The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.
Last week, I featured a set of landslides that were triggered by heavy rainfall at Franschhoek in South Africa on 23 – 24 September 2023. My good and very kind friends at Planet Labs have captured a high resolution SkySat image of the site, and have made it available to me to post.
So, here is the image of the site collected on 3 October 2023, showing multiple landslides at Franschhoek:-
The image shows four major landslide source areas, all of which appear to have initiated as shallow failures in thins soil / regolith cover. A more detailed view of the three higher elevation failures shows this very well:-
Indeed, the source area of the landslide to the west is remarkably small. In each case, this initial minor failure has entrained soil and rock from the slopes below to become a major flow. The middle two landslides appear to have merged, vreating a very long runout.
However, Sarah Bates at Planet labs correctly identified that there is greater subtlety in these landslides than might be initially apparent. This is an image collected before the rainstorm, in this case on 13 September 2023:-
Very suprisingly, this image shows that one of the two central landslides had already occurred when the September 2023 rainstorm impacted the area. This had caused substantial damage to the fields. We can use satellite imagery to collect information about when this first landslide occurred. It is not visible in an image collected on 6 June 2023:-
But it is present in an image collected on 22 June 2023:-
Planet Labs PBS 2023. Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/