Can trees and woods reduce flooding?

Credit: Ian Thwaites / Alamy Stock Photo
Credit: Ian Thwaites / Alamy Stock Photo

Storms are making their mark. Each year heavy, driving rain is bringing misery to thousands of people across the UK. And it's prompted serious interest in the role trees can play in keeping people and property safe.

One in six homes are at risk of flooding. February 2020 was the wettest on record, the fifth wettest month since 1862. 3,300 properties were flooded by Storm Ciara and Dennis, destroying people’s homes and livelihoods.

Climate change means we will see more extreme weather events. But using trees and woods we can improve flood risk management to lessen the devastating effects of flooding.

Why are we experiencing more flooding?

Climate change has increased the likelihood of the extreme rainfall events which are now considered a significant risk factor for the UK.

Changes to our rural and urban landscapes also mean that rain is hitting the ground at higher speeds and taking much long to drain away, leading to an increased risk of flooding.

These are the main types of flooding that affect communities.

Credit: James Osmond / Alamy Stock Photo

River flooding

When a river can’t cope with the amount of water entering it and overflows its banks.

Credit: Terry Whittaker / Alamy Stock Photo

Surface water flooding

Heavy rainfall onto already saturated or compacted ground or hard surfaces runs across the surface and collects in lower areas.

Credit: Philip Pickin / Alamy Stock Photo

Drain and sewer flooding

Following heavy rain when drains have become blocked.

Credit: DGDImages / Alamy Stock Photo

Coastal flooding

When weather and tidal conditions combine to increase sea levels, inundating coastal areas. This type of flooding can surge inland often affecting nearby neighbourhoods.

How can trees help with flooding?

Flooding poses a serious risk. But there is hope.

We can respond to flooding by planting more trees. They help to reduce flooding in a number of ways, all while sequestering carbon, adding value to our lives, and much more. Here’s how they do it.

Canopy cover

Rain hits the ground at higher speeds where there is a lack of tree cover. A canopy of leaves, branches and trunks slows down the rain before it hits the ground simply by getting in the way. We call this interception.

The interception of rainfall by trees can spread the effect of a rain storm over a longer time period. This allows some of the water (studies suggest up to 30%) to evaporate back into the atmosphere directly from the canopy without ever reaching the ground. All trees act in this way, but our native broadleaf trees do it best in summer when they’re in full leaf, but even in winter trees can intercept and re-evaporate rainfall.

Roots

Root systems help water penetrate deeper into the soil at a faster rate under and around trees. This means less surface run-off and more water storage in the soil.

In rural areas, the removal of trees and hedges and changes in cropping have increased run-off from fields. Planting trees and hedges can be effective in increasing water infiltration, and reducing and slowing run-off on farmland. Water infiltration rates can be 60 times higher within tree shelterbelts than adjoining farmland.

Urban trees

In towns and cities, the rise in impermeable surfaces such as roads, pavements and driveways has led to increased surface water run-off. Following UK-wide flooding in the summer of 2007, a report from Sir Michael Pitt identified the paving over of front and rear gardens as a major factor in surface water movement in towns and cities. Around two thirds of all the flooding in 2007 was as a result of surface water run-off.

80%
That’s how much more trees reduce surface water run-off compared to asphalt.

An increase in green space could drastically reduce run-off in built-up areas. There are other, smaller ways to make a difference too. For example, the small patches of bare ground around street trees, known as tree pits, are sometimes covered by asphalt. Removing the asphalt is an easy way to improve water infiltration in urban areas.

Dams and deadwood

Trees, shrubs and deadwood along riverbanks and on floodplains act as a drag on flood waters, holding back water and slowing the flow at times of flooding. But trees can help reduce flooding even when they’re not in the ground. Wood dams and deflectors are carefully placed and individually designed as a flood prevention measure.

These dams can be used to direct water into preferential areas, increasing temporary water storage and slowing down the flow of water. They are a low-cost solution that utilises natural materials and processes.

Fight against flooding: our work on the ground

Smithills Estate, Lancashire

There are more than 25 different flood prevention measures, including dams, at our Smithills Estate near Bolton.

It's all about discovering the effectiveness of flood prevention measures in the uplands and how these can help alleviate any flooding in the lowlands. Smithills rises to 456m above sea level and borders a large urban area.

One of the measures is leaky dams, which have been placed in strategic locations across the uplands. They are made of log piles each placed further down than the last, creating a stepped landscape. As the water fills each step and slowly filters through the gaps between the logs, it slows the flow and soaks up the water.

There are also wetlands and scrapes, which involves creating a shallow excavation to hold water in the landscape.

So far the measures are producing positive results, reducing levels of excess water. And there's a bonus. The new environments have attracted an elusive insect never before seen in Greater Manchester, the keeled skimmer dragonfly.

We need more trees

The message is loud and clear. The UK needs more trees.

Evidence and experience suggest that trees and green spaces play a central role in protecting us against flooding, offering cost-effective benefits. We must expand tree cover and protect and restore woodland on an unprecedented scale.

That's why our work is so important...

  • We expand, restore and protect native trees and woodland in the UK. In 2018 we planted, distributed or sold 3,254,048 trees, creating some 1,714 hectares of new woodland across the UK in rural and urban areas.
  • Our work lobbying government ensures that natural flood risk management solutions are considered as part of local and regional flood management schemes. 
  • We support the work of the Environment Agency, providing evidence, technical guidance and tools for flood and coastal risk management authorities.
  • To improve targeted tree planting, we continue to call for mapping so that we know where trees and woods are most likely to reduce flood risk in England.
  • Improving the resilience of our landscapes is crucial to us. We encourage government to do more to incentivise woodland creation and tree planting in countryside, towns and cities.

Our work will ensure the UK can tackle its climate and biodiversity crises at the same time. Find out how we'll make it happen in our Emergency Tree Plan for the UK.

Looking up into canopy, Tring Park

Fight for our planet

Support our work to plant trees and look after the ones we have.

Become a member today
Keep exploring