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Why London's dry ground will be slow to recover

Briohny WilliamsLondon
BBC Aerial view of a church surrounded by drought-hit grassland, with central London visible in the distanceBBC
All Saints church, Blackheath, surrounded by drought-hit grassland

London is still experiencing drought conditions and, despite recent rain, much of the capital's ground will take time to recover after weeks of record-breaking dry weather.

About 80% of London sits on clay, which extends to depths of more than 100m (328ft) in some places.

The clay has lost moisture, causing it to shrink and crack. In some cases, this can lead to subsidence, resulting in damage to buildings and homes.

Sustained rainfall over a prolonged period will be needed for the ground to fully recover, experts say. Rob Higginson, a geotechnical engineer, says it will be a "very slow process" and that "a single downpour is not going to do it".

Aerial view of dry, yellow grassland in a London park, with the city skyline in the background.
Parks and green areas across London have dried out owing to the hot weather

July was the driest on record for England and Wales, and a hosepipe ban remains in place for Thames Water and Affinity Water customers in London following prolonged dry weather, low rainfall and increased demand for water.

Higginson said that while "putting a hose on the ground will keep your grass growing, it won't affect the underlying clay at all".

He explained how recent heatwaves had affected clay deep underground.

"If the clay gets warm, it loses volume, and it's going to go down and contract.

"You get the cracking, but you also get downward movement. If a house is sitting on top of the clay, it will go down with it."

He said this predominantly affects homes built between the post-war period and the 1980s.

Rob Higginson standing in a dry London park where grass has turned brown during drought conditions.
Rob Higginson, a geotechnical engineer, says London's clay ground needs prolonged rain

However, even when rain does arrive, he said the recovery process would not be straightforward.

Higginson said: "The topsoil will absorb a certain amount of it, and then it will go down and sit on the clay layer.

"Then you've got surface runoff, so the excess water will just run off.

"What you really need is prolonged rain. It doesn't have to be intense, just a reasonable amount over a long period of time, so it has time to percolate into the very fine voids in the London clay."

Even when the grass begins to turn green again, the clay beneath may take much longer to recover from the effects of this summer's drought.

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