Why trees prevent flooding

This article may be stating the obvious to many people but it seems like most people in the media don’t seem to relate land use with flooding. A quick scan of the news leads to lots of people saying we need to spend more money on flood defences, no one every seems to specify exactly what that means but it is generally implied that we are going to build something, somewhere, at some point, maybe.

There are a great many “solutions” to flooding, mainly by restoring land to its natural state. As many of you know overgrazing, particularly due to livestock farming, hugely increases the risk of flooding. Why? Because A, the soil becomes compressed and firm which accelerates surface runoff and B, there are no trees or shrubs to absorb the water. When there are trees water is absorbed 67 times faster than it is absorbed through grass. 67 times! Depending on where you plant the trees you could also argue that they prevent soil erosion and make riverbanks stronger as well. This does not necessarily mean going as far as rewilding, it simply means planting more trees.

Vast swathes of grass and concrete have been flooded.


A key problem is the approach to sheep farming, highlighted in this article written by a conservative MP. It’s pretty long so I’ll summarise, he is upset that sheep farms and small farms are disappearing and suggests we do what we can to keep them before we lose the skills and traditions we’ve had for many years. Sheep farming is very destructive to the landscape, rendering it grassy, increasing soil compaction and removing vegetation. It is heavily subsided meaning people are being paid by the government to remove the vegetation in order to allow sheep farming. To argue that it is tradition and therefore acceptable is bizarre. If you actively set out to make land as floodable as possible sheep farming would tick all the boxes. I’m aware it is not just about sheep farming and that we can’t get rid farming entirely but there needs to be some common sense, we can’t remove our natural flood defences and replace them with anything that will be anywhere near as effective. The MP in question also argues that conservation is at odds with farming,

“dominant environmentalists quietly encourage farmland to be handed directly to the RSPB, or planted with trees, and the National Trust allows water to ruin the lowland pastures of their small tenant farms, apparently on the advice of the Environment Agency.”

He goes on to explain how conservative policy will keep the farms and not allow tree planting on the hills, going against the Environment Agency’s advice. That’s not a policy that has gone well. River flooding is about more than the area that floods, the upstream areas are important for mitigation, keeping a riparian strip (vegetation alongside a river) and having some targeted tree planting would reduce flooding massively.

Unsurprisingly the problem stems from politics where it is better to do something that looks good and is popular, rather than something that actually works. The former environment secretary Owen Paterson once said,

“I am absolutely clear that we have a real role to play in helping hill farmers to keep the hills looking as they do”

No, no we don’t need to keep the hills bare. If the water was absorbed before it reached the rivers then we wouldn’t have so much flooding. A research paper on the subject of tree planting and flooding found that if 5% of the land was reforested flooding would be reduced by 29%. Keeping the landscape bare is actively increasing the risk of flooding. Last year Owen Paterson decided that he would increase the amount of dredging, despite the Environment Agency warning that this would make downstream flooding worse, also not a policy that went well. As most of us are aware, dredging also removes most of the wildlife from a river and destroys an ecosystem. An all round bad idea and once again at odds with the Environment Agency.

Incidentally Owen Paterson’s degree was in history and the current Environment Secretary , Liz Truss, has one in mathematics. Not the people I would choose to manage the environment but there we go. Whilst we are on a political theme I would like to point out Mr Tim Farron, Lib Dem leader and all round hypocrite. He recently wrote a piece for the Guardian entitled “Where are our flood defences” after being caught in the floods. He also wrote a piece in 2013 after Natural England dropped their “Upland Vision” plan in which he said,

“I am delighted that Natural England are readjusting their approach to the uplands, with the recent dropping of their ‘Uplands Vision’ and the appointment of Will Cockbain to the board and making efforts to support our hill farmers.”

The “Upland Vision” plan called for more trees and less sheep in the hills. Mr Farron objected to it and now questions why there are no defences?

You have feel sorry for the Environment Agency, they’ve given lots of good advice that has been widely ignored and now they are being blamed for the flooding. If the government hadn’t supported the cutting down of trees, given permission for developers to build houses on floodplains and hadn’t ignored the advice of its own advisers then the situation would not be anywhere near as bad.

I did my undergraduate dissertation in relation to saltmarshes and coastal flooding and the best method in that case was natural defence approach. Those in charge seem to be focusing on temporary avoidance and response to floods rather than on a more long term approach. Sea walls and river barriers are popular at moment, ineffective and expensive, but popular.

We know the problem and we know the solution, it’s just that no one in government will support it.

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