The floods in the UK has triggered a storm of utter nonsense about what does and doesn’t help to prevent or reduce flooding. One of these claims is that dredging rivers will help with preventing flooding or at least will make them less severe. This is wrong.
I live in The Netherlands and we’re a country with a very long history fighting against the ocean and our rivers. It’s because of our constant battle with water that we have a vast network of defences, a lot of resources to help during a crisis, and contingency plans when things do go wrong. But despite all that nature still sometimes surprises us, it has learned us to never underestimate her. We got two such lessons in 1993 and 1995 courtesy of the river the Meuse.
I distinctly remember those lessons as I grew up right next to the river the Meuse. It’s a very fickle river as it is fed by rain water, the water level can vary a lot thanks to weather. What exacerbates this situation is that this river has its origin in the French and Belgium Ardennes region, a very rocky region that can’t absorb a lot of water. There’s also a very steep gradient between the origin of the river and it’s delta, making it a fast draining river. It’s because of all these factors that the river Meuse taught us the lesson that you shouldn’t dredge rain fed rivers.
What we had done since 1840 was to widen and deepen this river; it made more shipping possible. We also created an extensive network of dykes around it which restricted how far the river could spread out during a period of high water. These two things proved disastrous with this type of rain fed river.
What happens in such a situation is that dredging indeed does increase the flow rate in a river. But this comes at a price, when water rises this increased flow means the water will rise faster. This gives a region dealing with rising flood waters less time to assess the situation and respond. The other detail is that because a dredged river drains water faster the peak of the flood water will be higher. Basically the water moves as one big pulse down the main channel instead of a more gradual and spread out one.
When you then combine this with a river that’s restricted in how far it can spread out due to dykes you make this problem worse. You’ve created a channel where a very big mass of water can go through with very little to slow it down. Giving you even less time and it again can increase the height of the flood waters. You can see these effects on a small-scale if you dig a trench with mini dykes next to it. When you throw a bucked of water in it you’ll see the water move in one big mass. Which can easily defeat the little dykes put next to it, especially when this water mass slams into a turn or other flow restriction.
That’s what happened in 1993 and 1995, a big pulse of water moved down the river Meuse defeating a lot of our defences. They just weren’t designed to be able to deal with the amount of water and the speed at which it flowed. What made this pulse so big is that the month before this flood it rained almost continuously. The ground was so saturated with water that it couldn’t absorb the rain any more and all the water went into the river (in 1995 the ground in Belgium was also frozen making it even worse). This set the stage for the flooding disaster when the heavy rains hit, all the water went into the river and came rushing towards us in such an amount that it overwhelmed our defences.
These two flooding disasters caused a shift in policy for how we manage the river Meuse. We still have our dykes and we still dredge the river for shipping, but we changed something about the river. What we did was give the river room to spread out during high water events. We also made sure that these areas had a lot of vegetation and all kinds of little side channels and gullies. These are designed to slow down the flow of the river so it rises more gradually. It also gives the river more room during high water events making the water rise more gradually and peak at a lower level.
We also intentionally added regions that would flood during high water events. The goal of this is to catch water so that during a high water event the river peaks at a lower level. It also has as a side benefit that this again can slow down the flow in the river. You can see the same thing if you dig a meandering channel in your garden, make some small side channels, and create some small holes in the ground. Then surround that with a couple of small dykes. When you throw a bucket of water through that you’ll notice the water spread out and drain a lot slower. That’s the situation you want to have with a rain fed river. When it does go wrong and the river overwhelms your defences you’ll have a lot more time to respond and the flooding will be less severe.
A lot of these defences are in the form of small nature reserves. These natural defences are great for tourism and a lot of wildlife has its home there. Creating these areas also generated a lot of gravel, clay, and sand that industry in my country gladly used. The economy benefited from these projects, they’re low in upkeep costs, and a lot cheaper to build than flood defences designed for the types of floods we experienced in 1993 and 1995. Plus these regions are a lot prettier than most big flood defences.
It’s because of these lessons that experts say that dredging rivers won’t solve the flooding problems the UK is experiencing; you could easily make them worse. To handle the kind of situation the UK is experiencing you need water management solutions that slow the flow of a river and catch water. It gives you time to respond and deal with the water that’s coming down a river. The current situation in the UK isn’t that different from what we Dutch experienced in 1993 and 1995. The UK has had a lot of rain this winter making the ground saturated and unable to absorb water. All the rain that now falls in the UK goes straight into its rivers, any heavy rainfall in those conditions will cause trouble.
Blaming nature reserves is not what you should do in this case, or blaming the lack of dredging. It should now be obvious that there are some very good reasons why experts say that dredging wouldn’t have helped. They are also the people who you should turn to for advice for what does help. We Dutch listened to them and acted on that advice, I hope that the UK does this also.