Andrew Heald: Trees offer cheap and effective flood prevention

Planting more trees in our uplands could slow the flow of water and reduce flooding downstream. Picture: PA

The debate on flooding and how to prevent it is widening to embrace natural flood management, including large-scale tree-planting. Solely relying on engineering solutions, like flood barriers, means fighting nature rather than working with it.

Many communities across Scotland were badly affected by the recent floods, with existing defences proving inadequate in the face of extreme weather.

So rather than raising the barriers higher still, it’s time to go back to the source of the problem and look at planting more trees in our uplands to slow the flow of water and reduce flooding downstream.

The UK’s uplands are largely treeless, following centuries of clearance – they are poor at retaining water, particularly during very heavy rainfall. We have engineered our landscapes, through measures like river straightening and improved drainage, to move water downhill quickly, increasing peak flows when water reaches downstream communities.

Anne-Marie Trevelyan, MP for Berwick-upon-Tweed, suggested in a Westminster debate that 200 million new trees by 2020, mainly in the uplands, could “help nature to hold water and to reduce the risk of flooding in the long term. ”.

There are several ways tree planting reduces water flows:

• Trees pull more water from the ground than grass or other short vegetation and release it into the atmosphere via needles or leaves.

• Tree roots are much larger and deeper than grass roots, opening up the soil structure and allowing water to percolate into soil more quickly and deeply.

• Rain falling into woodland is intercepted by hundreds of branches, leaves and needles, taking much longer to reach the ground.

• Trees and woody debris along streams and river banks can slow the flow of water reaching a stream and the speed of a stream reaching a river.

• Silt in lowland rivers reduces their capacity during floods. This is largely soil and small stones washed into streams from fields. By planting trees, soil is protected and the chance of erosion reduced.

• Allowing rivers to flood into wooded areas reduces the speed and potential impact further downstream as woodland helps retain flood water.

Woodland not only reduces the overall volume of flood water, but more critically, slows the speed of the water, giving people longer to evacuate and lessening peak flows to reduce potential damage.

A number of experiments in the uplands of Scotland, England and Wales have investigated whether natural flood management is effective.

The Tweed Forum project on the Eddlestone Water has helped to significantly reduce the potential for flooding on the Tweed.

A mature woodland can be 60 times as effective as bare hill land in slowing water flow into rivers. But we don’t need to wait 40 years for trees to grow.

Current research shows a change from grazed pasture to woodland can deliver water retention benefits from just one year onwards, with increasing benefits as the trees take root and grow.

It is also very cost-effective compared to heavily-engineered, highly expensive flood defences.

It shouldn’t be a question of whether we plant more trees, but where we plant them as part of a longer-term, more natural approach to flood prevention.

• Andrew Heald is technical director of Confor: promoting forestry and wood www.confor.org.uk