Trevor Hughes

USA TODAY

A lethal mix of gravity, water and dirt combined to create an “avalanche” that tore through a small Colombian city, killing more than 200.

What happened in Mocoa is known as a “debris flow,” which came during an unusually wet season, topped off with a shorter period of intense rain, federal landslide scientist Jonathan Godt said. He said he reviewed images and video of the Mocoa disaster, noting the mountains surrounding the city.

“That very heavy rainfall makes the soil like goo, makes it easy to flow. It just starts sliding down the hillside,” said Godt, coordinator of the U.S. Geological Survey’s landslide hazards program. “My guess is that it was moving very fast and would have been full of rock and boulders and pieces of buildings. For someone experiencing it, an avalanche would be a very accurate description.”

Witnesses reporting hearing buildings shuddering and vibrating as the flow crashed through Mocoa. Scores remain missing since the deluge struck after midnight Saturday when many people were sleeping, washing away trees, vehicles, houses and everything in its path. Pictures posted to social media show bridges wiped away, piles of debris in the town center and overturned vehicles tumbled amid tree limbs, rocks and fencing.

The avalanche is the fourth-worst weather-related disaster to ever strike Columbia, said Jeff Masters, director of meteorology for Weather Underground. He said the damage is great because the city was built in a potentially dangerous location: at the bottom of mountains, alongside a riverbed.

Desperate search for Colombian flood survivors; death toll reaches 207

Godt, who has created small-scale landslides in Oregon for research, said debris flows like this one can be more devastating than a flood because the water gives the flow speed and the debris gives it extra punch. He said the flow would have been loud: Think water rushing, boulders crashing and trees being torn apart.

“Because it has all of this water behind it, it’s a really dense heavy flood,” he said. “That mixture can move at 35-40 miles an hour, and because it’s so dense it has a lot more momentum and destructive power than water alone.”

Godt said flows like this happen anywhere there are mountainous regions. He said the damage varies depending on how steep the surrounding hillsides are and whether trees or other vegetation help “anchor” the wet soil in place. Hillsides burned clean by forest fires or clear-cut via logging tend to be at a higher risk for slides, he said, because there’s less to retain the soil.

“Gravity is always working to drag the mountains down, and water changes the strength of the material,” he said. “A layperson's description of this as an avalanche of mud and rock would be absolutely accurate.”

Masters said the fact the disaster struck at night further compounded the danger: People waking up to such a calamity might have had no way to escape the flows.

“The debris impacts the houses, knocks them down, so you have a much lower chance of surviving," he said.