5 August 2019
Grandview Surf Beach: a fatal rock topple in California
On Friday a small rock topple occurred on sandstone bluffs on Grandview Surf Beach in Encinitas to the north of San Diego, in California, USA. The topple appears to have occurred without identified precursors. Three people were tragically killed in this landslide. This image, from Channel News Asia, provides an overview of the event:-
Reports indicate that the three victims were all women from the same family, who were local to the area.
The images above suggest that the failure mechanisms on these cliffs are quite complex, with rock toppling on the steep lower parts of the bluffs and shallow slips on the slightly lower angled upper sections. In this case the landslide appears to have been primarily the result of a large (some reports suggest about 10 m long) slab that underwent a toppling failure. Some of the debris from the upper part of the slope might also have been incorporated. This image, from Flipboard, gives a better sense of this failure event:-
The images above both suggest that the sea erodes a notch at the foot of the cliff at this site (such a notch can be seen on the left side of the image above), destabilising the face of the cliff. This sets up a classic toppling scenario. Note that the sandstones that form this cliff have little jointing, a situation that allows larger slabs to form and that sets up failures that can be rapid and catastrophic.
Coastal cliffs can be dangerous. There often seems to be a presumption that rockfalls occur primarily in periods of poor weather. I am not sure that there is much evidence to support that assumption.