Avalanche Incident
Avalanche Incident
*Unstable ground layer blamed for in-bounds "climax" release.
by Jim Stanford, Jackson Hole News
A sunny Thanksgiving powder day nearly turned ugly Thursday on Fred's Mountain at Grand Targhee. Shortly before 11a.m., a snowboarder descending a north-facing aspect of the mountain's headwall - a run known as The Good, The Bad and The Ugly - triggered a slow-moving, soft-slab avalanche. Measuring 300 yards wide, the slide ran about 300 yards and deposited three- to six-foot-deep piles of debris in the trees below the run and in an open area adjacent to Chief Joseph Bowl. Witnesses saw the boarder, allegedly known as "Speed" Wahker, ride out of the slide to safety. They saw no one else in the area at the time. Targhee's ski patrol quickly responded to the scene with two avalanche dogs and began probing and searching for possible victims. Several skiers with avalanche-probe poles joined the search.
The searchers first "spot-probed" areas of debris piled around trees, the most likely burial sights. Later, Search and Rescue Team members from Teton Country, Idaho and Teton County, Wyo., arrived with three more avalanche dogs and replaced the civilians.
An aura of calm presided over the scene as teams of 15 searchers lined up at the bottom of the remaining debris and moved up the slide's path one foot at a time. In all, about 40 people took part in the search. The Teton County, Wyo., Sheriff's Office pronounced the scene clear at 5:30 p.m. "There were no missing persons as of Friday, which is a good feeling," Targhee general manager Larry Williamson said on Friday afternoon. "With that kind of avalanche, you have to take every kind of precaution."
The slide is classified as a "climax" release because the slab of snow broke away at the ground level. The slab was three feet deep, although the upper portion - a cornice built up by wind-loaded snow - measured closer to five feet. Especially alarming is the fact that the slide occurred within the ski area's boundaries in an avalanche-controlled area. Targhee's ski patrol bombed the run three times Thursday morning, cut ski tracks across it and determined the slope was stable. In addition, "a lot of people skied it between 9:30 and 11 a.m.," Williamson said.
The slide's origin can be traced to an unstable layer of snow near the bottom of the snowpack known as depth hoar or temperature-gradient (TG) crystals. Depth hoar can form on any slope when a shallow snowpack buffers the relatively warm ground from the much-colder air. A series of clear, sunny days and cold nights during the early part of November created optimal conditions for the formation of depth hoar this year. The depth hoar layer, which bonds poorly to other layers in the snowpack, has been blamed for avalanches similar to the Targhee slide on Teton and Togwotee Passes.
"The large-faceted crystals have a lot of air between them and very little strength," Fadiman said. "They're hardly connected with each other and ready to collapse under a skier's weight. As long as the depth hoar layer persists, there will be significant avalanche danger, Fadiman said. More snow and warmer temperatures are needed to even the temperature gradient of the snowpack and to stabilize the snow.
In time, even the worst depth hoar becomes bonded with the other layers of the snowpack, Fadiman said. He estimated it will take probably at least another month before this process occurs around Jackson Hole. Jackson Hole News "Excursion" columnist and American Avalanche Institute instructor Ron Matous said in his Nov. 19 column that the depth hoar problem may pose a danger to backcountry users all season. He referred to the crystal layer as "The Dragon." "It's going to mean a lot of digging, as our snowpits will have to go all the way to the ground to see how things progress down there," Matous wrote. "I think I'll invest in a back brace."
The following information via our online form:
Email = dwitton@xbd.combr Date = 11/27/97br Time = 11:00br Location = Grand Targhee Ski Resortbr Elevation = 9800'br Aspect = nnwbr Incline = 30br Type = Slabbr Width = 300mbr Thickness = 1mbr Water = Drybr Avalanche Trigger = Snowboarderbr Sliding Layer = depth hoar/climaxbr People Caught = 0br Fatalities = 0br
Comments: First in-bounds slide @ Targhee during operating hours. Slide occurred after explosive control and 1 1/2 hours of skier traffic on popular run. Failure was due to weak growth-crystal layers at ground and at old surface hoar layer. Other backcountry paths have run, including some at popular Teton Pass areas. Continued cold temps since incident are increasing hazard.
|
|