A flash flood is a rapid flooding of geomorphic low-lying areas - washes, rivers and streams. It is caused by heavy rain associated with a thunderstorm, hurricane, or tropical storm. Flash floods can also occur after the collapse of an ice dam, or a human structure, such as a dam, for example, the Johnstown Flood of 1889. Flash floods are distinguished from a regular flood by a timescale less than six hours.

Causes

Flash flooding occurs when the ground becomes saturated with water that has fallen too quickly to be absorbed. The runoff collects in low-lying areas and rapidly flows downhill. Flash floods most often occur in normally dry areas that have recently received precipitation, but may be seen anywhere downstream from the source of the precipitation - even dozens of miles from the source. In areas on or near volcanic mountains, flash floods have also occurred after eruptions, when glaciers have been melted by the intense heat.

Hazards

The United States National Weather Service gives the advice "Turn Around, Don't Drown" in reference to flash floods; that is, it recommends that people get out of the area of a flash flood, rather than trying to cross it. Most people tend to underestimate the dangers of flash floods.

Flash floods are extremely dangerous because of their sudden nature. Being in a vehicle provides little to no protection against being swept away; it may make people overconfident and less likely to avoid the flash flood. More than half of the fatalities attributed to flash floods are people swept away in vehicles when trying to cross flooded intersections. As little as two feet of water (60 cm) can be enough to carry away most SUV-sized vehicles. In the United States, the National Weather Service (part of the National Oceanic and Atmospheric Administration) reported in 2005 that, using a national 30-year average, more people die yearly in floods (127 on average) than by lightning (73), tornadoes (65), or hurricanes (16).

The desert southwestern U.S. is especially dangerous for both hikers and vehicles from the sudden onslaught of water from isolated thunderstorms. These rains fill poorly-absorbent and often clay-like dry riverbeds. A moving flood will usually be headed by a debris pile that may have wood branches and/or logs. Deep slot canyons can be especially dangerous to hikers as they may be flooded by a storm that occurs on a mesa miles away, sweeping through the canyon, making it difficult to climb up and out of the way to avoid the flood. Valley roads frequently cross dry river and creek beds without bridges. From the driver's perspective, there may be clear weather, when unexpectedly a river forms ahead of or around the vehicle in a matter of seconds.

Historical examples

• 1952 The Lynmouth disaster.
• 1967 Flash flood in Lisbon, Portugal. 464 dead.
• 1971 Kuala Lumpur floods, Malaysia.
• 1976 The Big Thompson River flood, which killed 143 people in Colorado.
• 1990 June 14, Shadyside, Ohio flooding.
• 1990 The Quad Cities Duck Creek Floods of 1990.
• 1997 Flash flood kills eleven in Antelope Canyon.
• 1998 Flash flooding in San Marcos, Texas resulted from rains totaling from 15 to .
• 2004 Boscastle flood.
• 2006 Mount Rainier National Park Flooding.
• 2006 Flash flooding kills 125 in Ethiopia.
• 2007 Sudan floods.
• 2008 The June 12-13, 2008 Floods around Duck Creek in Davenport, Iowa.
• 2009 The 2009 Kentuckiana Flood resulted from 20-30 inches of rain falling in 75 minutes.
• 2009 Turkish flash floods.
• 2009 September 21-22 in 9 Georgia Counties, Killing 10 people
• 2009 September 26 in Metro Manila primarily Marikina city, Taguig City, and Pasig City; and many municipalities of the provinces of Rizal, Bulacan and Laguna taking more than a hundred lives and leaving thousands of affected residents homeless. It also submerged several municipalities under feet deep of water for several weeks.
• 2009 October 10-13 in Northern Luzon causing major landslides in Cordillera Mountains, and submerging 80% of the Province of Pangasinan.

See also

• Freshet
• Huayco

Further reading

References

External links

• Scottish Flash Flood Public clip of the Fochabers flood in Moray september 09
• Decision tree to choose an uncertainty method for hydrological and hydraulic modelling, choosing an uncertainty analysis for flood modelling.
• Flash Flood Alley movie and video clips.
• Flash Flood Safety information.
• Great footage of flash floods in the arid midwest heading down dry washes after heavy rain.
• Map of central Texas flash flood alley.
• Workshop Proceedings Flash Flood Management
• Workshop Proceedings Flash Flood Forecasting