July 2010
Boscastle, 2004. Carlisle, 2005. Midlands, 2007. Cumbria, 2009. Heavy rainfall was a key factor for all of these recent flood events. Is the UK entering a period of changing, heavier rainfall? And if so, is this a symptom of climate change? Or are the causes of recent floods and their accompanying rainfall patterns attributable to other physical influences?
This case study examines some of the theories that may help explain recent extreme rainfall events that have caused major flooding. We also present a summary a key human factors that also contribute to the increasing potential for flood disasters to be experienced by UK citizens.
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The Cumbrian floods of 2009
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Climate change and changing rainfall patterns
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21st Century flood disaster predictions for the UK
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A Level / IB Hazards activity
The rainfall that brought floods to the Cumbrian settlements of Cockermouth and Workington in 2009 was the heaviest precipitation recorded in the UK since records began over 200 years ago. Just how extreme was this weather event? What were its consequences for people, the environment and the economy of Cumbria?
In November 2009, storm-driven flooding devastated parts of Cumbria in northern England. Especially badly-hit were the Cumbrian town of Workington, which the River Derwent flows through, and Cockermouth which was flooded when the River Cocker exceeded bankfull discharge. A recent in–depth report by the Financial Times (05 February 2010) examined the characteristics and impacts of the Cumbrian floods. The newspaper’s main findings can be analysed as follows:
(1) Characteristics of the rainfall and of the river in flood
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This was “the greatest day of British rain since records began more than 200 years ago, a total of 316.4 mm.”
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The rain was frontal in origin – a strong depression moved inland from the Atlantic – but precipitation levels were further increased by the relief effect of the Cumbrian fells.
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At its maximum, the River Derwent was 10 metres wider than during normal conditions.
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At the Camerton gauging station, the River Derwent's peak discharge measured 573 cumecs, 25 times higher than its normal average flow level.
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The River Cocker’s flow exceeded anything seen in the last 30 years.
(2) The range of impacts of the flooding
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The regional economy was badly hit: the floods caused £100m damages.
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Many people were injured and PC Bill Barker was killed when a bridge in Workington collapsed. Luckily, the region has a low population density but nonetheless 1,500 homes were flooded, 885 of them in Cockermouth.
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There were environmental consequences too - whilst at peak discharge, fluvial erosion by the River Derwent’s triggered mass movement (such as landslides) along its banks. The river also tore loose and carried away hundreds of trees, damaging local ecosystems and habitats.
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Debris transported by the river – a river load comprised of boulders, trees and man-made lumps of concrete and cars – destroyed six bridges, bringing further devastation to the landscape of Cumbria.
Cumbria has struggled economically in recent years. Like many other rural regions of the UK, employment opportunities have become limited. In the UK’s post-productive countryside, there are relatively few jobs left in farming. Development agencies have often attempted to tackle this problem through a strategy called rebranding. This has usually involved giving a boost to tourism by devising new ways of marketing local heritage, food or other important rural resources.
Cumbria is a good example of a rebranded area that now derives much of its income from various forms of tourist consumption. Examples include:
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Westmorland farm shop
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Walby Farm Park
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Silverband Falconry
However, prominent negative reporting of the 2009 floods in British national newspapers left the West Cumbria Development Agency worried that potential visitors would reject Cumbria as a holiday or day-trip destination in 2010. As part of the on-going rebranding campaign, a new national newspaper campaign was therefore devised. Purposely designed to offer re-assurance to tourists, it announced confidently that Cumbria was ‘open for business as usual’.
How will climate change impact on rainfall patterns in the UK? Are some changes underway already? While there is a broad consensus among scientists when it comes to making 21st Century temperature-rise predictions, there is far less agreement when it comes to forecasting changes in rainfall patterns.
Reliable predictions of future changes in rainfall patterns and intensity for the UK are not easy to make. There are two key reasons for this difficulty:
The hydrological cycle Because higher temperatures bring higher rates of evaporation, it is not necessarily the case that a hotter climate need bring drier weather. (For instance, the equator and the tropics are both “hot” climatic regions. Yet some forested areas of equatorial Africa experience daily rainfall while neighbouring desert regions receive little moisture.) Predicting what the net effect of warmer weather will be on rates of evaporation, condensation and precipitation for a temperate maritime location such as the UK is a tough task. Understandably, there is still a great deal of uncertainty shared amongst meteorologists, despite the sophistication of their recent computer modelling.
The jet stream Most of the UK’s heaviest precipitation comes in the form of frontal (depression) rainfall. Depressions are low-pressure weather systems that travel over the UK heading in an easterly direction. The course they take is tied to the location of the polar jet stream, an upper atmosphere air current that circles the northern hemisphere. The energy flow of the jet stream is just one part of a highly complex planetary-scale air circulation system whose future behaviour is also very hard to predict in any warming world scenario and with any significant level of confidence. But changes in the position of the jet stream will be a critical factor determining whether different parts of the UK begin to experience storm rainfall more or less frequently.
What are jet streams
Current location of the Atlantic jet stream
What can be concluded? Some increased potential for heavier rainfall events is apparent, because:
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warmer air can hold more water – 6% more per 1ºC increase – so a correlation between rising air temperatures and increasing heavy rain can be expected.
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higher evaporation rates over the Atlantic can be expected to provide approaching weather systems with greater inputs of water vapour and energy.
But what about changes in the overall seasonal pattern of rainfall? One scenario that has won the backing of some IPCC (inter-governmental panel for climate change) scientists describes progressively stormier, wetter winters and drier summers as the 21st Century unfolds. Paul Davies, chief forecaster at the UK Flood Forecasting Centre, told the Financial Times (05 February 2010) that the recent Cumbrian floods “were entirely consistent with the climate change predictions for the UK”.
For instance, it is notable that the Cumbrian rains fell in November on account of a low pressure system arriving from the Atlantic where exceptionally warm sea temperatures were recorded in 2009 (the sixth-highest since 1880). “If this was a freak, then it was a freak that made an awful lot of sense.” (Financial Times, 05 February 2010)
However, the future scenario where climate change brings greater rainfall in winter but not summer to the UK is by no means a certain outcome. There are other possible scenarios such as the jet stream moving permanently further northwards. This would mean fewer depressions passing over southern regions of the UK, resulting in lower rainfall throughout the year.
Howard Wheater of Imperial College told the Financial Times: “ Climate models are pretty good at getting global temperature…but they are hopeless at getting rainfall right.”
Some commentators have been quick to present high-profile flood events striking the UK are harbingers of anthropogenic climate change. This hypothesis certainly merits further investigation given the cascade of recent headline-grabbing flood events, including:
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2000 Major floods across the UK, including River Severn
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2004 Village of Boscastle experiences a 1:400 year flood event
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2005 Major flooding in Carlisle
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2007 Heavy fluvial and pluvial (surface water) flooding in urban areas across the UK brings record losses of £3 billion for insurance industry
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2009 Cumbria experiences heaviest rainfall ever recorded
However, some experts believe there may be an alternate, natural, explanation for what has been occurring. Not all of these recent events are consistent with the anthropogenic climate change hypothesis. For instance, the floods of 2004 and 2007 were summer events, which is not in line with IPCC predictions for drier summers.
Another explanation runs as follows. The decades following the 1960s – when detailed flood records first began to be kept for the entire UK – are now believed to have been a naturally flood-poor period. As researchers have studied regional data for the earlier decades of the twentieth century, a picture has emerged of a contrastingly flood-rich period running from 1920-1960. It may therefore be the case that recent heavy rainfall events mark the natural, cyclical return to a flood-rich period that is independent of any longer-term modification of rainfall patterns linked with anthropogenic climate change.
The suggestion that long-term climate change may not be responsible for recent flooding is not good news, however. Most meteorologists and hydrologists are still in agreement that heavier rainfall events will become more common as the Earth’s climate warms in coming decades. If this predicted long-term trend should coincide with the “normal” cyclical return of an extended flood-rich period, then the potential for the UK to experience truly extreme rainfall events in the years ahead may be even greater than has hitherto been suspected.
Whether or not rainfall patterns are permanently changing, flood disasters that cost lives and money look likely to become more common in the UK for a range of reasons. These include sea-level rises and the increasing vulnerability of the population due to its growth in numbers, location and wealth.
A disaster is the materialisation of a hazard. Whenever people are found living near coastlines or along river flood plains there is also a hazard risk. Flood hazards are found in some inland areas away from rivers too, where local drainage causes problems – for instance at the base of steep slopes, in depressions or heavily urbanised areas with poorly-maintained storm drains.
Physical causes of increasing flood disaster potential
Several physical factors help explain why flood disaster potential is predicted to rise in the UK during the 21st Century.
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Climate change and sea-level rise Thermal expansion of the oceans and run-off from melting land-based glaciers and ice sheets is predicted to bring as much as one metre of sea-level rise by 2100
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Climate change and changing rainfall patterns In Cockermouth, the 1 in 100 years river defences have been breached three times in the last five years. Some experts have attributed this to warmer Atlantic Ocean temperatures providing depressions with greater inputs of moisture and energy – although the overall picture of change is still very unclear.
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Return to flood-rich conditions As explained in the previous section, many experts now believe that the UK experiences decades-long natural cycles of lighter and the heavier rainfall. A flood-poor period is believed to have recently ended; we may now be experiencing the onset of a flood-rich period.
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Regional ground subsidence In some parts of the UK, notably southeast England, the land is sinking at around 1-2mm per year. This is due to long-term isostatic rebound (a process whose operation is associated with the growth and later removal of enormous weights of ice that depressed parts of the earth’s crust during the last ice age).
Human causes of increasing flood disaster potential
There are additionally many interconnected human factors that help explain increasing flood disaster potential in the UK:
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Population growth Britain’s population has grown slowly but steadily since 1945 to reach over 61 million. Some growth has been due to natural increase; more recently migration has been a major factor. Inevitably, new housing has been needed which results in increased urbanisation and the accompanying loss of natural permeable ground surfaces and vegetation cover along river flood plains (factors which promote the interception and infiltration of rain during storms and work to naturally lessen river flood risk).
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Social changes and the housing crisis Social changes have accelerated the rate at which new housing has been built. Increased life expectancy has led to more houses being occupied by single pensioners; increasing divorce rates also drive property demand; more university places require greater numbers of student dwellings. These factors have led to new housing being needed even in areas where population numbers are not growing overall. Once again, the result is overall growth in impermeable surfaces in river-line and coastal settlements with a known flood risk.
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Underestimation of risk Some New Towns and housing estate developments dating from the 1960s – as well as the more recent Thames Gateway urban expansion – are located in areas where, despite provision of defences, there remains significant residual flood risk. The flood-poor nature of the period 1960-2000 may help explain why in some cases housing developments were approved despite being located in what are now more clearly understood to be risky locations
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Increased wealth and possessions A major flood event today is almost certain to be hailed as a greater disaster than a similar-sized event 50 years ago. The reason for this is that people have far more to lose and so economic costs and insurance claims will be higher. As the majority of the population have moved into well-paid tertiary (service sector) work, incomes and affluence have grown accordingly. People who have recently had home “makeovers” may have more to lose if their home is flooded
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Lack of flood and experience Younger people living in flood-risk areas may have little appreciation of the risks that exist due to the flood-poor nature of the 1980s and 1990s. As a result, they may be inadequately prepared for flooding and may have poor recovery powers / resilience if a flood strikes (see inset box).
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Urban land-use sectors Classic urban land use models, such as that devised by Hoyt, show “working class” housing sectors in close proximity to industrial areas. Such land uses often develop alongside rivers in urban areas thanks to industry’s dependence on rivers for a range of purposes. As a result, the poorest and most vulnerable members of society – who also frequently lack home insurance – have the greatest flood risk. This increases the potential for human disaster (housing markets were an important factor influencing outcomes in the US city of New Orleans when Hurricane Katrina struck).
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Insurance markets UK insurers may become less obliged in the future to provide household insurance in areas with a known flood risk. This would greatly increase the potential of financial disaster for households.
The vulnerability paradox
Taking action against small floods can increase long-term vulnerability by fostering a false sense of security. A coastal town that is protected by, say, a 1 in 20 year flood wall may very well not experience any flooding for 20 years or perhaps even longer (each year there is only a 1 in 20 chance of a flood occurring that is large enough to overwhelm the wall).
As long as the settlement remains flood-free, an increasing proportion of its population will come to consist of migrants and young people who have no personal experience or memory of flooding in their town. So they are less likely to take any personal responsibility for making flood plans of their own (such as flood-proofing their own property, or keeping an emergency flood kit). They are also less likely to take any kind of flood planning precaution, such as signing up for automated telephone warning services from the Environment Agency.
When, inevitably, a large flood with a greater return period than 1 in 20 years overwhelms the town flood defences, there will be more unprepared and defenceless people and properties present than would be the case if the population had more regular experience of the small, manageable flood events that the flood wall has insulated them from over the years. The lesson? Low-grade public defences may deter citizens from taking greater personal responsibility for developing their own hazard resilience.
[Source: Based on an argument put forward by Professor Stuart Lane at the 2010 Geographical Association conference in Derby.]
(a) Analyse the links between jet stream activity and extreme weather events for the UK. (10 marks)
[Tips - Discuss the way depression tracks move from west to east across the UK in tandem with high-altitude westerly air currents. The jet stream is a fast-moving belt of air within these high-altitude westerlies. The position of the jet stream therefore helps determine what parts of the UK are worst affected by extreme weather events such as heavy frontal rainfall. Conditions in the upper air flow – notably whether the flow is weakening and diverging – can help determine how deep a depression becomes and how severe the resulting storm conditions are. Discussing these two different but related aspects of the relationship between jet stream activity and extreme weather events for the UK (i.e. the storm patterns as well as their actual strength) is sure to bring you high marks if done accurately. ]
(b) Explain why the population of the UK may experience increasing flood hazards in the 21st Century. (15 marks)
[Tips – Make sure you can discuss a range of human and physical factors and that your response is carefully structured (begin a new paragraph when each new factor is introduced). Physical themes to include are climate change and increasing evaporation perhaps bringing more extreme storms; climate change and sea-level rise driven by thermal expansion of the oceans and, as time passes, the melting of land ice; a possible return to ‘flood-rich’ conditions after ‘flood-poor’ decades. Human factors to consider are: urbanisation of flood plains; population growth, migration and the social need for more new housing. The best answers will be able to bring some of these factors together to build a complex multi-causal argument. For instance, the flood-poor period of the 1960s and 1970s encouraged greater urbanisation of flood plains as the risks were perceived to be lower than they actually were!]
Written by Dr Simon Oakes, a Chief Examiner in geography. He worked at Middlesex University’s Flood Hazard Research Centre (FHRC) where he has since consulted on several flood hazard research projects.