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Insight | What engineers can do as London sinks

London victoria station flooded

As The Clash aptly put it back in December of 1979: “London is drowning, and I live by the river”.  

Well less drowning, more sinking, according to a Christian Aid report published this month, which claims several major cities are sinking into the ground, including London and Shanghai, China. 

London’s slow march into the mud comes as fallout from the last ice-age. The technical term is ‘‘glacial isostatic adjustment’’, which refers to the movement of land that was previously locked in place by glaciers. 

The glaciers that carved the impressive Scottish landscape also weighed the UK down like a see-saw, driving London and the southern counties upwards and out of the sea.  

Since these behemoths have now melted, the effect is reversed. Scotland is rebouding at a rate of 1mm a year, the report says, and London at 2mm a year. Coupled with rapidly rising tides, the water comes 3mm to 4mm closer every year to returning London to marshland it once was.

 

 

The IPCC predicts sea levels could rise as much as four meters.

The IPCC predicts sea levels could rise as much as four meters

Source: IPCC

The IPCC predicts sea levels could rise as much as four meters.

The growing threat is no more apparent than in the current use of London’s primary defence against the tides: the Thames Barrier. When opened in 1984, it was envisaged that the barrier would be raised to defend against a once-in-a-century high flood. Now it is closed six or seven times a year.  

 

Environment Agency London area manager for flood and coastal risk management Cantor Mocke said that the increasing pressure from the changing enviroment meant a changing threat to London which has led to the creation of the Thames Estuary 2100 Plan.

“London and the Thames Estuary benefit from a world-class system of flood defences, including the Thames Barrier and other major flood defences, walls and embankments, pumping stations, and flood gates,” Mocke said. 

“Increasing pressures, including climate change, mean that flood risk is increasing, however, the Environment Agency has worked in partnership to develop the Thames Estuary 2100 Plan to provide strategic direction for managing tidal flood risk in the Thames Estuary up until the end of the century.

“It sets out how we can work together with risk management organisations, infrastructure providers, private industry, and the public to continue to protect the 1.3M people and £275bn worth of property in the estuary, and maintain London’s reputation as a growing global city.”

 

Unlike London, Shanghai is sinking because of groundwater use and extraction, which can be reversed by pumping water back into the ground, an equally simple solution is not available for London. 

WSP head of water specialisms Hammish Hall says that because London’s rising water is very different to cities in Shanghai or Jakarta in Indonesia, money is better spent on flood resilience and some cases flood defence, instead of trying to stop the sink.  

“London’s situation is not like Indonesia, where ground water recharge has had some benefit in stopping sinking. The problem we have isn’t to do with ground water extraction, we can’t stop the whole of the south coast from sinking by this see-saw effect,” Hall said.

Hall explains that if engineers can’t keep the city from sinking, the next step is to keep the water out. Flooding to major cities like London is so costly that large, expensive infrastructure like the Thames Barrier is justified, says Hall, but smaller communities would be better off with homes designed to withstand a flood, not keep it out.

“For somewhere as economically important and densely populated as London, flood defence is the most viable option. It is too expensive and disruptive to allow our capital to flood, so the defence approach is appropriate,” Hall says. “But somewhere like Weymouth for example, resilience is a better option and people will learn to deal with some occasional flooding.” 

He adds: “Instead of keeping water out, which is very hard to achieve, you can replace carpets with hard flooring, move plug sockets to above waist height, and change cheap plaster board for one that doesn’t need to be replaced every time it gets wet.” 

Hall says that designing buildings to be resilient to flooding allows people to back in their homes in a matter of days instead of weeks. 

With good flood warning, valuables can be moved upstairs, and the flood can come and go, the rooms can be hosed down, and people can be back in their homes in a matter of days instead of the weeks and months it can take to re-establish homes now.” 

Cundall associate director Simeon Parker agreed that the way forward was designing buildings to withstand flooding, and that it should be part of engineer’s professional responsibility. 

“We need to fully protect our hospitals residential areas and infrastructure like sewage treatment works and power stations and generators, outside of that, with construction materials and the way we design them, we can allow water to pass through buildings,” he said.

“We should always look to use features and space in and around the building to manage water within the site, that way we are not blindly pushing the problem onto someone else, we are dealing with it there and then. This should be part of our professional responsibility.” 

Cundall is implementing this way of thinking in their current projects, said Parker, citing one residential project below a river’s flood level in London.  

“The way we are dealing with that is having habitable levels lifted above flood level, but we are not lifting the building up on slits with dead space below. That space has functions not affected by water ingress, like cycle stores, parking, and cleaner’s storage. We are making sure the buildings plant rooms and other critical systems are elevated above that flood level.” 

“That area essentially comes sacrificial because we use materials in the construction that are not susceptible or affected by temporary storage of water. “

According to the National Assessment of Flood Risk, published by the Environment Agency in 2009, “around 5.2 million properties in England, or one in six properties, are at risk of flooding”. Therefore it is vital future buildings are designed to withstand flooding, as Parker and Hall suggest. 

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