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. 

5 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.

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