Over just two hours, the Guadalupe River at Comfort, Texas, rose from hip-height to three stories tall, sending water weighing as much as the Empire State building downstream roughly every minute it remained at its crest.
The force of floodwater is often more powerful and surprising than people imagine.
Comfort offers a good lens to consider the terrible force of a flash flood’s wall of water because it’s downstream of where the river’s rain-engorged branches met. The crest was among the highest ever recorded at the spot — flash flooding that appears so fast it can “warp our brains,” said James Doss-Gollin, assistant professor of civil and environmental engineering at Rice University.
The Texas flood smashed through buildings, carried away cars and ripped sturdy trees out by the roots, dropping the debris in twisted piles when the water finally ebbed. It killed more than 100 people, prompted scores of rescues and left dozens of others missing. The deaths were concentrated upriver in Kerr County, an area that includes Camp Mystic, the devastated girls’ camp, where the water hit early and with little notice.
Water is capable of such destruction because it is heavy and can move fast. Just one cubic foot of water — imagine a box a bit larger than the size of a basketball — weighs about 62 pounds (28 kilograms). When the river rose to its peak at Comfort, 177,000 cubic feet — or 11 million pounds (5 million kilograms) of water — flowed by every second.
“When you have that little lead time … that means you can’t wait until the water level starts to rise,” Doss-Gollin said. “You need to take proactive measures to get people to safety.”
A small amount of water — less than many might think — can sweep away people, cars and homes. Six inches (15.2 centimeters) is enough to knock people off their feet. A couple of feet of fast-moving water can take away an SUV or truck, and even less can move cars.
“Suppose you are in a normal car, a normal sedan, and a semitrailer comes and pushes you at the back of the car. That’s the kind of force you’re talking about,” said Venkataraman Lakshmi, a University of Virginia professor and president of the hydrology section of the American Geophysical Union.
And at Comfort, it took just over 15 minutes for so much water to arrive that not only could it float away a large pickup truck, but structures were in danger — water as heavy as a jumbo jet moved by every second.
At that point, “We are past vehicles, homes and things can start being affected,” said Daniel Henz, flood warning program manager at the flood control district of Maricopa County, Arizona, an area that gets dangerous scary flash floods.
The water not only pushes objects but floats them, and that can actually be scarier. The feeling of being pushed is felt immediately, letting a person know they are in danger. Upward force may not be felt until it is overwhelming, according to Upmanu Lall, a water expert at Arizona State University and Columbia University.
“The buoyancy happens — it’s like a yes, no situation. If the water reaches a certain depth and it has some velocity, you’re going to get knocked off (your feet) and floating simultaneously,” he said.
The landscape created the conditions for what some witnesses described as a fast-moving wall of water.
Lots of limestone covered by a thin layer of soil in hilly country meant that when rain fell, it ran quickly downhill with little of it absorbed by the ground, according to S. Jeffress Williams, senior scientist emeritus with the U.S. Geological Survey.
A flash flood generally starts with an initial lead wave and then builds as rain rushes over the landscape and into the river basin. It may rise quickly, but the water still takes some time to converge.
