Instant Genius - How to combat flooding

Episode Date: December 16, 2024

Anyone who has switched on the news even briefly this year will no doubt have seen scenes of extreme flooding occurring in all corners of the globe. But are these events getting more frequent and more severe or are we just becoming more aware of them? In this episode, we catch up with Prof Trevor Hoey, a hydrologist based at Brunel University, London. He tells us about the various climate mechanisms driving these extreme weather events, how counterintuitively severe droughts are being caused by the same processes and he also details some of the measures we can take to minimise the damage these events cause. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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talking about the most fascinating ideas in science and technology today. I'm Jason Goodyear, commissioning editor at BBC Science Focus. Anyone who switched on the news even briefly this year will no doubt have seen scenes of extreme flooding occurring in all corners of the globe. But are these events getting more frequent and more severe? or are we just becoming more aware of them? In this episode, we catch up with Professor Trevor Hoey, a hydrologist based at Brunel University, London.

He tells us about the various climate mechanisms driving these extreme weather events, how counterintuitively severe droughts are being caused by the same processes, and he also details some of the measures we can take to minimise the damage these events cause. Welcome to the podcast. Thanks for joining us. Thanks for having me. Today we're talking about the recent spate of floods we've been

having. So first off, how widespread are they? Are they happening all over the globe? Well, it depends a little bit, I suppose, of what your perspective is, doesn't it? Because in the last couple of months, there have been floods all across the world in different continents. We've seen flooding in Southeast Asia, we've seen flooding in East Asia, we've seen flooding in Europe, we've seen flooding in Latin America, in Africa, and of course the floods that people have seen here in the UK over the last few weeks. It's autumn, and so we get floods in the autumn in the UK. It's not unexpected that we would see some flooding, but we have seen really extreme flooding, haven't we, in parts of Wales, the south-west, in the Midlands, perhaps slightly less

further north in Scotland, where we often get flooding, but there have been localised floods in Scotland too recently as well. So sticking with the global picture is the same storm system responsible for all of this? No, there are a range of different mechanisms in different regions. And there's some connection. The connection is that high-intensity rainfall events are generally driven by having a lot of moisture in the atmosphere, and a lot of moisture in the atmosphere is determined to some extent by having warm sea surface temperatures, so you get a lot of evaporation,

and the atmosphere itself would be a relatively warm, so that it holds a lot of moisture, holds more moisture when it's warm atmosphere than does when it's cooler. So the floods in Spain recently, for example, very warm conditions in the Mediterranean this summer, warm sea, warm atmosphere, very high moisture contents, and then cold air coming in and triggering very intense rainfall. And that happened not just once but twice in the last few weeks. you've seen those devastating floods in Valencia and other regions of Spain. Whereas in the tropics, for example, there have been quite severe a series of typhoons in the Philippines in the last few weeks. They've had six typhoons, the last of which was a super

typhoon last week that I was in the middle of, and it rained quite a lot. And that is unusual to get such a sequence of typhoons. It's quite late in the season to have those. Again, they're caused primarily by very warm sea surface temperatures, a lot of evaporation, warm atmosphere holding lots of water, and you get the rotation that gives rise to typhoons like hurricanes. This time of year, you might expect in the Philippines to get two or three, but they have six in sequence, just one straight after the other, after the other. So, regionally conditions, similarities, but not the same thing. You mentioned there that typhoons have become more prevalent, etc.

How do the current events compare to previous events? Are they getting more severe? Well, there are two parts of that, I suppose? Are they getting more severe and are the severe events getting more frequent? And I think the answers to those two go together. Clearly, we have always had very severe storms. I mean, there are hurricanes, major hurricanes, typhoons, people in the UK will remember large storm events that in 1986,

a storm that caused Michael Fish some embarrassment at the time. So we do always get these very high magnitude storms. And you can look into the past record and see the record of those storms. And you can see spikes where you get the largest storms. The frequency with which we're seeing those is definitely increasing. And you look at how many Category 5 hurricanes there are in the Caribbean in the last few years. You look at the number of typhoons in Southeast Asia. You look at the number of these weather events like they have in the Mediterranean recently

that generated the floods. And you see trends for these events to become more frequent. Quite widespread in different regions of the world, you're seeing the same sort of pattern. People will also note that in many parts of the world, droughts are becoming longer or more intense as well as increased storminess. And those two things can both be true. at the same time, partly because different regions are affected by those different phenomena, but equally, the processes that give rise to drought, very high temperatures, very stable weather

systems for long periods of time, lack of rainfall, those meteorological processes are simultaneously diverting the air masses, diverting the moisture to other areas. And so while one region of Europe is having droughts, others are seeing increased rainfall compared to. the norm and vice versa. So what do we know about why this is happening? I know that's a terribly complicated question. In terms of the controls over weather and climate, in very simple terms, we can go back to increased global temperatures

and particularly increased temperatures of the oceans. And the oceans are the source of nearly all of the moisture that falls as rain. And so increased oceanic temperatures mean, you've got more evaporation, you've got more moisture going into the atmosphere. A warmer atmosphere can hold more water than a cooler atmosphere, as we mentioned earlier. So you have the conditions under which you've got more moisture in the atmosphere. And then that will precipitate.

It will fall as rain or snow over high ground. You can trigger very intense and prolonged rainfall events by the interaction between the atmosphere and, for example, the land surface as the air is forced over mountains, you get a lot of very intense rainfall on the windward side of mountains, but equally you can generate large storms, the sort of depressions that we see here in the UK, hurricanes, typhoons, cyclones, and other parts of the world, that as temperatures are warming, those are becoming more intense and they're lasting for longer,

and so generating these large, long-lasting rainfall events. Yeah, so I don't want to be too gloomy, but can we expect these events to be more severe and more frequent as time goes on? It's an interesting question. There are almost certainly some limits, and so we won't necessarily see continuous increase. Having said that, the increased frequency of major storms, increased frequency of long drought periods, are entirely consistent with the predictions of climate models that are predicting climate. change over the next 10, 50, 100 years. And when you look at the predictions of those models, we are expecting to see a continuation of some of the trends that we're seeing now. So I think we do have to prepare for increased storminess. We also have to prepare in the UK, for example,

for droughts becoming more frequent, for periods of summer excess heat becoming more frequent and more intense. But on the other hand, many parts of the UK, we're predicting increased rainfall over the next 30 to 50 years and therefore the increased probability of flooding. So you mentioned Michael Fish and his disastrous prediction earlier. And you just talked about 30 to 50 year predictions. So we can be reasonably confident about those. Is that right? Yes. I mean, the way which predictions are done these days is not to make a single prediction. You don't. You don't. just set your computer up and say, tell me the future and then believe what comes out. What you do is you run your computer model many, many times, tens of thousands of times,

each with very slightly different settings. You can put slight different assumptions in there. You can put in very slightly different conditions. And what you generate is a series of predictions. And within that series of predictions, some will be predicting, for example, more rainfall, some less rainfall, and there'll be some sort of average in the middle. And you can look at the distribution of those predictions to say, well, we are the sort of midpoint of all the predictions is this, but there's a five percent chance it'll be above this level or a five percent chance. It won't be as high. It'll be below a certain level. So predictions have become more probability-based, and people will be familiar with that, because

You hear that from weather forecasters when they're on the television or radio presenting their forecast. They will quite often talk about probabilities. There's an 80% chance of snow today. And that is based on the same sort of approach of running your weather and climate models many times and then looking at the likelihood of different things happening. I might be getting this wrong, but it seems to me to be slightly counterintuitive that we can look far into the future with these sorts of things. But when it comes to the next day or two, it's much more difficult to predict.

That's a really interesting observation. I mean, one of the things people often say about weather forecasting is that weather forecasting, we know what is going to happen. We don't necessarily know exactly when it is going to happen. Weather systems develop at different speeds. Sometimes storms intensify very quickly. Sometimes they intensify slightly more slowly. sometimes high pressure systems become established as we have in the UK.

We had that period of three or four weeks of just continuous gloomy, cloudy weather because the high pressure system essentially got a bit stuck. Now, sometimes they get stuck like that, sort of blocking high phenomenon. Sometimes those high pressure systems just aren't quite as strong and they break down. So it's the same reasoning, I suppose, that we use probabilistic approaches to climate model. for the very long term, weather forecasting relies a little bit on the same sort of approach. And from a given set of starting conditions today, the weather as it is today, we can be really quite confident to what the weather's going to be like in three hours,

six hours, 12 hours time. By two days' time, we're a bit less confident. By seven days time, we're a bit less confident. As I say, we know what's going to happen, but maybe that storm will get here next Thursday or it won't get here at all Sunday. And so that's where you get the uncertainty from. So it's obviously a complex issue. Having said that, how have flood alert warnings and extreme weather warnings set?

I mean, very different in different parts of the world. And I think what's interesting here is to think about in those parts of the world where major weather-related disasters are a fact of life, how they deal with these things. That's a bit of a contrast maybe to in the UK where severe as the weather is. It isn't as severe as in certain other regions. And the ability to forecast with certainty, as we've just been talking about, is to some extent limited.

You can give a probability type forecast. Anybody making these sort of forecasts and setting warnings is very aware of the cry wolf effect. You don't want to be saying it's going to flood. and they're not the captain. Oh, it's going to be a flood, no, there isn't. So they try to set the warning levels at a level where there is a high degree of confidence that that flood is going to occur, and then you have a little bit of a safety factor to say, well, actually, it may not, but if it does, it's going to be serious, so we better issue a warning. And the other part of this, of course, is those warnings have to be very dynamic. They have to respond to what actually

happens in terms, for example, that a flood warning in a river system, you might be predicting 100 millimeters of rain, and 100 millimeters of rain might be fine, but 120 millimeters, rain might give you a flooding problem. And so you have to be responsive as the rain is falling to say, actually, this looks if it's going to be a little bit more than we thought, and therefore we've got to be ready to issue a warning. And that's where you can run into some of the of the discussions that we've heard in the news in the last week or so about when warnings should be given and shouldn't warnings have been given earlier. And the reason warnings often aren't given early is that there really isn't confidence that there is going to be a flood event until

quite late on because there are particularities of where the rain falls, which part of the catchment it falls, how intense it is, that change the circumstances on the ground very quickly. And I'll just give you a contrasting example. In Central Australia, for example, where you have very large rivers flowing over thousands of kilometres, the flood events are generated a long way from the places that may be flooded. So the rain is falling over the mountains, it then gets into the river, and then it flows. And so it's not unusual for people there to be given four or five days notice of a flood because the rain has fallen. It's on its way down the river. It's coming. You've just got to wait until it gets there. And so flood warnings in those circumstances can be issued

very early on. People know what situation is. They can move their possessions. They can move their livestock. They can do whatever they need to do to avoid it. In the UK, most of our land is quite steep. Our river's quite small, small, short. We don't have a lot of time between the rain falling and the flood occurring in most parts of the UK. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition for Citizens Bank. You said this place was steps from the water.

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So where do floods actually come from? Is it rivers breaking their banks or the drainage infrastructure getting overloaded or a bit of both? There are seven different sources of river flooding, and I think sometimes these get sort of lumped together a little bit. So a lot of river flooding is where you have excess rainfall or maybe the snow or the snow melts plus rainfall in a river, and the volume of water is simply more than the natural channel of the river can carry. And so you'll then get water filling the channel and then going over banks. Hydrologists don't really like the term breaking their banks. I mean, we would use the term overbank, so it's over the top of the bank.

Nothing's broken, it's just gone over the top. But what has happened over many years, in a country like the UK, places that have been settled and built up for centuries, you've had land drainage, you've had rivers being straightened, artificially modified. And so the speed with which water gets through the natural system has been accelerated. So water is being delivered downstream faster generating flooding. When you come to urban areas, we can also get flooding by the urban drainage, just being overwhelmed. We're seeing more of that as summer rains get more intense. In our cities, we see, you see the

pictures every summer day, you have the bus underneath a railway bridge somewhere with water up to its middle. And that is nearly always because the drainage system has been overwhelmed. It might have been blocked. There might be rubbish. in there. There might be shopping trolleys and plastic bags and things that are stopping the drains functioning as designed, or the drains may simply not have been designed to carry the amount of water that's being presented to it. So we refer to that as it's often called pluvial flooding. It's rainfall intensity is exceeding the capacity of the drainage system to take that away. And then, of course, the other type of flooding that we have in the UK is coastal flooding,

where you have the interaction that often between river water, downstream and seawater. Some of the most famous floods in the UK in 1953 in affecting Norfolk, Suffolk, Essex coasts were a result of a storm surge where the, essentially the water level in the North Sea was raised because the air pressure was so low at the same time as there was a lot of water on the land and there had been recent rain. And you've got a lot of coastal flooding as a result of that phenomenon. Let's have a look at things we can do about this then. So what role can engineering and technology play?

There are different roles, aren't there? I mean, one is the technological ability to monitor what is happening in real time and to become much better at what's sometimes called nowcasting. We can do forecasting, we'd also do now casting, what is actually happening, how much rain is falling, where is it falling, we can use computer models to simulate where that rain's going to go, make predictions about what that's going to mean for river levels. Is it going to mean that they go overbank?

Is it going to cause flooding? Is that going to coincide with high tide in a low-lying area near the sea, for example? So we can use technology to integrate data, which is increasingly available from satellites and other sources, with computer models, and then with warning systems so that we can automatically tell people what the risk is and provide the information that they need. So that side of technology, the sort of information technology side of it and how we integrate datasets. Lots has happened. The last 30 years, enormous strides have been made. Lots is happening at the moment. AI, for example, is helping in this area and we will continue, I'm sure, to make

advances. The other area that people would talk about using technology and engineering, of course, actually in how we manage the landscape and how we root water and get water away from our infrastructure and away from houses, homes, roads, industry, farmland, whatever. And traditionally, river engineering was often based on let's get the water away as quickly as possible. How do we accelerate the movement of the water away? Because once it's gone, it's no longer a problem. It's on its ways to see, problem solved. And what we've realized some time ago, and it's become increasingly well accepted over the last 30, 40 years, and it's now in a lot of engineering design approaches, is that that approach doesn't really work.

There are places where it does work and there are places where it is absolutely the right thing to do, but equally in many cases, you're simply transferring the problem to someone else. If you get the water off your land, it's arriving with somebody else's land faster. and therefore the problem may actually be growing. So the technological approach has actually slightly reversed to how do we slow the water down from how do we speed it up. How about so-called nature-based solutions? How do they work?

Yeah, in terms of flood management, there's quite a lot of different things that would come under the category of nature-based solutions. I mean, for a give example, let's go back to the slowing the flow down. Nature-based solutions are basically two types. One is how do you slow water down, how you make it take longer to work its way down towards the sea, and the second is how do you create more space for the water to spread into? The slowing the water down part, we see in upland areas, for example, in the Pennines, there's been a lot of work done in the last few years, in peatland areas, creating small dams.

They're not dams that trap the water, they're leaky dams, they allow water through, they just slow it down. to try to reduce the rate of which water is going further downstream. Many places we've seen rivers that in the past were made straight have now been restored to a meandering or more natural shape to slow the water down and also with benefits for biodiversity and habitats and other things. So there are a range of methods that can be used to try to slow the water down. And then creating more space for water, what we've done in the last couple of hundred years in the UK

is we've often isolated rivers from their floodplains. We've built balls, for example, to stop water spilling onto floodplains. We've built all sorts of stuff on floodplains, houses, cities, factories, all sorts of things on floodplains. Those have in many cases separated the river from its actual floodplain. So we can create space for water by restoring some of those connections. In places, that means taking flood banks away and saying to the river, do what you want? In other places it may mean trying to restore connectivity. You obviously don't want a road flooding, but you might build more culverts underneath the roads

to allow river water to get into some floodplain that has been isolated from the floodplain in recent years. Similarly, at the coast, you can do the same sorts of things, the idea of replacing hard coastal defenses with softer defenses, allowing salt marshes to grow and so on, creating good. natural barriers between the sea and the land. And those salt marshes will absorb some of the edges from the sea. They will provide space from the water to spread out. So you get less flooding of farmlands and other areas.

So say if I do live in an area that's vulnerable to flooding, is there anything I can do to protect my home and my property? Well, yes. And it's an interesting sort of question about where the balance should lie between individuals and the government or other agencies to protect from flooding. People have been writing about this for many years. I remember 40 years ago seeing a book that was written in New Zealand called Creating Flood Disasters. And the point was that flood disasters are really human created.

They are created by people choosing to do things in places that will flood. if we didn't choose to do things in those places that were floods, then we wouldn't have flooding problems. That therefore means that when we think about how we respond to flooding, we need to accept that our activities are sort of getting in the way of where rivers and the sea would naturally be putting water. So how do we stop that water getting into the highest risk activities? What can we do as individuals?

It depends on the level of risk. In many places, you will see floodgates have been installed, and many houses and flood-prone areas have these little barriers at the front of their houses. And when it's going to flood, they get a warning, and they go and put the barrier in place, act a bit like sandbags, but it's actually a firm barrier that should be almost completely watertight.

So you can do things like that. In many parts of the world, you see people have adapted their styles of buildings to cope with flooding. You see people will have seen pictures of houses in the Amazon on stilts, for example. In the UK, we don't have many houses built like that. But there are some, there are some where there is a garage on the ground floor and then you go up steps to the living area. There's no electricity on the ground floor. The electricity all comes in at floor one level and then props down from the ceiling so that if you are flooded, your electricity keeps working.

And we can design in the future to take account of flooding, we can to a certain extent, adapt and retrofit. We can also think about what we will do if there is a flood. So there's a concept in hydrology called the probable maximum flood. And the idea of the probable maximum flood is, okay, what if the worst happens? Because someday the worst is going to happen. So in the UK, for example, it might be it snows, there's a lot of snow lying around, a meter or so of snow, and then suddenly it warms up and there's very intense rain. So all the snow melts and there's a lot of rain. There's going to be a very, very big, large amount of water

running off the land and you're going to get a lot of flooding. So when the probable maximum flood occurs, what do we do? Well, we can't engineer our way out of that. We have to be ready to cope with it when it happens. We have to have civil defence in place so that we know how to issue warnings. We know how to get help to vulnerable people. we know how to make sure that critical infrastructure is protected. We have to have systems in place that are tested that we know work.

As individuals, we need to know what our level of risk is. Many of us live in places that are not at risk of flooding. We don't need to worry about flooding. We might just worry about other things. If we do live in a flood road area and a very large flood occurs, what are we going to do? Have we planned in advance? Are we signed up to the warning system?

have we a container full of fresh water in our bedrooms so that if the water supply goes off or we can't get downstairs to the kitchen, we've got some fresh water to drink. So there are a lot of things that we can think about at individual level and a lot of things that can be thought about and planned at sort of societal level to make sure that when the worst happens, that we're able to cope with that and come out the other side and rebuild. Thank you for listening to Instant Genius, brought to you from the team behind BBC Science Focus. That was Professor Trevor Hoey. If you liked what you just heard, then please do consider subscribing to Instant Genius on your preferred podcast platform.

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