How do flash floods occur?

For the period 1920-1997 there were 312 flooding fatalities in Colorado.

Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and the duration is how long the rain lasts. Topography, soil conditions, and ground cover also play an important role.

Flash floods occur within a few minutes or hours of excessive rainfall, a dam failure, or a sudden release of water held by an ice jam. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Furthermore, flash flood-producing rains can also trigger catastrophic mud slides. You will not always have a warning that these deadly, sudden floods are coming. Most flood deaths are due to flash floods and flash floods are the #1 weather related killer in the United States.

Nearly half of all flash flood fatalities are auto related. If you are in your automobile, look out for rapidly rising water or flooding at highway dips, bridges, and low areas. Even six inches of fast-moving flood water can knock you off your feet, and a depth of 2 feet will float your car. Never try to walk, swim, or drive through such swift water.

	Water weighs 62.4 lbs. per cubic foot and typically flows downstream at 6 to 12 miles an hour.
	When a vehicle stalls in the water, the water's momentum is transfered to the car. For each foot the water rises, 500 lbs. of lateral force are applied to the car.
	But the biggest factor is buoyancy. For each foot the water rises up the side of the car, the car displaces 1,500 lbs. of water. In effect, the car weighs 1,500 lbs. less for each foot the water rises.
	Two feet of water will carry away most automobiles!

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