Engineer: Floodwater Surge on Guadalupe River Rivaled the Flow Over Niagara Falls

The deadly flooding along the Guadalupe River on the Fourth of July has underscored the power of moving water for people who live in the Hill Country and along the I-35 corridor, an area often called "flash flood alley."

Experts used flow-rate measurements taken on the river at the height of the flood to work out how much water was moving downstream. Chris Combs, associate dean of research in the Department of Mechanical Engineering at the University of Texas at San Antonio, said the peak flow reached as much as 11 million pounds of water per second, or more than a million gallons every second.

To put that in perspective, Combs asked people to picture a gallon milk jug and then imagine a million of them passing by each second. He said the volume is "on par" with what goes over Niagara Falls at any given time.

According to Combs, the water's weight, speed and buoyancy force combined to make it both deadly and destructive. He noted that a gallon of water weighs eight pounds, while a cubic foot, roughly the size of a beach ball, weighs 60 pounds.

Even small depths of moving water can be dangerous, he said. Around six inches of fast-moving water can knock a person off their feet, and a single foot can carry away vehicles. Combs compared the force of one foot of fast-moving water to a football tackle, with about 500 pounds of lateral force applied per foot.

When the water is several feet deeper, it can push houses downstream, Combs said. That is what happened in Kerr County during the flood.

The river rose 26 feet in 45 minutes. Whether advance warnings could have prevented some of the deaths remains a matter of debate.

Combs called it a very unusual and rare event. "Anyone that's there is in a very bad situation, and I think this was a very unique and rare event," he said. "We hope we don't see anything like this for a very long time, but I do hope that we can learn lessons from this to develop emergency preparedness actions that help make sure this never happens again."