24 January 2023

A coastal cliff collapse at Blacks Beach, San Diego, California

Posted by Dave Petley

A coastal cliff collapse at Blacks Beach, San Diego, California

On 20 January 2023 a large section of coastal cliff failed at Blacks Beach in San Diego, CaliforniaThe landslide was captured on a video that has been posted to Youtube by Kent Ameyaro.  It is well worth viewing:-

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There is another video of a part of the landslide on the KTVB Youtube channel:-

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And there is a nice view of the aftermath of the failure on the CBS 8 San Diego Youtube channel:-

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The landslide at Blacks Beach is located at 32.885, -117.253.  I’ve found a nice description of the geology of this area, including this image of the sedimentary sequence at Black Beach.    The Ardath Shale is described as:

“…a mix of shale, silty shale, with some beds of sandstone and conglomerate that are similar in physical appearance to some beds in the other Eocene-age rock formations. However, the Ardath Shale is dominated by steep-slope-forming shale (not the nearly vertical cliffs of the sandstone and conglomerate beds). The soft shale beds erode easier forming alcoves beneath waterfalls and is susceptible of forming slumps.”

The landslide itself is complex.  The principal mechanism of failure is a deep-seated rotational failure, which goes through its main (most rapid) movement phase from about 3:00 for about 45 seconds.  During this time the uplift of a shale bed at the toe develops beautifully, and the slumping at the crown of the landslide is also clear (and is evidenced by the exposure of less weathered material at the base of the cliff at the top of the slump block in the image below):-

The 20 January 2022 landslide at Blacks Beach in California.

The 20 January 2022 landslide at Blacks Beach in California. Still from a video posted to Youtube.

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In the three minutes leading up to this point multiple smaller failures and collapses can be seen.  This is probably an indication that movement was occurring on the rotational shear surface, causing extensive internal deformation to the slump block, driving these smaller failures.

After the main phase of movement the landslide became more stable, but smaller collapses continued to occur.  This is common in such circumstances.

Whilst the landslide occurred on a sunny day, California has endured a succession of atmospheric river events that have been associated with heavy rainfall and large waves.  Thus, the landslide might be the result of higher than normal pore water pressures and/or toe erosion.

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Acknowledgement

Many thanks to loyal reader Jonas Van Rutte for highlighting this landslide to me.