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15 February 2006

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Flood risk

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Everyone should be aware of the dangers of flooding. It can endanger lives and property. Flooding happens as a result of heavy rainfall that can cause rivers or sewerage systems to overflow. Coastal storms and tidal surges can flood low-lying land in estuaries and on the coast.

Who is at risk from flooding? » River flooding » Sewerage flooding » How do we manage the risk from flooding? » Find out more

Who is at risk from flooding?
Flooding happens naturally and can’t be completely avoided. Some areas are at higher risk than others, for example river flood plains and low-lying coastal areas. Around 5 million people, in 2 million properties, live in flood risk areas in England and Wales. Changes in our climate, such as more severe storms and wetter winters, will increase that risk.

Roads, railways, housing and commercial developments are often built in floodplains because it is relatively easy and cheap to build there. In some areas, the rate of development on flood plains has more than doubled in the past 50 years.

Flood plain development reduces the space available to floodwaters. The speed and height of the flood increases as it moves down stream.

In some cases development in the flood plain acts like a dam, increasing flooding upstream. Drainage systems and hard surfaces, such as roads and car parks, can also increase flooding by quickly transferring rainwater into rivers.

Any new developments on floodplains increase flood risk. We advise local authorities on flood risk when they are considering planning issues. We also encourage using sustainable drainage systems in new developments to limit the chance of flooding.

Anyone who has experienced flooding will confirm the stress, disruption and grief that can be caused as well as the physical danger. Find out if your home or business is at risk from our interactive flood map. Click on the link below.

River flooding
River channels can only carry a limited amount of water. Heavy rain or sudden snowmelt can cause rivers to rise to the point where they overtop their banks. During a flood, the excess water flows onto the low-lying areas on either side of a river – the flood plains. Periodic flooding of low-lying areas nourishes the soil. Flooding has benefited farmers in this way for centuries.

A devastating river flood hit the village of Boscastle in Cornwall on August 16 after 200 millimetres (8 inches) of rain fell in 24 hours. Floodwater rampaged through the village taking cars, trees and other debris with it. Around 2 million tonnes (440 million gallons) of water flowed through Boscastle that day.

There is some evidence that the frequency of peak river levels is increasing (Indicator: Flood Levels). There is evidence that flooding comes in cycles and the rivers with a long data record will give a more reliable assessment of trends. The two rivers with the longest records are the Severn and the Wye. The Severn shows a constant trend, but the Wye has an increase in frequency of peak river levels.

Sewerage flooding
Very heavy rain can result in severe, but localised flooding. Some sewerage systems, for example London’s Victorian system, can be easily overloaded in heavy rain. Properties can be flooded and large amounts of raw sewage released into water bodies. The ecological consequences can be severe. In August 2004 heavy rainfall in London lead to appalling pollution events that killed thousands of fish, left sewage debris and a foul smell along the foreshore of the tidal Thames, and significantly increased E. coli levels in the river. A solution to the problem in London is being developed by the Thames Tideway Strategic Partnership. It will need substantial investment.

Flooding on coasts and in estuaries
Low-lying, open coastal areas are at risk of being flooded (Figure 1). In estuaries, flooding can occur as a result of ‘surges', caused by the combined effects of atmospheric pressure, high tides and high winds. In addition, sea levels around the UK are about 10cm higher than they were in 1900.

We can control the frequency, extent and size of flooding to some degree. We own and operate several tidal barrages, including the Thames Barrier, which protects 150km² of London that lies below the high tide level.

The Barrier was closed 55 times to protect London from tidal flooding between 1983 and 2004 (Indicator: Thames Barrier closures).

As well as using the Barrier to protect against tidal flooding, it can also be used to reduce flood risk from exceptionally high rainfall upstream. The Barrier is closed against the tide to keep water levels low in the down stream portion of the Thames so that increased water volume from high rainfall upstream has somewhere to go. For example, between New Years’ Day and 8 January 2003 high rainfall upstream and high spring tides increased London’s flood risk. During this period we closed the barrier a record fourteen times. This is double the previous record of seven consecutive closures during the floods of October 2000.

How do we manage the risk from flooding?
Our flood management activities can reduce the risk of flooding. We manage the land, river systems and flood and coastal defences.

We use land use planning, regulation, flood warning systems and emergency response procedures to limit the impact that floods can have.

We try to limit the dangers that flooding presents to people living in an ‘at risk’ area. However, flooding Is a natural hazard and society must accept that flood risk can not be eliminated entirely, only reduced.

This work is worth doing. Flood damage already costs about £1billion a year and our flood risk management prevents further yearly damages of £3.4 billion. But we can’t defend everywhere. Damage costs from flooding are predicted to be as much as £25 billion under a worst case climate change scenarios in the 2080s.

Since 1988 the cost of repairing the damage from extreme weather events and floods has increased by 60%. Climate change in the UK could bring more extreme weather conditions, including more frequent floods and storms

We estimate that over the next 80 years we will need to spend £22 to £75 billion on flood defences in England and Wales to combat the effects of climate change

For more information see managing flood risk below.

Find out more

To find out the current flooding situation or potential flooding risk in your area: Environment Agency information on: Further information:

Environmental indicators:

Thames Barrier

Thames Barrier

Baltmoor Wall strengthening

Baltmoor Wall strengthening

Tidal defence - Humber

Tidal defence - Humber

Flood Map

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Additional links: regional information


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©The Environment Agency 2006
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Author: Rachel Anning | rachel.anning@environment-agency.gov.uk