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A new research report on Landslide Dam Events of YarlungZangbo River issued by our lab ----2018-10-24


The glacial debris flow hit Sedongpu Valley in Milin County of Tibet, 6km downstream from Brahmaputra, in the small hours of October 17, 2018, blocking the main stem of Brahmaputra and forming barrier lakes. After analyzing evolution of glacial debris flow in the valley, our lab has submitted research report to relevant departments.And the reason why we could get research result within a short time was because we carried out specific research on the glacial debris flow of Sedongpu Valley in 2017.

 

Our lab undertook research to the flow immediately and followed up the information after it occurred. On one hand, satellite remote sensing data collected before and after the flow was used to analyze the disaster. On the other hand, our researchers assisted relevant departments to carry out emergency work in terms of scientific surveying, ice and snow analysis and further landslide prediction in the area, which provided a scientific basis for ensuring the safety of people’s lives and property in the relevant areas.

 

At the invitation of Prof. YaoTandong, one of the pioneers of studies on ice core in China and an academician of The Chinese Academy of Sciences, Prof. Yu Bin of our lab has joined the panel of experts of Yarlung Zangbo River Blocking on October 20 and they traveled to Yarlung Zangbo River for spot investigation on the accident. The group studied the existing data of the disaster as soon as they arrived Nyingchi. Li Weile, a researcher in our lab presented research findings of the glaciers over the past 20 years, whichcontributed to theresearch on the river-blocking.It is not a conventional landslide or debris flow. Based on the analysis on the seismic signal detection by Qinghai-Tibet branch of Chinese Academy Of Sciences and the heavy masses of ice on the surface of accumulation body, it is preliminarily determined that the very ice avalanche caused landslides, thus blocking the river. On October 21, Prof. Yao Tandong and other researchers flew to the Sedongpu Valley, and found new erosion of glacial moraine in the front and downstream of glaciers. Therefore they reached the following conclusions: Ice avalanche(glacial landslide or collapse) occur at the upper part of glacier, and the fast moving ice avalanches strongly scraped and eroded the moraines in the lower part of the glacier, as well as the slope wash along the valley, finally causing barrier lakes in Yarlung Zangbo River. 



Figure1: SepudongValley



Figure2: Aerial Photo


Figure3: Major glacier in SepudongValleyviaRemote Sensing Interpretation




Thus, researchers draw the following conclusions through analyzing multi-temporal satellite remote sensed data and images: 


(1) Analysis of historic records of the event

 According to the satellite remote sensing images recorded since 1984, researchers find that large-scale glacial debris flows have had hit Sedongpu Valley before 1984 and formed a barrier dam on Yarlung Zangbo River, which burst floods from its left side. Under a stable environment in 1984~2013 as well as in 2014 ~2016, few of large-scale debris flow taken place in this area. However, in 2014, a large-scale debris flow blockade occurred in Sedongpu Valley. The solid material in the flow covered nearly one-second of the original blockage area, and then collapse came from the left side (Figure 4). And two large-scale landslide dam events happened between October 20 and 27 and December 21 and 23, 2017 (Figure 5 and 6). 

Figure 4: Satellite image of Sedongpu valley before and after events in 2014


Figure 5: Satellite image of the mouth ofSedongpu valley in October, 2017 



Figure 6: Satellite image of the mouth of Sedongpu valley in December, 2017




(2) Preliminary analysis of the causes

According to multi-period historical satellite remote sensing data, the glaciers around the valley has shrunken, with a maximum retreat of 3500 meters since 1984, (Figure 7). After glacier retreat, a large number of moraines stacking in the way, could be the key driver for large-scale debris flow. In addition, many tensile cracks can be seen in the glaciers (Figure 8), which could lead to glacier avalanche. Once a large-scale avalanche occurred, the avalanche eroded moraines and moved them to the downstream, and finally resulted in a large-scale debris flow. From the high-resolution satellite imagery after the blocking in October 2017, obvious erosion at the scene can be seen (Figure 9). After the two river-blocking in October and December 2017,abundant porous solid mass containing ice and water accumulated in the main area of the valley (Fig. 10). Therefore, we infer that the October 17th 2018 river-blocking was also due to a large-scale ice collapse in the back edge of the glaciers, which caused large-scale glacial mudslides and formed a barrier lake. 



Figure 7: Changes in glaciers around theSedongpu valley



Figure 8: Ice cracks of the glacierin Sedongpu valley


Figure 9:  The back edge of the glacier after the river-blocking in October 2017


Figure 10: Planet image of the valley on September 19, 2018