I wondered if the next wave of floods would reignite the idea that dredging prevents flooding. When your living room is ruined and your sofa is floating off down the street, it’s natural to feel that things could have been done to prevent it and to want to see action to prevent it happening again.
Dredging very much feels like the answer, especially when you need an answer quickly, which is why the idea is so catchy and why politicians will sell the idea to victims of flooding. It’s why local MPs will call for dredging. It all feels like an insistence on the solution. But dredging is not the answer!
To put it in really obvious terms, as in Somerset a couple of years back, we have flood-plains, miles wide, completely under water. Just think: how deep or wide would you have to make our rivers to accommodate all that water? More or less as wide as the flood-plain. Dredging the Eden would make no difference at all.
The first issue is the amount of rain: it’s breaking records. Even a natural landscape, without farming and roads and land-drainage, would have flooded under that amount of rainfall. Dredging would do nothing in the face of it. Dredging might even make the problem worse.
Once you’ve got a massive amount of water piling in to the lower end of a river system, the problem is not one of how much water you can fit inside the river: the sheer volume of water on the landscape ought to show that you simply couldn’t make the rivers big enough. The problem is one of flow, the volume of water that can flow along the channel and the rate at which it flows.
Just think of it as traffic on match day. Flow is limited by restrictions in the channel (or road) much more than it is limited by channel (or road) size between the restrictions: as anyone who has funnelled from three lanes to one on a busy motorway will understand. The channel restrictions in lowland valleys are such things as weirs, bridges and … towns: these are the road-blocks on a river. Dredging only creates more lanes between the road-works. It does nothing about the road-works themselves. Who thinks that would solve congestion on a road?
If dredging wouldn’t help, what would?
Dealing with the restrictions would make far more difference. By-pass channels around towns and bridges and weirs would get the water away, and help prevent or lessen rises in water level far more effectively than dredging ever would.
Such schemes are more expensive than dredging, however. So you might not hear the government agreeing to them. But if flooding is the ailment, calls for dredging are medieval witch-hunts and flood alleviation schemes are modern medicine.
However, the fact is is we can’t actually stop it raining. And we can’t even prevent flooding. But we can influence where that flooding happens.
At the moment we make it so that all the rain floods downhill in a hurry, drowning lowland valleys and towns.
All the ways in which we modify the landscape make flooding much worse. The biggest culprit is – ironically – land drainage. Over the last few decades we’ve turned once absorbent landscapes into kitchen draining boards. The best way to lessen the impact of flooding on lowland farms and more importantly, on lowland towns, is to make the highland / upland landscape more absorbent, and then to find places where you allow flooding to take place. You could build two-stage river channels. You could pay farmers to allow their land to flood. Etc etc.
The simple fact is this: to alleviate flooding where you really don’t want it to happen, you have to manage flooding somewhere else: somewhere better than a town centre.