Abstract
Abstract. Coseismic avalanches and rockfalls, as well as their simultaneous air blast and muddy flow, 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, as well as sequence of the multiple events, we conducted an intensive in situ observation in October 2015. Multitemporal 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 ± 1.54 × 106 and 0.84 ± 0.92 × 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 collapsed glacier ice could not be dominant source for the total mass. Succeeding events were multiple rockfalls which may have been triggered by aftershocks. From the initial deposit volume and the area of the upper catchment, we estimate an average snow depth of 1.82 ± 0.46 m in the source area. 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 between October 2014 and the earthquake on 25 April 2015. 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. The anomalous winter snowfall may have amplified the disastrous effects induced by the 2015 Gorkha earthquake in Nepal.
Highlights
The avalanches and rockfalls in Langtang Valley, about 70 km north from Kathmandu, resulted in one of the most remarkable tragedies in Nepal triggered by the 25 April 2015 Gorkha earthquake
By analyzing the differences between all digital elevation models (DEMs) with the differential GPS (dGPS) data in the off-debris area, we assess the accuracy of the DEMs
The mean deviations of the unmanned aerial vehicle (UAV) DEM (23 October) is ±0.26 m over the off-debris area and 0.34 m over the whole domain where elevation data are available
Summary
The avalanches and rockfalls in Langtang Valley, about 70 km north from Kathmandu, resulted in one of the most remarkable tragedies in Nepal triggered by the 25 April 2015 Gorkha earthquake. As well as their simultaneous air blast and muddy flow, killed more than 350 people and destroyed the village of Langtang (Kargel et al, 2016). Utilizing a set of high-resolution SPOT6/7 images, Lacroix (2016) estimated a deposit volume of 6.95 × 106 m3 over the village, with a larger amount of avalanche material deposited above 5000 m a.s.l. Differences between two digital elevation models (DEMs) created from images on 21 April 2014 and 10 May 2015 showed distribution of the deposit and destroyed forest on the opposite slope (Fig. 5a in Lacroix, 2016) with errors not exceeding 3 m over the unaffected terrain. The contributions and sequences of avalanches (snow and ice) and rockfalls are not yet fully understood, though some of the event was described in Kargel et al (2016)
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