18 September 2018
Drone images of the Barangay Ucab landslide
The loss of life from the Barangay Ucab landslide has now reached 54 people; the search for victims continues. There is now agreement that there is no possibility of further survivors. Sadly, it is thought that 42 people remain missing, potentially making this one of the most deadly landslides of the year to date.
On Twitter, Mohammed Abdiker (Director of Operations and Emergencies at the International Organization for Migration – the UN migration agency) has posted a set of images collected by drone of the Barangay Ucab landslide. These reveal for the first time the nature of the landslide:-
This image shows that the landslide was a large, comparatively deep-seated slide in deeply weathered regolith. It appears to me that the slide transitioned into a channelised flow once it reached the channel; it is this flow that appears to have destroyed the accommodation block. This image shows the channel downstream of the landslide source, including some of the destroyed structures:-
Notably, most of the debris appears to have left the channel and entered the main river. This may explain why so many victims have not been recovered as yet. The image below shows the channel in more detail. Note the people for scale – this provides a sense of the magnitude of this landslide. The vulnerability of the buildings is also clear, as is the presence of many other landslides in addition to the one that caused such high loss of life:-
In terms of the cause of the landslide, I suspect that the investigation might like to look carefully at the road that crosses the slide. This part of the first image provides more detail:-
The intact section of road suggests deep undercutting of the hillslope, with no apparent slope protection. We have previously seen a failure mechanism in which such road cuts destabilise the upper portion of the slope (NB PDF), which loads the lower portion, triggering large-scale failure that transitions into a flow. To verify this requires further investigation, and there may be other explanations, but it would be a good initial working hypothesis.