The National Weather Service flood watch for the Grizzly Creek burn zone came shortly before noon on Tuesday. When the watch became a warning shortly after 3 p.m., the Colorado Department of Transportation closed the interstate through the canyon for about three hours.
The closure was among several for Glenwood Canyon since late June. Three times the canyon has closed for extended stretches after rain washed countless tons of soupy mud and debris onto the interstate. And the closures are going to keep happening.
“It has not taken a whole lot of rain to move what has moved so far so I anticipate there will be more movement in some of those same drainages but it’s hard to measure and know exactly so much,” said Elizabeth Roberts, an ecologist with the White River National Forest who has spent most of the past year planting grasses in the burn scar to stabilize soil and restore damaged terrain.
The seeds Roberts and her team sow will eventually become the rooted plants that keep soil from moving in the dozens of debris fields that funnel into Glenwood Canyon’s Colorado River. But since the Grizzly Fire burned into winter last year, she’s racing to get seeds into every path of scorched earth. Many of the Grizzly Creek Fire’s 32,631 acres are in steep, rocky chutes where seeds would not take anyway.
Everyone knew the runoff and rains of 2021 would pose a threat to Glenwood Canyon. The City of Glenwood Springs spent more than $10 million on emergency watershed protection projects that included replacing and upgrading water intakes and filtering systems in the No Name and Grizzly Creek drainages where the city collects its water.
Swift protection for the highway from rain-loosened debris was much more difficult, if not impossible.
“We are dealing with lot of different drainages, upwards of 20 different drainages that have elevated risk of debris flow, so trying to treat the number of areas is challenging,” Bob Group, the manager of the CDOT’s geohazards program, said on July 4, the day after several feet of mud buried the highway.
The U.S. Geological Survey created a landslide hazard map following the Grizzly Creek Fire that identified dozens of drainages where the likelihood of debris flows was increased if the area saw only 15 minutes of rain that fell at a rate of roughly an inch an hour. That map was spot on. Debris flows that shoved tons of mud onto the highway have come from three separate areas where the USGS estimated the chance of debris flows was between 40% and 100%.
Forest and transportation officials were working with models, so the actual amount of mud coming down and where it might end up was impossible to predict.
“When it came down to actual measurements and volume, I was surprised that some of the small drainages went as quickly as they did,” Roberts said.
Roberts has been doing most of her seeding work on the rim above the canyon. She’s been surprised to see lots of natural vegetation coming back in the first year. The growth of herbaceous shrubbery — known as forbs, which are neither grassy nor woody, like snowberry, chokecherry and fireweed — has been “quite significant,” Roberts said.
That’s been helpful because forest botanists are generally speeding native grasses, which can take a couple years to firmly establish, depending on the health of the soil.
Predicting the intensity of storms is just as challenging as predicting where and how much debris may flow from the canyon walls. The storms since late June over Glenwood Canyon have been strong and quick. The cells form swiftly and they dump torrents of rain in a matter of minutes, sometimes across a very small area. That rain, especially up on the rim of the canyon, can funnel into a gully-washing deluge as it plummets down the 2,000-foot cliffs above the highway and Colorado River.
“We can see more debris coming down depending on where these storms are sitting,” Roberts said. “It’s just so challenging. So very challenging.”
The July 3 slides that buried about two miles of interstate east of the Hanging Lake tunnel came from five identified debris flows. The June 27 and 28 mudslides came from debris flows farther west, near the No Name exit. Roberts said eventually those smaller debris flow paths will be scoured clean and rain will simply flow as water, not mud and rocks. There are many more debris flows that have not been hit with heavy rain.
Mitigation in the narrow canyon is complicated. The stretch of interstate built between 1980 and 1992 is an engineering marvel, heralded not only for its ingenious efficiency but how its minimal footprint protected as much of the canyon as possible. When a fire hit perhaps the worst place on Interstate 70 for a burn scar, there just isn’t much room for barriers and other strategies for protecting roads from rain-riding debris. That isn’t stopping CDOT from trying to find ways to divert flows of mud and rock.
“We are in the process of … taking aerial imagery from the recent handful of events, measuring volumes of material generated and going through the process of revising previous assumptions and see if there is a way for mitigation,” Group said.