How Just a Few Inches of Water Can Trigger a Flash Flood

Sometimes flash floods end in mass tragedy, as was the case when the Guadalupe River overtook summer camps in Texas. But more often, people are killed by flash floodwaters when they try to drive or walk through them and get swept away.

“I worked as an expert witness for a case in San Diego County that cost the lives of the driver and a 5-year-old boy, because the driver went over a flooded stream in an SUV and got swept downstream,” says flash flood expert David T. Williams. “It was heartbreaking.”

To help spread awareness about not underestimating the power of flowing floodwaters, the National Weather Service (NWS) created a campaign called “Turn Around, Don’t Drown.” The department also issues various flash flood warnings to alert people of potential dangers.

Here’s what to know about the causes of flash floods and strategies for keeping yourself safe.

What Is a Flash Flood?

A flash flood is a sudden rise or surge of water, such as a river overrunning its banks. Causes can include heavy rainfall, melting snow and sleet, mudslides into a body of water, dam or levee failure, or an ice dam’s sudden breakup.

“Flash flooding typically creates dangerous water flows that can quickly destroy property and lives, as tranquil creeks and streams rapidly turn into deadly torrents,” says Bruce Jones, Chief Meteorologist at Midland radio.

How Much Water Causes a Flash Flood?

How much water causes a flash flood varies depending on the terrain of an area, but even a small amount of water can quickly become dangerous depending on its velocity and depth. According to NWS, six inches of fast-moving water can knock over an adult, and 12 inches can carry away most cars.

“In the hot desert, one inch of rain will quickly run off the baked soil, causing creek beds to flood,” says Jones. “In central Texas, limestone hills with shallow topsoil shed water so, there, a four-inch rain can create catastrophic flooding.”

How Does a Flash Flood Happen?

Flash floods are often caused by intense rainfall over a short period (the technical definition is six hours or less).

The topography of the land and waterways also determines the intensity of the flood. For example, urban areas with large swaths of impervious pavement are likely to have more intense flooding, as are areas below mountainous regions recently burned by a wildfire, since there is no vegetation to collect and slow the water. Floods are also more likely to occur if the ground is already saturated by previous rains.

Even if it’s not raining in your area, rain upstream can cause a flash flood. That’s why it’s essential to carry a weather radio or other device that can alert you about flash flood conditions, especially when hiking and camping in flash flood-prone areas, says Williams.

“Often, flash floods make loud noises as they go downstream, due to turbulence and the movement of debris, so be alert to these noises,” says Williams.

How Long Do Flash Floods Last?

It depends. Often flash floods last for just a few hours, but sometimes they can persist for more than a day. Again, this depends on the amount of rainfall and the terrain.

What Is a Flash Flood Warning?

The National Weather Service (NWS) issues Flash Flood Warnings when a flash flood is imminent or occurring. If you receive an alert in your area, take immediate action by moving to higher ground.

NWS issues a number of other flood alerts, too. Common ones include Flood Advisories and Flood Watches, both of which signal that conditions are favorable to flooding, so be prepared for evacuation and continue to monitor the the weather.

Depending on the resources of the area, warnings are issued in various ways, including sirens, radio and television announcements or reverse 911 texts.

About the Experts

• David T. Williams has more than 45 years of experience in the water resources industry, and currently runs DTW and Associates Engineers, Inc. in Commerce City, Colorado. His certifications and degrees include PhD, PE, PH, CFM, CPESC, F. ASCE, F. EWRI, and BC.WRE.
• Bruce Jones has worked as Chief Meteorologist at Midland, the leading NOAA weather radio brand, for more than 17 years. Prior to that, he worked as Chief Meteorologist for KSNT-TV in Topeka, Kansas.

Related

• Emergency Survival Kit Checklist for Severe Weather 
• How To Keep Your Family and Your Home Safe During High Winds 
• How To Prepare an Emergency Evacuation Plan 

The post How Just a Few Inches of Water Can Trigger a Flash Flood appeared first on Family Handyman.

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