How trees reduce the risks caused by flooding
On a random evening to write this piece, I looked at the BBC’s Flood Warnings page. The evening I looked (November 10, 2023), there were 110 flood warnings in England, Scotland and Wales.
As rain falls, it collects together and follows the pull of gravity. From the hills, rainwater runs into the valleys it has carved, and from the valleys it joins rivers. As rivers swell with excess rainwater, the banks burst, and the water flow continues picking up pace. Both of these (overflowing banks and faster-moving water) cause damage to homes and towns.
There are several ways that trees can alter the “run-off” of water from our uplands, breaking up the impact of flooding:
Intercepting rainfall
Storing more water in the soil
Slowing the flow
Reducing silt in waterways
Retaining water on floodplains
Let’s look a little closer at what each of these do.
Trees are the first line of natural defence against flooding (image from Heart of England Forest).
How trees intercept rainfall
Trees, with their lofty canopies, have a greater capacity to evaporate water than lower vegetation like grass, resulting in less rainwater making it to the ground. This process is known as interception. Interception happens when the tree's crown catches rainwater, with some raindrops slipping through the gaps between the leaves and stems, falling directly to the ground as throughfall. This intercepted water is held temporarily on the surfaces of leaves and bark. Eventually, it either evaporates into the atmosphere, trickles down the trunk to the ground as stem flow, or falls off the surfaces to the ground as further throughfall.
Trees help to store more water in the soil
When you think of plants, which are the tallest in the UK? Trees! And, were you to look at plant roots, tree roots are the largest and deepest in the ground.
Large and deep tree roots punch through the soil, and rainwater can flow deeper into the soil before being flooded and looking for another path.
This means that for any given area of land, the soil under trees has the potential to store much more water and delay its passage downstream than the same area under grass.
Trees slow the flow of water
Trees that are planted alongside streams and rivers and the presence of fallen trees and branches work to slow the speed of flood waters.
Where rivers have had trees planted on their banks and the presence of tree debris, water flow has slowed significantly enough during flooding to prevent damage to the built environment, such as bridges.
Trees reduce silt going into waterways
Silt comprises soil and small stones that are washed into waterways.
Silting in rivers causes shallower waters and blockages. With shallower water and blockages, excess water looks for another way to flow, resulting in burst banks.
Tree roots hold soil firm and in place. The more soil held in place:
The healthier the ground is
The less silt runoff there is
The better downstream water flow is
Planting trees in upland areas and along river banks is a tremendously effective way to contain silting and keep potentially healthy soil in place, benefitting agriculture and natural habitats. This natural approach of planting trees to prevent silting is far more beautiful and longer-lasting than the alternative, which is dredging waterways and trying to find a place to put displaced river muck.
How trees retain water on floodplains
The UK's flood-prone regions have mostly been deforested to develop either agriculture or create more land for building.
Yet, the re-establishment of forests in these flood zones, where it's feasible, can significantly slow down the speed of floodwater, thereby reducing the destruction that flooding brings. Wet forests are particularly effective in absorbing floodwater on the floodplain and gradually releasing it downstream.
Relieving pressure on water treatment plants
There is another upside to controlling areas prone to flooding. 2023 has seen the media focus on water companies underperforming, highlighting a particularly stinky problem that water users have known about for years. Raw sewage is released into rivers when higher river levels tax drainage and sewer systems. Welsh Water has recently admitted that human waste levels in waterways have been in excess for a decade. This problem of discharge can be managed with better overall floodwater management.
To sum up
Tree planting has emerged as a modern and effective strategy in flood management, with the capacity to mitigate soil erosion and water flow. Despite some criticism that this approach isn't effective in the short term - only as trees mature in up to 20 or more years, research indicates that transitioning from grazed meadows to woodlands can deliver significant water retention advantages within a year. These benefits only grow along with the trees.
Numerous scientific studies corroborate the effectiveness of tree planting in upland regions as a critical component of natural flood management measures.
The ability of trees and woodlands to slow down flood flows not only helps to reduce the flood's size but also gives people more time to respond to flood warnings and move belongings upstairs, reducing flood damage.
