Colorado debris flows pose threat as intense rain meets dry, vulnerable terrain

Colorado's dry landscape and steep terrain can create a threat for mudslides, more accurately described as debris flows, when intense rain moves through the state.

"It can happen really quickly," said Peter Nelson, a professor in the Department of Civil and Environmental Engineering at Colorado State University in Fort Collins.

As seen recently, photos from Pitkin County show what a quick rainstorm can do to the landscape.

"Mudslides, debris flows, are happening when it rains hard enough that the water just can't infiltrate into the ground fast enough," Nelson said. "I think we're having the mudslides and debris flows happening now, just because we have more frequent high-intensity rainstorms happening."

Even in normal years of precipitation, the ground can only absorb so much liquid at a time. When it has been extremely dry or land has been impacted by a fire, sloped terrain can become even more vulnerable.

"High-intensity precipitation falling on steep, sloping terrain that has limited infiltration capacity, whatever doesn't infiltrate will run off," Nelson said. 

In burn scars, there are fewer plants for the water to be caught by. In a less-dramatic way, drought has had similar impacts across the state.

"With the drought that's been occurring, perhaps the vegetation has fallen back a little bit. There's been some die-off of plants. The soil structure might have changed a little bit, and so that tends to reduce the overall roughness of the surface that the water and mud can flow over, leading to faster flows, which can cause problems downstream as well," Nelson said.

For those who live around the mountains, Nelson said there is a simple step they can take when severe weather moves in.

"It's just important to pay attention to the weather and be ready to get out of the way if it looks like something serious is about to happen," Nelson said.

With how dry the landscape is, a strong storm that would not cause a debris flow in previous years may be able to this year.

And with forecasters monitoring what could become a super El Nino by the winter season, Nelson said there's a chance Colorado may see several strong storms in the region in the months to come.