Avalanche Incident

Official Reports

Bull River Avalanche Fatalitybr Denali National Park, April 25, 1998

Prepared May 1, 1998 by Doug Fesler and Jill Fredston,,BR. Alaska Mountain Safety Center, Inc.

Synopsis: On April 25, 1998, at approximately 3 p.m., snowmachiner Mark Slimmen of Fairbanks triggered and was killed by a hard slab avalanche while playing highmark on an unnamed peak in the headwaters of the North Fork of the Bull River in the eastern foothills of Denali National Park.

Report: As his two companions, Bob Tolman and Mark Springer, watched from a nearby knoll, Slimmen ascended the right flank of a SE facing bowl in an attempt to see how high he could climb with his snowmachine. When he reached a point approximately 200 ft. upslope and was in the process of turning his machine, the entire slope fractured and started to move. His companions said it appeared he was aware of his situation and tried to outride the avalanche by pointing his machine downhill and "gunning" it. Unfortunately, as he accelerated over the leading edge of the moving slab (a wave 8-12 ft. high), his machine was tumbled and he was tossed off. His friends observed him (from the torso up) trying to swim to stay on top, as the machine veered off to his right. When the gradient of the slope decreased abruptly from 37( to 17(, the debris piled up and Slimmen disappeared from sight along with his machine.

Slimmen's companions responded toward the debris, even as it was still moving. Not having any probes, beacons, or shovels, they searched the surface for clues, called out, and dug in likely places with their helmet visors. Unsuccessful in their efforts, they snowmobiled the 12-14 miles out to the highway and reported the accident to the Alaska State Troopers who, in turn, contacted the National Park Service (NPS).

That evening, at approximately 7:30 p.m., the NPS launched a helicopter to the site with two rangers. They searched the debris for visual clues and spot probed likely areas but did not linger due to deteriorating weather and poor visibility obscuring the starting zone. Not wanting to put rescuers at risk from secondary releases and unable to see the starting zone, the rescue was suspended until the weather cleared the next morning. Meanwhile additional rescuers (two Alaska Search and Rescue Dog dogs and handlers and four Alaska Mountain Rescue Group members, including two avalanche specialists) were called in from Anchorage along with approximately a dozen NPS personnel from the surrounding area. Once on site the next morning, these teams found the snowmachine within about one hour and had a positive probe strike of the victim about a half hour later. It then took more than an hour to dig the victim free. Both the snowmachine and the victim were completely buried in the toe of the debris.

Critical Data:

Terrain Factors:

• Elevation: +- 6,300 ft.;
• Aspect: Mainly southeasterly;
• Starting Zone Configuration: Generally planar, with some slight convexities;
• Lower Track and Runout Zone: Generally concave;
• Slope Roughness: Generally smooth and free of vegetation, scree evident in places, some undulation north of center;
• Path Width: +- 300 ft. (estimated);
• Path Vertical Drop: 350 ft. (estimated);
• Starting Zone Slope Angles: 37 to 39 degrees (measured);
• Runout Zone Angle: 17 degrees (measured);
• Runout Angle (Alpha angle): 29 to 30 degrees (depending upon location, measured);

Snowpack/Avalanche Factors:

• Type: Hard slab, human triggered;
• Classification: HS-AO-3-O/G;
• Debris Depth: Typically 7-12 ft. deep, with some areas estimated at 25 feet;
• Fracture Depth: > 6 inches to < 39 inches (estimated)
• Slab Depth: Average = 1-1.5 ft. (estimated); Fracture Length: ( 550 ft.;
• Slab Layer: Composed of 4 major storm layers interspersed with 6 distinct ice lenses, all well-bonded (see fracture profile) Note: Profile was done at the fracture line on southern side of path where the snowpack was deeper and is not necessarily representative of the shallower snowpack immediately above the victim. Subsequent investigation in the trigger zone revealed the presence of weak, fully developed facets which failed to the ground.
• Weak layer: Advanced faceted snow which likely failed in collapse and shear under the weight of the snowmachine and rider;
• Bed surface: Old snowmachine tracks visible. Depending upon location, intermediate-advanced facets, ground, or wind-affected snow surface.
• Instability Indicators: Numerous similar slides evident throughout the area; Air temperatures warmed considerably during daylight hours on this aspect, especially within the bowl area. Solar warming influence was evident in the upper snowpack layers (moist surface snow, buried sun crusts observed). Warming was important in that increasing flow stress may have also increased the elastic energy within the slab and as the slab weakened, there may have been less bridging of the weak layer;

Weather Data:

No data exists, but reconstruction from general weather knowledge of the area and snow stratigraphy indicates a three week period prior to the accident in which several significant precipitation/wind events occurred. These were interspersed with periods of warmer, clear weather during which ice lenses were formed. The day of the accident was warm (probably in the 50s F in the bowl), with broken clouds.

Victim Data:

• Sex/Age: Male, 34
• Burial Depth of Victim: 6.5-8 ft.
• Burial time: Approximately 19.5 hours (3 p.m. ,4/25 to ~10:30 am, 4/26)
• Burial Depth of the Snowmobile: 2-3 ft.
• Position of the Snowmobile: Upright, facing downhill diagonally, windshield ripped off.
• Body Position: Prone, face down, arms extended, perpendicular to fall line;
• Vital Signs: No vital signs, no ice mask (lens) or air pocket next to face other than a uniform inch airspace molded around upper torso/shoulders due to melting from body heat. Victim likely died almost immediately of suffocation;
• Victim Location: Found ( 25 ft. upslope from the machine (see figure 1);
• Snowmobile Location: Found midway +- 10 ft from the edge toe;
• Distance Victim Traveled from ALS (area last seen): +- 150 ft.

Rescue Response Data:

Locating the Victim: Three factors contributed to finding the victim relatively quickly:

• A witness was able to reconstruct the location of the victim when caught and the area where last seen (ALS);
• From the witness information and flow patterns, avalanche specialists were able to reconstruct the likely trajectory, identify the primary search area, and deploy rescuers to the appropriate area for searching;
• Well-trained avalanche SAR dogs, with handlers who knew how to read their dogs, identified two likely scent areas (the snowmobile and the victim), and persisted in having these areas probed and dug out. Both the machine and the victim were found by probing after dogs had alerted on the area. All teams worked well together.

Major Contributory Factors of the Accident:

• Prime-time slope angles for slab failure: 37 to 39 degrees;
• Sunny aspect: Caused weakening of the slab through solar warming during the preceding hours;
• Classic terrain trap: a steep slope ending in a depression where a deep burial is likely;
• Inexperience in evaluating snow stability or terrain from an avalanche perspective;
• An aggressive, risk-taking attitude;
• No rescue equipment or avalanche training.
• Possible alcohol-impaired judgement: 12 empty beer cans were found in the victim's snowmobile but it is unknown whether the victim had consumed any alcohol.

Other observations:

The fact that the other two snowmachiners were parked in a safe spot prevented all three men from being involved in the avalanche. The witnesses did an excellent job of watching the victim as he was carried downslide which later helped establish a primary search area.

Because the victim was at the leading edge of the moving debris, he was carried nearly to the toe of the debris.

In a large number of accidents involving snowmachiners, the victims have been found within 40 feet of their machines, often upslope. Slimmen's accident fits this pattern.

Highmark is a risky game as steep slopes are being approached from the bottom. Highmarking in concave bowls increases the probability of deep burials. Safety can be maximized by good timing (playing highmark during stable snowpack periods); choosing lower angle slopes, paths that have already released, or windward (and preferably planar) slopes over leeward terrain; climbing near the edge of a slope rather than centerpunching it to increase the chances of escape, and having only one snowmachiner at a time on the slope with all others watching from a safe zone.

[Figures not currently available.]

Figure 1: The Slimmen Avalanche (as viewed from the witnesses' location): Looking west from same spot the witnesses viewed the avalanche, the "X" marks Slimmen's approximate location when initially caught. Note Slimmen's entry tracks at lower right and the likely trigger zone (the bare spot to the left of the X), composed of weaker faceted snow. The "wave" mark shows the approximate location where he was tossed from his machine as the tried to overrun the leading edge of the moving debris. The "O" represents the ALS (area last seen). The "triangle" indicates Slimmen's approximate burial location and the "square", the approximate location of his buried snow machine. (Photo by Doug Fesler)

Figure 2: The Slimmen Avalanche (as viewed in profile): Looking N-E from the fracture line visible to the extreme left of the photo in Figure 1. Note the same bare trigger zone as seen in Figure 1 to the left of the X and the flow pattern of the debris (downslope from the trigger zone) which carried Slimmen nearly to the toe. The knoll to the right is the location from which his companions witnessed the avalanche and from which the photo in Figure 1 was taken. (Photo by Doug Fesler)

Media Reports

Man dies in Denali avalanche

Monday, April 27, 1998

By MAUREEN CLARK, The Associated Press

The body of a Fairbanks man was recovered Sunday after he was killed in an avalanche in Denali National Park and Preserve.

Mark Slimmen, 34, was riding his snowmachine in the park Saturday afternoon with two friends when he was swept 150 feet down a mountainside by an avalanche.

His body was found at about 10:30 a.m. Sunday, buried beneath almost 7 feet of snow and ice, park spokeswoman Jane Tranel said.

Slimmen had been riding along a mountainside, about 3,000 feet above the Bull River drainage near Broad Pass shortly after 2 p.m. when the avalanche began.

"He turned and the avalanche started behind him. His companions could see it as it happened. He was thrown from the machine and was able to swim through it at first but then was buried," Tranel said.

Slimmen's friends, Bob Tolman and Mark Springer, searched for him immediately after the avalanche but were unable to locate him. They traveled back to the Parks Highway, 13 miles east, and notified the Alaska State Troopers.

Troopers and National Park Service rangers flew to the area at about 6 p.m. Saturday night but were unable to locate Slimmen during a brief search. The search was postponed until Sunday morning because of unstable snow conditions, gusty winds and dwindling daylight.

The Alaska Mountain Rescue Group and Alaska Search Dogs assisted in the search.

Several inches of snow had fallen in the park recently and snowmachiners were out in droves over the weekend, Tranel said.

"This is a very dangerous time of year. Folks have to be really cautious. With temperatures warming up and more sunshine, we definitely have a high avalanche alert in the back country," she said.

Rangers are advising all back-country travelers to use caution and carry personal locator beacons, shovels and probes.

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