New storm surge scenarios deliver greater certainty over UK flood risk impact

Gallagher Re’s Iain Willis presents new research on UK storm surge risk.

Exactly 70 years on from the devastating 1953 North Sea floods, new joint research by the Gallagher Research Centre (GRC) and HR Wallingford highlights that 60,000 households may be at risk from a UK storm surge event, with climate change-driven sea-level rise exposing a further 17,000 properties by 2050.

These new realistic disaster scenarios will support UK insurers in stress testing their exposure to coastal flood risk, while helping explore the impacts of potential UK flood defence breaches and how this risk will increase with climate change.

Two extreme but plausible UK storm surge scenarios were developed separately for the country’s east and west coasts. The choice of scenario regions was based on a combination of historic surge activity and potential insurance impact. An additional analysis of the same events estimated the impact of climate change on these events in 2050 through a Representative Concentration Pathway (RCP) 4.5 emissions scenario.

The performance of coastal flood defences is critical to mitigating storm surge risk – particularly here in the UK. This is the first study where we've really explored in any detail what the realistic parameters of property exposure and insurance loss could look like for different UK coastal communities if defences are breached. More than that, it provides a glimpse into how climate change-driven sea level rise will steadily increase the loss potential of these UK surges in the future.

Historical losses

The UK has experienced several notable coastal floods over the past century. As Table 1 shows, the most devastating of these were the North Sea floods of 1953, which resulted in 307 fatalities and damage to 24,000 properties. It was the single biggest loss of life from flooding in the UK and Europe in the 20th century and caused levels of damage that today would cost more than £1bn.

Though UK surge events in recent years have been mitigated by effective flood protection, the 2013/2014 UK winter season resulted in £500mn of damage. Fifty flood defence breaches occurred and 2,800 properties were flooded.

Origin of UK storm surges

In the northern hemisphere’s winter months, it is common for low pressure synoptic weather systems to track across the North Atlantic to Europe. The combination of a low centre of pressure, diurnal tidal ranges and wind-driven waves can lead to large masses of water being pushed against UK coastlines. The phenomenon is commonly known as storm surge, and can result in flooding as water levels far exceed expected tidal ranges.

The North Sea is particularly susceptible to large tidal surges as wind, atmospheric pressure and tidal actions can combine at the same time along the length of the east coast.

Exposure dataset

A geospatial residential and commercial property database was used to locate properties at risk. Insured values were then disaggregated from a Gallagher Re UK market portfolio to assign residential and commercial insured values within the property portfolio. Low-lying areas of the UK coastline are particularly at risk of storm surge. By aggregating the exposure database within areas that have an elevation ranging between 0 metres and 8 metres above sea level, it showed that 13 percent of households in the UK may be at risk of coastal flooding, with a significant concentration along the east and west coasts.

Hazard simulation

Scenarios were chosen from a catalogue of extreme surge events, focusing on the severity and potential impact they could have on heavily exposed coastlines. The next step was to take the initial sea-state (i.e. surge) of the events selected and simulate their interaction with the coastal regions using 2D hydrodynamic modelling. This was undertaken by HR Wallingford using high-resolution Lidar data.

Flood breaching was based upon flood defence reliability analysis comprised within a national set of fragility curves created by HR Wallingford’s flood defence model. The analysis was implemented across a number of known flood defence locations with alternative widths and sections breaching in different model simulations.

To account for climate change, the UK climate projections were used to assign the projected RCP4.5 sea level rise estimates for 2050. This scenario was selected as it most closely matches the CBES regulatory requirements of climate stress test scenarios for the UK.

The simulated flood footprints were then compared to Gallagher Re’s insured market portfolio and UK coastal flood vulnerability curves, producing estimates for the number of impacted properties in each scenario, as shown in Figure 1.

The worst-case scenario of breaching for the east coast of the UK was shown to result in a tenfold increase in the number of flooded properties. The west coast event shows a similar trend but as defences are lower on the west coast of the UK, the impact of breaching of defences is less pronounced.

Impacted areas of the west coast scenario include Blackpool (Lancashire) and Bristol, while the east coast event impacts Hull (Yorkshire), Skegness (Lincolnshire), King’s Lynn (Norfolk), Southend (Essex) and Sandwich (Kent).

Climate change

The storm surge scenarios also provide a valuable tool for simulating the effects of climate change on UK coastal flood risk for insurers. The research shows how anticipated sea level rise by 2050 will only increase loss potential from UK storm surges. Based on the RCP4.5 2050 climate scenario, an additional 16,000 insured properties could be impacted on the east coast. Climate change significantly increases the flood footprint across towns such as Great Yarmouth and Hull.

It is important to note that climate change has two impacts: it impacts the number of properties flooded, but also the depth of flooding experienced by properties already at risk – further exacerbating losses. It is estimated that an additional 24 percent and 6 percent of the exposure will be flooded with a depth that exceeds 0.5 metres and 1 metre, respectively.

The above is an excerpt from the Gallagher Research Centre’s paper ‘New Realistic Storm Surge Scenarios for UK Insurers’, published by Gallagher Re on 9 August, 2023. For the full paper, please visit: New storm surge scenarios developed for UK insurers

Iain Willis is research director, Gallagher Research Centre at Gallagher Re