Urban umbrella: how trees stem flash-flooding

This post was written by Stephen Gray, Press Coordinator for Trees for Cities, an independent registered charity. -Ash

As the Xynthia storm starts to move away from western Europe and England, recent studies compiled by the urban tree charity Trees for Cities have been finding that even the familiar London plane is protecting us from flooding more than previously thought.

The urban umbrella

Trees work to protect urban areas in two simple ways: by intercepting water at the canopy, and by absorbing surface water through their roots. Water is drawn up through the tree in the process of transpiration and it is eventually lost to the atmosphere through stomata on the underside of the leaves.

By slowing the rate of sudden, heavy rainfall, trees reduce the peak volume of water needing to be displaced during the storm, and so avoid the need for expensive man-made floodwater containment systems, and the risk of sudden flooding.

The average cost to a UK resident of a flooded home is between £20,000 and £30,000 i.

Studies

In England, a study in Northumberland calculated the value of woodland areas around a river, in terms of the offset engineering costs of flood control, at £1,200 per hectare. In our changing climate, woods around urban areas and the trees within them are in especial need of protection for them to carry on providing these benefits ii.

In the city of Atlanta, USA the American Forests organisation calculated that if trees were removed, the cost of building containment facilities for the floodwater runoff would be $2 billion.

This means that for every $1 spent on trees, $5 can be recouped in money saved iii.

Trees for Cities

Trees for Cities, a UK charity which undertakes projects nationally to improve green urban spaces, has been campaigning for and planting new trees in cities since the charity was launched in 1993. In London, the mayor, Boris Johnson, recently announced a new target for the city of an increase in canopy cover from 20 to 25% by 2025 iv - which would make a quarter of the city green from above.

Sharon Johnson, Chief Executive said: "Over recent years we have seen a real difference in the attitude taken to trees in UK cities. They're becoming much more valued as an asset to the community, providing a wealth of financial and cultural benefits.

"As a charity, we're committed to increasing awareness of these benefits: our streets are community spaces, and so many are an untapped resource for the people who live on them. We've been proud to be planting trees across the country, through individual donations and tree planting schemes, so that residents can enjoy their benefits for hundreds of years to come."

Other benefits

Protection from storms is not the only reason for increased interest in urban vegetation: A house sheltered by street trees can have its heating and air-conditioning costs reduced by up to 10%, as the trees insulate the area and reduce wind speed in winter, as well as providing shade and reflecting heat away from the ground during warm summers.

Trees also provide a natural aesthetic benefit to streets, and when a suitable mix of species is chosen, should pose little threat to the integrity of surrounding buildings.


References

i www.publications.parliament.uk/pa/cm200910/cmselect/cmenvaud/uc113-iv/uc11302.htm
ii www.treeworks.co.uk/downloads/s15speakers/Isabel_Dedring_Mayors_Office.pdf
iii www.treesforcities.org/files_reports/tfc_treesMatter.pdf
iv www.milliontreesnyc.org/downloads/pdf/nyc_mfra.pdf


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