Anomalous winter snow amplified earthquake induced disaster of the 2015 Langtang avalanche in Nepal

Abstract

Abstract. Co-seismic avalanches and rock falls, and their simultaneous air blasts, which were induced by the 2015 Gorkha earthquake in Nepal, destroyed the village of Langtang. In order to reveal volume and structure of the deposit covering the village, and sequence of the multiple events, we conducted an intensive in-situ observation in October 2015. Multi-temporal digital elevation models created from photographs taken by helicopter and unmanned aerial vehicles reveal that the deposit volumes of the primary and succeeding events were 6.81 × 106 m3 and 0.84 × 106 m3, respectively. Visual investigations of the deposit and witness statements of villagers suggest that the primary event was an avalanche composed mostly of snow, while the contribution of collapsed glacier ice could account for a few percent of the total mass. Succeeding events were multiple rock falls which may have been triggered by aftershocks. From the initial deposited volume and the upper catchment area, we estimate an average snow depth of 1.56 m in the source area using density assumptions of snow and ice. This is consistent with anomalously large snow depths (1.28–1.52 m) observed at a neighboring glacier (4800–5100 m a.s.l.), which accumulated over the course of four major snowfall events since October 2014. Considering long-term observational data, probability density functions, and elevation gradients of precipitation, we conclude that this anomalous winter snow was an extreme event with a return interval of at least 100 years, which amplified or even caused the disaster induced by the 2015 Gorkha earthquake in Nepal.

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“…Large co-seismic events occurred during the 2015 Gorkha earthquake that caused numerous casualties in the Everest region (Bartelt et al, 2016). A multicomponent avalanche destroyed a village completely and killed more than 250 people in the Langtang catchment (Fujita et al, 2017;Kargel et al, 2016). These two recent events in the central Himalaya were avalanches that included not only snow but parts of hanging glaciers, debris, and underlying rock.…”

“…Similarly, poorly documented are heavy snowfall events or blizzards. For example, Cyclone Hudhud claimed multiple lives within a few days in 2014 and the huge amounts of accumulated snow possibly contributed subsequently to the large Langtang avalanche in 2015 triggered by the Gorkha earthquake (Fujita et al, 2017).…”

“…At least in the western Himalayan region, Ballesteros-Cánovas et al ( 2018) suggest, based on an extrapolation of reconstructed trends, that climate warming will continue to increase the hazard of avalanches, as warmer air temperatures in winter and early spring favour the more frequent occurrence of large, wet snow avalanches, which are able to reach populated valley bottoms. Devastating high elevation snow, or mixed snow-ice avalanches in Langtang, Nepal and Gayari, Pakistan (Fujita et al, 2017;Saif et al, 2022) highlight the particular threat of earthquakes occurring during periods of heavy snow loading. Extremely heavy precipitation events are expected to become more frequent in the future.…”

Quantification of different flow components in a high-altitude glacierized catchment (Dudh Koshi, Nepalese Himalaya)

“…This is particularly true for the glaciological year 2014-2015, when the ice melt contribution decreases from 60% with v0 to 41% with v3 and the snow melt contribution increases from 29% to 47%. This can be explained by the fact that 2015 is a particularly snowy year (Fujita et al, 2017), which highlights the importance of a correct representation of snow processes in the model. This also shows the need to use as much validation data as possible to assess the coherence between the ice, snow and hydrological processes and reduce the uncertainty on the flow components estimation.…”