I get the impression the media is catching up. Two mornings running now we’ve had people on the Today programme talking sense. Today Professor David Balmforth, former President of the Institution of Civil Engineers and their spokesperson on flooding. Yesterday, Harold Van Waveran principal expert on flood protection at the Dutch government’s Ministry of Infrastructure and Environment. Both advocating a measured and intelligent approach to building our capacity to absorb and cope with floods.
But UKIP supporters and Daily Mail readers seem particularly unable to get their heads around it all. The pitchfork wielding throng still want their bonfire. Dig it deeper! Build it higher!
But I figure, these good folk must like their tea. So here’s a storm in a tea-pot experiment that I hope will make it clearer.
Get a tea pot and hold it under the tap. Turn the tap on and let the tea-pot fill until water is spilling out the spout. Adjust the tap so the amount going in to the pot is the same as the amount going out and the level in the pot stays the same: that’s a river under normal conditions.
The tap is rain, the tea-pot is the River Foss, the spout is York city centre.
Now throw in a storm. Turn the tap on harder, until you get to the point where there is more water going in to the tea-pot than can get out the spout. The tea-pot fills to the brim and then overflows.
It’s obvious, surely, that even if you doubled the size of the tea-pot (dredged the river), it would still – eventually – overflow.
And even if you raised the rim of the lid of the tea-pot (built higher flood-defences), it would still – eventually – overflow.
So what can you do?
You can try to make the spout much bigger. This is what everyone wants to do because it’s like the most obvious solution. The problem is that the landscape-scale equivalent would be to move York city centre. And you’d have to move not only York but every other bridge and constriction all the way to sea, making way for a massively oversized river that would cost zillions to excavate and millions to manage on an annual basis.
You could add another spout or two. This makes more sense because the landscape equivalent would be to build high-flow channels that by-pass York. When rivers reached a given height they’d spill into the channel and not the city and the extra channel(s) would add to the river’s flow capacity. It’s flow capacity that matters once the water is at York.
So, it’s clear if you do nothing about the constriction – York – then dredging the channel or building higher flood walls buys you as much time as it takes that extra space to fill with water. In the case of these recent storms that’s no time at all. The amount of storage space dredging or flood walls provide is miniscule.
But if you use the flood-plain upstream you can add thousands of times more storage space. It would be like adding another massive vessel between the tap and the tea-pot. That space would eventually fill too, but if it’s large enough to buy you the duration of the storm, then you’re winning.
Finally, you can slow down the tap. Not by stopping it from raining – obviously – but by slowing down the rate at which the water leaves the hills and rushes downhill towards York. There are examples of where this has been done and where it’s working, so I’ll post an article about that in due course.
Basically, you’re going to need a combination of all the above: innovative flood-relief channels and by-passes, flood storage capacity on upstream land, and river and landscape management that buffers the flow and increases the landscape’s capacity to hold water.
But passive flood-defence structures and dredging are the leeches and dung-pat prescriptions of medieval quacks.