Intensifying rainfall under climate change:
what does this mean for flood?

Over Monday 3 and Tuesday 4 February, the Royal Society hosted a two-day discussion meeting on the intensification of short-duration rainfall extremes and implications for flash flood risks. The meeting brought together global academic and industry representatives to present research into current and future intense rainfall events and the different types of associated risks.

Monday opened with an interesting presentation by Baroness Brown of Cambridge on the UK’s approach to mitigating against, and adapting to, climate change. It highlighted that flooding is the UK’s number one risk, which was supported by evidence that changes in short-duration rainfall (rainfall which falls in less than 6 hours) will increase by 20% under a 2°C warming scenario and 50% with 4°C warming by 2080. It also emphasised that the UK is far from adapted for 2°C warming, let alone 4°C.

The theme of day one was the intensification of short duration rainfall and how models have been used to investigate these extremes. There was a clear and consistent message across all talks that rainfall will become more intense in a warming climate. Each speaker used the Clausius Clapeyron relationship (for every 1°C increase in temperature there is a 6-7% increase in the atmosphere’s water vapour) to support their message. For some areas in the world, the scaling for this relationship is ‘super’ - up to a doubling of this rate.

The importance of resolution was also highlighted throughout the day. A discussion on storm-scale models reinforced that modelling should not necessarily be undertaken at the finest resolution to obtain a good representation of the hazard and a model’s resolution is dependent on many factors, including the time and spatial scale of the hazard modelled.

A poster session closed day one, showcasing a range of innovative research into the intensification of short-duration rainfall and flood risk. JBA Risk Management was represented by David Cross who presented his PhD research on modelling intense rainfall in a changing climate (pictured right). The research focused on single site rainfall and presented a new approach for extreme rainfall estimation at sub-hourly scales using models which construct the rainfall profile from a clustering of rain cells in storms.

There was another relevant poster to JBA’s flood mapping work which investigated potential future changes in surface water hazard. The research adjusted climate models regionally and then used the derived rainfall extremes as input into JFlow (JBA’s 2D hydraulic model) to produce a climate change-adjusted hazard map, allowing the severity of surface water flooding under the new climate to be investigated.

Tuesday’s sessions steered more towards research involving flood risk. Topics included urbanisation, future flood estimation and industries responding to climate change. Some key challenges in these areas of research included accounting for seasonality in intense rainfall, data scarcity and incorporating Sustainable Drainage Systems (SuDs) into models, all of which are familiar topics for JBA.

A panel discussion between industry representatives Murray Dale (JBA Consulting) (pictured below), Dr Harriett Orr (Environment Agency) and Nalan Senol Cabil (Willis Towers Watson) offered insightful opinions on various climate change and flood risk questions from the floor. It was interesting to hear how slight differences in their areas of work provided unique perspectives.

For example, the usefulness of a single average loss number was discussed. Nalan explained that this would suffice for the insurance industry whereas Murray explored the difficulties it would pose for the water industry; organisations are required to conform to regulations and using a single average loss number could mean too much or too little is spent on the creation and maintenance of flood defences, sometimes up to millions of pounds.

However, the overall take home message from all was the need to act on climate change. Harriett emphasises that we need to react to the changing climate now in order to adapt to a warming, wetter country.

This conference demonstrated how academia and industry can, and should continue to, come together to work collaboratively to help raise awareness, and mitigate the effects of, the intensification of short-duration rainfall extremes and its implications for flash flood risks.

If you’re interested in learning more about JBA’s work in flood mapping, catastrophe modelling and analytics, and how we can help you manage the risk, please get in touch.

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