Podcast: UK flooding - Past, present and future

River Thames in flood

10 December 2014 by Sue Nelson

This week in the Planet Earth podcast, Terry Marsh and Mike Acreman from the Centre for Ecology & Hydrology (CEH) talk about their work on UK flooding.

They talk to Sue Nelson about how we're getting a better understanding of the processes responsible for recent severe floods - and how we might be able to limit the harm they do in future.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Sue Nelson: This time on the Planet Earth podcast - winter floods and what we've learnt from them. Hello, I'm Sue Nelson and I am at the Centre for Ecology & Hydrology near the river Thames at Wallingford in Oxfordshire, a place that often suffers from flooding. A recent review investigating the potential influences on UK winter floods was published in the journal Nature Climate Change, so with another potentially wet winter ahead for all of us I am with Professor Mike Acreman and the co-author of the paper, hydrologist Terry Marsh.

Terry Marsh: Well, we're standing here on the flood plain of the Thames, probably 300 metres away from the river itself.

Sue Nelson: And yet we're three hundred metres but there is a stone set in the grass just beneath our feet that says "flood level", with a little arrow "January 2003". So in January 2003 the water from the Thames 300 metres away actually reached the middle of this parkland.

Terry Marsh: Yes, but that is because the parkland is a flood plain and the Thames fills into it. A flood of this magnitude you might expect probably once every ten years or so.

Sue Nelson: But during last winter's storms in 2013, 2014 which is the subject of the paper that you were apart of - how far did the water come up then?

Terry Marsh: That was the wettest winter that this country had ever experienced. It came to a similar level here but there were places in the Thames which were flooded for a very sustained period. The difference about last winter was the flooding wasn't extreme but it was very sustained. So communities that could tolerate two or three days of inundation really struggled when that stretched beyond three or four weeks.

Sue Nelson: Now, the paper was a review looking at the possible influences of the flooding during that winter. Did you look at any specific traits?

Terry Marsh: Obviously the background was to attempt to understand the climatic drivers which were impacting on the sort of conditions that we experienced last winter, and that would range from the rainfall itself but more to the generating mechanisms what is influencing our rainfall patterns and their variability through time. And the sort of factors that would be of interest there would be solar radiation, melting of the ice caps, the El Niño effect and other teleconnections. What we've learnt perhaps over the last twenty years is that it is a lot more complex than we first imagined, but monitoring and modelling coming together are beginning to give us some mechanism to get to grips with what's happening now, and that would provide a much firmer basis for predicting what's going to happen in the future.

Sue Nelson: Now you did this review with the Met Office and also with a number of different universities including Oxford, Exeter and Reading. So you were using meteorological data as well?

Terry Marsh: There was a great deal of meteorological data used and this is always the driver. The difficult bit is moving from rainfall to actual river flow and understanding how things like evaporation, land use and - in this country in particular - the management of the rivers influence flood risk.

Sue Nelson: And what conclusions did you come to in terms of the influences bearing in mind, as you said, that it was a very unusually wet winter?

Terry Marsh: The conclusions were that it was a remarkably wet winter. We have had wet winters before, this one was outstandingly so. The frequency of low pressure systems crossing most of the country was remarkable. The intensity of the depressions which brought most of the rainfall was also remarkable. Getting beyond that to link that to the teleconnections that I mentioned, to solar activity and to perhaps to the melting of the ice sheets is now a focus of continuing research.

Sue Nelson: Mike Acreman, you have got an environmental perspective on flooding. How do you view the conclusions of this review?

Mike Acreman: I think there are two things to say. First of all we are looking out here on the flood plain and you can see that it is being used for recreation - there's a cricket ground and football ground here and I think it is important to remember that flooding is a natural process and many of the areas of the flood plain along the river Thames are actually designated for important wildlife plants and animals, and that's because of the floods that bring natural nutrients and sediments to the river. An important point is to make sure that those areas are used in sympathy with flooding. So the problem is if you build houses on the flood plain then of course you are going to create a flood risk. Whereas as I say in front of us we have a recreation park, so when it is flooded, okay, people can't play cricket for a while but as soon as the flood has gone it goes back to its recreational use.

Sue Nelson: We've just been passed by a jogger there - somebody who is enjoying the recreational uses. So, you have a certain amount of sympathy really then for flooding being a natural process that perhaps people, in general, we always see flooding as it is negative and something to be avoided.

Mike Acreman: Well, the flood water has got to go somewhere and of course it is terrible when people's houses are flooded or lines of communication are disrupted, roads and so on, but flooding in the right place can be very, very positive.

Sue Nelson: So how do we reduce the amount of flooding in a more natural way perhaps?

Mike Acreman: So, Terry has given you some examples of the meteorological drivers of floods and they obviously dominate. If you've got heavy rainfall you are likely to get some flooding. But we've seen over the last 50 to one hundred years a lot of changes in the environment and we have had the national ecosystem assessment recently which highlighted the loss of wetlands and other natural areas, which are being more and more managed and things like houses being built on them. There's a concern that this is changing the way in which rainfall is converted into river flow and creates floods in the landscape. So one idea would be to try and restore some of those natural environmental areas like woodlands and wetlands. They're not going to be the panacea; they're not going to cure floods, because as I was saying flooding is a natural process- but every little bit helps. And also with restoring those sorts of environments you get multiple benefits, so you may get a bit of help with flood management but you also get the biodiversity and other benefits that come along with it.

Sue Nelson: Terry, what evidence have you got that flooding is on the increase?

Terry Marsh: Well, as background, the Natural Hydrological Monitoring Programme is operated jointly by CEH and the British Geological Survey, and we have records going back in some cases over one hundred years of river flow and ground water levels. The evidence from that is that there is no compelling trend in flood magnitude across the country. There is some tendency towards an increased frequency of medium magnitude floods. The one beneficial, in a sense, element of climate change in a warmer world is that the risk of snow melt-aggravated flooding, which was a real problem in the 19th Century is tending to decline. So that is beneficial in terms of fluvial floods; of course, as temperatures rise so sea levels rise and that's aggravating tidal flooding, so it is a complex picture.

Sue Nelson: Now the review couldn't come to a conclusion about the relationship between human activity and flooding, why not?

Terry Marsh: Because it is very difficult. If you look at the recent flooding and if we compare it with 1947... in 1947 the flooding was the biggest that we saw across England and Wales in the whole of the 20th Century. If we had the degree of development then that we have now the number of properties flooded would probably have been in excess of 150,000. How many were flooded last winter? Probably around 7,000. So this is a tribute to the flood-alleviation measures that have been taken. We must continue to do more, but equally people need to understand the risk at personal level. Fifty years ago people tended not to move around the country every much. Now we're very mobile at a community level and people living by the Thames wouldn't have seen it flood for twenty or thirty years. They don't carry that as a personal memory so the solution is not only organisational or even political, it's personal as well.

Sue Nelson: Do you agree with that Mike?

Mike Acreman: Yes. I think we need an integrated approach to flood management. We need to recognise the overall drivers of things like climate change, sea-level rise. We need to improve our direct management of the rivers, as Terry was saying, by having flood relief channels, a sensible building policy, but we also need to heighten awareness about floods and what people can do to help mitigate them or move away from them or to react when a flood actually occurs.

Sue Nelson: Can you use any of the material and the information that you've got to make predictions about what's going to happen this next winter, 2014 to 2015, and whether flooding is going to be more or less likely or the same?

Mike Acreman: Well the short answer is no! There is some evidence that flooding tends to cluster, so if we're in a wet domain, if you will, then there's a slightly increased chance that one wet winter will follow another. It depends on a number of factors. The outlook for this winter is going to depend primarily on the intensity and the magnitude of the rainfall, as always, so it is a challenge for the modelling community to improve their modelling capability. We're never going to stop flooding so we need to think about our planning both in the urban areas and in the natural landscape so that we try and work in a way which adjusts to floods so that we work in harmony with them and don't pretend that we could ever stop flooding.

Sue Nelson: Mike Acreman and Terry Marsh, thank you both very much indeed. This has been the Planet Earth podcast for the Natural Environment Research Council. You can see some photos of our interview today on our Facebook page and probably no doubt on Twitter. I'm Sue Nelson, thanks for listening.