13 January 2021
The 7 August 2020 landslide in Gokseong County, South Korea
In early August 2020 a period of heavy rainfall in South Korea triggered multiple landslides, killing several people. In terms of loss of life the most significant event occurred on 7 August 2020 in the village of Osan in Gokseong County, South Jeolla Province, in which five people were killed. There is an initial report on this landslide in the journal Landslides (Choi et al. 2021), which is really quite interesting.
The image below is taken from the paper, providing a vertical aerial photograph of the aftermath of the landslide:-
The landslide initiated at the bottom of the image and moved towards the north. It traveled a total distance of about 680 metres, with a volume in the order of 20,000 to 50,000 cubic metres. In the deposition zone it inundated five houses, killing all five of the victims.
As noted above the landslide was triggered by heavy rainfall. Choi et al. (2021) note that the cumulative rainfall in Gokseong County for the 3 days preceding the landslide was of 277 mm, with a peak intensity of over 50 mm per hour.
The most interesting aspect of this work though is the initiation of the landslide. As the image above shows, the crown of the landslide is adjacent to a road, and close examination of the image shows that road widening work was underway at the time of the failure. Choi et al. (2021) have observed that initiation was through the failure of a valley-filling embankment with embedded geogrids supporting the road, National Route 15. The failure surface of this fill slope removed a section of the under construction northbound lane of the highway. After failure it appears that the fill went through static liquefaction and transitioned into a debris flow that struck the houses. The debris flow included the concrete blocks that formed a part of the embankment, increasing its destructive power.
So, although his was a rainfall induced landslide, it is an anthropogenic slope that was responsible for the fatal landslide. Choi et al. (2021) speculate that during the temporary works the embankment might have lacked adequate drainage, allowing the landslide to occur.
Choi, S.K., Ramirez, R.A. & Kwon, T.H. 2021. Preliminary report of a catastrophic landslide that occurred in Gokseong County, South Jeolla Province, South Korea, on August 7, 2020. Landslides. https://doi.org/10.1007/s10346-020-01616-y