Abstract
Abstract. In low-lying coastal regions, flooding arises from oceanographic (storm surges plus tides and/or waves), fluvial (increased river discharge), and/or pluvial (direct surface run-off) sources. The adverse consequences of a flood can be disproportionately large when these different sources occur concurrently or in close succession, a phenomenon that is known as “compound flooding”. In this paper, we assess the potential for compound flooding arising from the joint occurrence of high storm surge and high river discharge around the coast of the UK. We hypothesise that there will be spatial variation in compound flood frequency, with some coastal regions experiencing a greater dependency between the two flooding sources than others. We map the dependence between high skew surges and high river discharge, considering 326 river stations linked to 33 tide gauge sites. We find that the joint occurrence of high skew surges and high river discharge occurs more frequently during the study period (15–50 years) at sites on the south-western and western coasts of the UK (between three and six joint events per decade) compared to sites along the eastern coast (between zero and one joint events per decade). Second, we investigate the meteorological conditions that drive compound and non-compound events across the UK. We show, for the first time, that spatial variability in the dependence and number of joint occurrences of high skew surges and high river discharge is driven by meteorological differences in storm characteristics. On the western coast of the UK, the storms that generate high skew surges and high river discharge are typically similar in characteristics and track across the UK on comparable pathways. In contrast, on the eastern coast, the storms that typically generate high skew surges are mostly distinct from the types of storms that tend to generate high river discharge. Third, we briefly examine how the phase and strength of dependence between high skew surge and high river discharge is influenced by the characteristics (i.e. flashiness, size, and elevation gradient) of the corresponding river catchments. We find that high skew surges tend to occur more frequently with high river discharge at catchments with a lower base flow index, smaller catchment area, and steeper elevation gradient. In catchments with a high base flow index, large catchment area, and shallow elevation gradient, the peak river flow tends to occur several days after the high skew surge. The previous lack of consideration of compound flooding means that flood risk has likely been underestimated around UK coasts, particularly along the south-western and western coasts. It is crucial that this be addressed in future assessments of flood risk and flood management approaches.
Highlights
Flooding is the most dangerous and costly of natural disasters (Pall et al, 2011)
The dependence is stronger for total sea levels for sites linked to tide gauges in the northern Irish Sea (e.g. Portrush [20] and Bangor [19] in North Ireland and Portpatrick [21] and Millport [22] in Scotland), and this is most likely because tidal range is small here and not such a dominant factor on total sea levels compared to other sites
We found that the joint occurrence of high skew surges and high river discharge occurs more frequently during the study period (15–50 years) at sites on the south-western and www.hydrol-earth-syst-sci.net/23/3117/2019/
Summary
From 1980 to 2013, floods accounted for more than USD 1 trillion in losses and resulted in at least 220 000 fatalities globally (Munich Re, 2017). More than 50 % of these deaths and a large proportion of the economic losses occurred in densely populated low-lying coastal regions. Coastal areas are home to more than 600 million people and constitute strategic economic centres (McGranahan et al, 2007). Coastal flooding is a growing threat due to sea-level rise and changes in storminess (Nerem et al, 2018; Church et al, 2013), ongoing vertical land movement (Brown and Nicholls, 2015), and rapid population growth and accompanying development in flood-exposed areas (Brown et al, 2018; Hallegatte et al, 2013)
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