MadSci Network: Earth Sciences

Re: How are the magnitudes of flash floods classified? Is there a scale?

Date: Tue Jan 27 12:53:00 2004
Posted By: Tony Gaglierd, Assistant Professor , Natural Science and Engineering Technology
Area of science: Earth Sciences
ID: 1073676601.Es

Dear Will,Vito and Alvin

You asked how are the magnitudes of flash floods classified? Is there a 

I spoke to the people at the National Weather Service our Emergency 
Management Agency, and FEMA.

Aside from the Weather Services Flash Flood Watch and Flash Flood Warning, 
there is no system to classify flash floods like those used to measure the 
force of tornados and hurricanes.

Flash floods usually result from intense storms dropping large amounts of 
rain within a brief period.

Flash floods occur with little or no warning and can reach full peak in 
only a few minutes.

Floods and flash floods occur within all 50 states. Communities 
particularly at risk are those located in low-lying areas, near water, or 
downstream from a dam.

Did you know that?

Floods are the most widespread of all natural disasters— except fire.

Most communities in the United States can experience some kind of flooding 
after spring rains, heavy thunderstorms, or winter snow thaws. 

Floods can be slow, or fast rising but generally develop over a period of 

Dam failures are potentially the worst flood events. A dam failure is 
usually the result of neglect, poor design, or structural damage caused By 
a major event such as an earthquake.

Floodwaters can be extremely dangerous. The force of six Inches of swiftly 
moving water can knock people off their feet.

The best protection during a flood is to leave the area and go to shelter 
on higher ground.

Flash flood waters move at very fast speeds and can roll boulders, tear 
out trees, destroy buildings, and obliterate bridges.

Walls of water can reach heights of 10 to 20 feet and generally are 
accompanied by a deadly cargo of debris. 

The best response to any signs of flash flooding is to move immediately and
quickly to higher ground.

Cars can easily be swept away in just 2 feet of moving water. 

If floodwaters rise around a car, it should be abandoned.

Passengers should climb immediately to higher ground.

Mitigation pays. It includes any activities that prevent an emergency, 
reduce the chance of an emergency happening, or lessen the damaging 
effects of unavoidable emergencies. Investing in mitigation steps now such 
as constructing barriers such as levees and purchasing flood insurance 
will help reduce the amount of structural damage to your home and 
financial loss from building and crop damage should a flood or flash flood 

I hope I answered your question and gave you some helpful information 
about Flash Floods

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