Flooding in the UK: Understanding the past and preparing for the future

On the 16th of October 2019, Ivan Haigh ­Associate Professor in Coastal Oceanography at the University of Southampton – gave a presentation on the “characteristics and drivers of compound flooding events around the UK coast” at the BRIDGE research seminar in the School of Geographical Sciences. He began by outlining the seriousness of flood risk in the UK – it is the second highest civil emergency risk factor as defined by the Cabinet Office – before moving on to the first section of the talk on his work with the Environment Agency on its Thames Estuary 2100 plan (TE2100)[1].

Thames Estuary 2100 plan: 5-year review

The construction of a Thames barrier was proposed after severe flooding in London in 1953, and it eventually became operational 30 years later in 1983. Annually, the Thames barrier removes around £2bn of flood damage risk from London and is crucial to the future prosperity of the city in a changing environment.

The Thames Barrier in its closed formation. Image source: Thames Estuary 2100 Plan (2012)

Flood defences in the Thames estuary were assessed in the TE2100 plan, which takes an innovative “adaptive pathways management approach” to the future of these flood defences over the coming century. This approach means that a range of flood defence options are devised and the choice of which ones to implement is based upon the current environmental data and the latest models of future scenarios, in particular predictions of future sea level rise.

For this method to be effective, accurate observations of recent sea level changes must be made in order to determine which management pathway to implement and to see if these measurements fit with the predictions of future sea level rise used in the plan. This work is carried out in reviews of the plan at five-year intervals, and it was this work that Ivan and his colleagues were involved with.

There is significant monthly and annual variability in the local tide gauge records that measure changes in sea level, and this can make it difficult to assess whether there is any long-term trend in the record. Using statistical analysis of the tide gauge data, the team was able to filter 91% of the variability that was due to short term changes in atmospheric pressure and winds to reveal a trend of approximately 1.5 mm per year of sea level rise, in line with the predictions of the model that is incorporated into the TE2100 plan.

Compound flood events around the UK Coast

In the second half of his presentation, Ivan went on to discuss a recent paper he was involved with studying compound flood events around the UK (Hendry et al. 2019)[2]. A compound flood occurs when a storm surge, caused by low atmospheric pressure allowing the sea surface to rise locally, combines with river flooding caused by a large rainfall event. These can be the most damaging natural disasters in the UK, and from historical data sets stretching back 50 years at 33 tide gauges and 326 river stations, the team were able to determine the frequency of compound floods across the UK.

Along the west coast, there were between 3 and 6 compound flooding events per decade, whereas on the east coast, there were between 0 and 1 per decade. This difference between east and west is driven by the different weather patterns that lead to these events. On the west coast it is the same type of low-pressure system that causes coastal storm surges and high rainfall. However, on the east coast different weather patterns are responsible for high rainfall and storm surges, meaning it is very unlikely they could occur at the same time.

Number of compound flood events per decade at each of the 326 river stations in the study. Triangle symbols implies rover mouth on West coast, circles East coast and squares South coast. Image Source: Hendry et al. 2019 [2]

There is also significant variability along the west coast of the UK as well, and the team investigated whether the characteristics of the river catchments could impact the possibility of these compound flooding events occurring. They found that smaller river catchments, and steeper terrain within the catchments, increased the probability of these compound flooding events occurring as water from rainfall was delivered to the coast more quickly. From the improved understanding of the weather patterns behind compound flooding events that this work provides, the quality and timeliness of flood warnings could be improved.

From the question and answer session we heard that current flood risk assessments do not always include the potential for compound flood events, meaning flood risk could be underestimated along the west coast of the UK. We also heard that Ivan will be working with researchers in the hydrology group here at the University of Bristol to further the analysis of the impact of river catchment characteristics on the likelihood of compound flooding events, and then extending this analysis to Europe, North America and Asia.

References

[1] Environment Agency (2012), “Thames Estuary 2100 Plan”.
[2] Alistair Hendry, Ivan D. Haigh, Robert J. Nicholls, Hugo Winter, Robert Neal, Thomas Wahl, Amélie Joly-Laugel, and Stephen E. Darby, (2019). “Assessing the characteristics and drivers of compound flooding events around the UK coast”, Hydrology and Earth System Science, 23, 3117-3139.