Submitted by masa on Wed, 2017-03-29 16:28
23-29 minutes
On 27 March 2017, a group of high-school students in Japan lead by teachers encountered an avalanche near the Nasu ski-resort, and 8 people (7 students and 1 teacher) died, and 40 were injured. Here I discuss what happened and how from a viewpoint of the winter mountain activity.
The deadly accident happened during the 3-days Spring-mountain safety course for students of mountain-walking clubs in 8 high schools in Tochigi prefecture, run by the prefectural high-school PE council. Reportedly(link is external), the council had notified the clubs that any one who considered to participate the major (mountain-walking) competition in May must attend the course. Eleven high-school teachers in the prefecture took a lead and instruction role for the event, and reportedly some were highly experienced. 51 students attended the course.
On 27th March, the final day of the event, 8 teachers and 40 students attended. The original plan on the day was to summit Mt Nasu (Mt Chausu). However, because of the foul weather, the teachers on site decided to change the plan to snow-wading walking practice on a slope near Nasu-Onsen Family Ski Resort in Yumoto, Nasu. They started the training at around 8 am. Allegedly it was in the blizzard and the visibility was poor at that time. The site of the training was a 30+ degrees slope above the piste of the ski resort. According to a civil servant of the town council(link is external), Avalanches have been of regular occurrence every year at the slopes above the piste. The ski resort had been already closed by that day after the winter season, and was not operating (lifts etc).
Note: Japanese mountains are so snowy, it is an important skill to progress efficiently in deep powder snow, called "Russell", named after the snow-plough Russel Plow invented by the American James Russell.
At around half eight am, an avalanche happened [two avalanches as reported later] and almost all the group members were involved. It is thought to be a surface avalanche (as opposed to a full-depth one). The rescue team arrived at the site 3.5 hours after the avalanche. They found some victims to be buried, and although one was miraculously rescued alive, 8 people were dead (all the fatality was due to suffocation by the pressure of burial in snow). Reportedly, the deepest depth of buried victims was 2 metres. Note the initial call for rescue to the police was made by a teacher of the group, who descended to a hotel at the foot of a mountain, where he phoned, and it was about an hour after the avalanche. Allegedly there was a separate ground-team at the foot of a mountain, and the ground team and group on site had a radio link. The reason why the radio link was not used, and it took as long as 1 hour before the initial contact to call for a rescue is not yet understood well at the time of writing.
At around half 10 am the previous day, the local Utsunomiya meteorology office had issued warnings for heavy snowfall, avalanches, and snow-whirl. It forecast the 24-hr accumulated snowfall by the morning of 27th in the mountains in the northern part of the prefecture to be 30 cm, and issued the warning. According to the national meteorology office, the local observation point in Nasu Town saw 0 cm of snowfall accumulation at 1 am on 27th, and 33cm by 9 am on 27th.
(References: Shimotsuke Shimbun (δΈιζ°θ)(link is external), J-Cast(link is external) etc)
None of the group members was reported to be equipped with avalanche transceivers. All the members were brought back down (live or dead) from the site by the rescue team that hurried to the site.
Avalanches are categorised into many classes, and each of them has a different mechanisms and conditions to be triggered. Here I consider about the dry loose-snow surface avalanches by fresh snow, which was the one that supposedly happened on the day.
The mechanism and conditions of triggering of surface avalanches are complex. They depend on the complex combination of history of snowfall, wind (strength and direction), temperature, sunlight, as well as the local geography However, there are some typical textbook conditions that induce such avalanches. Here is the list.