How is climate change affecting river and surface water flooding in the UK?
Climate change is increasing the likelihood of heavy or intense rainfall in many parts of the UK, particularly in the winter months. This is leading to higher risks of river and surface water flooding.
Factors in the risk of river and surface water flooding
Besides the quantity and intensity of rainfall, there are other factors in the risk of river and surface water flooding. River flooding is more likely if a lot of water drains into water courses rather than being absorbed by the ground. The permeability of the ground (determined by factors such as the presence of vegetation or man-made impermeable surfaces, the soil and rock type) and local relief will affect this – if the ground is less permeable and steeper, more water will reach streams and rivers. If the ground is already saturated by previous rainfall, or if it has been baked hard by the sun, there will also be more surface run-off into water courses. Whether or not the floodplain is protected by flood defences will also play a role in the extent to which flood waters become a hazard.
Surface water flooding occurs when rainfall sits or flows on the surface rather than running away through drainage systems or soaking into the ground. Therefore, the risk of this hazard occurring depends on the capacity of drainage systems and the permeability of the ground. It can be common in urban areas where there are lots of man-made impermeable surfaces like concrete and tarmac, and where drainage systems may be overwhelmed or inadequate.
Changes to the UK’s weather and climate that are increasing flood risk
Climate change is increasing the risk of both flooding and drought in the UK. As the atmosphere warms, it can hold more water, which is then released during rainfall events. A drought may end with heavy rainfall that cannot drain away as the ground has dried out and flooding may occur as a result.
Research carried out by staff at the UK Met Office in 2017 found that climate change means there is “a high chance of exceeding the observed record monthly rainfall totals in many regions of the UK”. Their analysis shows the chances of winter rainfall exceeding current records, which for Southeast England are 7% in at least one month in any given winter, while there could be a 34% chance of breaking a regional record somewhere each winter in some other regions of England and Wales.
Another study from 2017 concluded that heavy rainfall events like Storm Desmond, which affected Northern England and Southern Scotland in December 2015, have been made about 40 per cent more likely by climate change.
Analysis by the Met Office from 2020 shows that on average, for the decade 2010 to 2019, UK summers were 13 per cent wetter, and winters 12 per cent wetter, than in the period 1961 to 1990. Seven of the 11 wettest years on record – since 1862 – in the UK have occurred since 1998. The five wettest winters have been from 1990 onwards.
Climate projections for the UK indicate that winters should continue to be warmer and wetter. Rainfall events during summer and autumn are also expected to become heavier.
What is the risk of flooding in the UK and how is it being managed?
The UK Climate Change Committee warned in 2019 that the most recent climate change risk assessment revealed 1.4 million people in England currently face a risk of 1:75 or greater of flooding of any kind (including coastal) – this means there is a 1.33% chance of flooding in any given year and the current associated damages to homes cost £270 million annually. The number at this level of risk could increase to 1.7 million if global warming reaches 2˚C above the pre-industrial temperature.
Measures to increase resilience against flooding include investments in more and better flood defences along rivers, in the form of both permanent and temporary structures. In 2019 research for the Association of British Insurers found that £1.1 billion a year of flood damage is being prevented by the UK’s current network of river barriers and defences. ‘Natural flood management’ is another approach and includes restoring bends in rivers, changing the way land is managed so soil can absorb more water, building ‘leaky dams’ that slow the flow of river water, and planting trees in a river catchment to intercept rainwater. To be effective, flood management, including natural approaches, needs to consider an entire river catchment.
The Climate Change Committee and other experts consider that progress in increasing resilience to flooding in the UK is not keeping pace with the rising risk. Another aspect of resilience, the Flood Re joint initiative between the Government and insurers that helps make flood risk insurance more affordable, has been criticised for not being designed “to support the necessary increase in resilience for current and future flood risks”. In urban areas, the risk of surface water flooding is also being made worse by the increase in the area covered by impermeable surfaces that prevent the absorption of rainwater. And new building and property development in flood-prone areas, including on greenfield sites, is exacerbating the risk of flooding from climate change in many parts of the UK. Over the last decade more than 120,000 new homes in England and Wales have been built in flood-prone areas. Research by the Grantham Research Institute has found that “there is very little evidence that developers, planners and financiers are taking into account climate change when deciding how and where to build”.
Each of the devolved administrations of the UK has developed plans and strategies for managing flood risk. In July 2020, the Environment Agency published its National Flood and Coastal Erosion Risk Management Strategy for England, with a vision for “a nation ready for, and resilient to, flooding and coastal change – today, tomorrow and to the year 2100”. It states: “In the face of a changing climate, we need to also make our places more resilient to flooding and coastal change, so that when it does happen it causes much less harm to people, does much less damage, and ensures life can get back to normal much quicker.”
This Explainer was written by Bob Ward and Georgina Kyriacou.
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