How Heavy Rain After Heatwaves Can Lead To Flash Floods

Following a spate of heatwaves in parts of the UK this summer, we’re finally getting some below-average temperatures and rain.

This includes areas of the parched south and southeast, like Wales and East Anglia. These regions have declared drought following an unusually dry season and increased water usage (sadly, sudden downpours don’t seem likely to change those conditions any time soon).

And while the wet weather is very welcome, it may also pose some risks.

Yesterday, the BBC wrote: “Flash flooding remains possible, especially in areas where the ground remains very hard after the prolonged hot, dry spell”.

Flash floods have already taken place in areas like Edinburgh.

Per NASA, it’s a “rapid rise of water along a stream or low-lying urban area” that usually occurs within six hours of heavy rainfall. When the volume and speed of its fall means water can’t be drained away quickly enough, it creates flood conditions.

The UK government describes flash flooding as “very dangerous”, advising people not to walk or drive through the water resulting from it. Even “shallow flood water (around 15cm or 6 inches) that does not look dangerous can knock a person over or conceal hazards”.

Flash floods, they explain, can create fast-moving and deceptively deep water that may injure or kill people and can happen anywhere, including towns, cities and the countryside.

It has a variety of possible causes, but if flash flooding comes from heavy rainfall, it’s sometimes also called surface water flooding.

To clear away sudden rainfall faster, the ground needs to be able to drain efficiently.

And though you might think that dry soil would soak up rain the fastest, that’s not actually the case. A video filmed by Dr Robert Thompson from the meteorology department of the University of Reading after the 40ºC heat of 2022 illustrated why.

The clip, shared on X, showed three clear cups of water upended onto wet, “normal summer”, and post-heatwave grass.

While the first two conditions drained the container quickly, the heatwave-struck ground took far longer to absorb the liquid.

In the comments underneath the post, Dr Thompson said that after very long spells of hot weather, soil “gets compacted through the lack of water.

“But there’s a big factor from the small soil particles becoming hydrophobic (like the coating on waterproof coats) – which combines with water’s very strong surface tension, meaning water molecules can’t fall through the small gaps.”

Then, he added, there’s the fact that “warmer air holds more water, so heavy rain is heavier… that’s a pretty bad combination and makes flash flooding an increasing threat.”

A combination of compacted, unabsorbent ground and a greater mass of already-drenching water makes flash flooding risk higher.

Flash floods can be hard to predict. But some warning signs include:

• Sudden, heavy rain, i.e., from thunderstorms,
• Dark water that moves very rapidly,
• Severe weather warnings,
• Damage to buildings and structures near the flooding.

The government advised that staying up-to-date with local weather warnings and signing up to flood warnings (via this link) can help.

But they added that if you suspect flash flooding, you may need to act quickly before any official warning is given.

Try to get to high ground safely if possible, don’t travel through flood water either by foot or vehicle, and call 999 if you need to. If you do speak to the emergency services, follow their instructions carefully.

“Do not use flimsy or temporary structures – like a tent or a caravan – for shelter during flash flooding,” the government added.