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Updated: Wed, 10 Apr 2013

In detailBack to top

Experts warn that climate change and economic development - particularly building along rivers and flood plains - will leave more and more people vulnerable to flooding in the years to come.

Death tolls from floods have generally fallen in recent years as countries become more adept at dealing with the health implications.

Two decades ago, floods in Bangladesh would kill thousands, but improved sanitation and public education now mean those affected are much better informed.

However, experts say there's a need for long-term planning to deal with floods since preventing them is all but impossible.

Floods destroy homes, crops and livestock and cause soil erosion. Their impact on urban areas can be devastating, especially in cities which have large areas of informal settlements, usually with no basic infrastructure.

Floods are triggered by severe thunderstorms, tornadoes, tropical storms, monsoons and melting snow. Ice jams can also create floods. These occur when a combination of rain and snowmelt rapidly swell frozen rivers, and the layer of ice on top of the river breaks into large chunks which can pile up further downstream and dam the river, causing floods.

Coastal areas are subject to storm surges caused by tropical storms, tsunamis, or rivers swollen by exceptionally high tides. Rising sea levels have made many communities more vulnerable to flooding, especially in low-lying regions in South and Southeast Asia.

The El Nino oceanic and atmospheric phenomenon – which affects global weather every three to eight years – can also increase the risk of flooding, especially in South America and East Africa. Elsewhere it can create drought.

The impact of heavy rains is made worse by deforestation and intensive cultivation which cause serious soil erosion and decrease the ability of soil to retain water. In Haiti, for example, deforestation means there is nothing to stop heavy rainfall from washing down hillsides, carrying with it large amounts of mud. In 2008, the city of Gonaives was buried in mud after a series of severe storms.

Building homes on natural floodplains both increases the risk of floods to those houses, and means floods may no longer be contained within those areas.

Pakistan’s 2010 floods – the country’s worst in recorded history – that submerged one-fifth of the country, were caused by many factors including unusually high rainfall, massive deforestation, building on flood plains, and poorly maintained infrastructure.

Climate experts say global warming could make flooding worse, but warn against attributing individual floods or disasters to climate change.

Types of floodsBack to top

Flash floods are usually the most lethal. Heavy downpours, often in mountainous highlands, can create surges of water that turn dry river beds or flood plains into raging torrents in minutes.

Flash floods can also occur after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate.

Local communities usually have little time to flee to higher land, and homes in the water's path can be totally destroyed. Roads and railways often become impassable, making delivery of aid much more difficult.

The waters usually swiftly subside, leaving behind widespread damage.

Slow onset floods, like those that affect Bangladesh every year, can also be lethal but tend to give people much more time to move to higher ground. When deaths occur, they are much more likely to be due to disease, malnutrition or snakebites. In 2007, floods in China displaced tens of thousands of snakes into neighbouring areas, increasing the risk of attacks.

Slower floods are also less likely to sweep away property, although it may still be damaged or destroyed. Areas remain under water for weeks or even months.

Coastal areas are vulnerable to storm surges caused by storms, tsunamis and rivers swollen by exceptionally high tides. The surges overcome defences and swamp coastal areas, which happened in New Orleans in 2005 after Hurricane Katrina.

Improved preparationBack to top

Huge strides have been made in dealing with the consequences of flooding. Two decades ago, floods in Bangladesh used to kill thousands, almost all from disease. Now, cholera outbreaks after floods have almost been eradicated, mainly through better access to sanitation and public education.

Aid agencies and governments are now much more aware of the risks of disease, and have become better at providing medical services and clean water, as well as giving out information. Treatment for cholera has also improved.

But aid workers say they need to raise the spectre of mass disease outbreaks – even if they rarely occur – to prevent people becoming complacent and sparking an epidemic.

Flood waters may contain sewage and dangerous chemicals from industries. Flood survivors may drink dirty water, and even wading in flood waters can cause infection. Survivors often live in overcrowded camps, schools and other public buildings. Health centres may be swamped with children suffering from respiratory, skin and eye infections.

Early warning systems have also improved with some countries using text messages to warn communities of predicted flooding.

When floods hit Mozambique in 2000-2001, few were prepared and some 700 people died. Many survivors had to be plucked from treetops by helicopter. When floods hit in 2007, aid workers said the government was swift to broadcast radio warnings and evacuate people from vulnerable areas - only 45 people died.

State responseBack to top

Responding immediately to flood disasters almost invariably comes down to local and national governments. The United Nations and other external agencies usually lack immediate capacity on the ground.

In China, authorities generally use the army to move sometimes millions of people from affected areas.

Helicopters are usually needed both for rescue and food distribution.

The 2005 Hurricane Katrina disaster in New Orleans showed that even wealthy countries such as the United States struggle when faced with widespread flooding.

Living with floodsBack to top

Rural communities from the Zambezi in southern Africa to Bangladesh have traditionally used small mounds of raised ground to escape floodwater. But rapid urbanisation and reliance on dykes and embankments built by European colonisers have reduced the emphasis on traditional methods of coping.

Raised railway lines and roads can limit drainage and stop water escaping – which is why they are so often swept away, experts say. And yet post-disaster Western aid frequently concentrates on rebuilding them exactly as they were before.

In many flood-prone regions, drainage is inadequate and building is carried out without regard to flood patterns. Often, there are no plans to prevent or cope with flooding at all.

Experts recommend building houses that are more durable and survivable, and which can be re-used within a couple of months of a flood disaster. Many houses cannot be used for over a year after being flooded.

If pumps bringing up water from the underground water table are raised above the level of likely floodwater, they can be used even in the event of flooding.

In parts of rural India, families store food in places that are not easily flooded.

Small boats can be pre-positioned to help with evacuation and rescue, and local staff trained to carry out these operations. Teaching people to swim can also save lives.

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