Original article written by Asher Minns and reviewed by Dr. John Boardman, both of the Environmental Change Institute. A comprehensive guide to the basics of flooding, flooding costs to the UK, causes of flooding and types of flood.

Introduction

Eleven years ago my soil science tutor pointed to a housing development and predicted that it would be flooded in the future.  He stated that councils were folly to allow developers to build on floodplains. A recent Planning Policy Guideline now states that flood risk should be considered in all stages of the planning process. Here I review a little of what is known about the causes and consequences of flooding.

The River Thames at Windsor in November 2000 - the swan is swimming above the promenade. Photo courtesy the Royal Windsor Web Site by Thamesweb

Sections in this article

• What is flooding?
• The costs of flooding
• Flooding and floodplains
• Causes of flood
• Types of flood
• Flooding and climate change
• Predicting floods
• Where next?

What is flooding?

Flooding is a natural feature of the environment. Floodwaters stimulate fish breeding and migration, bring moisture and nutrients to soil to make fertile agricultural land, and are important for replenishing and recharging water tables and wells. For example, Bangladesh is one of the world's most fertile countries because floods regularly recharge the soil with nutrients. Water scientists (hydrologists) estimate that about a quarter of the 100,000 cubic km of rainwater that falls annually on the earth runs off as flood flow.

The costs of flooding

The impacts of floods are often high in human costs. In autumn 2000, Bangladeshis experienced their worse floods for over 50 years, affecting over 20 million people. In the UK's autumn floods, eight people lost their lives and 3000 homes were effected. The Association of British Insurers estimates the cost of these floods as £1.3 billion (£860m for domestic property and £440m for commercial).

The Royal Institute of Chartered Surveyors state that in the south east of Britain there are 1.7 million homes and 130,000 business properties valued at £200 billion at risk of flooding. In addition, 13 million hectares of farmland worth £7 billion are in danger of being flooded.

New Brunswick, New Jersey USA, 17 September 1999, a day after the passage of Hurricane Floyd. The image is courtesy of the National Weather Service, Philadelphia, and photo by Shaun Baines, a graduate of nearby Rutgers University.

Flooding and floodplains

Floodplains are the level tracts of land over which a river or stream spreads when it floods. Humans settle floodplains because of the nearby water, flat fertile land, and easier transport and communications. It is no coincidence that some of the greatest civilisations have their foundations in fertile floodplains - Mesopotamia was centred on the Tigris-Euphrates and the Egyptians depended upon the Nile.

Among other essential roles, floodplains provide water purification and provide physical protection against floods. In developing countries, loss of floodplain habitats is seriously diminishing the living standards of rural livelihoods, increasing disease in humans and livestock and causing loss of biodiversity.

Causes of flood

Increased winter downpours and storms might well be an indicator of climate change, but land use and management are also contributory factors in flooding.

Urban and runoff floods

As land becomes increasingly paved and tarmac-covered it loses its ability to absorb rainwater. The rain becomes runoff, flooding rivers, drains, roads, houses and other urban infrastructure. Land development means that more and more of the floodplain surface, because of roads, housing, drains, and packed earth etc, becomes impervious to water. This increases the volume of water runoff and the hazard of flood.

Thirsk, North Yorkshire, on the 3^rd of November 2000. Image courtesy of www.thirsk.org.uk

Muddy floods and overgrazing

Muddy floods are soil and water runoff from steep fields that are bare of crops in the winter. Until the relatively recent introduction of crop subsidies, farmers would have used steep-sloping fields for permanent pasture to graze livestock. A permanent cover of grass stabilises the soil, absorbs, and traps rainwater. Soil erosion caused by soil wash-off is a major factor in degrading the quality of farming land.

In upland Britain, overstocking means that sheep overgraze the vegetation. The grass plants and their roots are too short to trap and absorb rainwater. Overgrazing and agricultural drainage also causes upland peat bogs to dry out. Kept wet, peat bogs are natural sponges that absorb huge amounts of water, as well as absorbing carbon dioxide from the atmosphere.

In a unique judgement in 2001, the Devon County Court established that a farmer did not use reasonable care in preventing muddy flooding of a highway. The court established that the cause of the flooding was arable planting rather than grass for grazing and the farmer was aware of the wash-off risks with arable planting. His lack of care had resulted in violent rain washing 400 tonnes of silt and topsoil onto a road, which the council had to clear.

Types of flood

Coastal floods

These floods happen when hurricanes and storms produce heavy rains or drive ocean water onto land. Coastal flooding can also be produced by tidal waves created by storms, earthquakes or volcanoes. Beaches and houses are often swept away by the force of the water.

The Thames Barrier, the world's largest movable flood barrier, spans a third of a mile across the Thames at Woolwich Reach, South East London.  Image courtesy of http://www.tellmeabout.co.uk/

River floods

Some floods are seasonal because of snow melt or heavy rains. The water fills the river basin too quickly and the river flows over its banks. The floodplain becomes covered with water, often making important habitats for wildlife, especially birds.

Flash floods

Flash floods are caused when small and powerful fast flowing rivers are quickly formed because of excessive rainfall or dam failure, sometimes triggering catastrophic mudslides.  Flash floods can be powerful enough to carry away roads, bridges and other structures, and can occur with very little warning. Mudslides drowned or buried at least 600 people living on the Gulf of Mexico in October 1999.

Flooding and climate change

The autumn of 2000 was the wettest for Britain since measurements began in 1766. Widespread floods throughout low-lying areas of the country occurred at the time of the UN climate change negotiations in The Hague. Journalists and climate change campaigners cited the storms and flooding as proof of global warming and climate change. The Chief Executive of the UK Met Office, Peter Ewins, described this reasoning as "too simplistic." He commented to The Times newspaper that "Ministers' remarks about climate change were often so wrong that they made his scientists wince."  The Head of the Climate Analysis Group at the University of Reading, Dr David Stephenson, ascribed the floods to flipping of the North Atlantic Oscillation (NAO). The NAO is a pattern of atmospheric pressure that forms because of the differences between the high pressure over the Azores and low pressure over Iceland. Dr Stephenson states that the North Atlantic Oscillation has flipped between its two states more or less at random since the 1800s.

The body of opinion of the International Panel of Climate Change (IPCC) agrees that extreme weather events that often result in flooding will become more common with global warming.

Predicting floods

The Environment Agency has an important role in warning people about the risk of flooding, and in reducing the likelihood of flooding from rivers and the sea. Around 5 million people in 2 million properties live in flood risk areas in England and Wales. See the links below for maps of the flood risk areas of the UK.

Where next?

On ClimateX.org

For more information about extreme weather, see 'Climate Change and Extreme Events'

The economic costs of flooding are considered in an article by insurance and climate change consultant, Andrew Dlugolecki, 'Climate Change Hits Your Pocket'.

Other sources of information  

The following organisations are responsible for flood management and research in the UK:

The Environment Agency is the UK government organisation that is charged with managing and developing flood defences. Flood risk maps from the Environment Agency can be viewed at Environment agency Flood risk online maps .

The Centre for Ecology and Hydrology, Wallingford specialises in issues of water management

Also see the EU Wise-Use of Floodplains website

Staffordshire Learning Net has information about the causes and consequences of flooding

For more information on flood risk areas of the UK, see:

Homecheck.co.uk

For worldwide floods:

For an archive record of global flooding up to the present (2007) see the Dartmouth Flood Observatory

For a history of floods:

John Vidal, Environment Editor of the Guardian writes about mankind and legendary flood myths in 'Still adrift on Noah's Ark' More about the search for Noah's flood at National Geographic