Articles | Volume 17, issue 11
Nat. Hazards Earth Syst. Sci., 17, 1907–1921, 2017
Nat. Hazards Earth Syst. Sci., 17, 1907–1921, 2017

Research article 13 Nov 2017

Research article | 13 Nov 2017

Multiple remote-sensing assessment of the catastrophic collapse in Langtang Valley induced by the 2015 Gorkha earthquake

Hiroto Nagai1, Manabu Watanabe2, Naoya Tomii1, Takeo Tadono1, and Shinichi Suzuki1 Hiroto Nagai et al.
  • 1Space Technology Directorate I, Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki, 305-8505, Japan
  • 2School of Science and Engineering, Tokyo Denki University, Ishizaka, Hatoyama-machi, Hiki-gun, Saitama, 305-0394, Japan

Abstract. The main shock of the 2015 Gorkha Earthquake in Nepal induced numerous avalanches, rockfalls, and landslides in Himalayan mountain regions. A major village in the Langtang Valley was destroyed and numerous people were victims of a catastrophic avalanche event, which consisted of snow, ice, rock, and blast wind. Understanding the hazard process mainly depends on limited witness accounts, interviews, and an in situ survey after a monsoon season. To record the immediate situation and to understand the deposition process, we performed an assessment by means of satellite-based observations carried out no later than 2 weeks after the event. The avalanche-induced sediment deposition was delineated with the calculation of decreasing coherence and visual interpretation of amplitude images acquired from the Phased Array-type L-band Synthetic Aperture Radar-2 (PALSAR-2). These outline areas are highly consistent with that delineated from a high-resolution optical image of WorldView-3 (WV-3). The delineated sediment areas were estimated as 0.63 km2 (PALSAR-2 coherence calculation), 0.73 km2 (PALSAR-2 visual interpretation), and 0.88 km2 (WV-3). In the WV-3 image, surface features were classified into 10 groups. Our analysis suggests that the avalanche event contained a sequence of (1) a fast splashing body with an air blast, (2) a huge, flowing muddy mass, (3) less mass flowing from another source, (4) a smaller amount of splashing and flowing mass, and (5) splashing mass without flowing on the east and west sides. By means of satellite-derived pre- and post-event digital surface models, differences in the surface altitudes of the collapse events estimated the total volume of the sediments as 5.51 ± 0.09  ×  106 m3, the largest mass of which are distributed along the river floor and a tributary water stream. These findings contribute to detailed numerical simulation of the avalanche sequences and source identification; furthermore, altitude measurements after ice and snow melting would reveal a contained volume of melting ice and snow.

Short summary
We demonstrated an assessment of the sediments caused by a catastrophic avalanche, induced by the main shock of the 2015 Gorkha Earthquake in Nepal. A Japanese space-borne sensor, PALSAR-2, have a high potential for delineating the hazardous zone. Comparison of pre- and post-high-resolution topographic data estimates the avalanche-induced sediment volume as 5.51 × 106 m3. High-resolution satellite imagery revealed that it has multiple layers of sediment with different physical properties.
Final-revised paper