Researchers take deep dive into large, intense March avalanche cycle in Aspen, state
SLIDES BY THE NUMBERS
Researchers say the avalanche cycle in the Colorado mountains March 1-14 was historic for the number and intensity of slides. Here’s one eye-opening stat from the Colorado Avalanche Information Center:
Number of recorded avalanches in March 2019 rated D4 and D5, the most destructive: 87
Number of D4 and D5 slides from 2010 through 2018: 24
The avalanche cycle that struck the Colorado mountains in March destroyed a mining structure that had survived since 1881, produced three slides among the most destructive ever recorded in the state, obliterated the sheriff’s house in Hinsdale County and blanketed roadways in places that had never been covered.
Colorado Avalanche Information Center researchers are just starting to get a grip on the magnitude of events from March 1 to 14.
“Nobody alive remembers anything like this happening,” Brian Lazar, deputy director of the avalanche center, said Nov. 6 at a presentation at Cripple Creek Backcountry in Carbondale.
A standing-room-only crowd of about 100 crammed into the store to watch a slideshow featuring images that Lazar called “avalanche porn.”
CAIC recorded nearly 1,000 avalanches between March 1 and 14. In reality, at least five times that many actually occurred, Lazar said. CAIC uses satellite imagery for avalanche research. The satellites focus on areas where timber was knocked down. It indicated there were nearly 5,000 sites in the Colorado mountains last winter. Sometimes, CAIC is learning of massive slides from pictures provided of the carnage remaining this summer.
It wasn’t just the numbers but the sheer destructive force that made the March cycle so memorable.
The D-scale is an assessment of the destructive potential of avalanches. A D1 slide is relatively harmless. D3 can knock down timber. D4 can destroy a structure and alter a section of forest. D5 is the highest, with the force to alter the landscape and even destroy a small village.
CAIC has documented 87 avalanches at or above D4 during the two-week March cycle, Lazar said. To put that into perspective, there were 24 slides of that magnitude from 2010 to 2018 combined.
Two of the three biggest slides in March occurred in Pitkin County. The first occurred in Conundrum Creek Valley on March 9. The slide broke along 1 mile of the ridge south of Aspen Highlands, including the Five Fingers area. It dumped debris as deep as 200 feet in the creek.
“This was the biggest avalanche I’ve ever seen in the state of Colorado, probably the biggest avalanche I’ve ever seen in the lower 48 states,” Lazar said. “It wiped out thousands of trees.”
One house was spared only because a protective concrete wedge diverted snow, tree trunks and debris to avoid a direct hit. Lazar and other researchers used chainsaws to cut disks out of tree trunks left in the slide path to estimate of their age. That helps determine how long it has been since a slide of that magnitude struck the area.
“From some of the tree coring and some of the tree disking we’re doing, this particular event may be more in the order of one in 300 years,” Lazar said.
Another of the D5 slides occurred March 14 along a 2-mile ridge starting at Garrett Peak just outside of Snowmass Ski Area.
“You’ve often heard you can’t get D5 avalanches in Colorado,” Lazar said. “I’ve often said that myself because the terrain’s just not big enough.”
He predicted that after more information is assessed, additional slides from the cycle will be reclassified as D5, most likely some in Gothic and Lake City.
“So much went down in this (two-week) period, there’s no way I can capture it in one talk,” Lazar said at the start of his presentation.
In a nutshell, here’s what happened. Average to slightly above-average snowfall during the first half of the winter built a strong and stable snowpack throughout the Colorado mountains. March brought a series of exceptionally wet snowstorms.
“We got hammered for two weeks straight,” said Lazar, a resident of the Carbondale-area and a CAIC forecaster for the Aspen-Marble zone.
The Aspen zone was among the hardest hit. While many mountains received 4 to 6 feet of snow, Schofield Pass near Marble received 12 feet. The storms added at least 4 to 6 inches of snow water equivalent — the amount of water in the snowpack.
The snowpack couldn’t handle that amount of loading with wet, heavy snow. In addition, a lot of snow was built up since there hadn’t been a lot of slide activity earlier in the winter. As a result, the slides in March were huge.
“When things finally started failing, they went catastrophically,” Lazar said.
There are indications that wet storm periods might become more common.
“With climate change and that kind of stuff happening, we’re seeing 3- to 4-inch water storm events more frequently,” Lazar said.
CAIC’s research of the historically large and intense cycle is heading in several directions. For example, some of the largest and most destructive timber was carried to the toe of the slide path. Conventional wisdom had been that it settles out earlier. The pattern displayed in March might force reassessment of damage potential.
Also of interest is the somewhat random nature of slides. While many avalanche paths slid — some for the first time in decades and even more than 100 years — not all slopes did. Lazar showed a picture of a road sign saying “End of Avalanche Area” knocked over by an avalanche. That displays the uncertainty around big cycles, he said.
Of greatest concern to CAIC is how to warn people in isolated mountain areas about avalanche danger. As population grows, fewer people have telephone landlines so they aren’t accessible by reverse 911. The destruction of the Hinsdale County sheriff’s house, which was occupied by three people at the time, demonstrates a need for greater communication with people in endangered areas, Lazar said.
“Having gone down there, I can’t believe anyone lived through this thing,” he said.
The March storm cycle was destructive enough. Avalanches that month killed two people — one in the backcountry and another in an accident while clearing a roof. Ten structures were hit statewide. Gas and power lines were damaged. Highway 550 over Red Mountain Pass south of Ouray was closed for 18 days. Slides even closed Interstate 70 and Highway 91 near Copper Mountain. Ten people were trapped in vehicles on open highways throughout the state.
Once the snow stopped, avalanche conditions quickly returned to mostly stable for the remainder of winter and spring.
“We went from pretty stable to the world falling apart to pretty stable just like that,” said Lazar, who added that analysis of the intense two-week period will continue. “This is going to be years of research in the making.”
Story from aspentimes.com
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