29 March 2023
The Novato landslide in California: digital elevation model data demonstrates a much older, larger landslide
Posted by Dave Petley
The Novato landslide in California: digital elevation model data demonstrates a much older, larger landslide
Loyal reader and long time online friend Funkenbeachin has extracted the 1 metre digital elevation model (DEM) data for the site of the Novato landslide in California. He has made a a video that shows the site, which he’s posted to Youtube:-
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This video really demonstrates the power of DEM data, which highlights beautifully that the most recent slide is a reactivation of the lower portion of a much larger, ancient landslide. I wonder whether the engineers who designed and constructed the slope on the uphill side of the road were aware of this. This landslide can clearly be seen in the still from the video below:-
1 m DEM data for the site of the Novato landslide in California. Still from a video posted to Youtube by loyal reader funkenbeachin.
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This is landslide country – there are many other landslides visible in the image above, a consequence of active tectonics that have created a dynamic landscape, weak rocks, regular high intensity rainstorms and occasional earthquakes.
There are certainly lessons to be learnt from the Novato landslide.
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.
the creepy soils and deep weathering horizon in Franciscan Complex rocks means that many unstable hillslopes can be active with or without clear geomorphic expressions
The affected frontage road was built to replace a notoriously short and dangerous old style off/on-ramp to the Park once US 101 was widened through this area. It worked well until about 20 years ago when traffic from burgeoning residential development in the region increased tremendously. I’m not sure who funded the new road – CALTRANS (the State Highway department) or State Parks, which has had tight budgetary problems for many years.
In the still frame of slope, derived from and draped on the 1-m DEM, above, I see a head scarp and body of an earlier (Holocene?) slump at the site. That old slump is a small portion of a yet larger, presumably older slump complex. These features are clear in the 1-meter/pixel lidar, though subtle in the old 7.5 minute (~30-meters/pixel) topographic map.
Google Earth has a time-series of aerial photography since 1985. There seepage in the cut of US-101 is evident as gullies in images as early as 2003. In the 2005 image, the cut appears to have a bulge with gullies on it? In 12/2007 and 9/2008 photo resolution is much better and scarps of small debris sides and gullies are apparent. The 3/2015 shows newly constructed Redwood Boulevard, road elevation 55 feet, maximum cut elevation 85 feet, 30 foot high cut, with a concrete lined V-ditch. Topical surface erosion treatment is apparent. I don’t see evidence of horizontal drains that would reduce seepage force in the slope. an open question. In the 8/2019 image Redwood Blvd has been realigned, the cut advanced into the slope, during the drought. In the 2/2021 image Redwood Blvd appears to be back to the original alignment? The 2019 image mosaic suffers radial distortion, right at the cut slope? I’m confused, another open question.
The failure surface of the active slump is close to sea level, making it difficult to drain.
James, can you answer the question about ~2018 realignment of the local road that appears in 8/2019 Google Earth imagery? Has any attempt been made to control subsurface drainage?
Version 2 of this movie, zoom to higher resolution; smooth, slow panning: https://youtu.be/bP_OWq3VX6Y
Version 3 of this movie includes 2022 USDA NAIP high-resolution aerial imagery: https://youtu.be/eKS3qC1hX-Y