What are Flash Floods?

The sudden and intense flooding known as “flash flooding“, or surface water flooding can be caused by excessive rainfall, runoff, or as a result of blocked drains.

Urban areas tend to be most susceptible to flash flooding due to the vast impermeable ground cover, scale and proximity of development restricting the natural infiltration into the ground. This results in an increased risk of overland flow as drainage networks become overwhelmed by intense rainfall events. As a result of climate change and land management practices, the prevalence of flash flooding in the U.K has increased, and will continue to do so as weather events become more severe.

London Flooding in 2021

A notable example of flash flooding was in July 2021 in London, where, in some areas, more than a month’s worth of rain was recorded as having fallen in a single day. This set a rainfall record for the month, and ultimately resulted in widespread flooding, impacting many homes and businesses across West and South London. The flash flooding also disrupted transport networks, with Bermondsey and Pudding Mill Lane tube stations being severely impacted. At these stations, water flowed onto the tracks, closing the stations. The signalling systems were also impacted and infrastructure damage was significant.

Basements and cellars were particularly at risk, with them naturally situated below ground level, such that when overland flow occurred, they were the first to be impacted.

The flash flooding in London made it clear that the city’s infrastructure is not designed to cope with such extreme weather events, and more focus should be given to climate change adaptation. One such strategy would be the introduction and expansion of Sustainable Drainage Systems to mimic natural processes and reduce the likelihood of flash flooding.

Following the flooding, the Mayor of London, Sadiq Khan, demanded more financing for uprating infrastructure and flood defences in order to help manage and mitigate future flood risks. He also encouraged property owners and managers to implement Property Flood Resilience measures to help protect their own properties. This could include aspects such as flood barriers or non-return valves.

What precautions could be taken against flash flooding?

• The first step is to know your flood risk. A recent study showed that 67% of people are unaware of their flood risk. You can check your risk of surface water flooding on the relevant government website (Environment Agency, Natural Resources Wales or the Scottish Environment Protection Agency).
• Whilst the Environment Agency have a robust flood warning system, this is focused on river flooding, and therefore we recommend signing up to a severe weather alert app alongside any flood warnings.
• Install Property Flood Resilience measures such as flood doors, or anti-flood airbricks to reduce ingress into your property. Any such installations should follow a Flood Survey.
• Prepare an evacuation plan: Knowing where to go and how to get there is crucial in the event of a flash flood. Identify safe exits from your immediate area and prepare a place to remain in the event of an evacuation.
• Prepare a flood evacuation kit: Create an emergency supply kit containing nonperishable food, water, first aid supplies, medicines and necessary paperwork.

If a flash flood warning is issued for your area, you should be prepared to act swiftly, and avoid entering flood water where possible, due to the hidden dangers and potential for harm.