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Consultant warns of London's hidden groundwater flood risk

London faces a serious threat from groundwater flooding and officials are failing to act to mitigate risks, a leading water consultant said today.

WSP head of water Ola Holmstrom said that although the capital faced the possibility of river and tidal flooding, the risks were known and as a result, London is well prepared compared to other parts of the country.

But the scale of the groundwater flooding threat is unknown and is a cause for concern, said Holmstrom.

“Tidal and fluvial flooding are going to be limited issues [in London in the future],” said Holmstrom.

“Surface water flooding is a major risk, but it has big question marks surrounding how big a risk.

“It is there, although [the flood levels] are often shallow. We can handle it, we can improve things.

“Groundwater flooding is still an unknown. I warn you that what has been built over the past 20 years and what has gone into soakaways is a major liability.”
Holmstrom said that while sustainable drainage systems (Suds) yielded benefits, there was a concern that, in the capital, relatively impermeable underlying clays mean water is restricted to the upper strata, potentially making groundwater flooding as it seeks to escape more of a risk than people are aware of.

He said new property developments should consider the full impact of Suds and other interventions in the ground such as piles, which might affect groundwater levels.

“We need to start building schemes that can be adapted as they go along,” said Holmstrom. “We must stop buildings that are static.”

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