Avalanche Incident
Avalanche IncidentFrom: ChabotDoug/r1gallatin@fs.fed.us
On the 26 March 1998 one snowmobiler was buried in an avalanche on the south flank of Scotch Bonnett Peak outside of Cooke City, Montana. The following is a brief description of the events.
Setup:
The week preceeding the avalanche we experienced warm and sunny weather which resulted in a substantial melt freeze crust on most south and east aspects at most elevations. Following this period of high pressure the mountains around Cooke City received appoximately 6-10 inches of new snow on the 24th and 25th of March. These snows were accompanied by 10-20 mph winds out of the west. On the morning that the avalanche occurred the avalanche danger was rated Moderate, except on wind loaded slopes steeper than 35 degrees where it was Considerable.
26 March 1998:
A snowmobiling group from Iowa, consisting of 20 members, were out sledding on the south shoulder of Scotch Bonnett Peak. The group was taking turns highmarking this slope when Mike Grevegoed got stuck. A friend sledded up the slope to assist him when the slope released. His friend was able to snowmobile to safety, but Mike was swept away and buried. The group witnessed the event and initiated a rescue. No one was wearing beacons, and out of the entire group they had 2 shovels and one probe. A ski from the snowmobile was sticking out of the snow, so this is where they started the probe. On their first try they hit his foot and dug him out in 8 minutes. He was buried immediately downhill of his snowmachine. His helmet was packed full of snow and he was turning blue, but was still conscious and breathing.
Comments:
I went down to Cooke City on March 28th to investigate this incident. Unfortunately I haven't been able to talk to the victim, so most of the avalanche account is from third parties. On the day of my investigation I had good visibility, but unfortunately the wind has been blowing the past 36 hours which filled in most of the avalanche path.
The crown face was 18-24 inches deep and had an angle of 38 degrees. The average slope angle was approximately 34 degrees. It was 200 feet wide and ran 600 vertical feet. The avalanche started at an elevation of 10,400 feet. The slope is a southeast aspect and is subject to wind loading off the ridgetop as well as crossloading lower down. The crown was about 150 vertical feet from the ridgetop on a steep rollover.
The slope failed on a layer of graupel that was sitting on the melt freeze crust from the previous week. The slab consisted of a hard wind blown snow. The slope was triggered either by the snowmobiler trying to assist or by the combined weight of the two riders and their machines on the slope. Reports indicate that the avalanche occurred as the rider coming up to help got about 3/4 of the way up the slope.
It is unclear how deep the victim was buried. Some accounts claim 5 feet, but this is unsubstantiated. Because of the recent wind loading I was unable to see where they dug him out. Debris depth seemed to be in the neighborhood of 4-5 feet.
There was no other evidence of avalanche activity in this area.
Douglas Chabotbr Gallatin National Forest Avalanche Center
Yesterday, a snowmobiler was caught and buried in an avalanche near Cooke City. He was high marking on a slope when his sled got stuck. Another member of his party rode up the slope and the avalanche released. The snowmobiler with the stuck sled was buried several feet deep. The other rider was able to ride out of the avalanche. There was a large number of folks in the group.
They didn't have any avalanche rescue beacons or shovels. One person had a probe pole. The buried person was located with the probe and was uncovered in about 8 minutes. He was very lucky and didn't suffer any injuries.
The avalanche occurred on Scotch Bonnet Mountain. It was about 100 yards wide, 200 yards long and about 2 feet deep. It sounds as though only new and recently wind transported snow was involved.
Yesterday, Doug was in Cooke City. He checked out the slope where a snowmobiler was buried in an avalanche on Thursday. The avalanche was about 200 feet wide, ran about 600 vertical feet and the crown fracture was 18-24 inches deep. The slope has a southeast aspect and an average slope angle of 34 degrees. The slope angle at the crown fracture is 38 degrees. This slope had a significant deposit of wind transported new snow, which failed on an ice crust. He did notice some collapsing of the snowpack on lower angled south facing slopes and saw a recent avalanche on a wind loaded slope that had been triggered by a falling cornice.
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