As winter fades toward spring, it’s time for another season: rockfall season.
The Vail Valley was hit last week by a pair of rockfall episodes — one along Interstate 70 in Dowd Canyon, and another on U.S. Highway 24 on Battle Mountain.
While locations can change, transportation officials prepare for rockfall season each year as the snow melts around the High Country.
Bob Group is the Colorado Department of Transportation’s geohazards manager. Group noted that moisture levels increase in the fall and spring. That can loosen soils, leading to slides. In the spring, melting snow seeps into cracks in rocks, then freezes at night, expanding and sometimes breaking loose rocks and boulders.
Group noted that a lot of highways in western Colorado are flanked by sedimentary rock, including shale and sandstone.
Along the I-70 and Highway 24 corridors, maintenance crews keep a watchful eye for falling rocks. Snowplow drivers can push smaller rocks off the road. Group noted that during warm spells, plow drivers will patrol the roads looking for fallen rocks.
Larger events require moving front-end loaders and excavators to a site. Really big rocks have to be drilled and blasted to break them into truckable pieces.
Working two angles
Group said his team works in a couple of directions.
The first is an emergency response to active events. Those are slides that transportation officials expect but don’t know exactly where the rocks will fall.
That response includes site cleanup and adding additional protection measures — including roadside rockfall barriers — when possible.
The other direction is “proactive risk reduction.” Group said work is concentrated in areas where there’s been more activity. That work also includes obtaining funding for mitigation efforts including mesh screens and rockfall barriers for slide zones in the higher reaches of canyon walls.
“We go through a process of trying to assess where the highest risk levels are,” Group said.
One of those risky areas is along Highway 24. That stretch of road “has been an issue,” Group said. There were some mitigation measures in place, but a rockfall in the summer of 2022 destroyed the barriers that were in place at the time.
But Glenwood Canyon is the best-known hazard zone in this part of the state.
And while a pair of long closures have come in the years since the 2020 Grizzly Creek fire, Group said the canyon has always been a challenge.
Large boulders have punched holes in the road deck, and slides have been part of the canyon’s geography just about since the highway through the canyon was finished in 1992.
Group said the days after the Grizzly Creek fire brought slides due to the burn-off of vegetation.
The summer of 2021 brought another lengthy closure when a massive debris flow closed the highway, the recreation path and affected the course of the Colorado River in the canyon.
That debris flow required essentially three projects: road repair, debris cleanout and dredging debris from the river.
“We had concerns about the new river elevation,” Group said, adding that flooding could be one offshoot.
It’s better — for now
After all that work, the region had a decent summer in 2002, at least from a wildfire and rainfall perspective.
Work in the canyon now includes more mitigation measures to protect from debris flows. Existing barriers, as many as 20 of them, had to be cleaned out and repaired to ensure they’re properly functioning now.
Some of those debris flow barriers stand as tall as 12 feet. In addition, there are more wire mesh and other rock barriers, along with storage space for material that comes down from the canyon walls.
Group said the result of all that work is that the canyon’s defense mechanisms — and natural conditions — are about where they were in the summer of 2019.
Still, he added, the canyon closures aren’t over.
The I-70 Coalition is a nonprofit consortium of businesses and local governments dedicated to improving traffic flows through the mountain corridor.
Coalition director Margaret Bowes praised transportation officials for keeping a close eye on potential danger zones.
But, she added, “Mother Nature always rules.”