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 [1]. Coastal flooding is rated as the second highest risk for causing civil emergency in the UK, after pandemic influenza [2]. Combined with fluvial flooding, it is responsible for at least £0.25bn in annual economic damages [3]. Coastal flooding is a growing threat due to accelerating average sea-level rise and possible changes in storminess associated with climate change [4] as well as continued population growth and development in flood-exposed areas [5]. Continuing to improve the understanding of extreme sea level and coastal flood events is therefore of utmost importance.

Waves batter the coast at Chesil Beach during the storm of 4-6 February 2015 (Credit: Tim Poate and Gerd Masselink).

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.

Figure 1: The four broad footprints of sea level events that occur around the UK and the position and tracks of the storms, and associated winds (arrows), that resulted in these spatial footprints.

 

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.

Figure 2: How water level changes with neap and spring tides (Courtesy of AztecSailing).

 

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.

 

Future research

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 streets of Old Portsmouth flooding during the storms of 14-18 December 1989 (© Portsmouth City Council / East Solent Coastal Partnership).

A family evacuated in Whitstable, Kent, during the ‘Great Storm’ of 1953. These floods killed 307 people in eastern England and were the catalyst for the construction of the Thames Barrier. (© Canterbury City Council 2015)

 

 

 

 

 

 

 

 

 

 

 

 

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).

  1. Environment Agency, 2009. Flooding in England: A National Assessment of Flood Risk. Environment Agency report. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/292928/geho0609bqds-e-e.pdf.
  2. Cabinet Office. National Risk Register of Civil Emergencies. Available at https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/419549/20150331_2015-NRR-WA_Final.pdf (Cabinet Office, 2015).
  3. Penning Rowsell, E. C. (2015). A realistic assessment of fluvial and coastal flood risk in England and Wales. Transactions of the Institute of British Geographers 40, 44–61, doi:10.1111/tran.12053.
  4. Church, J. A. et al. (2013): Sea Level Change. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Cli- mate Change, edited by: Stocker, T. F. et al., Cambridge University Press, Cambridge, UK and New York, NY, USA.
  5. Hallegatte, S. et al. (2013). Future flood losses in major coastal cities, Nature Clim. Change, 3, 802–806, doi:10.1038/nclimate1979.