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.


An overview of the flooding. ( Image credit,The Daily Mail)


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 massively reduce flooding.

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 also a 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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Scott Thomson

Scott Thomson

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7 Responses

  1. Avatar Mike Hamblet says:

    As someone with a long term interest I feel you’ve got it right here. There’s a conflict building between land-owners and downstream residents. Land-use changes are required to ease the strength and speed of rainfall run-off.

  2. Avatar Tom Allison says:

    Well said Scott there seems to be a wholesale vendetta against trees. I am right behind anything that promotes more trees.
    Farmers are more than happy to take their subsidies and have wiped out river weed with field run-off.
    I propose a tax on domestic wood burning the proliferation of which will see further habitat loss.


    Trees were cut down to timber small early coalmines. Also to shore up trenches in the Great War. Parts of Scotland if not elsewhere were “seeded” with white heather instead of trees.Relatively small numbers of sheep (check the selling price that a farmer gets for sheep…pitifully…extremely small ) are kept by farmers against all economic logic. The sheep and rabbits without fox predators make our “countryside” far less than it could be. Trees and shrubs are eaten to the very root before they can establish.
    One writer commented:
    ” Farmers don’t actually love the land, they only want it for sex ” For all that, they’re doing it wrong.

  4. Avatar Peter Ness says:

    Utter nonsence that only gives the undeducated false hope. You are implying that flooding never happened when land was in its ‘natural state’. This is nonsense. A plethora of geological and geomorphological evidence shows floods have always happened. They are a natural process. The key to flooding is intensity and overall volumes of rain, snowmelt or both with respect to river flooding. Forest Rearch website has said for years that trees do not stop floods – look it up!

    • There are enough houses built on clay soil and fortunes lost ,where the surrounding trees were later chopped down to show actual physical proof of the ability of trees via root systems, maintenance of the crumb structure of the soil and transpiration to add to the argument for the planting of trees to aid flood defence. The houses cracked, in some cases fell down. The trees were suddenly not there to remove the stored water within the clay and assist in capillary action in the soil. Micro-organisms and insects too, attracted to the eco-systems of the trees also marginally increased soil temperature,This tree-factor is a known fact to all responsible house builders and house insurance companies. The building of social housing close to trees have laws relating to it with guidelines of which particular trees “suck out” water at the highest rates.
      Of course you are correct that flooding is a natural process and it will not be eliminated. But it can be used positively and manipulated.

  5. Dear Mr Ness
    I have looked up this issue as you so kindly suggested.

    I am not saying that trees alone will stop floods entirely but you are also misinterpreting the Forestry Research Website. They do say that forestry has limitations in relation to flooding and quote the relative interception rates of water whilst pointing out that the forest sponge effect can only cope with so much. This is all true however they also point out that:

    “If soil compaction from grazing is found to contribute to increased flood flows, the planting of native woodlands could help to counter this, as well as provide a range of other benefits such as improved water quality”

    This has since been proven and thus they are essentially agreeing with my article.

    They are arguing that tree planting will have only small effects on flooding using the data they had when writing that page. Much of the research they are using however is more than 10 years old and significant steps have been taken in that time, see the links in my article. That being said, even the study which they commissioned (Nisbet and Thomas 2006) says:

    ”Overall, there appears to be significant scope for using woodland to help reduce flood risk, “

    Tree planting may only provide a slight benefit over “natural land” i.e grassland and small shrubs, but it provides a very large benefit over grazed farmland due to the compaction of the ground by the hooves of sheep and other animals. The problems of soil compaction were not known when they wrote that page and it has since been proven to be a serious issue (again see the paper I quoted in the article)

    The website is very cautious about making claims and point out there are potential negatives. I agree that their general tone is discouraging but their research is actually quite positive. They refer to the DEFRA project of 2008 and say

    “The final report on this Defra funded project was published in July 2008. The work provides further support for the potential of floodplain woodland to alleviate downstream flooding.”

    They also commissioned and published a study in 2013 in relation to rural land management that said

    “The results suggest that there is considerable scope for using strategically placed floodplain woodland to alleviate downstream flooding. In particular, it offers a means of tackling the increased flood risk associated with climate change.”

    The whole issue revolves around what the land use is currently, what sort of vegetation is planted, where it is planted and the effects of climate change on rainfall.

    The DEFRA report refers to this saying:

    “It is not necessary to plant a continuous stretch of floodplain woodland either across the full width or an extended length of the floodplain; a series of smaller blocks spread out along or across the floodplain will be just as effective at flood attenuation and may be easier to achieve in practice. Results indicate that even a 20 m wide block of woodland would generate a lag effect.”

    Also you have to remember the broader ecology of the tree planting is a good thing as forests have many key functions that benefit the environment and reduce the effects of climate change (From the DEFRA report)

    “Floodplain woodland offers a range of benefits, including for water quality, carbon storage, nature conservation, fisheries and recreation.”

    I do not agree that this is “Utter nonsence that only gives the undeducated false hope.” I think it is an idea which is backed up by evidence and indeed by the group whom you argue is against it.

  6. Dave Blake Dave Blake says:

    Hi Scott, while I am with you on your broad point (that politicians tend to share the prejudices of their electorate, rather than follow science), I’m not sure that you have really got into the detail of some of these issues.
    Trees increase the infiltration rate of the soil, so can play an important role across watershed in getting water from local rainfall off the surface and into the soil (on limestone areas, it may go into the rocks below). This stops the water being a nuisance on the surface to agriculture and transport, for instance. However, this is a slow process so it does not ‘stop flooding’. FR are rightly cautious on this.
    Trees transpirate large quantities of water and can cause significant drying of soils in their immediate area, but it is not enough to point to this effect as one that could ever stop floods or cause droughts.
    Trees, even newly planted ones, slow down surface flows during flood conditions, causing entrained sediment and flotsam to be dropped across a wide area rather than building up lower down the stream profile or in nuisance areas such as under bridges. This is a much more impactful effect – reducing the harmfulness of flooding – but does not ‘stop flooding’.
    Natural defences such as ‘leaky dams’ are fine in normal and higher than normal flows, doing good work in slowing water down. But in exceptional flows of the kind that cause significant, damaging floods, these may just supply a huge amount of material that is entrained in the torrent and will accumulate lower in the stream profile, causing worse damage (I saw this in the floods in Boscastle, Cornwall).
    Holding water back in the upper reaches of a catchment is certainly helpful (with reducing both floods and droughts) and you are dead right that land management changes are needed. But trees are just a small part of a complex and highly political suite of responses to water management challenges in a changing climate.

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