words and pictures by charles rangeley-wilson

To Tame or Conquer the Flood?

At the request of Peter Devery via Twitter I have posted below the reply I wrote to a daft letter about flooding published in The Times (2nd January). My reply was published on the Saturday two days later, along with another in sort of agreement and two more chorusing the original daftness. Since then many more letters and opinions have been published in all the papers, some advocating good sense, some not.

“Sir,

I don’t agree with Professor Brinkworth (Flood Remedy, Letters Jan 2nd). The restrictions to flow in most British rivers are bridges, weirs and other man-made structures. Dredging between them makes no difference to flow, only to capacity. But no matter how much capacity you try to create by dredging, heavy rain will soon fill it. The only way to manage flooding is to do exactly that: manage flooding. For decades we have tried the opposite, to eliminate flooding according to the logic expressed by Professor Brinkworth. It doesn’t work. Instead it turns our rivers into ugly ditches and makes the problem far worse into the bargain. We should instead look for ways to slow water down, to harness the value of wetland. Taming nature through understanding is possible. Conquering her through ignorance is not.

Yours etc.”

It is really simple: the capacity of a river (no matter how much it is dredged) is tiny compared to the volume of rain that can fall over a catchment during a cycle of storms and torrential rainfall. The limiting factor in any river system is conveyance and that will be limited most of all by bridges, weirs and towns not by how much we dredge the rivers.

Since I wrote this I have been trying to think of a useful image that will help clarify this easy confusion between capacity (how big we make a river) and conveyance (the rate at which water can flow down that river). Perhaps it is easier to see it as traffic?

Say we have a vast amount of traffic moving along a large number of feeder roads all descending on a motorway, all the traffic moving in the same direction. We want to avoid congestion – flooding – on the motorway. Anyone can see that the best way to avoid a jam would be to stagger the rate at which traffic arrives onto the motorway so that it never exceeds the rate at which traffic can pass along the motorway.

This is more or less exactly comparable with rivers. Since the 1950s we have widened the feeder roads (with canalisation and dredging), increased their number and length (by cross-hatching the landscape with drains and ditches) and increased the volume of traffic which flows down them (by draining all the wetlands and floodplains which once held water and released it slowly). We have widened the motorways from three lanes to six (more dredging) but only between the bridges; under those bridges (real bridges, weirs or towns) the motorways remain three lanes wide.

It would be a start if we stopped talking about flood defence and started talking about flood management – because what is needed is a shift of mindset. We manage traffic and we manage congestion, but we defend against flooding, as if by launching civil engineers against it as so many King Canutes this inevitable event can somehow be defied.

Flooding will happen: it’s a consequence of rain and gravity. The issue is not preventing it but more where it happens. How we restore wetlands and create landscape storage, and how we encourage landowners to allow flooding where we want it and thus avoid it where we don’t … that is what we should be discussing. 

8 Responses to “To Tame or Conquer the Flood?”

  1. John Richardson

    Bang on Charles, a great analogy.

    I remember driving into London down the M3, 30 miles in 25 minutes then four lanes becomes two and the speed limit 40mph, then 30. Just like a river, everyone who can divert diving down avenues, lanes and roads and jamming them up
    A news bulletin interviewed a farmer in the Thames valley who said the problem was a lack of dredging. What? It’s an excess of rain the fools. If an inch of rain falls on an acre of land in an hour how many gallons is that to be absorbed and drain away? If that land is concrete and tarmac…

    Pearls before swine I’m afraid.

    All the best, John

  2. Mark Sixsmith

    Wise words. Alas, the opposition to it shows how much so many people in Britain have lost touch with nature and natural processes.
    I came to your site via Richard Harrabin’s article on the BBC:

    http://www.bbc.co.uk/news/science-environment-25931847

    Back in the 1990’s I worked for the Pang Valley Project in Berkshire; re-wilding the river was the main objective and was embraced by the local community (including farmers).
    Regards
    Mark

  3. rangeley

    All rivers are the same in that physical processes of erosion and deposition determine a stable channel size and shape. Dredging – and most other forms of modification – create instability. Unstable states are not sustainable in that they require constant intervention. So, yes, in terms of the points I am making, all rivers are the same.

  4. rangeley

    The Pang – a lovely chalk-stream and the inspiration to Wind in the Willows, I have believe. I’ll look up that project: thanks for the tip.

  5. Waterside Tales

    The people that have lived alongside the river their entire lives have seen first-hand the difference that dredging, and the consequent lack of it over recent years, made to the flooding situation each year on large rivers like the Thames.

    Enough said.

  6. rangeley

    Dear Waterside, I’m sorry I didn’t reply to your comment sooner. I’m all for memory as a form of legitimate evidence. However, rivers flood as a result of a very, very complex set of interractions and attributing flooding to a single given land management action or lack of it is too simplistic. My comments on flooding have all been aimed at the idea that we should manage flooding by doing just that managing flooding. Rivers like the Thames have periodically flooded since the beginning of time. Human land management (forest clearance, land drainage) has only made them more prone to flooding. Flooding is as inevitable as the tide. Dredging is one way of allowing a river to accommodate more water. But you have to factor in other things. A river flows, so it is not just a question of capacity, but also of passage: the limits to passage on rivers are weirs and bridges. Also dredging has to be repeated to be of any use at all: so we have to look at how sustainable and effective it is against the cost. And by that I mean the cost of dredging versus other, potentially more effective methods of managing flooding. And finally there’s simple physics. In this past winter’s floods no amount of dredging would have prevented the floods, simply because no amount of dredging would have created the capacity to store all that water in the channel. When a floodplain is a metre deep under water and flooded over hundreds or thousands of square kilometres anyone can see that all that water would not fit in the river. So, maybe there are other things we should be thinking about? Like for example encouraging upstream land-owners to store water rather than drain it off their land in a hurry, or finding areas of land closer to towns which can be temporarily inundated, so as to slow the rate at which the rivers rise in built-up areas. Schemes like these might be much more effective than dredging at keeping waterside houses from flooding. To an extent the public has done itself a disservice by asking government for the wrong thing. And the government has been happy to oblige, by throwing an inadequate amount of money at a catch-cry word “dredging” in the cynical hope that the headlines will die down and that next time the floods come someone else will be in charge.

  7. Waterside Tales

    The government sold virtually all our dredging equipment to Sweden, that’s why it has done no dredging for many years. I can assure you that I am extremely well connected with those in the council to whom the matter of flooding is a management issue. Little is more tiresome than people blaming inefficient management and control systems on anything other than just that. Flood Plains flood, that’s what they do. I know, I grew up living on one. The amount of time the water takes to drain away is the issue and common sense and simple mathematics (volume calculation) should answer most of the basic questions people that seem to grapple with, usually with misplaced intensity and passion. The government allowed environmental groups, campaigning against dredging, small victories solely because it suited their agenda. It really is time for people to pull their heads out of their backsides and start expecting a little bit more. This isn’t the Nile Delta or the Zambezi, it is a perfectly manageable issue, it simply costs. I can assure you that dredging was not just restricted to the Thames, but also every other river, such as the Wey and so forth. The combination of the complete neglect of this responsibility will be seen time and time again until the dredging resumes. No amount of wetland or holding back of water will help, in fact recently this seriously exacerbated the problem as more and more rain fell, proving what a daft idea it was in the first place as more and more floodwater ended up with nowhere to go and consequently everywhere. Everywhere.

    The water needs to drain away and as fast as possible. Period.

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