FWN39 Spring 2023 – New Guidance for Using Trees to Combat Flooding

Land managers across Scotland are being urged to take note of new forestry guidance on using trees as natural defences to minimise the risks of flooding. UK Forestry Standard Practice Guide: Designing and managing forests and woodlands to reduce flood risk gives practical advice to landowners, managers and authorities involved in flood risk management, on how best to use and manage trees to reduce flooding.

The guidance outlines key considerations for flood risk during planning and design, including:

• Site location and conditions
• The scale of planting
• Design of the forest or woodland
• Timescales of effectiveness in relation to
  tree age and water usage.

The guide also provides advice on operations including cultivation, drainage, and road construction and harvesting, as all these factors can have negative effects on flooding if not managed properly. Employing effective management practices during these processes can slow, intercept and reduce runoff to minimise the impact on flood risk.

How trees help reduce flooding

Trees and woodlands can act as a form of natural flood management (NFM), from woodlands throughout a catchment area, to trees along the banks of watercourses.

• Trees evaporate more water than any other type of vegetation. Moisture is released back into the atmosphere.
• The large surface area of a woodland canopy intercepts rainwater. All the leaves and branches collect water droplets, slowing down how fast they reach the ground.
• Roots help water to infiltrate deep into the soil, meaning less runs off the surface.
• Trees and woody plants on floodplains help to slow water flow, reducing flood peaks. Deadwood also helps to slow the flow.
• Trees along watercourses help to stabilise banks, reducing erosion and siltation. Reduced siltation in turn maximises the capacity of the watercourse to contain water.

Natural flood management uses woodland and trees to slow the flow as part of a whole catchment approach to managing flood risk

© Crown Copyright. Reproduced with the permission of the Forestry Publishing Group from UKFS Practice Guide Designing and managing forests and woodlands to reduce flood risk.