7 April 2011
An interesting new video of a landslide in action
Posted by Dave Petley
Thanks to Tom Hodgson for pointing out this one. A new Youtube video of a landslide in action has appeared in the last couple of days. Whilst it is not the largest slide, the quality of the video is excellent – it should be visible below:
This slide occurred on Nelson Road in Santa Cruz County of California. There are a few interesting things to note. First, the ground is dry, as is the landslide itself. Second, note how the main event was preceded by precursory activity, with an increasing event rate. Finally, note how abruptly the movement ceased, and the smoothed out morphology that resulted. If there is a lesson from this it is that trying to pass by a slope that is spalling rock in the manner that was occurring at the start of the video is highly dangerous.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Great video.
It is interesting to see that the road has been closed in advance. Could anyone say why and when the road was closed?
At the start of the video, it looks like there was a lot of loose debris at the base of the slope. Could this suggest the precursor activity was long enough to close to road? Or was it closed by a previous event?
FWIW, It looks dry, but news reports say there was a lot of rain in the area.
From http://capitola.patch.com/articles/large-tree-crashes-into-home-on-nelson-road: “The home at 480 Nelson Rd. is about a half mile from a massive landslide that occurred on Monday. …….Skalland said that with the rain continuing …..’It’s just so messy out there right now,’ she said. ‘Nelson Road is like a river. There is just so much water coming down.’ Another storm system is expected to arrive Friday night, bringing more rain and wind.”
From http://scottsvalley.patch.com/articles/wet-weather-preventing-landslide-from-being-cleared , 24 March
With another storm pounding the area, a geologist surveying the extent of the slide Wednesday said it was too unstable to begin to clear at this point, because of the level of saturation.
From http://abclocal.go.com/kgo/story?section=news/local/south_bay&id=8026106 21 March
The rockslide is fallout from the wet weather that has been pounding the Bay Area and there is more to come. A 200-foot wall of trees, rocks and dirt came down across Nelson Road in Scotts Valley. ……. Residents along Lockhart Gulch Road say they noticed parts of the cliff breaking off Monday morning. Then, it all came down around 2 p.m. and resident Jackie Maurer was waiting with her iPhone.
Thanks for your blog incidentally – it’s unfailingly fascinating & a great resource.
Great video, impressive precursors! Just amazing to see the increasing activity. As the person runs, one can see that the road is wet.
Looks like most people were wearing rain gear, plus the guy wearing the cowboy hat upperback was wet.
Looks like a man-made cut slope failed, rather than an natural slope
I found it most interesting how the large chunks of ‘rock’ that were rolling downslope ‘broke up’ into lots of small chunks as they rolled, indicating the ground is a ‘fractured shale’ if I can invent a term.
The west end of the San Fernando Valley in Los Angeles has hills with that kind of composition in places.
And then at the end it’s a big pile of ‘gravity sloped’ small chunks of shale type soft rock, almost like a sand dune in terms of slope angle due to gravity vs. resistance.
Could there have been earthquake activity at a level below human sensitivity going on as the stimulant for the slide ? On a seismic scale of 1 or 2 for strength ? Just enough to shake the shale loose, but not so much as anyone would notice it.
This looks like it was caused by three factors. First is inherently weak structures associated with rapid uplifting (which occurs all over the place in California). Second is the heavy rainfall stressing those slopes – a lot failed recently in NoCal. Third is the road cut which took out the support at the toe of the slope, and allowed the overall downward creep force created by one and two above to become a landslide. In some places in California, they now just reroute the road around the toe of the failure if they can since trying to stabilize the failure after removing the new toe created by the landslide is very difficult (there are several good examples of this on Pacific Coast Highway below the Palisades in the LA area). They are also putting in far beefier retaining walls with post-tensioned anchors drilled far into the hillside.
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In turn, may I thank my brother-in-law, Peter Valtin, for passing the link on to me. He knows, through a friend, the people whose access was cut off by this slide.