Tuesday 9 February 2021

Looking for an explanation for the 1959 Dyatlov Pass Incident.

During the night of February 1, 1959, nine Russian hikers died under unexplained circumstances during a skiing expedition in the northern Ural Mountains. The group had decided to set up their camp on the slope of the Kholat Saykhl; the name means 'Dead Mountain' in the local Mansi language. Something unexpected happened after midnight that caused expedition members to cut the tent suddenly from the inside and escape towards a forest, more than 1 km downslope, without appropriate clothes, under extremely low temperatures (below −25°C), and in the presence of strong katabatic winds induced by the passing of an arctic cold front. Twenty-six days to three months after the tragedy, search teams found bodies in the forest and on their way back to the tent. According to the 1959 Soviet criminal investigation, 'a compelling natural force' led to the death of the Dyatlov group. However, the nature of this force has not been identified. The mystery arises from numerous unexplained observations. While hypothermia was determined to be the main cause of death, four hikers had severe thorax or skull injuries, two were found with missing eyes and one without tongue; some were almost naked and barefoot, traces of radioactivity were found on some of their clothes, and signs of glowing orange spheres floating in the sky were reported that night.

Several theories have been proposed to explain this incident, including infrasound-induced panic, animals, attacks by Yetis or local tribesmen, katabatic winds, a snow avalanche, a romantic dispute, nuclear-weapons tests, etc. The originally popular avalanche theory has been questioned due to several contradictory pieces of evidence: (1) no obvious signs of an avalanche or debris were reported by the search team that arrived 26 days later, (2) the average slope angle above the tent location was not sufficiently steep for an avalanche (lower than 30°), (3) the hypothetical avalanche released during the night, at least nine hours after the cut was made in the slope, and (4) the thorax and skull injuries were not typical for avalanche victims.

In 2015, the Investigative Committee of the Russian Federation re-opened the investigation and in 2019 concluded that a snow avalanche was the most probable cause of the accident. The results of this investigation have been challenged recently by the office of the Prosecutor General of the Russian Federation, which in 2019 started its own investigation and in July 2020 came to the same conclusion as Investigative Committee of the Russian Federation. Both investigations have not, however, disclosed scientific explanations for the four counterarguments listed above and therefore keep being challenged by the relatives, public, and researchers. In particular, a 2019 Swedish-Russian expedition disagreed with the Investigative Committee of the Russian Federation conclusions, instead proposing that the direct impact of katabatic winds on the tent was the main contributing factor.

Based on the significant amount of published material, it seems that previous investigations lack an important ingredient: a quantifiable physical mechanism that can reconcile the avalanche hypothesis with seemingly conflicting evidence. Identifying such a mechanism may provide new insights into the nature of stormtriggered snowpack instabilities.

In a paper published in the journal Communications Earth & Environment on 28 January 2021, Johan Gaume of the School of Architecture, Civil and Environmental Engineering at the École polytechnique fédérale de Lausanne, and the Institute for Snow and Avalanche Research, and Alexander Puzrin of the Institute for Geotechnical Engineering at Eidgenössische Technische Hochschule Zürich, show that, even though the occurrence of an avalanche at this location is unlikely under natural conditions, the combination of four critical factors allowed the release of a small snow slab directly above the tent. 

Pictures taken at the tent location shortly before and weeks after the incident, and a sketch of the slope before the incident. (a) Last picture of the Dyatlov group taken before sunset, while making a cut in the slope to install the tent. (b) Broken tent covered with snow as it was found during the search 26 days after the event. Gaume & Puzrin (2021).

These factors include (1) the location of the tent under a shoulder in a locally steeper slope to protect them from the wind, (2) a buried weak snow layer parallel to the locally steeper terrain, which resulted in an upward-thinning snow slab, (3) the cut in the snow slab made by the group to install the tent, (4) strong katabatic winds that led to progressive snow accumulation due to the local topography (shoulder above the tent) causing a delayed failure. Furthermore, the possible construction of a parapet above the cut (a classical safety procedure to protect the tent from the wind) could have accelerated the failure process. The proposed physical mechanism couples the onset of dynamic shear-fracture propagation in the weak snow layer with wind-induced snow transport. Provided a realistic wind deposition flux, our model shows that the conditions for avalanche release can be met after a delay of 7.5 to 13.5 hours from the moment the hikers made the cut in the slope, in agreement with the forensic evaluation of the time of death. Dynamic avalanche simulations suggest that even a relatively small slab could have led to severe but non-lethal thorax and skull injuries, as reported by the post-mortem examination.

Configuration of the Dyatlov tent installed on a flat surface after making a cut in the slope below a small shoulder. Snow deposition above the tent is due to wind transport of snow. Gaume & Puzrin (2021).

The mountain slope at the location of the tent is highly irregular. Around 100m above the tent, there is a shoulder which separates a rather flat plateau and a steeper slope below. This slope consists of 4–6 m high steps and the tent was installed below one of them, where it was easier to make a cut in a locally flatter slope. The choice of the tent location was also likely driven by the fact that the larger scale shoulder would protect them from the strong winds. In reality, as Gaume and Puzrin show, this choice of location could have contributed to the accident: small scale topographic variability resulted in a locally steep weak snow layer while the larger shoulder contributed to significant wind-driven snow accumulation above the tent, eventually leading to an instability.

Schematic of the Dyatlov group’s tent installed for heavy weather and high winds i.e. deepened down the slope with a parapet for wind protection. Evgeniy Buyanov in Gaume & Puzrin (2021).

Major arguments against the avalanche hypothesis include insufficient signs of the occurrence of an avalanche (no apparent deposit or crown fracture) and the relatively mild slope (about 23°). It appears, however, that the ground surface in the vicinity of the estimated tent location was steeper (up to 30°) than the average snow slope. Subsequent snowfalls smoothed the irregular terrain to the observed average 23° slope. This implies directly that the slab above the layer of depth hoar was thinner uphill, which has three main consequences: (1) It results in the average slope of the buried weak layer being about 28°, increasing the probability of slab release. (2) It reduces tensile support at the top of the slab, considerably limiting the avalanche volume. (3) It makes it more difficult to trace avalanche signs, especially 26 days after the incident.

Local terrain in the Dyatlov Pass, reported snow dynamic friction values, and typical slope angles in Human-triggered avalanches.(a) Terrain map of the Dyatlov Pass, indicated by the red circle, blue triangle and black crosses indicate locations of the tent and of the hikers’ bodies, respectively. (b) Slope angles reported for 139 accidentally triggered avalanches. (c) Dynamic friction-angle distribution. Green, faceted types of crystals (depth hoar, faceted crystals, rounded facets, and surface hoar); black, new snow, decomposed and fragmented crystals, and rounded grains. Gaume & Puzrin (2021).

While a 28° slope may still be perceived as being too mild to cause an avalanche, based on the often-quoted and implicit rule of thumb that a minimum of 30° is required, in reality, the critical slope angle can be as low as 20°, provided the dynamic friction angle (sometimes called crack-face friction) of the snow is sufficiently low. In fact, field measurements have shown that the dynamic friction angle of snow can be as low as 15°, especially for very low temperatures. In particular, the buried crystals of depth hoar, which were reported by the investigation on the site, tend to exhibit rather low friction values.

(top) Snow cover (blue) and terrain (red) profiles near the tent. (bottom) zoom of the terrain profile above the tent. Evgeniy Buyanov in Gaume & Puzrin (2021).

In spite of the fact that the 28° inclination of the buried weak layer was higher than the angle of friction, the snowpack did not fail for at least nine hours after the slope was cut, due to cohesion in the weak layer. In principle, overcoming cohesion does not necessarily require additional loading. Recent work on delayed snow avalanches has proposed a release mechanism driven by rate-dependent processes in the snow slab and the weak layer, which can develop under constant loads. It cannot, however, accommodate a nine-hour delay, due to the relatively short extent of the slab. It follows that the true mechanism must involve additional loading of the slope. Given the extremely low temperatures and strong katabatic winds, it is unlikely that anyone would have climbed above the tent during the night, disturbing the weak layer. In the absence of significant snowfall, the only way to accumulate additional load is through wind transport.

Snow accumulation above the tent resulted from katabatic winds and the presence of a shoulder located above the tent. Possible construction of a small snow parapet by the Dyatlov group (classical safety measure for snow camping), could contribute to additional loading.

Gaume and Puzrin present an analytical model for a thinning snow slab gradually loaded by wind-transported snow above the cut in the slope, which evaluates the wind deposition flux necessary to reproduce the forensic estimate of delay. Subsequent numerical modeling confirms that the observed injuries of the Dyatlov group members are consistent with the failed-slab dynamics.

In addition to explaining the delay, Gaume and Puzrin's proposed mechanism provides the pre-failure slab geometry, which can help in understanding how a relatively small slab caused the severe but non-fatal injuries reported. Gaume and Puzrin address this question by combining a novel numerical model with existing data for Human-thorax injuries from impact tests performed by the automotive industry.

Three-dimensional numerical simulations based on the Material Point Method and finite-strain elastoplasticity show that this small-slab avalanche impacted the hikers lying on the tent floor and filled the excavated space but did not have a significant runout, consistent with the reported lack of clear avalanche signs. The predicted length of 5.0 m for tensile failure of the slab is in agreement with the analytical model. The simulated snow slab reached a velocity around 2 m per second upon impact. At this velocity, an impact on a Human thorax of a typical snow block with a volume of 0.125–0.5 m³ and density 400 kg/ results in a maximum thorax deformation between 28% and 34%, corresponding to the lower range of values reported from crash tests for a 10 kg mass impacting the thorax at a speed of 7 m per second. According to the Abbreviated Injury Scale, these deflections would mostly lead to non-fatal thoracic injuries from moderate to severe, in agreement with the autopsy report of the Dyatlov-incident criminal investigation. Such injuries are not usually observed in avalanche victims, because impacts rarely occur against stiff obstacles. In the Dyatlov case, the victims were trapped between the falling slab and the tent floor, which was placed on compacted snow reinforced by skis.

Material Point Method simulation of the dynamics of a snow-slab avalanche and its impact on a Human body. Blue snow particles represent regions in which the yield condition has been met. The inset shows a simulated (3D Material Point Method) thorax deflection (normalized by an original thorax height of 20 cm) for a crash test with a 10 kg rigid mass impacting the thorax at a 7 m per second velocity (black curve) and for impacts by 0.125, 0.25, and 0.5 m³ elastoplastic snow blocks of 400 kg/m³ density and velocities of 2 m/s (orange dashed–dotted curve, red dashed curve, and red solid curve, respectively). Injury levels correspond to the Abbreviated Injury Scale. Gaume & Puzrin (2021).

Significant progress in snow and avalanche research over the past two decades has allowed better understanding of avalanche dynamics and of the processes related to snow-slab avalanche release. Nevertheless, no mechanism similar to the one inspired by the Dyatlov mystery has been explored in the literature, and its physical quantification required new theoretical developments.

Spatial distribution of the velocity norm before the impact of the wind snow slab. This figure also illustrates the typical sizes of snow blocks before the impact. Inset: maximum value of the velocity norm of the wind snow slab as a function of time during the Material Point Method avalanche simulation. Gaume & Puzrin (2021).

In Gaume and Puzrin's analytical model, these developments include a snow slab with a spatially variable thickness and its evolution due to sintering of the wind-transported snow, which affects the instability of a buried weak snow layer. This is highly relevant for the study of natural storm-triggered slab avalanches because its application is not limited only to wind-blowing snow events but can also account for additional loads due to a snowfall. The variable slab geometry resulting from irregular local topography and the cut made in the slope play a critical role in determining whether or when an instability will occur. Gaume and Puzrin's simulation of the impact of a snow avalanche on a Human body constrained by an obstacle combines advanced elastoplastic constitutive models with large-deformation dynamic numerical analysisand biomechanical modeling of the Human body. This opens new perspectives for research on the effects of snow avalanches on Human health and safety.

Animation of the Dyatlov Pass avalanche. Gaume & Puzrin (2021).

Needless to say that Gaume and Puzrin's models are based on a number of assumptions, which can be justified for this particular case study and relaxed for future research. For example, given the very low reported temperatures, Gaume and Puzrin assumed a brittle behavior for the weak layer which allows neglecting the effects of the process zone. Moreover, weak layer volumetric collapse did not have to be accounted for in our approach because this layer remains completely intact before the onset of instability. Furthermore, the analytical model assumes a 2D geometry which in this case can be justified by the fact that the length of the shoulder controlling snow deposition is much larger than the length of the tent. Yet, the 2D profile of the deposited snow has been simplified for the sake of obtaining a closed-form solution. An important source of uncertainty lies in the dependency of the wind deposition flux on the average wind velocity. The available research shows a very wide range of measured deposition fluxes for a relatively narrow range of the average wind velocity. Nevertheless, the range of wind velocity back-calculated using the analytical model and the forensic estimation of the delay is in good agreement with the range reported in nearby weather stations during the night of the accident.

Concerning the numerical models, because Gaume and Puzrin's main focus was the global thorax response, the skeleton and individual ribs were not analysed. Focusing on the thorax impact, the Material Point Method simulation is initiated at the onset of slab release and the weak layer is not explicitly modeled. In spite of these simplifications, both the analytical and numerical models independently predicted a similar size of the failed slab, providing additional validation for the new mechanism. In addition, while our simulations show that in principle, the observed injuries could have resulted from the avalanche impact, the impact-induced deformations of the thorax would be rather sensitive to the size of the disintegrated slab blocks and thus to the relative positions of the bodies with respect to the cut and slope direction. Given this uncertainty, it is also possible that the thorax injuries were the result of a later snow impact in a very steep ravine where the bodies of the victims, escaping the avalanche area, were found.

Solving the Dyatlov Pass mystery is an enormous task, which is far beyond the scope of Gaume and Puzrin's paper. Gaume and Puzrin hope, however, that their work may contribute to determining the plausibility of the avalanche hypothesis. More importantly, it allows the quantification of conditions that can help to prevent similar incidents. Clearly, for a cut in the snow slope to cause a delayed slab release, it requires a relatively rare combination of: (1) a sufficiently steep, weak layer at the base of the snowpack, (2) a cut in the slope, and (3) significant snow accumulation after the cut due to wind transport. However, once these conditions are present, the occurrence of a delayed release requires fairly common values of geometrical and mechanical parameters, and Dyatlov-related investigations have indeed reported a non-negligible number of similar accidents. This implies that building a tent even on a relatively mild slope (less than 30°) can be dangerous and should not be recommended when combined with a cut in the slope. Instead, digging a snow cave may be a safer solution, as confirmed by the increasing use of this practice for winter camping in recent decades. 

In conclusion, Gaume and Puzrin's work shows the plausibility of a rather rare type of snow slab instabilities that could possibly explain the Dyatlov Pass incident. Yet, Gaume and Puzrin do not explain nor address other controversial elements surrounding the investigation such as traces of radioactivity found on the victims’ garments, the behavior of the hikers after leaving the tent, locations and states of bodies, etc. While possible explanations are given in multiple published sources as well as by both the Investigative Committee and the Prosecutor General of the Russian Federation,  Gaume and Puzrin believe that this will always remain an intrinsic part of the Dyatlov Pass Mystery.

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