Flash floods, as the name implies, are highly unpredictable and can be equally deadly.

Thankfully, they remain rare and, when they do occur, often drain away before anybody realises what has happened.

Flash floods result from:

• massive and sudden rainstorms;
  
  
• a rapid snowmelt in the mountains;
  
  
• failure of natural or man-made water defences.

Most inland floods occur relatively slowly with water levels reaching a peak over a period of days.

However, in mountains or cave systems, the "discharge volume" of streams changes significantly from month to month, and is subject to seasonal factors.

It is this unpredictability and rapid change that makes flash floods extremely dangerous.

As the water builds up, it displaces into tributaries and streams with an accelerated run-off from the land. Within minutes, the water level will rise dramatically, bringing with it mud and natural debris from the land.

In the worst cases, areas that are normally quite dry can be suddenly overrun with waves of rapidly moving muddy water.

Hydrologists have measured almost instantaneous rises in flood waters from virtually nothing to up to 10 metres (30 feet) in the worst cases.

The waters reach their most dangerous state further down a mountainside as narrow gorges and canyons provide acceleration. At this stage the water is carrying enough force to move rocks, tear trees from the ground and destroy buildings and bridges.

Once it has reached peak levels, the water can move at such a speed that it will travel several kilometres in a matter of minutes, often leaving other areas of a drainage basin relatively unscathed.

Tragedies

In one 1998 tragedy in Oregon, US, at least two whitewater rafters lost their lives when they were swept away by flood waters in an infamous stretch of the Illinois River known as the "Green Wall".

The survivors clung on to the high mossy canyon sides overnight as water travelled at more than 560 cubic metres (20,000 cubic feet) per minute down the gorge - seven times faster than normal.

A year earlier, 10 hikers were swept to their deaths when a flash flood, caused by a storm 50 kilometres (30 miles) away, hurtled through "The Corkscrew" in Antelope Canyon, Arizona.

The Corkscrew is a dramatic twisting channel of smooth sided rock. But with walls up to 15 metres (50 ft) high and only metres apart, the European hikers had no way to escape.

Witnesses described seeing silt, driftwood and boulders rush through the channel. Some of the victims were swept into nearby Lake Powell; the others were buried under a metre of mud.

Recent flash floods

May 2001 Thailand: 30 people killed
May 2001 Iran: 32 people killed
March 2001 Portugal: 70 killed
Sep 2000 India: 400 people killed
Jul 2000 China: 200 people killed

Floods in caves

Flash floods are as lethal below ground as they are on mountainsides.

One of the main dangers associated with caving and potholing is the nature of the water table. Cavers often find that the water level can change so dramatically within a system that routes become blocked during a trip of just a few hours.

Experienced cavers will often avoid entering a system if there has been heavy rain or snowmelt in recent days.

Dry and warm surface conditions often hide the fact that the ground is holding large quantities of water which, at some point, will be released into cave passageways with lethal force.

Surviving

The best way to avoid a flash flood is to take local expert advice before venturing into mountainous areas.

All popular mountain areas provide daily weather advice, which will include information on the potential danger of flash floods. Take the advice and remain wary.

While hiking, storm clouds up ahead can be read as a sign that a flash flood may occur at some point in the area.

Once these warning signs are recognised, it is best to avoid gorges and dry stream beds, and move to higher ground.

Following a stream downhill may appear a good idea at first, but it could result in tragedy.

Even a car offers little safety: many motorists have lost their lives as their car engine stalls and they are swept away in less than a metre of fast-moving water.

This article was extracted from BBC news (www.bbc.com)