Flood safety: what have historic flash floods taught us?

The scenes depicted in the newspaper photos, on the television, or in the Internet video clips are often quite similar. Helpless people are suddenly in terrible danger as their seemingly safe haven is engulfed by a deluge of water. Often the victims are surprised by how quickly the water rose. Many are equally surprised by the seemingly small amount of water it takes to sweep their vehicles off the ground. Often flood victims question why flood control programs didn't work, or why forecasts weren't more specific.

Flash floods are one of the deadliest meteorological phenomena in the United States and around the world. A flash flood is a particularly dangerous type of flood that occurs within 6 hours of when the rainfall begins. From the parched deserts to the humid plains to the steep mountain ranges, nature is able to occasionally combine intense rainfall with specific ground conditions to produce a rapid-onset flood along small streams, down mountain canyons, or through city streets.

Many studies have looked at the reasons why flash floods evolve in such a rapid and localized manner. Rainfall intensity appears to be more important in many flash flood events than the actual amount of rainfall. Flash floods often occur in small drainages, that is, those drainages associated with small creeks and streams. Steep terrain enhances the chance of flash flooding, but flash floods do occur in relatively flat areas as well. Factors such as urbanization and forest fires can dramatically decrease the amount of rainwater that enters the soil resulting in more water that immediately runs across the ground surface toward the nearest stream.

Vehicles are particularly prone to the dangers of a flash flood. Depending on the vehicle, between 18 and 30 inches (45-75 cm) of water is enough to lift almost any passenger vehicle off the road surface. Once floating, vehicles move toward the faster-moving flow, which is typically the deeper part of the flood.

We will look at a number of flash flood events. Some will be in rural settings, some in urban environments. Some will be in steep canyons, some in relatively flat terrain. Some occurred during extended periods of wet weather, some occurred during severe drought conditions. We will look at what they all have in common and what you should know about how these floods form and how you can stay safe during a flash flood.