Coastal floods, driven by extreme sea levels, are a major hazard both nationally and globally, with wide-ranging social, economic and environmental impacts.
Nationally, it is estimate that £150 billion of assets and 4 million people are currently at risk from coastal flooding in the UK . Coastal flooding is rated as the second highest risk for causing civil emergency in the UK, after pandemic influenza . Combined with fluvial flooding, it is responsible for at least £0.25bn in annual economic damages . Coastal flooding is a growing threat due to accelerating average sea-level rise and possible changes in storminess associated with climate change  as well as continued population growth and development in flood-exposed areas . Continuing to improve the understanding of extreme sea level and coastal flood events is therefore of utmost importance.
We have just completed one of the most detailed assessments of extreme sea levels and coastal flooding ever carried out for the UK. We examined extreme sea levels and storm surge events over the last 100 years using tide gauge records to assess how unusual the 2013/14 winter season was from a coastal flooding perspective. We also set out to assess the spatial footprint of coastal flooding events to determine what types of storms lead to simultaneous flooding along extended coastline stretches and to examine the temporal ‘clustering’ of the flooding events (i.e. events occurring one after another in close succession). These two issues have important financial and practical implications for the risk management and construction sector, such as flood management, (re-)insurance, infrastructure reliability and emergency response; but understanding of these issues before now has been limited.
Extreme sea levels mostly caused by moderate storms surges
One of our most important findings is that the majority (86%) of extreme sea level events that have occurred around the UK over the last 100 years, occurred as a result of moderate storm surges, combined with high spring tide, rather than extreme storm surges. Interestingly, we find that most of the largest extreme storm surges have, by chance, not occurred on high spring tides and hence haven’t led to flooding. It is important that we therefore improve understanding and prediction of moderate storms and how these might change in the future, not just focus on extreme storms. Our results also suggest that much higher extreme sea levels than have been observed could potentially occur if the very large storm surges occur at high spring tide.
Footprint over the UK
Our analysis identified that most storms that approach and cross the UK generate extreme sea levels along one of four main stretches of coastline (see Figure 1). Which stretch of coastline will be impacted is determined by the pathway the storm takes. Importantly, we identified that there have been occasional events, when extreme levels have occurred along large lengths of coast and even along two unconnected stretches of coastlines during the same event. The broad footprints we have identified could be used to better inform flood management, the insurance sector, and national emergency and infrastructure resilience planning.
Clustering of flooding events
Clustering of storms, such as what happened during the 2013/14 or 2015/16 season, is an important issue as it can lead to large socioeconomic impacts and cumulative insurance losses. Crucially we find that these close succession events typically impacted different stretches of coastline. We found no recorded instances of extreme sea level events happening within 4-8 days of each other. This is because if storms are separated by 4-8 days, one will always occur during neap tide (see Figure 2). The combined sea level, even with a large storm surge, is unlikely to be high enough to lead to extreme levels.
How unusual was the 2013/14 season?
We also considered how unusual the 2013/14 season was in the context of the last 100 years, from an extreme sea level perspective. We found that storms during the winter seasons of 2013/14 generated the maximum-recorded sea level at 20 of the 40 tide gauge sites around England, Wales and Scotland; and the largest number of extreme sea level events in any season in the last 100 years.
Moving forward it is important further research is undertaken to better understand the drivers of extreme UK winters and, due to their rare nature and high impact, how they may be affected by climate change. To continue to improve understanding of coastal flooding we have developed a coastal flood database and online tool called SurgeWatch, the first step to provide a systematic record of coastal flood events around the UK since 1915.
The study is described in detail in the following journal paper:
Haigh, I. D. et al. Spatial and temporal analysis of extreme sea level and storm surge events around the coastline of the UK. Sci. Data 3:160107 doi: 10.1038/sdata.2016.107 (2016).
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