Natural defences

Rather than invest in ‘grey’ infrastructure to tackle urban flooding, planners are turning to nature to mitigate the effect of severe storms

Leaf skeleton

Photograph by Giles Revell

In 28 June 2012, two hours of torrential rain caused flash flooding across Newcastle, and power cuts to 23,000 homes; the storm also caused lightning strikes to the Tyne Bridge. 'Thunder Thursday' as it came to be known, caused an estimated £8m worth of damage to homes, roads and local businesses.

When a storm of that magnitude hits a city, the run-off from impermeable surfaces – roofs, pavements and roads – quickly overwhelms the drainage system, causing flooding and transport chaos. The clean-up costs for a severe rainfall event like Thunder Thursday can run into millions of pounds.

Increasingly, planners and local councils are coming to recognise the role that nature can play in preventing flooding. So-called 'blue green infrastructure' – green roofs, green walls, street trees, pocket parks, ponds and water channels – mimics the way that nature handles water, reducing the rate and volume of run-off that would otherwise inundate an area. It can be extremely effective – using Newcastle city centre as a case study, researchers at Newcastle University compared the effectiveness of blue green infrastructure with traditional engineering measures, such as culverts or flood walls, in reducing transport disruption during extreme rainfall events like Thunder Thursday. Research published by the University in the Royal Society Open Science Journal found that greening every roof in the city could reduce disruption to transport systems by just over 25%.

Green infrastructure is now rising up the UK political agenda. As part of his London Infrastructure Plan, Mayor Sadiq Khan established a Green Infrastructure Task Force; Birmingham and Liverpool are also developing citywide green infrastructure plans.

While few would dispute the benefits of greener cities in terms of flood prevention, mitigating urban heat islands, reduced pollution and a better quality of life for the people who live in them, the economic value of green infrastructure has been harder to prove. Attaching a specific investment cost to many overlapping outcomes is tricky. Some methodologies already exist for measuring the economic benefits of natural assets; the most widely used of these is a suite of tools, i-Tree, which is used to evaluate the benefits of the 'urban forest', including carbon storage, reduction of air pollution and reduction of stormwater management. The i-Tree assessment of London that was carried out in 2015 calculated that the capital's urban forest had produced the following savings:

= 3,414,000m3 per annum,
worth £2.8m

= 2,367,000 tonnes per annum,
worth £146.9m

= 2,241 tonnes per annum,
worth £126.1m

The Natural Capital Committee, the government's advisor on the economics of the environment, is testing new accounting frameworks that also take into account less tangible benefits such as recreational enjoyment and aesthetic experience. As cities become smarter, data science can also be used to measure the impact of green infrastructure, such as green roofs and green walls, more effectively.

In Washington D.C., where urban stormwater run-off is the fastest-growing contributor to pollution, the government has created a new marketplace where green infrastructure is a tradeable asset. Property owners who build green roofs, rain gardens and the like, are given stormwater credits that they can sell to others who need to offset the run-off from their developments. In London, the borough of Camden is trialling a Parks Improvement District, funded by levies from local businesses that benefit from their proximity to green squares in Bloomsbury.

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