[H-DS10] Geohazards in humid, tectonically active countries and their precursors

[HDS10-11] Characteristics of slope disasters triggered by sudden topographic deformation in the Himalayan region

*Hiroshi YAGI¹, Daisuke Higaki², Ching-Ying Tsuo³, Akihiko Wakai⁴, Yusuke Yamamoto⁵ (1.Faculty of Art, Science and Education, Yamagata University, 2.Nippon Koei Co.,Ltd., 3.Faculty of Agriculture and Life Science, Hirosaki University, 4.Graduate School of Polytechnics, Gunma University, 5.Geoscience Research Laboratory Co.,Ltd.)

Keywords:Rock slide, rock avalanches, rock flour, debris flow

Debris flow occurred in Chomoli district, the watershed of Alakananda river. It’s source area is located on steep rock slope of high relief near Mt. Nanda Ghunti, it’s altitude is ranging 5200-5600m a.s.l. It initiated as rock slide at first and then turned into rock avalanches and debris flow, considering from distribution of Rock Flour due to fracturing of the rock mass slid on steep slope over 35 degrees. Same phenomenon accompanying Rock Flour due to rock slide occurred in the Seti river watershed in 2012 and Langtang Himal area triggered by Nepal Earthquake 2015. Their source areas are located in high relief and high altitude areas in Nepal Himalayas and their mass-movement type are rock slide. Rock avalanches traveled very long distance over 50km.
There distributed many remnant of landslides and landslide dams in Manag area behind Annapurna Himal, Nepal, the upper reach of Marsyandhi river. And very thick debris deposit over 100m are distributed along the middle course of Marsyandhi river in the southern mount foot area of Annapurna Himal, forming higher terraces of the last glacial age. That implies debris flow deposits which were turned from rock slide and rock avalanches reached to the the Midland area traveled long distance across the Himalayan range in the last glacial age.