Development pressures: Is climate change to blame for the increase in flooding?

Climate Change and Sustainability March 05, 2020

Colin Whittingham, RSK Associate Director

It has been widely published in the press that building tens of thousands of homes on flood-prone land is compounding the effects of flooding in the UK. Tory backbenchers called on Boris Johnson to review the government’s housing policy over concerns that new homes were either not flood-proof or were exacerbating issues in neighbouring communities. But are the goalposts on flood risk changing because of climate change?

Following storms Ciara and Dennis, and to some extent Jorge, there were up to 250 flood warnings and alerts across England, from Devon to Cumbria, with a further ten in Wales, following one of the wettest Februarys on record for more than 250 years. With the flood events still affecting people across the country, it is too early to fully assess the effects and their severity. But we do know that many major rivers have reached their highest levels on record, and “unprecedented” is used to describe the event in the media.

These floods occur only months after similarly devastating flooding in Yorkshire and the Midlands. The summer of 2019 also saw more localised, but dramatic, flooding in the Peak District and Lincolnshire.

With the use of social media and easier access to news feeds, it seems that floods are more widely publicised than before. But the winter of 2015–16 saw devastating, record-breaking floods in northwest Britain, with the highest recorded daily rainfall and the highest peak river flows recorded in England. In the winter of 2013–14, sustained flooding affected southern England – most notably the Somerset levels and the Lower Thames. While this event was less exceptional in terms of flood peaks, the duration and extent were remarkable.

Generally, the first two decades of the 21st century have been characterised by many major floods affecting the entire UK. Inevitably, the current floods have prompted widespread discussions about flood risk management and claims that floods are increasing because of climate change.

There are many forms of flooding, but it is fluvial (river) flooding that typically causes the most damage owing to its extent, depth, velocity and duration. With intense rainfall events, pluvial (surface water) flooding inevitable affects more localised areas, but I will focus on river flooding risks.

Although the most recent flooding is still to be fully assessed, a history of peak flows is recorded by the National River Flow Archive (NRFA) from more than 1500 gauging stations at various UK locations. The data held by the NRFA enables assessing live data and analysing the longer-term trends in peak flows. While I have not assessed the data from all available records, for obvious reasons, luckily the NFRA has, and the results indicate the same conclusion. Over the past four or five decades (the length of most typical river-flow records), there has generally been an increase in river flow across most of the UK. More importantly, regarding floods, there have been increases in peak river flows (commonly, studies analyse trends in the “annual maximum” – the highest peak river flow in each year), and floods have become more frequent.

The work is also consistent in terms of the geographical picture. The map below shows trends in annual maximum flow for 346 UK gauging stations. Increases in high river flows (indicated by blue triangles) are most noticeable in the north and west of the UK. There are also significant increases in some other parts of the country, such as southern England and East Anglia.

Data from the UK National River Flow Archive. The map shows the strength of trends (triangle size) for the annual maximum (“AMAX”) flow across the UK from 1967–2016. Trends expressed as the percentage change over the period relative to the long-term average. White shading denotes statistically significant trends. Blue symbols indicate increases; red symbols indicate decreases.

Acknowledgement: Data from the UK National River Flow Archive. The map shows the strength of trends (triangle size) for the annual maximum (“AMAX”) flow across the UK from 1967–2016. Trends expressed as the percentage change over the period relative to the long-term average. White shading denotes statistically significant trends. Blue symbols indicate increases; red symbols indicate decreases.

The graphs below are examples of long-term variability in floods in some of the areas affected in 2019–20. The rivers Don, Calder and Eden, with records going back to the 1960s and 1970s, show increasing trends, with recent floods, in 2007 and 2015–16, standing out. Importantly, the graphs show trends in annual peak flows using data up to 2017, and do not include the 2019–20 floods. Early estimates, however, suggest the recent peaks will further reinforce these trends.

However, the record from the River Severn – a rare example of a record extending to the 1920s – portrays a different story.

Although the graphs indicate a weak increase since the 1960s, they reveal more extensive flooding in the pre-1960s period. The record reveals a weak downward trend, albeit with the most prominent feature being notable variability between years and decades, with more flood-rich periods in the early 2000s contrasting with flood-poor periods, such as in the early 1970s.

Data from the UK National River Flow Archive. A time series of annual maximum flow (“AMAX”) for four selected rivers in areas affected by flooding in 2019–2020. The black dashed line shows a linear trend; the red line shows a smoothing filter, which emphasises the variability between decades.

Acknowledgement: Data from the UK National River Flow Archive. A time series of annual maximum flow (“AMAX”) for four selected rivers in areas affected by flooding in 2019–2020. The black dashed line shows a linear trend; the red line shows a smoothing filter, which emphasises the variability between decades.

In principle, there is a fundamental link between climate change and flooding. A warmer atmosphere retains more moisture. Correspondingly, climate change projections suggest future increases in flood severity are likely for the UK. But are we seeing an impact of climate change in the increased flood trends we have witnessed over recent decades?

Research in the UK, using historical flood records, such as for the Severn, points to pronounced variability between decades, leading to flood-rich and flood-poor periods that are driven by several factors. These variations make it difficult to interpret trends, so we cannot say with confidence how much of the observed increase in flooding over recent decades is because of human-caused warming and how much is because of natural variability.

What we can say with certainty, however, is that there has been an increasing trend in flooding over the past four or five decades in parts of northern and western Britain, and this is consistent with what we expect in a warming world.

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