Scott Anderson and Suzanne Anderson climbing up a landslide above the highway in Boulder Canyon after the 2013 floods.

(Photo courtesy of Bob Anderson)

When the historic rains hit the Front Range in September 2013, riverbanks burst, roads and houses flooded, and dirt and debris cascaded down hillsides. 

The catastrophe led University of Colorado geomorphologists Suzanne Anderson and Robert Anderson to consider how such big, rare events reshape the landscape. They found that the erosion was equivalent to a thousand years or so of weathering.

Click on the audio link above to hear their conversation with host Ryan Warner. Edited highlights are below.

On how daily weathering and big storms shape the landscape

“You can think about this landscape as operating with a two-step process. Step one is the very slow everyday weathering of rock that generates debris. That takes place at rates that are extremely slow, about the rates of a human hair per year, or an inch per a thousand years. This debris then moves downslope very slowly. Then it takes these big events apparently, like the big storm we had in 2013, to trigger landslides, which then perform step two to get the debris out of the mountain slope.”

On where most of the damage happened

“We compared the locations of failure sites with the surrounding topography and we found the failures seemed to happen with equal probability on slopes facing different directions, but they had to be steep.”

On why there weren’t landslides on all of the steep slopes

“You can think of the slow everyday processes as slowly loading up the gun that is the top of the channel. Then the storm is acting as a trigger that fires the gun. If there is no debris in the channel, the gun is not ready to fire. So it may well be that some of the channels were ready to be fired, they had enough debris in them to trigger a landslide, whereas others had insufficient loading of the gun. Those might be sites that might go in the next big storm.”