Why trees are essential for our flood defences
When rain falls on woodland, it does not all hit the ground at once. Leaves, branches and bark intercept the rain, allowing a significant proportion to evaporate back into the atmosphere. Studies of woodland in the UK suggest that around 25–30% of rainfall can be intercepted during storms, significatnly reducing the amount of water reaching the soil throughout the heaviest downpours.
Once you get below ground, tree roots then transform how soils behave too. Roots create pores and channels that open up the soil and allow it to absorb the water rather than flow across the surface – this is known as soil inflitration. Research by the Environment Agency shows that woodlands can boost soil infiltration up to 60 times compared to grazing pasture. Trees turn a potential flood rush into a slow, deep soak.
In fact, trees and woodlands are so good at this job that the Government has estimated that they are worth over £400 million annually in protecting communities from flooding.
Trees that work well in Sussex
Different parts of Sussex call for different trees. Along rivers and streams, alder and willow thrive in wet soils, stabilising banks and bending rather than breaking during high flows. On slopes above floodplains, oak and beech develop extensive root systems that improve soil structure and reduce runoff from heavy rain.
At Knepp and other rewilding sites, fallen branches and root systems form natural “leaky dams”, holding back water upstream and releasing it slowly over days rather than minutes. These features mimic how rivers functioned historically, before channels were straightened and cleared.
Riparian woodland (strips of trees lining waterways) combine shade, root strength and fallen wood to both slow water during storms and reduce erosion over the long term.
Beyond flood defences
There is a useful side effect too. Slower water drops its sediment. That means less silt choking rivers and fewer nutrients washing off fields and into the sea. This improves water quality, supports fish and invertebrates, and reduces pollution reaching Sussex’s chalk streams and coastal waters. Woodlands also store carbon, support biodiversity and provide shade that keeps rivers cooler during summer heatwaves.
Across Sussex, trees are not a single solution to flooding. But placed well, and allowed to grow old, fall, rot and regenerate, they form a living system that works quietly and effectively to protect our communities.