Avalanche expert says Conundrum slide likely a 300-year event
A massive slide that swept down from Highlands Ridge into Conundrum Creek Valley last weekend was probably a 300-year event, a leading avalanche consultant said Tuesday while touring the site.
“This is definitely a big one for Colorado,” said Art Mears while surveying the volume of snow and debris the avalanche deposited. The only comparable slide he has seen in Colorado struck the Gothic area near Crested Butte in 1995, he said.
Mears is an engineer from Gunnison who consults with governments and individuals to locate infrastructure and buildings where they will avoid avalanches. When structures are built in avalanche-prone areas, he helps create designs to mitigate the risk. He has worked on 1,100 projects in nine states and eight countries. He has designed several avalanche mitigation systems for homes in Pitkin County, including a “splitting wedge” concrete wall that probably saved a house at 1053 Conundrum Creek Road from getting flattened by last weekend’s slide.
The wall suffered no visible damage and it deflected most of the high-density lower layer of snow and debris in the avalanche. Some damage was sustained on the west end of the house. Reinforced glass held but an entire window frame was dislodged. Part of an upper-story wall was punched in but the house apparently didn’t suffer any structural damage.
The splitting wedge was designed to protect the house from a 100-year avalanche, per Pitkin County’s requirements. That means there is a 1 percent risk of an avalanche that size happening every year. With a 300-year event, there is a three-tenths of a percent annual probability of an avalanche that size occurring.
The concrete wedge is 21 feet at the top and tapered down to 17 feet on the lower ends. The south leg is 110 feet long and the west leg is 90 feet.
Tons of snow, trees and rock fell from the Five Fingers and K-Chutes on the west side of the valley. The avalanche fell with such force that debris shot more than 200 feet up the east side, wiping out additional trees. Then the debris spilled downvalley or to the north, where it buried the Conundrum Creek Trailhead and U.S. Forest Service facilities there. Much of the debris wrapped around the house to the east.
However, it was clear from broken aspen and conifer trees surrounding the house that debris also ran into the wedge. Mears said there was likely enough material flowing that it bent the trees and caused some of them to break. Smaller aspen trees were smashed flat.
Mears said the house would have been in a red or high hazard area if it didn’t have the splitting wedge. The special wall dropped it to a blue or moderate hazard area. Without the wall, the house probably would have been heavily damaged by last weekend’s slide, he said. The house was unoccupied at the time of the slide.
The house was built in 1987, according to records in the Pitkin County Assessor’s Office. An avalanche damaged it in 1996.
The current owner, listed as MW III Aspen LLC, bought the house and 25.8 acres in December 1998. The new owner applied to Pitkin County in 2001 to build the splitting wedge to protect the 5-bedroom, 5½-bath structure of nearly 5,000 square feet.
In a report prepared for that application, Mears described the potential destructive force of a 100-year avalanche in that area.
“The Five Fingers path produces a major avalanche which can involve more than 100 acres of snow during (a 100-year event) and falls about 3,500 feet to the Conundrum Creek Valley floor,” his 2001 report said. “Maximum velocities on the steep slope will exceed 110 mph.”
He accurately predicted that the blast from the avalanche would ascend roughly 300 feet up the east wall of the valley.
Mears said Tuesday that despite its size, the avalanche probably only lasted about one minute. The powder cloud kicked into the air probably settled within another 30 seconds or so, he said.
There are additional avalanche chutes downvalley or north from the house. A slide in what’s known as the Teepee Chute killed a man living in a teepee on adjacent property in February 1995.
Mears said Pitkin County has a lot of areas of high avalanche hazard because of its numerous valleys with steep slopes. The risk of damage to property is high because of the amount of development in the prone areas.
While Pitkin County has a design standard for a 100-year avalanche event, Gunnison County has a 300-year standard, he noted.
“I believe, in my opinion, Pitkin County should (do) that,” he said.
Pitkin County is a leader in land-issues in a lot of ways, Mears said, and avalanche mitigation should be another.
The latest Conundrum avalanche demonstrates why a great design standard may be needed. The width of the crown was estimated at 5,000 feet by investigators from the Colorado Avalanche Information Center.
“Highlands Ridge released naturally. Not a path or two off the ridge but everything from the Five Fingers to the K-Chutes,” CAIC’s report said.
Multiple feeder paths share a common run-out, making the avalanche particularly destructive.
CAIC rated the slide a 4.5 out of 5 on the avalanche destruction scale. Mears said the destructive potential is difficult to assess. One way would be to return when the snow melts and date the age of the large conifers the avalanche took out, he said.
While many avalanche experts don’t feel it’s possible to have a D5 avalanche in the Continental U.S., Mears said the Conundrum slide could well qualify.
At least one boulder will need to be blasted as part of the cleanup effort.