021; Zhang & Najafi, 2020).…”

“…Conventional approaches to flood hazard assessment are based on the assumption that the drivers of flooding are independent of one another. However, recent studies show strong evidence for the interactions between drivers of floods, especially in coastal areas around the world (Eilander et al., 2020; Hendry et al., 2019; Moftakhari et al., 2017; Nasr et al., 2021; Robins et al., 2021; Wahl et al., 2015; Ward et al., 2018). Different mechanisms can trigger flood events simultaneously or successively, leading to an extreme impact even if the contributing drivers are not extreme (Masson‐Delmotte et al., 2021).…”

Compound Flood Events: Analysing the joint occurrence of extreme river discharge events and storm surges in Northern and Central Europe

“…First, we limited the time frame of the data sets to the late fall and entire winter season, as storm surges mostly occur in the winter season in northern Europe, see for example (Liu et al, 2022). For the winter season, we used a time frame from December to February, such as also done by Robins et al (2021).…”

Assessment of the impact of climate change and flooding on bridges and surrounding area

“…In addition, the effect of compound flooding is more severe on bridges due to the short time period between two or more events. Compound flooding is more frequent during winter/ autumn than summer/spring (Robins et al, 2021). Therefore, engineers must be able to design the bridges to withstand the forces of pre-evaluated shock sizes and consider urban planning by identifying areas that are at risk of flooding, and then developing strategies to reduce the risk of flooding in those areas.…”