Why Dredging Makes Flooding Worse
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.
charlesrangeleywilson.com 
21 Responses to “Why Dredging Makes Flooding Worse”
Sounds like you work for the Govenment with all that propaganda.
Propaganda: “information, especially of a biased or misleading nature, used to promote a political cause or point of view”.
What political cause am I promoting? I don’t hear any political parties – especially not the current government – arguing for the point of view I am promoting. And what is misleading about what I have written? That dredging rivers does not protect cities and towns from floods? That allowing the river to spill out of its banks in certain places would help protect other places? Sounds like you didn’t read it.
If you personally have been flooded, I am genuinely sorry about that. But dredging rivers wouldn’t have protected you and it won’t protect you in the future. We’ve been dredging and draining for centuries, but only in the last fifty years or so has science shown us how rivers actually work. Our landscape management has simply not caught up with what we now know and its high-time some alternatives ideas were looked at. Those who have been flooded should be the first to demand it.
But they are misled by the genuine propaganda of all political parties, an ignorant media and various vested-interests engineering conglomerates and agencies who want bigger budgets to deliver more of the same stuff that didn’t work before .
A well reasoned article, much of which confirms what knowledgeable anglers have been saying for years.
While I sympathise with the ideas, you will have to provide more evidence if you are to influence policy. There are probably no ‘natural’ rivers in England and few in the whole of the U.K. as we have been messing with them since the Neolithic, so finding a fully balanced hydro system will be a challenge to begin with.
You’re right. But with £1.3 billion of damage (this year alone) would it not be worth investing a million or two in one valley and studying the difference? That’s a decision for government to make, but to me it’s a no-brainer.
Easy to blame “the engineers” and sometimes they deserve it ; – especially when trying to implement oversimplified concepts of people who don’t really have the depth of knowledge they purport to have – and deep down they know it.
Excellent article. There has been re-meandering (ie trying to imitate their natural shape and allowing it to happen) of rivers done in England and here in the Scottish Borders – a short length of the Eddleston Water which is said to have reduced flooding in Peebles at the Cuddy recently. Evidence for good effects of restoring upland peat bog and re-wooding too is accumulating.
Well, I’m not quite blaming them. I’m saying the civil-engineering lens through which they look at rivers is inadequate. But the temptation to blame does rise when the BBC and other media give endlesss air-time to civil engineers and drainage authority executives who – of course – prescribe more of what’s gone before, whilst blame any inadequacy in the function of their flood-defences entirely on a lack of funding. That’s disingenuous, to say the least.
Sarah, thanks for that. I know the Eddlestone Water. We’ve been doing some work like this here in Norfolk too. But to make the case effectively we need the chance to restore one whole catchment – from the headwaters to the city that gets flooded – and study the difference. It would need money, government-support and co-operative farmers. But I keep reading of the enormous scale of damages: £5 billion was bandied about yesterday, and thinking surely it makes cost-effective sense to compensate farmers who allow their land to be included in flood-plain restoration. I mean the maths cant even be that hard. A flood-plain stores miles more water than a flood-defended river. Raising the banks of a river by a meter or two through a city centre costs millions of pounds and stores very little extra water. Allowing the land upstream to flood? Miles cheaper and miles, miles more efficient.
While there’s public concern for homes and property calling for defensive strategies, dredging is certainly not the answer.
Dredging ruins an ecosystem and it fills back in again due to the increase and speed of new flow. I’m surprised there’s no mention of fish spawning grounds and gravel banks being spoiled. But then I suppose this is the last thing on most people’s minds when their homes are been flooded, and besides, fish are not cuddly or seen much by the average public to care about.
Floodplains are important but are only part of the answer. As long as man wishes to live in lowlands next to the waters edge, we need a more balanced approach to the problem. Higher banks and relief channels along the rivers around towns and villages will divert a lot of the deluge but also natural floodplains need to be utilised as part of any flood relief scheme too. In areas of particular risk, vast reservoirs open or underground/covered can also be dug strategically so as to avoid excessive crop spoilage on farming land. These would also provide refuge for birds and wildlife as well as being used as a source of water to lessen the burden of abstraction in times of drought in some lowland areas.
As a practicing civil engineer in the highways field, I have to say that I agree with the article for the most part. Highways also cause similar issues in that when there are floods, the roads can cause a natural barrier The knee-jerk reaction is to raise the road, instead of allowing the water to cross over/under/through the road to create balance.
When the floods happen, a series of ministers, EA and others are dragged out of the woodwork to give a sound-bite, but behind the scenes nothing really changes. The road exacerbates the effects of the rivers, and so the cycle begins again.
As civil engineers, we are well aware that what we are doing could be better, but because of the companies, agencies etc. that we work for we are generally asked for a quick-fix to the problem in events such as these. We are rarely allowed to ‘waste’ our time with logical long term thinking, as it isn’t cost effective at the time.
Believe me when I say that we find it as frustrating as yourself.
Terry, thanks for your note. I suppose I should be careful about what I imply. Civil engineers have a lot to add to the debate. But it is frustrating that fluvial morphologists are so rarely involved. I’ve not heard even one being interviewed on the news. Our understanding of rivers has advanced greatly over the last few decades, but this knowledge appears to be ignored by the media and government. No doubt there will be quick fix solutions announced again this time: another meter on the flood walls, bigger pumps, etc. And everyone will hope this rain is the worst we’ll see for fifty years. Something tells me it might not be, however. Thanks for sharing the frustration! Charles.
This should help.
As I have already said I am against flood defences for one reason alone and I will explain in lay mans terms, if you fill your bath to the top and let the plug out you are controlling the flow of water hence you can refit the plug anytime but with that bath full of water and you put a hole in the side of the bath then you have no control and you will be flooded no matter how you try to contain the water with your defences I.E towels etc.
So on this basis alone it proves that flood defences do not work and when you have a flood defence manager from the EA telling us that they won’t work then why are we set on building these defences?
The bigger picture is quite simple massive volumes of water originate in the mountains all river originate from a high point to low point.
We should be looking at how to prevent the water coming from those mountains hitting the rivers and at least you can the control the amount of water you allow in each river depending on the amount of rainfall on already saturated ground.
To give you an idea. Back in 2005 carlisle was flooded with 3 million cubic meters of water this time I reckon with 4 million lets say 5 million to be on the safe side when Ullswater alone could hold an extra 520million cubic meters when it is on flood by only 1 meter and a dam being perforated you will have less chance of the river Eamont being on flood so this alone would save pooley bridge Armathwaite Warwick bridge and carlisle.
I would use the same solution for Cockermouth Keswick and Workington.
Not forgetting Glennridding.
I understand making alterations within the national park will upset a few people but if this is what it takes to prevent future flooding for the whole of cumbria then so be it and I’m sure that everyone that was affected by flooding would agree. This includes the government and all the services involved in the aftermath as well as the cost implications.
I first came up with these plans back in 2005 after being flooded myself and I took them to a professor a government advisor on flooding who corroborated the whole idea of containing water with perforated dams at source and that flood defences will exasperate the flooding situation.
I did take my ideas to the then flood defence manager Glynn Vaughan but they were set on building the defences because there models proved they could protect us!
This has now proved unsuccessful throughout the whole country where these defences Do NOT work.
We should have a full new strategy on flood prevention and I find the people who are making these decisions are not coming up with the solution to the problem of flood prevention as all they can see are walls around each city.
I do understand that my solution may not work for the whole country but my concentration has been for cumbria first and foremost but I have taken a look at York and the same solutions can apply there too.
Just to clarify the dams they do not need to look like dams and you would not even know they were there so I’m not talking about a big concrete wall that you can see. Most people I have told think it will look like the Hoover dam but that’s not the idea.
The emphasis is around concealment of the dams in the national park and a rising perforated dam at Ullswater lake that rises with the height of the water.
Many people have made comments on planting trees and hedges and making flood plains and there is no doubt that tree roots will help or having a flood plain but we are talking 100s of millions of cubic meters of water here that the ground can not cope with so this option is not practical and you must have another solution hence my ideas in flood prevention.
I hope I have given you some food for thought and look forward to discussing my proposals.
Your sincerely
Phil Cochrane
Sent from my iPhone
Sent from my iPhone
Hi Phil,
I was looking at Cockermouth flood pictures the other day. I suspect the upstream lakes already have quite a significant buffering impact on flows and also that a lot of water arrives from the catchment downstream of the lakes. Technically what you describe would make a big difference, as you are using the lake like a sponge, or a natural flood-plain. However, I’m guessing the national heritage value of the lakes would make the idea a very tricky one to sell to a public beyond those with wet houses in Cockermouth.
From what I could see of the Cockermouth images there are a lot of problems within the town and these ought to be addressed first: factory buildings on over-flowing parts of the floodplain opposite the town; a foot bridge and a road-bridge that are too low and act like weirs in the flood (you can see the water cascading over them and backed up significantly above each one); and another weir downstream on a hairpin meander. All of these will back the water up in the main river, which means the tributary cannot drain and I suspect this is what floods the town. The proposed and existing flood-banks look like they should be secondary measures after dealing with the bridges, weirs and the construction of high-flow relief channels which could by-pass high water levels across the two big meanders. C.
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Your “one size fits all” approach to drainage doesn’t apply to all waterways. I live 1/2 mile from a bayou that drains a 32,000 acre flood plain. Being close to the Gulf of Mexico, this bayou is tidally controlled and goes up and down with the tide. This bayou was dredged from 1934 to 1959 fairly regularly then “stopped”. That was 63 years ago. In that period of time, this bayou has silted-in, has debris, logs and trees in it. This bayou will now drain less than 1/2 of what it should. Property floods during a summer rain. If we have a “wet” hurricane, hundreds of homes flood. The County has directed waterways to this bayou and is presently digging huge ditches that will empty into the bayou. Our local government has decided to make us the sacrificial lambs and not return this waterway to it’s historical depth, especially in light of the fact that they are routing other waterways and digging new ones that empty into this small bayou. When confronted, our “leaders” point to articles like yours that say “dredging doesn’t work”. They can’t seem to see that the extra water they are routing is a form of dredging, it’s digging out existing ditches and digging new ones that will empty into our inadequate drainage channel. By adding additional water to a waterway that has filled in by 1/2, the only possibility to facilitate this water reaching the Gulf without flooding a bunch of people along the way is to dredge. Your “one size fits all” might work some places but it doesn’t work here.
Hi Tom, I sympathise although I don’t quite agree with all you say. As you point out, the fact that all the ditches leading to the bayou are being treated to speed up drainage is liable to lead to flooding. That’s the heart of your problem. The wiser policy would be to try and slow the flow of floodwater from the catchment to the bayou. Once the water is down with you, if the bayou is tidal, then for many hours in the day it’s going to be full of water anyway, no matter how deep you make it by dredging. The capacity of the bayou to accommodate flood flow from upstream will be compromised by the tide and must have more to do with spatial width than channel depth. We’ve had the same arguments here in the UK when the Somerset levels flooded: this is land that is lower than sea level. Dredging cannot physically increase channel capacity when the tide fills that space with seawater twice a day. On the Somerset levels there is one channel behind raised banks and when too much floodwater meets the incoming tide, there is just nowhere else for it to go than sideways. On the Fens, however, there are two river channels, over a mile apart, with flood banks to the east of one and west of the other: that mile-wide strip is designed to flood and it provides an enormous amount of extra capacity. You can find this spot if you look for Welney, Wisbech, UK, on googlemaps. This was done by clever Dutch drainage engineers hundreds of years ago: they understood that at these altitudes the tide is the ultimate control of water levels and that the only way to accommodate extra flow is width, not depth. The real issue for you, I feel, must be flow and not depth. Too much floodwater arriving too quickly in the face of an incoming tide can only ever back up and spill sideways. You could make the bayou miles wider, but I doubt that would work if there is property on and around it. You could build really high flood-banks, but without spatial width they would have to be very high indeed. Or you can slow the flow from upstream. My guess is that the drainage of the catchment has coincided with this more hands-off management of the bayou and that both appear causing flooding: my argument would be that it is really the drainage of the catchment that is the cause of the flooding, especially if, as you say, that has been accelerated in recent years. Politicians very rarely understand all of what is actually very simple physics to do with flow, sea level and capacity and I hope my argument is not misused, as you say.