The challenge facing Colorado transportation officials in Glenwood Canyon this summer is staggering: Dozens of basins ravaged by last summer’s Grizzly Creek fire drain down toward Interstate 70, and all it takes is a short burst of intense rain in the right spot — a downpour lasting as little as 15 minutes — to send a soupy mix of sludge and rocks flowing onto the highway.
Already this summer, two mudslides have covered the highway, and geology and engineering experts say the danger could last another couple years. The greatest control the Colorado Department of Transportation has, they said, is over when to close the highway proactively to protect travelers — not the prevention of the slides themselves.
That’s in large part because the steep terrain of the canyon, along with the difficulty of accessing high basins that were burned in last summer’s 32,631-acre fire, make typical debris-flow mitigation projects difficult, too costly or impossible.
In gentler terrain, crews could dig out flat areas in slide-prone drainages to slow the flow of water or install barriers to catch debris.
The upshot is that it will take years in some areas for soil burned during the fire to sprout new ground cover and recover normal levels of water absorbancy. Other agencies have evaluated the burn area and begun making plans to help along that process, which can take longer in areas where fire burned the landscape more severely.
In the meantime, much of the highway is at varying risk for slides. Even if the fire didn’t burn close to a given section of roadway, it may have burned forest or brush higher up the canyon walls and beyond the rims, on both sides.
“This is a huge area to cover, and there are a lot of potential sources” of slides, said Paul Santi, a professor of geology and geological engineering at the Colorado School of Mines in Golden who has studied post-fire debris flows. “So there’s not a lot (CDOT) can do, besides stay aware of the (risk) map, keep equipment ready and respond as they need to. Like with rockfall, there’s too many potential sources to be able to treat all of them.”
Since last fall, CDOT-hired crews have concentrated on spreading mulch on burned soil close to the highway, and seeding some areas, to help stabilize what they can near I-70.
But the slide risk map developed by the United States Geological Survey for each basin — the one Santi referred to — makes clear that burn areas high up in the canyon’s drainage basins pose widespread risk.
CDOT’s emerging practice has been to close rest areas and the canyon’s bike path whenever a flood watch is issued so that it’s easier to clear the highway if a storm becomes imminent. Once meteorologists issue a flash flood warning, CDOT closes I-70.
The result this summer is greater uncertainty not only for drivers, who face lengthy detours during closures, but also for the rafting companies in the canyon. The closures adds a potential wrench for people who’ve obtained permits to hike to Hanging Lake, since the trailhead is among rest areas closed during watches.
In Glenwood Springs, hotels and other tourist-oriented businesses are wary of the disruption, but so far they’ve been able to navigate it.
“We have not had many cancellations,” said Lisa Langer, director of tourism promotion for Visit Glenwood Springs. “There have been a few. People who generally cancel are the ones who are not familiar with driving in mountain conditions, and if they don’t know the road, then they’re not sure.”
“Left to the whims of nature”
The first two slides, caused by storms on June 26 and June 27, occurred less than a quarter-mile apart and emerged from different basins in the western part of the canyon, said Bob Group, CDOT’s geohazards program manager. CDOT already was on high alert, since the post-fire vulnerability of the slopes was well-known.
“We’re sort of left to the whims of nature and what specific paths storms take,” Group said.
In the early going, he’s keeping an eye on how where the slides happen compared to that USGS risk map. The first two were in basins with higher risk ratings for the kind of summer soaker that’s bound to happen occasionally, he said.
“It’s definitely an uphill battle, given the size and the scope and the criticality of that highway,” Group said of I-70. “I think that’s kind of unique, in that most of the fires that we’ve had to deal with previously have been on lower-volume roads … than I-70 through the canyon.”
Group says the level of rain-season danger could diminish in coming years.
But how long this will be the canyon’s new normal is the “million-dollar question,” said Santi, the School of Mines professor.
“There’s no single answer,” he said, “but generally you need something in the range of a few years to reestablish vegetation well enough so it can absorb moisture and break off whatever (water) is flowing on the surface and accumulating.”
After particularly severe fires, experts say it can take even longer.
“We’re taking the good with the bad”
In Glenwood Springs, Langer takes an optimistic view of the slide risks.
“The biggest thing to remember is that we’re thankful for the moisture,” she said. “In a perfect world, if it would rain softly between 2 and 4 in the morning, that would be great. However, that isn’t going to happen. We’re taking the good with the bad, and really the silver lining is that CDOT is on top of it. They know what they’re doing — and keeping on top of the weather forecast.”
Santi also took a wider view, noting that there’s a reason the Glenwood Canyon segment was the final piece of I-70 built.
It’s a tangle of tunnels, bridges, viaducts and retaining walls aimed at taming a wild canyon. Sometimes, he said, nature has the upper hand, whether in a raging wildfire or its aftermath.
“I think we take our access to it for granted,” Santi said of the canyon. “I think we just have to accept that it’s going to be a continuous battle in there, and they’re doing everything they can to keep the population safe — but these processes are going to continue to occur.”
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