Planting trees on floodplains can reduce the height of flooding in towns downstream by up to 20 per cent, researchers have found.
Scientists studied a whole river catchment in the New Forest over 100 square kilometres in order to understand how tree planting, river restoration and log jams (man-made dams) might affect the peak height of a flood.
Using a digital terrain model of the landscape and a hydrological model simulation the researchers from the universities of Birmingham and Southampton found planting trees on the floodplain and increasing the number of logjams, across 10-15 per cent of the total river length, could reduce the peak height of a potential flood in the town by six per cent – once the trees had grown for 25 years.
The scientists found that more extensive floodplain forest and river restoration (for example in 20–25 per cent of the total river length) resulted in a reduction in flood peak height of up to 20 per cent. As the trees age and the forests become more mature and complex up to 100 years post planting there are larger reductions in flood peak height.
The research, which was funded by the Environment Agency (EA), has been published in the journal Earth Surfaces Processes and Landforms.
The EA has been looking at the potential for river restoration techniques to be incorporated into wider flood risk management programmes.
Dr Simon Dixon, from the University of Birmingham’s Institute of Forest Research (BIFoR), lead author of the study, said: “We believe that tree planting can make a big contribution to reducing flood risk, and should be part of a wider flood risk management approach, including conventional flood defences.
"Tree planting would represent an extra element that helps to slow down the arrival of rain water to vulnerable locations.”
Welcoming the study, the Woodland Trust said farmers had a vital role to play in planting trees to reduce the scale of some flood events.
Diane Millis, the trust’s senior adviser on water and ecosystems said: “Farmers have a valuable role to play in understanding the problems within a catchment and the role that land use and management can play, alongside the expertise of flood risk professionals.
In order to achieve the best results we would like to see more innovative incentives provided to farmers and land managers to modify land use to maximise flood risk reduction."
Information on grants to plant trees
The Woodland Trust offer farmers subsidised, and in some cases free trees, with the farmer providing the maintenance.
Grants are also available through the Countryside Stewardship Scheme for tree planting and hedgerow maintenance. For advice email firstname.lastname@example.org
Farmers and landowners in Tebay, Cumbria, have been working with the Woodland Trust and Natural England to plant 61,000 trees to help reduce flood risk and improve water quality.
The trees have been planted on marginal land, enabling commoners to graze the majority.
The Woodland Trust said that over time, tree roots will break up the compacted earth, allowing soils to absorb more rainfall.
Water running off the hills will be slowed down, reducing the potential for flooding in the valleys below, and cutting the amount of sediment making its way into the Lune, a major salmon river.