An earthquake-triggered avalanche in Nepal in 2015 was exacerbated by climate variability and snowfall anomalies

Large rock-ice avalanches, dangerous due to their high velocity and long runout, are exacerbated by climate change in earthquake-prone regions. While climate change's influence on mountain instability is recognized, its contribution to avalanche destructive potential, particularly runout and air blast formation, remains largely unquantified. This study focuses on the 2015 Langtang avalanche, triggered by the Gorkha earthquake. The research question is: How did climate-related phenomena, specifically snowfall anomalies and warm temperatures, influence the dynamics and destructive power of the Langtang avalanche and its associated air blast? This is important because understanding the interplay between climate variability and avalanche dynamics is crucial for improving risk assessment and mitigation strategies in high-altitude regions. The study's purpose is to reconstruct the avalanche event using field data and numerical modeling to quantify the impact of snow cover and air temperature on avalanche runout and air blast dynamics. The importance of the study stems from the need to incorporate climate-related factors into hazard assessments for high-altitude rock-ice avalanches, improving the accuracy and reliability of future predictions and reducing the risk to human lives and infrastructure.

Previous research has established the link between climate change and mountain instability in glacial and periglacial areas. Studies have documented changes in snowfall patterns in high-altitude regions, including decreased duration and increased intensity. The impact of thick snow cover on avalanche volume, lubrication, and powder avalanche formation has also been investigated. However, existing rock-ice avalanche models often treat the sliding mass as a thermally insulated system, overlooking the influence of ambient air temperature on meltwater production and flow dynamics. The Langtang avalanche (2015) serves as a compelling case study, with existing literature attributing its destructive power to snow cover anomalies. This study builds upon this existing knowledge by integrating field observations and numerical modeling to provide a more comprehensive understanding of the interplay between climate factors and avalanche dynamics.

The study reconstructs the Langtang avalanche's evolution through a combination of field investigations and numerical modeling using the RAMMS::RockIce model. Field investigations provided data on the avalanche's extent, deposit volume, and the impact on the Langtang village and surrounding forest. Meteorological data from nearby stations provided air temperature information. The RAMMS::RockIce model, a depth-averaged model, simulates the flow dynamics of the avalanche core and the generated air blast. The model incorporates processes such as entrainment of path materials and meltwater production. The avalanche core is described by a granular ensemble of rock, ice, and snow particles, allowing for dispersion and compression, influencing the interstitial air space and the interaction with ambient air. The air blast is modeled as a turbulent flow, including suspensions of ice and rock dust transferred from the avalanche core. The model uses well-established finite volume schemes and is calibrated to match field observations of avalanche volume and deposit areas. Numerical simulations were conducted with varying snow cover depths and air temperatures to investigate their individual and combined effects on avalanche dynamics and air blast characteristics. The snow cover was adjusted based on slope angle, curvature, and elevation gradient. The air temperature was varied at the Kyanjing meteorological station, with a constant gradient applied to account for elevation differences. The model includes equations for mass and momentum balances, dispersive movement, heat energy balance, and meltwater production. The heat transfer between the avalanche core and the ambient air is calculated using an experimentally based heat transfer relationship for sphere particles. Tree-breakage calculations followed the method proposed by Feistl et al. (2015), considering the dynamic pressure of the air blast and the strength properties of the tree species involved. The study specifically analyzed changes in runout and flow regimes under different snow cover and air temperature conditions, rather than the variability in avalanche occurrence due to climate change.

Modeling results indicate that a 3-meter snow cover at the release area, consistent with field observations, accurately reproduces the observed avalanche dynamics. This includes the avalanche's final deposit distribution, maximum velocity, and the extent of the air blast impact. Without snow entrainment, the avalanche would have stopped before reaching the opposite mountain. The air blast's impact area and dynamic pressure decreased significantly with thinner snow cover; with no snow cover, the damage would have been minimal. Regarding temperature, simulations showed that higher air temperatures (19°C at 3862 m a.s.l.) increased meltwater production more than twofold compared to colder temperatures (-1°C at 3862 m a.s.l.). This resulted in a more fluid-like flow regime in the Langtang Valley and a longer runout distance. The increased meltwater acted as a lubricant, reducing frictional resistance. The maximum water content in the avalanche core reached over 1800 mm m⁻³ in the warm scenario, significantly higher than the 600 mm m⁻³ in the cold scenario. The high dynamic pressure of the air blast, exceeding 15 kPa in the village, and reaching up to 28 kPa near a specific point, is consistent with the observed damage. The model accurately predicts the area of tree breakage, which matches field observations. Snow entrainment facilitates the formation of a dispersed powder avalanche, amplifying the air blast’s destructive potential. The warm air temperature intensifies heat exchange, enhances meltwater production, and further increases avalanche mobility. The combination of these factors significantly contributed to the catastrophic nature of the event.

The findings strongly support the hypothesis that both anomalous snowfall and warm temperatures contributed significantly to the Langtang disaster. The substantial snow cover increased the avalanche volume and acted as a lubricant, while the warm temperature enhanced meltwater production and reduced frictional resistance. The model's ability to accurately reproduce the observed avalanche dynamics and air blast characteristics confirms the importance of these factors in determining the avalanche’s destructive potential. The study highlights the need to consider these climate-related variables when assessing high-altitude rock-ice avalanche risk. The results underscore the complex interplay between geophysical processes and climate variability in shaping extreme events. Even seemingly small variations in temperature or snow cover can lead to significant changes in avalanche behavior and destructive potential. The study's findings are relevant for improving risk assessment and mitigation strategies in similar high-altitude regions and underscore the need for considering both geophysical and climatic factors for more accurate and robust risk assessment methodologies.

The 2015 Langtang avalanche, exacerbated by anomalous snowfall and warm temperatures, demonstrates the significant impact of climate change on high-altitude rock-ice avalanche hazards. The study’s findings emphasize the need to incorporate these climate-related factors into future risk assessments. The use of advanced numerical modeling in conjunction with field investigations provides a powerful tool for understanding and predicting these types of events. Further research should explore the long-term impacts of climate change on avalanche frequency and intensity, as well as refining the model to incorporate additional factors like particle size distribution and more sophisticated heat transfer models.

The study relies on a depth-averaged model, which simplifies the complex three-dimensional nature of avalanche flow. The heat transfer relationship is based on experiments with spherical particles, ignoring the complexities of real-world particle shapes and sizes. The model's accuracy depends on the quality and availability of input data, including meteorological data and digital elevation models. The study primarily focuses on the impact of snow cover and air temperature on the avalanche's dynamics, and does not address how climate change influences the frequency of avalanche occurrence. The study assumes the worst case for the calculation of the vertical profile of total air blast pressure; future research can improve this area.