High-Resolution Topographic Data and Cross Sections of Recent Coastal Landslide


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High-Resolution Topographic Data and Cross Sections of Recent Coastal Landslide

June 2007

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Above: Panoramic view of the January 1, 2007, Northridge Bluff landslide. Bluff height in this view is about 140 m. View southeastward. [larger version]

Just a few months after a large landslide tore away part of a coastal bluff that forms the west side of Daly City, California, the U.S. Geological Survey (USGS) published detailed topographic data from the slide area, including map (overhead) views, oblique views, and cross sections. The landslide occurred on January 1, 2007; data collection began on January 9; and the report—"Terrestrial LIDAR Investigation of the December 2003 and January 2007 Activations of the Northridge Bluff Landslide, Daly City, California"—was posted online just 10 weeks later, on March 23. Shortly before publication, the report's first author, Brian Collins, was informed by engineers in Daly City's Public Works Department that tension cracks 9 m back from the crest of the current slide had recently appeared, making the report especially timely. The engineers used graphics from the report to show other city personnel and owners of land near the slide what happened, what is going on now with the slide, and what may be expected in the future. They also found the explanations of the slide history and recent events quite helpful and have incorporated the report into the department's archived files.

Above: Lidar data collected after the January 2007 landslide. This image is a composite of six lidar scans performed from the numbered locations (AC01-2007 through AC06_2007). Color indicates relative elevation, from red (lowest) to blue (highest). View southeastward. [larger version]

The data were collected by using the terrestrial lidar (light detection and ranging) technique, also known as "3D laser scanning," which consists of sending and receiving laser pulses to build a set of three-dimensional coordinates of virtually any surface. Data collection occurred at a rate of 8,000 points per second, generating a "point cloud" of three-dimensional coordinates. The point files generated from data collection were transformed into three-dimensional surfaces for cross-section and volumetric analyses. Data collected after the January 2007 slide were compared with similar data collected shortly after a massive slide in the same area in December 2003.

Above: Cross section C-C' through the Northridge Bluff landslide, showing land surface shortly after the 2003 event (red) and shortly after the 2007 event (blue); green is assumed geometry of landslide toe before the 2007 event. [larger version]

Above: Location of cross section; base map is land surface after the 2007 landslide. [larger version]

Results show that the 2007 event was a reactivation of the 2003 landslide, about one-third the volume of the 2003 slide, and almost entirely contained within the footprint of the existing 2003 landslide scarp. However, some new earth material was mobilized in the 2007 event, notably in the central head scarp area, where the crest retreated more than 9 m. The slides occurred in an area of steep bluffs extending from sea level to approximately 150 m in elevation, composed of weakly consolidated sediment. Residential development, utility lines, and roads occupy the land immediately east of the slide site.

First author Brian Collins is a Mendenhall Postdoctoral Research Fellow working with research civil engineer Rob Kayen in the USGS Western Coastal and Marine Geology Team. The full reference for the new report is:

Collins, B.D., Kayen, Robert, Reiss, Thomas, and Sitar, Nicholas, 2007, Terrestrial LIDAR investigation of the December 2003 and January 2007 activations of the Northridge Bluff landslide, Daly City, California: U.S. Geological Survey Open-File Report 2007-1079 [URL http://pubs.usgs.gov/of/2007/1079/].

Terrestrial LIDAR Investigation of the December 2003 and January 2007 Activations of the Northridge Bluff Landslide, Daly City, California - USGS Open-File Report 2007-1079
USGS (U.S. Geological Survey)