How do Flash Floods Occur?

Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration. Topography, soil conditions and ground cover also play important roles. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from tropical storms or hurricanes. Six inches of fast moving flood water can knock you off your feet, and two feet of rain will float your car. Never try to walk, swim or drive through swift water. If you come upon flood waters, stop! Turn around and don't drown!

Flash flooding occurs within 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 two to six times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers.

Flash Flooding in Arroyos/Washes

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

Environmental Clues

Listen for distant thunder - runoff from a faraway thunderstorm could be heading your way. 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 of Water Could Cost You Your Life

When a vehicle stalls in the water, the water's momentum is transferred to the car. For each foot the water rises, 500 lbs of lateral force is applied to the car. Buoyancy has the biggest impact. For each foot the water rises, 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.