The Impact of Climate Change on Flood Risks

Dr Louise Slater

Hydrologist and Associate Professor Louise Slater from the School of Geography, University of Oxford gave an overview of the changes in flood risk affecting Oxford. She confirmed flood risk has been increasing and the city needed to become more flood resilient. To illustrate the magnitude of change, she ran a statistical analysis that showed a flood from the 1960’s that would have occurred every 100 years, could under today’s conditions happen with much greater frequency. She said there were different ways of calculating, 
“But it tells us that we should expect these very large floods of 1 in 100 years to occur with greater frequency now than they did in the 1960’s. A one in a 100-year flood is not a static concept, it is a dynamic concept.”
She said the likelihood of extreme floods would change over time with factors like urbanisation, deforestation and changes in water management and climate variability. 
“The way to address flooding is usually with a multi-level approach. You do need hard engineering solutions, like flood defences. But you also need softer methods like natural flood management and green infrastructure. This is part of a combined package and should set the scene for all discussions about flood management.”

 
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Oxford 2007

 
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The Complex Challenge of Protecting Oxford from Floods

How and why flood risk is changing in Oxford?  These pictures illustrates why Oxford is such a complex situation: in the foreground we have major flooding of multiple channels in a low-lying floodplain, right beside these high value historic assets in the background, and a dense population which needs to be protected. Hence, it's a difficult challenge.

 

How Floods are Changing in Oxford

What the data tells us

So how are floods changing in Oxford? To answer this question, we need to look at the data. We're lucky in the UK to have high quality records of river flow. Here, these are data that are measured by the Environment Agency and curated by the UK Centre for Ecology and Hydrology, made publicly available online through the National River Flow Archive. Upstream of Oxford, we have two gauges on the River Thames, which are Farmoor and Eynsham. As we travel downstream through Oxford, we have two more gauges on the Thames, which are Sutton Courtenay and Days Weir. I thought we’d look at the longest river flow time series that we have for Oxford, which is Days Weir, with almost 80 years of data. 


These are continuous river flow measurements that are made over a period of 80 years, which allow us to understand whether floods are getting bigger or happening more frequently than before. Here on the left is the Days Weir gauge, and then the Google Maps aerial image on the right. 


So, let’s look at Oxford's river flow measurements since the 1960s which are based on the largest river flow measured in every year. The time series shows that floods have been increasing since the 1960s. This is happening not just in Oxford – in many rivers across the British Isles, we're seeing these patterns of increases in high river flows. 


We can also look at the longer record. We're lucky to have 80 years of data; it's an impressive record. Here we see a very different pattern that emerges: in the 1940s, we had some very large floods too. And if we apply statistical analysis, it actually tells us that floods have decreased relative to the 1940s, which is not what some people might be expecting, when we're talking about floods in Oxford. 


So, the changes in flooding are a little more complex than at first glance. We actually find oscillations in the data. These patterns are due to climate oscillations like the North Atlantic Oscillation. This is something well recognised, which has led to multiple flood rich periods in the British Isles. We saw a flood rich period in the 1940s, and another one again since the 1990s. And in contrast to these flood rich periods, we also have flood poor periods, which is what occurred in the 1960s, as you can see. So relative to the 1960s, yes, there has been an increase in flooding. 


To perform a quick experiment, I thought, let's see, how often would the 100-year flood as it was in the 1960s now occur under today's conditions? If we apply statistical modelling, we can see that a flood from the 1960s, which would have occurred every 100 years in 1965, under today's conditions might occur every 12 years on average. Now, there are different ways of calculating these types of statistics, and you'll obtain slightly different results depending on the method you choose. But it's still quite an important message: yes, we should expect these very large 1 in 100 year floods of the 1960s to occur more frequently now than they did before. 


So how and why is flood risk changing across Oxford? We can see that it is partly tied to these climate oscillations that we have. But the main message I wanted to put forward is that the idea of a one in 100-year flood is not a static concept. It's a dynamic concept. These extreme river floods will change over time. They change with factors like urbanisation, deforestation, changes in water management, climate and climate variability. These are all topics that we're actively working on at the University of Oxford. 


The way that we need to address these challenging issues is usually with a multi-level approach. We do need hard engineering solutions, like flood defences. But on the other hand, we also need to incorporate ‘softer’ methods like natural flood management, like green infrastructure. This is all part of a combined package. And I think this sets the scene.

 
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"The main message I wanted to put forward is that the idea of a one in 100-year flood is not a static concept. It's a dynamic concept. These extreme river floods will change over time. They change with factors like urbanisation, deforestation, changes in water management, climate and climate variability."

Dr Louise Slater