What are Britain’s flood defences?
In England the Environment Agency is responsible for managing the two kinds of flood defences: hard and soft.
Hard defences are designed to prevent water breaching a river bank, and may include dams, barriers, walls, weirs, sluices and pumping stations. An example of this is the 520-metre (1,700ft) wide Thames barrier, which protects London and low-lying areas of the south-east from tidal streams moving up the river.
Soft flood defences use environmental planning to minimise the risk of flooding to properties, for example leaving low level areas next to a river undeveloped. Clearing uplands for grazing and growing crops such as maize contribute to flooding by increasing rapid runoff after rainfall, so reversing these policies is a means to protect against floods.
How do they work?
Hard defences work simply by keeping the water out and diverting it downstream. In Cockermouth, one of the Cumbrian towns currently affected, the UK’s first self-closing barrier opened in 2012. Stretching 120 metres, the £4.4m project was launched after hundreds of people had to be evacuated from their homes in the floods of 2005 and 2009.
In Keswick, also flooded in Storm Desmond, the primary defence is a reinforced concrete wall made out of local slate and a 122-metre glass panel. Further floodgates have been installed across roads in Cumbria, rising to 1.8 metres in the most vulnerable areas.
But across large swaths of the UK is the traditional way to protect low-lying communities against flooding: a combination of floodwalls and embankments stretching for thousands of miles alongside rivers and streams.
How much do flood defences cost?
The government has pledged £2.3bn for flood defences over the next six years. Individual schemes can range from tens of thousands of pounds to hundreds of millions or more. The Thames barrier cost £500m when it was built in 1984.
How do they go wrong?
Flood defences fail when water levels are simply too high and water spills over the top. But even then the defences usually prevent the worst of the flooding and provide more time for people to evacuate. This happened in some of the areas struck by Storm Desmond, when flood defences in place could not cope with the unprecedented amount of rainfall.
Who decides where they should be built?
Assessments on managing flood risk are taken using various evaluations, chiefly by the national flood risk assessment (NaFRA), which allows Environment Agency analysts to spot trends, determine the effect of climate change, track development on floodplains and keep abreast of the condition of flood defences across the UK.
