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Trees, woodlands and flooding

Tom Nisbet, Science Group Leader of the Head Physical Environment Research Group at Forestry England, discusses the role of woodland creation and management in natural flood management solutions.

Despite Covid-19 continuing to dominate the headlines, the issue of flooding remains much in the news linked to the climate emergency. Faced with this threat, the Government is investing £5.2 billion in flood and coastal defences to reduce flood risk. A growing part of the mix is the use of nature-based solutions to mitigate flooding, with a Government commitment to double the number of Natural Flood Management (NFM) projects over the next six years.

Woodland creation and management are a core part of NFM and expected to feature strongly in flood management schemes. The most obvious way that woodland can reduce flooding is through the greater water use by trees. Trees can evaporate more rainfall than shorter types of vegetation due to the tall and well-ventilated nature of their canopy.

The evaporation of rainwater as a rising cloud above the forest canopy following rainfall, known as canopy interception. Image: Forest Research

An exposed tree root system showing how rooting can create an intricate network of large pores allowing rainwater to quickly infiltrate into the soil. Image: Forest Research

Secondly, woodland soils can readily accept heavy rainfall due to their undisturbed and well-structured condition. Regular inputs of organic matter from leaf fall and the action of tree roots creates an intricate network of large pores, allowing water to pass down into the soil, rather than quickly run off the surface.

A third way that woodland can contribute to NFM is by acting as a physical barrier to surface runoff. This is greatest for mature floodplain woodland, where the thick undergrowth, fallen trees and branches, and the formation of ‘leaky woody structures’ within streams act to slow flood flows.

The role that woodland can play in contributing to NFM is being tested and demonstrated in case studies, a key example of which is ‘Slowing the Flow at Pickering’ in North Yorkshire. This Forest Research led project was established in April 2009 to look at how changes in land use and management could reduce the incidence of flooding in the town. You can find out more about the 'Slowing the Flow at Pickering' project here.

A leaky woody structure reducing flow downstream during a flood. Image: Forest Research.

The aim was to reduce the chance of flooding in Pickering from 25% to 4% or less in any year using a combination of engineered and natural approaches. These included the construction of a large flood storage reservoir, the installation of many leaky woody structures and heather bale check dams, plus targeted woodland planting within the Pickering Beck catchment.

A strong local partnership was formed to deliver the set of measures within the original 6-year project timescale, the placement of which had to balance a range of factors and interests in the sensitive and designated landscapes within the catchment.

The measures remain to be fully tested by a large flood but an analysis of the effects of the Boxing Day 2015 storm, when 50 mm of rain fell over a 36-hour period, concluded with a relatively high degree of certainty that they prevented flooding to a small number of properties in the town. It was estimated that the measures reduced the flood peak by 15-20%, with around half of the reduction due to the NFM measures and half due to the flood storage reservoir. The results show that NFM measures are most effective when used in combination as part of a wider catchment-scale approach to managing flood risk

For further information on NFM, visit the UK Government's evidence directory on working with natural processes to reduce flood risk.