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A Field Investigation on the Catastrophic Azijue Debris Flow in Eastern Margin of Tibet〇Kongming YAN, Fawu WANG, Gonghui WANG

Introduction

To understand the triggering mechanisms, long-The Azijue debris flow, with an estimated volume ofrunoutmovement,

andmulti-stage

depositing

3.2 million m3, occurred at about 20:00 on 30 Augustmechanisms of the Azijue debris flow—a chained2020

geohazard—ain

GanluoCounty,SichuanProvince,

detailed

fieldinvestigation

wassouthwestern China, as shown in Fig. 1. The debris

conducted using an unmanned air vehicle (UAV) andflow dropped from 2,983 m a.s.l. to 770 m a.s.l. at

remote sensing technologies. During the investigation,

downstream part of the Niri River, travelling 2,213 m

wevertically and 8,700 m horizontally. It killed 3 persons,

topographical features, and precipitation data for theburied 2 villages with over 30 houses and 2 bridges,

debris flow event, as well as detailed photographs anddamaged a local middle school, and formed a barrier

descriptions of the debris flow gully. The aim of thisdam in the Niri River. Meanwhile, the rapidly flowing

articledebris mass interrupted Chengdu-Kunming Railway,

environmental effects on large-scale geohazards and to

an important transportation line in southwestern China,

support mitigation strategies.

for about 14 days.

Geological settings and precipitationcollected

istoregionalimprovegeologicalunderstanding

settings,

ofgeo-Ganluo, a county on the Tibet’s eastern margin which
is a transition zone from the low-altitude Sichuan Basinto the high-altitude Tibetan Plateau, is tectonicallyactive and surrounded by several active fault zones.
The densely distributed active faults have contributed
to many high-intensity earthquakes around the study
area, including five major ones (Mw > 7.0) and ninelarge ones (Mw = 6.0~6.9). The formations observed
along the investigated debris flow gully were theKaijianqiao (Zak), Suxiong (Zas) and Ebian (Pt1eb)
Formations, consisting of sedimentary, igneous, andmetamorphic rocks, respectively
Ganluo has a typical subtropical moon climate, withan average yearly rainfall of about 880 mm. However,the distribution of seasonal precipitation is uneven, as
it is dominated by tropical marine air masses from theFig. 1 Pre- and post-hazard overview of the final
deposition area. Images (a) and (b) were provided byresidents, whereas image (c) was collected using a UAV
camera.

Indian Ocean during the hot summers, and by polar
continental air masses during the dry, cold winters(Chen et al. 2020). Moreover, the precipitation inGanluo has obvious difference on temporal and spatial

shows a ‘V-shape’ in the narrow, steep, and curved

distribution. Most rainstorms occur at night comparedsegments, as at S-1 and S-5, with a slope of 18°. It

to the daytime, and concentrates on top of mountain but

shows a ‘U-shape’ in the wide, gentle, and straight parts,

not at the lower valley.

like at S-3, S-4 and S-6.

Fig. 2 Hourly rainfall from 30–31 August 2020 at LHstation.

Results and discussions
The Azijue debris flow had a long movement pathfrom its source area to the final deposition area, asshown in Fig. 3. Overall, according to the longitudinal
profile of the gully (Fig. 3), the flow path had a smallapparent friction coefficient of 0.25, representing agentle apparent friction angle of 14.3°. In this case,there was a long transition zone, from point A to point
C, with a straight and steep gully floor. Following it, acomplex deposition zone consisted of two subhorizontal platforms (points C to D and points D to E),
a major secondary landslide gully (points E to F), anda final deposition area in the Niri River (points F to G).
In this transition zone with an average slope of 20°, thedebris flow originated from a source area with a slopeof 31°, which gradually decreased to 8° at the end of
the zone (point C). Within the upper part of thetransition zone, a normal fault cut the gully 2,100 mhorizontally from the source area (point A). Following
the steep transition zone (average 20°), the gentler
deposition zone (points C to G, average 7°) included
two sub-horizontal platforms (points C to D and points
D to E, 3°) and the final deposition area (points F to G,1°). The two platforms, connected by a naturallandslide dam (point D, 18°), were separated by anarrow gully (points E to F, 16°) from the final
deposition area. A cross section of the debris flow gully

Fig. 3 Longitudinal profile of the Azijue debris flow path.
The geological formation was estimated according togeological information provided by Li et al. (2019): (a)location map of the longitudinal profile; (b) longitudinal
profile.

Acknowledgements
The research was also collaboratively completed by Dr.
Gang Fan, Dr. Yang Liu, Dr. Kounghon Nam, Mr. Bo
Zhang, Mr. Qiuling Li and Zichen Shu. The detailed
results have been published on the journal of Landslides(https://doi-org.kyoto-u.idm.oclc.org/10.1007/s10346022-02022-2).

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