Flash Floods
#1 Weather-related killer in the United States

How do Flash Floods Occur

Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration. intensity is the rate of rainfall, and duration is how long the rain last. Topography, soil conditions and ground cover also play and important role. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from tropical storms or hurricanes. 6-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 swift water. If you come upon flood waters Stop! Turn around and go another way.

Flash flooding occurs with in 6-hours of the rain event. Flooding is a longer term event and may last a week or more.

River Flood

Flooding along rivers is a natural and inevitable part of life. Some floods occur seasonally when winter or spring rains, fill river basins with too much water too quickly.

Urban Flood

As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. urbanization increases runoff 2 to 6 times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving river, while basements can become death traps as they fill with water.

Flash Flooding in Arroyos/Washes

An arroyo is a water-carved gully or normally dry creed bed. Arroyos can fill with fast-moving water very quickly

Environmental Clues

Listen for distant thunder -- runoff form a faraway thunderstorm could be headed your way. Look out for water rising rapidly. Nearly half of all flash flood fatalities are auto related. In your automobile look out for flooding at highway dips, bridges and low areas.

A Foot or Water Could Cost You Your Life.

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 waters momentum is transferred to the car.
For each foot the water rises, 500 lbs of lateral force are applied to the car.

But the biggest fact is buoyancy. For each foot the water rises up the side of the car, the car displaces 1500 lbs of water. In effect the car weights 1500 lbs. less for each foot the water rises. Two feet of water will carry away most automobiles.

source: US Department of Commerce, National Oceanic Atmospheric Administration.