At least 15 people are known to have been killed by the debris flow and flood from the landslide dam breach that was triggered by typhoon Ragasa.
The extremely intense rainfall brought by super typhoon Ragasa triggered the overtopping of the landslide dam at Matai’an in Taiwan on 23 September 2025. This was not a surprise – indeed I posted that this was a distinct possibility earlier this week, and evacuations were ordered for the downstream communities. Tragically, however, at least 15 people were killed by the outburst flood in Guangfu Township (光復). The Minister of the Interior, Liu Shyh-fang (劉世芳), informed a government briefing that a further 17 people are missing, but other reports suggest that the number of missing may be as high as 31 people.
It appears that the majority of those killed or missing were elderly people for whom evacuation was a challenge. This is causing much soul searching in Taiwan.
Weather conditions in the area of the landslide remain poor as typhoon Ragasa dissipates, so good imagery is not yet available. However, some infom,ation is now available about the sequence of events.
The breach started at about 15:50 local time on 23 September 2025. It was initiated by overtopping followed by extremely rapid downcutting. This image, released by the Forest Protection Department, shows the aftermath:-
Note the extremely deep channel that has been excavated by the flow. It is estimated that this reached a maximum depth of 80 metres, a remarkably high value. The volume of water released is estimated to have been 15.4 million m3 (6,000 Olympic-sized swimming pools), with most of the release occurring in about 30 minutes. There are suggestions that the peak discharge was about four times greater than had been anticipated.
This footage provides some perspective on the scale of the resulting flood:–
It is estimated that 85% of the storage volume of the lake was released in the flood, so there is some residual risk at this site. There is also a large volume of landslide debris remaining, much of which may be over-steepened, so this catchment is going to be very dynamic in future rainstorms.
Hopefully, in the coming days the skies will clear, allowing good satellite and aerial imagery of the Matai-an landslide site to be captured. One can only hope too that those reported missing are safe but out of contact.