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Trees to combat flooding, study shows

Groundwater flood

A new study from the universities of Birmingham and Southampton has shown that strategic tree planting on floodplains could combat flooding.

Targeted tree planting could reduce the height of flooding in towns downstream by up to 20%, according to the research, funded by the Environment Agency.

Scientists studied a river catchment in the New Forest over an area of 100km2, upstream of the town of Brockenhurst. The team looked at how tree planting, river restoration and “log jams” – man-made dams which have been shown to locally slow the flow of rivers – might affect the peak height of a flood in a downstream urban location.

University of Southampton professor David Sear, who supervised the project, said: “With increasing interest in alternatives to conventional hard flood defences, there is an urgent need for evidence that these alternatives can work. This research reminds us that natural processes, when targeted carefully, can reduce downstream flood risk alongside other societal benefits including biodiversity and recreation.”

Using a digital terrain model of the landscape and a hydrological model simulation, the researchers found that planting trees on the floodplain and increasing the number of log jams across 10-15% of the total river length could reduce the peak height of a potential flood in the town by 6%. However, this level of flood prevention is only reached once the trees have grown for 25 years, according to the findings.

The scientists found that more extensive floodplain forest and river restoration – for example, in 20-25% of the total river length – resulted in a reduction in flood peak height of up to 20%. The researchers found that 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.

Simon Dixon, from the University of Birmingham’s Institute of Forest Research and lead author of the study, said: “As our research shows, targeted tree planting and restoration can contribute to reducing flood risk. 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.”

The research team also studied engineered log jams. It has been reported that log jams are a positive intervention and could form part of a general strategy for alleviating flooding. However, the scientists found that although log jams slow the flow locally, this did not always translate to reduced flood risk at the catchment scale. The study showed that although log jams reduce downstream flood risk in some locations, in others they had no effect, or even increased flood risk. The researchers have recommended detailed site studies to identify the best locations to install them for flood mitigation.

Dixon added: “Log jams contribute to slowing the flow by backing up water and pushing it onto the floodplain. In locations where the floodplains are meadows or crops the water may still be able to flow over the surface quickly. To make the best contribution to flood mitigation, it is important they are used in locations with complex bankside vegetation to slow water flowing over the floodplain.”

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