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Avalanche Accidents on the Manti-La Sal National Forest

      Although many parts of the Manti-La Sal Forest are avalanche prone and numerous incidents occur each year, generally only fatalities are reported.

1991-2 SEASON:

Avalanche Accident Report, Utah Avalanche Forecast Center

February 12, 1992: Talking Mountain Cirque of Gold Basin in the Central Massif of the La Sal Mountains in San Juan County, Utah

Conclusions and Recommendations:

      I would like to first look at the four main factors which create an avalanche accident. first of all, you need to have weather (1.) which builds an unstable snowpack (2.), which rests on a slope that in steep enough to cause an avalanche (3.). Lastly, you need to have a person (4.) in the way of the avalanche. There is no avalanche hazard until people are involved. Avalanches do not happen by accident, but rather for particular reasons, in particular places, at particular times. With the benefit of 20-20 hindsight we can analyze these four factors and their role in this accident.

      (2.) The snowpack prior to 2/7 was weak throughout the La Sal Mountains, and forecasts from the La Sal Avalanche Forecast center reflect the knowledge of that fact. It was also known, and accurately forecast, that any additional load of new snow could raise the instability dramatically. The forecast of 2/8 stated that the critical amount of snow was 6", while on 2/10 the amount was 5".
      It appears that the rate at which new snow fell in the La Sals was deceiving. If the 10" of snow which had accumulated by 2/12 had arrived all at once, in one storm, instead of spread out over 6 days, this would have served as a red flag. As it was, the snowpack responded in the same way regardless of the rate of loading. Depth hoar does not respond to compressive stress by settling, but rather stores that stress until a critical level is reached and collapse occurs.       Collapse is one of the major indicators that a depth hoar snowpack has reached a critical level. The forecast of 2/11 notes that collapsing was already occurring, but the interpretation that the snowpack had not reached a critical level was incorrect. Collapsing of the snowpack was noted at several points along the ski tour, but apparently was not considered important or relevant to the slopes the group was planning to ski.
      There are other indications that the snowpack had reached a critical level. On 2/11 Jeremy Hopkins and Brett Sutteer had been skiing In Gold Basin and had experienced several large collapses of the snowpack. On the same day, 2/11, Hopkins had ski cut a short slope along the Gold Basin road, and this resulted in shooting cracks and a small fracture 12" deep and 10' long. The party skied past this slope on 2/12, arid the party also noticed a recent avalanche high on the north face of Mt. Tukuhnikivatz.
      The party did not dig any snowpits on 2/12, and after skiing a southwest facing slope into Gold Basin felt that the snow was stable. This southwest facing slope was later bombed with explosives on 2/14 and did not produce any avalanche activity, and in fact there was very little avalanche activity on any southeast, south, or southwest facing slopes. In many parts of the western U.S., south facing slopes have some of the stronger and more stable snowpacks because or the amount of solar radiation they receive. In the La Sal Mountains, south aspects are often wind scoured as the prevailing winds are from the south and west.
      One interesting comment that was made by Meleski is that as the party climbed into Talking Mountain Cirque the snow under the overlying the weak depth hoar. Snow that is quite hard on the surface is often mistaken as being stable, but it is not the strength of the strong layer that is critical but the weakness of the weak layer.
      The party felt some collapsing or the snowpack soon after starting up from lunch, yet did not experience other collapses as they progressed up the bowl. It is possible that the party interpreted this fact as meaninq that the collapsing was only occurring at the lower elevations, not in the wind-affected areas above timberline. This is sometimes the case, as strong winds effectively strip the weak snow away. However, in this instance the majority of the snow on the ground consisted or cohesionless faceted crystals, and the wind had only put a cap of wind slab over this weak snow.
      Collapsing is a potent mechanism for triggering avalanches, and is a progressive failure which travels through the snow in a process known as propagation. Collapsing of the snowpack occurs when the fragile substructure consisting of porous and cohesionless grains, is overloaded to the point of failure. The failure is essentially a catastrophic compression of the pore space that exists between grains. On an inclined slope, this collapse has a resultant downslope vector which initiates shearing and loads the crown region with high tensile stresses. On a steep enough slope, the collapse will be the initial failure which starts the slab avalanche in motion.
      A collapse travels through the snow as an advancing wave as adjacent regions are overloaded and collapse occurs. The rate of collapse propagation is not known, and is probably variable, but may exceed 100 feet per second. Collapses of the snowpack have been known to trigger avalanches at a distance of up to 1 mile from the initial point of failure. The party did experience a large collapse shortly before the avalanche started above them, and there is no question that this collapse triggered the avalanches that hit them and a third slide, which did not. There seem to be no other extraneous factors which could have triggered the slide. The party did have time to comment on the collapse, and given that the slides started up to 1000' feet away, this lag time would be explained by the time necessity for the collapse to travel to the starting regions.
      (3.) That the party was in avalanche terrain was known to most, if not all, of the members of the party. There are very few trees in Talking Mountain Cirque, and those that are in the cirque and along the base show the signs of avalanche damage. The Avalanche Terrain Master Map created by the La Sal Avalanche Forecast Center labeled all the terrain in Talking Mountain Cirque above the 11,000' level as having a high Avalanche Potential, while the base of the cirque down to 10,840' was labeled as having a Moderate Avalanche Potential. The party was at 11,300' when they were hit by the avalanche.
      The slope the party was on at the time of the avalanche was not remarkably steep. The slope angle for the first 1600' of the climb into the cirque was 25 degrees or less. Avalanches are uncommon on slopes of 25 degrees or less, while they are most frequent on slopes of 30 to 45 degrees. If the entire slope had been no steeper than 25 degrees all the way to the ridge, it is very unlikely that there would have been an avalanche. It is likely that the party felt ceertain that they would not trigger an avalanche on a slope no steeper than 25 degrees, and they were not intending to go up onto the steeper slopes above them where the forecast for 2/11 accurately predicted the hazard was high. What the party did not realize is that they were connected to those slopes above them via the snowpack. It is likely that the party knew that avalanches could run to their position but they did not expect that they could trigger a slide at such a long distance. The colllapse they caused traveled through the snow to release the avalanches on the steeper slope above.
      Far more important than the angle of the slope you are standing on is the angle between you and steeper slopes above. This angle, which indicates the distance an avalanche can be expected to run, is called the alpha angle. Typical alpha angles are in the low to mid 20's, and in this case the alpha angle from the starting zone to the toe of the debris was calculated to be 26 degrees.
      Because the party was on a low angle slope they were able to climb directly uphill without the need for switchbacks. This meant that the party members were nearly in a vertical line when the slide hit. The slide was not large relative to the whole bowl, but did not have to be to catch all of the party. It probably would not have mattered if there had been more distance between the members because they would still all have been in the same trajectory.
      The question remains of what exactly triggered the collapse of the snowpack. Two answers are possible: either the person in the lead or the group of three skiers close together. It is unlikely that the second or last skiers would have triggered the collapse if they were climbing in the same track as the preceding party members. Likely either Meleski, in the lead, or the group of three of Turk, Hopkins, and Loveridge, with their combined stress on the Snow in a small area caused the collapse.
      Traveling safely through large avalanche-prone areas is a difficult process, made more so by larger groups. Often even if you are spread out you are still in the same avalanche path. One of the most basic rules of thumb is to only expose one person at a time, but as the terrain size, and the group size, increases the problem becomes complex. Perhaps it would have been possible for one party member to remain at the base or the slope while the others climbed into the cirque. At least this would have left one person to initiate the search.
      Incredibly, all six party members were buried, with no one to do the search. By the time Meleski had himself dug out, nearly 5 minutes had elapsed, and it took Meleski another l5-25 minutes to uncover Yates from 5' of snow. Bigler and Meleski were familiar with the use of their beacons and were proficient, yet by the time they had reached the others they had expired.

1997-8 SEASON:

1998-9 SEASON: