Piers Morgan tells Liz Truss MP that she needs a ‘department for the unprecedented’, while all the government – or the opposition – can talk about is spending on flood defence. Either ‘we really did spend a lot’, or ‘you clearly didn’t spend enough’. Our approach is defined by and reflected in our language. Flood defence. Flood prevention.
This is all playing Canute.
We desperately need new thinking and new policy based on flood management, more than defence. Floods are natural and cyclical events. And every so often one will come along that redefines everything and blows our defences to pieces. Especially because of the way we manage and engineer the landscape. Our two-tier, engineering-based approach of dredging and defence is mutually contradictory and has disastrous consequences.
We channel all the flow into a concentrated maelstrom, then try – and fail – to keep the maelstrom out. Does this sound daft? If it does, that’s because it is.
I wrote this line in my last blog, and I’ve written it a few times before: “dredging makes flooding worse”.
But I can’t help wondering, whenever I tap out the words, if it sounds more activist than physicist. Whether the politicians and farmers and town planners who need to get their heads around the idea will gloss over a statement like that and dismiss it as impassioned hyperbole from someone who really doesn’t want to see a return to the days of dredging.
It’s true though: dredging makes flooding worse.
Post Cumbria 2015 Liz Truss is assembling a task-force of experts to look at the issue of flooding. Good. But I wonder who will be on the team. Will it include the fluvial geo-morphologists who understand how rivers work? Or will it include the civil engineers who don’t? The kind who told David Cameron and Owen Patterson after the Somerset floods of two years ago that dredging would have alleviated and shortened the duration of the floods.
Given that these people – whose mindset has dictated the direction of land-drainage and river management for the past fifty years – are engineers I’m puzzled by how static and simplistic are the models and thinking processes upon which their prescriptions are based.
It might be counter-intuitive, a little tricky even, to get your head around why, when you make a river channel deeper you make flooding worse. But it’s not impossible.
You’ve only got to look at a river for a few seconds to realise that it is not standing still – a river flows and it moves stuff (mud, sand, pebbles, rocks) too. Basic ‘this fits into that’ physics will not apply in a straightforward way. Successfully managing a river has to be based on an understanding of it as a dynamic system.
You’ve only got to look at a river for a few more seconds to also realise that of all the potential physical states it could occupy, a natural, unmodified river flowing through a natural landscape has almost certainly found its most stable state. After all, it’s been doing its thing for thousands of years. If there was a better shape to be in, the river would surely be in it.
Think a little longer still and you’ll also realise that if those two things are true (the natural river as a non-static physical process occupying its most stable state) then any change made to the stable state will cause instability. It’s a moving, physical process remember. It had already found its most stable state and we have now changed it.
When we dredge rivers we obviously change them: we make them deeper thinking that will make them hold more water (when we should be thinking about capacity for flow). If the river was a lake this mindset might work. A deeper lake holds more water than a shallow one. But the river is not a lake. The river is a dynamic system that has just been de-stabilised. So what happens?
First, we need to think about what determines the natural, stable shape of a river channel and valley. It’s kind of obvious but a river’s flow varies through the year. There are times of low flow and times of high flow and times in between. The natural shape of a river is, unsurprisingly, a shape that accommodates all these flows, but here’s the thing … that shape includes the flood-plain. This is where the trouble begins – because in a busy, highly populated landscape we don’t want the river to use its flood-plain.
But regardless of what we want, most of what determines the shape of a river happens at high flows where the water is close to or at the top of the river bank. This is the stage of flow, which occurs every year or two, when the erosive forces of the water in the channel are at their most powerful, when the river is most capable of moving the stuff it carries. Every river has a natural bankfull stage of flow, and therefore a natural, maximum channel size.
Flows above this are rarer (if they weren’t the river would be naturally bigger) but when they occur they should start to seep out over the flood plain. A good thing too. Because the flood-plain is, in the natural system, a pressure-relief valve. Just think about it: if the flood-plain didn’t exist then when you get floods the like of which we have seen in Cumbria this December, all that extra flow would have to go down the river channel!
Water is powerful stuff. A channel that for most of the time handles flows up to a certain limit would be blown apart by flows many, many times that limit. There would be massive erosion and destruction and knock-on consequences the like of which would be hard to imagine … unless you watched the news in December 2015.
Because ‘this into that’ civil engineers, who have not understood the dynamic physics at work in river systems have tried to engineer our rivers to take all flows, including the really high flows that should be seeping across a flood-plain. In doing so they have inadvertently built natural time-bombs and then asked us to live beside them. Because where do these time-bombs go off? In towns where the bridges and weirs and the constricted channels back up the maelstrom until everything snaps. As Robert Plant sang ‘when the levee breaks, mama you’ve got to move’.
These places are usually at the lower end of river systems and get the brunt of everything that has been done wrong in the upper system.
Okay, says the civil engineer. So we just haven’t done it enough. Dredge the rivers all the way to the sea, move all the bridges. What’s to stop that working?
The numbers? We’d have to make rivers many, many times their natural size … all the way from the hills and mountains to the sea!
Apart from the amount of land you’d have to dig out the way, apart from the entire cities you’d need to move and the hundreds of enormous bridges you’d need to build, apart from the fact that you would completely destroy all the wildlife in and around the river, what do you think would happen for the 99% of the time that the river’s flow is back to normal and now far too low for the enormous channel you’ve dug? The river is carrying material, remember: mud, sand, gravel. All this material would drop out of the now almost static flows of a river in a channel that is miles too big and begin to fill it in again. Unless you dredged it every year, top to bottom, eventually you’d have the naturally-sized channel nestled inside the enormous dredged channel and you’d be back where you started.
Dredging doesn’t work. Dredging is logically and financially unsustainable. Dredging makes flooding worse.
We need to develop a different way of managing our rivers, one that takes into account their dynamism and that is based on the physical model rivers have shown all along to anyone observant enough to take notice.
Rivers must have pressure-relief valves: this is a non-negotiable when the country is experiencing record rainfall and when storm frequency is so clearly on the increase. If we want some of the flood-plain to live on and farm, we have to re-naturalise the rivers everywhere else and give them room to be rivers.
Anything else is daft, hubristic and playing Canute.