Detailed Accident Report

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Date: 2008-12-29
Submitted By: JHMR
Place: The Headwall; Jackson Hole Mountain Resort
State: WY
Country: USA
Summary: 7 patrollers involved, 4 buried, all rescued



Date: 2008-12-29

Submitted By: Jackson Hole Mountain Resort

Place: The Headwall; Jackson Hole Mountain Resort

State: WY Country: USA

Fatalities: 0 Activity: SKI

Summary: 7 patrollers involved, 4 buried, all rescued

Location:

The Headwall is located above the Bridger Gondola Area of the Jackson Hole Mountain Resort on Rendezvous Mountain. This area is not accessed by a ski lift. Access is attained by a boot packed trail maintained by the ski patrol.

History:

Conditions are monitored for the development of problem layers as soon snow begins to accumulate on the Headwall in October. Avalanche hazard mitigation began on December 6th, when critical avalanche starting zones were boot packed in an attempt to disrupt November rain crusts and weak faceted layers above and below these crusts. In the ten days leading up to this incident avalanche hazard reduction routes on the Headwall were run on December 19th, 20th, 22nd, 23rd, 25th, 26th and the 28th. These efforts included the deployment of hand charges, execution of ski cuts and the deployment of avalauncher rounds on the 25th and Howitzer rounds on the 25th and 26th. Avalanche hazard reduction routes were run on the Headwall and other areas of the resort on the morning of the 29th. The initial round of avalanche reduction work produced a slide on the lower portion of the Headwall which deposited up to 8 feet of debris against the southwest wall of the Bridger Restaurant building.

Accident and Rescue Summary:

Due to the number and magnitude of avalanches on the initial round of avalanche hazard reduction routes, avalanche hazard reduction leaders decided it was necessary to perform a second round. At this time all guests standing by, inside the restaurant awaiting the work to be completed, were sent back down the mountain via the Bridger Gondola. After the first round of avalanche hazard reduction routes had been completed on the Headwall, clearance was given to a standby crew of two waiting on top to proceed with a four pound air blast on the area known as “The White Spider”. The White Spider avalanche path impacts the uphill boot pack access trail and does not run to the restaurant building. At 9:32, the standby crew announced this charge had been ignited. All employees had been cleared from the outside of the building except for four ski patrollers on the south corner of the building and two patrollers who were about 20 feet directly downhill of the building. A seventh patroller was inside the ski patrol station in the base of the restaurant building. When this air blast detonated it remotely triggered a large slab avalanche in the Shot 18 area of the Headwall. The eastern flank, closest to the point of detonation, was 200 feet away . This slab ran downhill towards the restaurant. One of the four patrollers near the corner of the building then observed a large volume of avalanche debris rapidly approaching the building and yelled. These patrollers ran along the downhill side of the restaurant building under an enclosed deck. Avalanche debris wrapped around the southwest side of the building entraining patio furniture into the debris and caught these four patrollers. One was buried up to his neck. The other three were buried but were able to clear a space around their faces on their own. The two patrollers downhill of the building also ran and were knocked over but not buried. The avalanche debris covered the entrance to the ski patrol station, trapping the seventh patroller inside.

The burial of the four patrollers on side of the building was witnessed by other employees. They were immediately dug out with very minor injuries. First notification of the avalanche came at 9:35 from one of the partially buried patrollers. A phone call confirmed that the patroller trapped in the building was unharmed. He was later evacuated from that portion of the building by other members of the patrol through an interior wall. Rescue personnel knew that all public had been sent down the gondola prior to the avalanche. Actions were immediately undertaken to determine if there were any missing employees. Meanwhile, transceiver, visual, and spot probe searches were ongoing. At 10:00, a transceiver search was complete and no other signals were found. By 10:14, the first avalanche dog was working the scene, and eight minutes later two more dogs and a Recco device were added to the search. Several small probe lines were assembled, searching likely areas. By 10:35, all area employees had been accounted for. At 11:25, after the scene was cleared by the dogs and the Recco, and after final checks were made on all employees, the site was declared clear.

Avalanche Data:

The Headwall is a broad avalanche path with multiple starting zones. This avalanche released from an area of the Shot 18 area of the Headwall. The starting zone of this avalanche had a southeast aspect and an average slope angle of 38 degrees. The elevation of the crown was 9,610 feet above sea level. The slide descended approximately 690 vertical feet to an elevation of 8,920 feet. The track of the avalanche was approximately 1750 feet long, 200 feet wide at the crown, and 275 feet wide at the toe. The depth of the crown face varied from three to five feet, with an average depth of four feet. In most areas, the bed surface was a rain crust. Failure of the slope most likely occurred on faceted snow above this crust and below a subsequent rain crust. The avalanche, classified as HS-AB-R2/D3-O, deposited debris to a depth of 10 to 20 feet.

Weather and Snowpack History:

In November 2008, there were two storms that created thick rain crusts up to an elevation of at least 10,500 feet. The first of these events occurred on November 13th, and the second occurred on November 29th. Weak layers of faceted snow developed above, between and below these slick crusts. During the first 21 days of December, 40 inches of snow containing 2.9 inches of moisture fell at the Raymer snow study plot (9,360 feet) at the Jackson Hole Mountain Resort. In the eight days prior to the incident 63 inches of snow containing 4.5 inches of moisture fell at this plot, including 5 inches of new snow containing 0.95 inches of water on the day of the event. Significant warming also occurred prior to this incident with a 30 degree Fahrenheit increase in the 48 hours leading up to this event. This scenario loaded a snowpack with persistent deep instabilities and initiated a deep slab avalanche cycle in the backcountry and in the resort that began on December 22nd. Avalanche hazard reduction efforts at the resort on the morning of this incident produced five class III avalanches and four class IV avalanches. Based on these results the avalanche hazard reduction leaders decided not to open the resort and to conduct additional avalanche hazard reduction efforts. The general avalanche hazard in the backcountry was high on December 29th.