Researcher: Climate change may make flash floods more likely

Gants Hill Roundabout flooding

Vehicles navigate flooding near Gants Hill Roundabout on Sunday, July 25. - Credit: Saleh Ahmed/@SalehAhmed_SA

Flash floods like those which hit London last month could become more frequent as a result of climate change, according to an Imperial College researcher.

Dr Barnaby Dobson, research associate at Imperial College London’s Faculty of Engineering, explained to The Recorder how extreme weather events overwhelm urban drainage systems and what can be done to mitigate such risks.

The researcher works in the field of water systems integration, which tries to study the water system holistically and the interaction between different parts of it.

He said that unlike rural flooding, where soil absorption is typically the problem, urban flooding occurs when the drainage network is overwhelmed by an unusually large quantity of rain.

“The UK standard of designing a drainage network is you should ensure your drainage network will not flood for a storm that is less than a thirty-year event.

“For various reasons it's probably likely to flood more than that, but it is still a reasonably performing drainage network.”

He noted that the 40mm of rain which fell on Sunday (July 25) was “quite extreme” – a higher level of rainfall in London would be expected on just 0.035 per cent of days.

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Dr Dobson said that while there was always a large margin of error with such calculations, the recent flash flooding in London might be expected to occur once every 10 to 20 years.

However, such events may become more likely with the changing climate, he believed.

The 0.035pc figure is based on London’s historic climate – the city’s cumulative climate data up to the year 2000.

The data for London’s climate between 2000 and 2020 yields a figure of 0.05pc, up 35pc, according to Dr Dobson.

Using a 'business-as-usual' projection, and once again stressing the large “error bars” for such calculations, Dr Dobson calculated that rainfall on the scale of Sunday July 25 will occur 60pc more frequently by 2060.

“If you look at all the different climate projections that were made 10 to 15 years ago, the one that has tracked what has happened is the business-as-usual one,” he said.

In the wake of the floods, many on social media linked the flooding to the lack of green space and gardens in London, and according to Dr Dobson, this topic of ‘urban greening’ is a hotly contested issue in his field.

“You have some people who swear by it and some people who say you just need bigger pipes,” he said.

“I don’t feel that either group has been proven right.”

He said that researchers were moving toward the idea that greening had some role in flood mitigation, but that for really extreme storms the effect would be marginal.

“The counterargument to just greening everything is will the soil be able to absorb it quick enough?”

Dr Dobson said he would be "surprised" if fatbergs had any connection to the July 25 flooding, as in much of London the stormwater and foul systems are separate.

Fatbergs are masses of waste matter formed by a mix of non-biodegradables in sewers such as wet wipes as well as deposits of oil and grease.

He felt that there are always actions that water companies could take to minimise flood risks – building bigger pipes, more frequent repairs, installing more tanks – but that there were no “free wins” and that these costly fixes would end up being paid for by the consumer.

His research group is currently looking into the possibility that water butts – household storage tanks for rainwater – could be used as a cheap and efficient way to mitigate flooding.

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