Compound Flooding in the UK: Past, Present and Future Co-occurring Extreme Flooding Hazard Sources

Abstract

Flooding is one of the most dangerous and costly natural disasters with wide ranging social, economic and environment impact. In low-lying coastal areas, flooding typically arises through four main sources; at the coast as a result of (1) storm surges and (2) waves, and terrestrially, through (3) fluvial (river) and (4) pluvial (surface water) flooding. When two or more of the sources combine, compound flooding can occur, and the catastrophic results can be magnified greatly, leading to some of the worst flood events (such as Hurricane Harvey in Houston, USA). Despite the increased consequences, compound flooding is far less well understood compared to the individual sources, particularly in the UK. The overall aim of this thesis is therefore to determine which regions of the UK coast are most subject to compound flooding, which combination of source variables are most apparent, and what variables (e.g., meteorological conditions, catchment characteristics) control the spatial patterns evident in compound events, to help inform future compound flood defence and response strategies.
The first objective is to assess the potential for compound flooding arising from the joint occurrence of extreme sea levels and river discharge around the coast of UK and investigate the driving mechanisms involved. The west coast is identified as having a far greater number of joint occurring events compared to the east coast (3-6 events per decade compared to 0-1). The primary cause of this spatial variability is found to be the meteorological conditions leading to extreme surge and river discharge on the west coast are similar storm types, whereas on the east coast, the storms typically follow different weather patterns and tracks. The strength and phase of dependence between extreme surge and river discharge is compared to river catchment characteristics (i.e., flashiness, catchment size and elevation gradient). It’s found 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.
The second objective is to quantify the risk of underestimating compound flooding around the coast of the UK arising from the joint occurrence of all possible pairs of the four main flood sources. The most extreme events joint occurrences were found between surge and waves (at Liverpool with 17 events per decade) whilst the strongest dependence was between wave and river discharge. All flood combination pairs showed a stronger dependence on the west coast compared to the east (for example, for wave vs river discharge, on the west coast, τ =0.35-0.5, whilst on the east coast τ = 0.05-0.2). Furthermore, it is shown that ignoring the relationship between the flood sources can lead to an underestimation of the flood risk by 5-6 times when compared to considering the flood sources dependent on each other.
Finally, a novel methodology is developed to extend compound flood records beyond the observed overlapping datasets, using the weather patterns likely to lead to compound flood events. In a historical meteorological reanalysis (1851-2017), no increasing or decreasing long-term trend was found in compound flood events (involving extreme surge and river discharge). Similar interannual and decadal variability was seen in the meteorological record compared to long-term individual surge and river discharge records. Clustering of surge events found in the periods 1925-1945 and 1995-2005 and fewer events in the mid 20th century (1945-1985) which matched to the compound events identified using the meteorological approach.
Based on these findings, the areas of the UK most at risk to compound flooding have been located and therefore, flood defences and responses can be designed to cope much more effectively with compound flood events. The identification of the meteorological conditions leading to compound flood events can be implemented into forecasting capabilities, reducing emergency response reaction times and potentially reducing deaths, injuries and damages. Furthermore, these weather patterns could be used to better understand the future risk of compound flooding, by examining their occurrence in future climate predictions.

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ORCID for Ivan Haigh: orcid.org/0000-0002-9722-3061

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