Good Question: What are the risks of another big landslide?

COLLBRAN, Colo. (CBS4) – The landslide that skidded down out of a bowl shaped drop-off from the Grand Mesa Sunday night had enough material to stretch out for three miles, and it was hundreds of feet deep in some spots.

It was so powerful it had enough force to go uphill.

“It actually jumped it. It made us pay attention,” said Dr. David Noe, a senior research geologist for the Colorado Geological Survey.

“It came around the corner and did what we call a NASCAR turn,” he added about part of its path.

An image from a drone of the Grand Mesa landslide (credit: CBS)

An image from a drone of the Grand Mesa landslide (credit: CBS)

The scale of the slide was stunning, but not unexpected.

The Grand Mesa has been loaded with snow this winter and spring. A waterfall pouring off the mesa drenched the mountainsides in the bowl area on the northern edge, south of Collbran.

Even though rock does not float, that much water changes the buoyancy. As water loads into the area, things move.

“It’s not like the hard rock you drive through on the I-70 road cut, the Dakota formation; this is a formation called the Green River formation where a lot of the oil shale comes out of,” Noe explained. “It’s got clay in it, it holds itself together just a little bit, but it’s very, very prone to failing.”

Noe and his colleagues have mapped out some of the areas of the state that are prone to slides, but not all. The amount of slide-prone territory is enormous and a significant chunk of the state’s geologic history.

“It’s not all that uncommon. Most of Western Colorado’s slopes have some type of landslide or failure mechanism, where they’re wearing away over time.”

One slope is so fluid, CDOT moved a road on the south side of McClure Pass north of Paonia several years back to the other side of Muddy Creek. Above the road, the saturated ground won’t stop moving.

New infrastructure can change the water flow beneath and weaken new areas.

“An irrigation ditch gets cut across the lower part of the landslide, all of a sudden that water shows up again,” said Noe. “Whenever we cut a slope to put in a foundation or a road that could change the whole stability of that landslide slope.”

Marble is one of the towns built on material from a past slide. How long ago is difficult to tell.

A landslide path near Marble (credit: CBS)

A landslide path near Marble (credit: CBS)

“It comes down and spreads out in the valley and the town of Marble is built on it.”

Above the town, Noe can see a gully where water seeps into the mountain. Someday there will be another slide. It’s hard to say whether there’d be enough time to evacuate people if one of the towns in dangerous slide areas were ever hit. The only thing that may be protecting them is that it will happen on a geologic time scale, not a human one.

“What I can’t say at this point is that this would happen within our lifetime or the next 200 years or the next 20 years.”

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