Detailed Accident Report

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Date: 2000-03-16
Submitted By: Ron Johnson, GNFAC
Place: Frazier Lake
State: MT
Country: USA
Summary: 1 snowboarder caught and injured

Frazier Lake Avalanche Incident

March 16, 2000

Submitted By:

Ron Johnson

Avalanche Specialist

Gallatin National Forest Avalanche Center

406-587-6984

rjohnson@fs.fed.us

Synopsis:

A snowboarder triggered, was caught in, but not buried in an avalanche on March 16, 2000. He had made a couple of turns off the top of a steep chute. A hard wind slab fractured above him.

He was flushed through a narrow rocky section of the chute. He stayed on his board and came to rest about 20 meters below the toe of the debris. He sustained a compression fracture of his

lumbar spine (L1). He was flown to Bozeman Deaconess Hospital and is expected to have a full recovery from his injuries.

The Story:

On Thursday, March 16th, five snowboarders and skiers, snowmobiled into the northern Bridger Range of southwest Montana. They intended to ski and board some of the steeper slopes and

chute in a cirque above Frazier Lake. Frazier Lake is located about 8 miles (13 km) north of Bridger Bowl Ski Area.

At about 11:00 am Nel Boshoff, 24, dropped into the top of a steep chute. This chute is shaped like an hourglass with a narrow section midway down the slope. The chute widens onto a steep open slope. The slope angle at the top of the chute is close to 50 degrees. During Boshoff's second turn, a hard wind slab fractured. The fracture propagated upslope and the slab released about 10 meters above Boshoff. The slope angle at the crown was about 45 degrees. The crown fracture was about 15-30 cm deep and 10-15 meters wide. He was swept down the chute

and was engulfed in debris as he went through the narrow, rocky section of the chute. He was able to stay on his board and ride out of the debris. Boshoff came to a stop approximately 20 meters downslope from the toe of the avalanche debris. In addition to being "banged up" he sustained injuries to his hip and back. His snowboard was "trashed".

Two members of his party went for help. They went to Bridger Bowl and notified Gallatin County Search and Rescue. Scott Schmidt (ski patroller, member of Gallatin County Search and

Rescue and an avalanche specialist at the Gallatin National Forest Avalanche Center) responded with the reporting party. He was first on scene. He assessed the patient and was able to stabilize Boshoff's spine with a full body vacuum splint. I arrived on the scene and a decision was make to attempt to land the Billings Deaconess Life Flight Helicopter on a small snow dome, located about 50 meters from the patient. The helicopter operation was successful and Boshoff was transported to Bozeman Deaconess Hospital.

Boshoff sustained a compression fracture of his lumbar spine (L1). He is expected to recover fully from his injuries.

Weather and Snowpack Details:

The avalanche occurred on Thursday, March 16th. On Saturday, March 11th, the Bridger Range received 20-30 cm of new snow. Little wind was associated with this storm and most avalanche

activity reported by the Ski Patrol at Bridger Bowl occurred on the new/old snow interface. The old snow surface was comprised of an icy layer on slopes that had been exposed to the sun and

some near surface facets on shadier slopes. Another storm on Monday, March 13th, deposited another 15 cm of new snow. Once again any avalanches related to this storm occurred on either

the old/new snow interface or on the layer that was buried by the March 11th storm. On Tuesday, March 12th, strong west winds blew along the ridgetops. Wind speeds exceeded 40 mph, transporting much of the new snow onto the lee side of the Bridger Ridge, which runs generally southeast to northwest. During Tuesday, several small wind slabs were triggered by cornice fall. None of the naturally released avalanches had fractures which propagated much wider than the width of the cornice that triggered them.

No new snow fell in the Bridger Range, on Wednesday, March 15th or prior to the avalanche on Thursday, March 16th. During this period, ridgetop winds from the west averaged less than less

than 15 mph. At the time of the avalanche, skies were partly cloudy and the temperature was about +2 C. Some wet snow "snowballing" was observed by the party involved in the avalanche, especially around sun exposed rocky cliffs.

Because they planned an early departure, the party had called the Avalanche Advisory during the evening of Wednesday, March 15th. The avalanche danger for this area on Wednesday was "Considerable on all recently wind loaded slopes steeper than 35 degrees and

Moderate on slopes steeper than 35 degrees without wind deposits". The avalanche danger for Thursday was "Moderate on all recently wind loaded slopes steeper than 35 degrees".

Members of the party had attended avalanche classes, carried all the appropriate avalanche rescue equipment, had skied and nowboarded in this area numerous times and were expert riders.

In this area, there was evidence of avalanche activity that had occurred on Tuesday, March 14th. There were no signs of avalanche activity that had occurred more recently than March 14th.

Several of the adjacent slopes had recent snowmobile "highmark" tracks, a boot track had been established in an adjacent gully, and there were snowboard tracks in that gully. A small

cornice was on the top of the chute in which the avalanche released. The aspect of the chute is southeast and there was evidence of recent wind loading. The wind had deposited a hard slab on the top 10 meters of the slope and a soft slab existed lower in the chute. These observations were made by one of the more experienced members of the group. This person had previously measured the slope angle at the top of the chute. It was 52 degrees. He estimated the slope angle at the crown face to be about 45 degrees. The avalanche ran about 250 vertical meters and the elevation at the ridgetop is 2800 meters. I measured the alpha angle and it was a whopping 38 degrees.

Summary:

Steep wind loaded slope, sunny warm day, decreasing avalanche danger, eager snowboarder, small avalanche, fast ride down a narrow rocky chute, not buried, broken back, successful rescue.