Rains that triggered last week’s Glenwood Canyon mudslides approached a 500-year event
A devastating wildfire that scorched Glenwood Canyon last summer followed last week by what the National Weather Service said was a 500-year event over parts of that burn scar made for the perfect storm that now has a major travel corridor closed indefinitely.
The multi-agency Burned Area Emergency Response (BAER) team that inspected the Grizzly Creek Fire burn area early last fall predicted what’s now played out in the canyon in recent weeks.
Heavy monsoonal rains was the main ingredient needed to trigger the extreme mud and debris flows that have occurred on and off since late June, culminating with two super events the evenings of July 29 and 31.
It came in sheets and waves, with some of the heaviest rain measurements in exactly the worst-possible spots above the canyon.
“Based on some of the rain gauge data, we saw anywhere from a 50- to 100-year event in spots, with some intensity around a 500-year event,” said Erin Walter, meteorologist with the National Weather Service in Grand Junction.
“There was some pretty substantial rainfall in some of those isolated locations, which resulted in the extreme flash flooding and debris flows.”
The heaviest of the rains on July 29 came around 9 p.m., about two hours after I-70 through Glenwood Canyon had been reopened to traffic following an earlier flash flood warning. The resulting mud and debris flows trapped close to 30 vehicles and more than 100 people altogether in the canyon overnight.
Fortunately, no one was injured, though several of the trapped motorists had to wait it out overnight in their vehicles, in the Hanging Lake Tunnels command center and at Bair Ranch.
Walter said one rain gauge site just east of the Grizzly Creek burn scar measured more than 2 inches in an hour’s time that night.
“Some of our measurements were even higher within 15-minute intervals,” Walter said.
Jason Kean is a research hydrologist for the U.S. Geological Survey in Golden and is part of the team who monitors rainfall within the Grizzly burn scar.
“We have at least five locations where there is a potential for substantial flows during a heavy rain event,” Kean said.
He said the July 29 storm dumped 0.52 inches of rain in 15 minutes in the East Deadhorse Creek drainage near Hanging Lake on the north side of the canyon.
One spot at Coffee Pot Springs Road just outside the burn scar measured 1.1 inches in 15 minutes, he said.
On the south side of the canyon, within the Deadman’s Creek drainage, a rain gauge measured 0.66 inches in 15 minutes.
Then, two nights later on July 31 after I-70 had already been closed for two days, another monsoonal rain event produced 0.7 inches in 15 minutes at Deadhorse Creek.
Blue Gulch, which was highlighted in drone footage released by the Colorado Department of Transportation earlier this week, had the biggest flow, Kean said.
That same night, Deadman’s Creek to the south saw 0.37 inches in 15 minutes, he said.
The heavy rains were apparently enough to knock some of the monitoring equipment out of commission.
“We are having some firmware problems at a couple sites that cannot be resolved remotely, and we are working with the manufacturer to address the issue,” said Cory Williams with the USGS Colorado Water Science Center in Grand Junction. “As part of the solution, we will need to visit the sites in-person and will be doing so as soon as equipment and firmware updates are available.”
Walter added that what had been a long-term forecast for an abnormally dry summer changed when a high pressure system shifted to the south and east.
“That brought a long duration of tropical monsoon moisture stretching across western Colorado,” Walter said.
A drying-out period is now forecast into next week at least, she said.
Even with the heavy rains recently, Garfield County and most of Mesa and Rio Blanco counties remain in Extreme Drought, according to the current U.S. Drought Monitor. Parts of the Western Slope are still in Exceptional Drought — a step up from extreme drought — including most of Moffat County.
“Any rain does help, but we’re still not out of the woods yet,” Walter said.
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