There can be flooding in England at the end of winter – here’s why

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During the week of February 2022, England and Wales were hit by three strong storms (Dudley, Eunice and Franklin). Persistent torrential rains flooded around 400 properties and severe flood warnings were issued for several major rivers, including the Severn River. Now the UK’s Met Office is predicting that England will experience severe flooding again in February 2023 – a forecast forecasters attribute to a global weather phenomenon called La Niña.

El Niño and La Niña are two separate phases of the Southern El Niño Oscillation (ENSO). This is the name of the phenomenon of irregular annual fluctuations in sea surface temperature (up to 3 ℃), atmospheric pressure and precipitation over the Pacific Ocean. The La Niña event is characterized by much lower sea surface temperatures in areas of the Pacific.

More research is needed on how global weather systems affect northern hemisphere winter weather. But large variations in Pacific sea surface temperatures could trigger a chain reaction of extreme weather around the world. La Niña could destabilize atmospheric pressure in the Atlantic Ocean and lead to heavy rainfall in western Europe and the southern United States.

But other factors may also contribute to the Met Office’s prediction of flooding. These include the long-term effects of climate change and urban development in flood-prone areas.

Global weather patterns

During La Niña, cooler water lowers the temperature of the air directly above sea level and causes it to sink. This creates large areas of low pressure in the Pacific Ocean that generally lead to increased precipitation in the surrounding region.

However, these vast areas of low pressure push the high pressure basins northwards towards Europe. This manifests itself initially as drier and cooler weather in the UK as seasonal rains brought by low pressure depressions off the Atlantic are blocked by persistently high pressure.

For example, the first half of December 2022 was the coldest start to winter in the UK since 2010. The average monthly temperature was 1.3°C lower than the December average from 1991-2020.

Severe storms are likely later in the season. As the high pressure subsides and La Niña pushes the jet streams north, the usual pattern of western depressions is allowed to resume. Colder-than-normal Pacific Ocean temperatures in recent months have led scientists to predict there is a 76% chance La Niña will last until the end of February 2023.

Favorable conditions

La Niña conditions can produce heavy rainfall in late winter. But the UK experiences increasingly extreme weather throughout the year. The long-term effects of this can create flooding conditions.

The south of England suffers from long periods of drought every summer. Last year, water supply regions in England, the Thames and Wessex recorded the fifth driest summer since 1836.

This has increased the risk of flooding as ground surfaces become less permeable to precipitation infiltration. Despite recent low-intensity rainfall, the risk of flooding in drought-affected areas may still be high. Low temperatures, such as those experienced in December, could also return later this winter and further reduce the soil’s ability to absorb water.

Underground, chalky aquifers dominate the central and southern parts of England. These aquifers, like sponges, have a limited ability to absorb and pass fast-flowing water. Heavy rains can therefore occur over land where they can flow quickly. Research indicates that above the ground water can flow at speeds up to 100 times faster than it flows through the rocks of the aquifer.

This water flows into sewers and rivers and can overwhelm their natural or operational capacity. The rivers then break their banks and cause flooding.

A fence submerged in a flooded river.

People don’t help much

Several other factors also increase the likelihood that heavy rains this winter will cause flooding in parts of England.

More than half of England’s major urban flooding in early 2022 was caused by underground blockages of outdated sewerage systems. Their insufficient capacity meant that they were quickly overwhelmed by debris floating in the floodwater.

Some cities in Britain, such as Hull, Bristol and parts of London, were also built on river floodplains. Floodplain land is often cheap, flat and, as such, easy to build on. But that makes these cities prone to flooding. Flood risk mapping showed that 19% of Gloucester, a city in south-west England, is at risk of regular flooding.

Climate models now predict climate change and global weather patterns with greater accuracy. However, mitigating their environmental impact often proves to be a challenge.

England requires extensive changes to infrastructure to reduce flood risk. One option is to ban housing in floodplains. However, this approach to urban planning requires overcoming legal and regulatory barriers.

Another approach would be to improve sewer capacity to take account of population growth and the associated pressure on water consumption. However, it takes time to complete large infrastructure activities. Various stakeholders, including the public, need to be consulted, while competing projects need to be assessed by experts and their impacts modeled. For example, as originally conceived, London’s Thames Barrier took 15 years to complete.

Thames Barrier, spanning the River Thames in London.

The implementation of flood management measures also requires political will. This has not always been possible, especially when flood management is considered excessively costly or environmentally damaging.

For example, river dredging was long considered an unsuitable flood mitigation technique in the Somerset Levels in south-west England. However, the winter flood of 2013-14 led to its immediate and ultimately successful implementation.

In the absence of these changes, the UK should prepare for a new wave of flooding towards the end of winter. Fueled by the La Niña event but exacerbated by urban development in areas prone to flooding and the effects of climate change, the effects could be severe.

This article has been republished from The Conversation under a Creative Commons license. Read the original article.

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Jonathan Paul does not work for, consult, own or receive funding from any company or organization that would benefit from this article, and has not disclosed any significant affiliations beyond his academic position.

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