Franschhoek: an interesting set of landslides in South Africa – GWC Mag

The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.

Over the years, I have rarely featured landslides from South Africa. However, over the weekend of 23 – 24 September 2023, several slope failures were triggered in South Africa by a storm that brought high levels of rainfall. The highest precipitation total (299 mm) appears to have been reported at Franschhoek, which is located at [-33.897, 19.152].

As a result, a number of landslides were triggered in Franschhoek, affecting both roads and buildings.  Franschhoek life has tweeted a set of images collected from the air of these landslides, which were taken by Jean Tresfon and posted to Facebook:-

The most interesting of these sites, with multiple landslides is also shown in the Facebook images below:-

The multiple debris flows shown above appear to have occurred at [-33.880, 19.091]. The site is shown in the Google Earth image below:-

These seem to be very good examples of small, shallow landslides in weathered rock that have entrained material downslope. In Hong Kong these would be termed open hillslope flows as they have not channelised. Note the extensive runout distance. It is clear that the slope has had trees removed in recent years. The initiation of the landslides occurred above this point, but I wonder if this deforestation contributed to the runout distance. There is extensive entrainment on this part of the slope.

Landslides occurred elsewhere in Franschhoek. News 24 has an account of a celebrity being forced to evacuate a building when it was struck by a mudslide at the MontMartre estate. The post includes images of the damage to the chalet.

Text © 2023. The authors. CC BY-NC-ND 3.0
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