Greg Stock Ph.D, Yosemite National Park.
Eric Hanson, xRez Studio, Inc./University of Southern California
Greg Downing, xRez Studio, Inc.
We present quantitative analyses of recent large rock falls in Yosemite Valley, California, using integrated high-resolution gigapixel imaging techniques. Rock falls commonly occur from the glacially sculpted granite walls of Yosemite Valley, modifying this iconic landscape but also posing significant hazards. Two large rock falls occurred from the cliff beneath Glacier Point in eastern Yosemite Valley on 7 and 8 October 2008, causing minor injuries and damaging structures in a developed area. We used repeat gigapixel photography using Gigapans and terrestrial laser scanning data (lidar), collected at a distance of >1.2 km, to characterize the rock-fall detachment surface and adjacent cliff area in high resolution. Three dimensional image analysis of these integrated data sets reveals that the cumulative failure consisted of a near-planar rock slab with a maximum length of 69.0 m, a mean thickness of 4.1 m, a detachment surface area of 2,409 m2, and a volume of 5,663 ± 36 m3. Shear (sliding) failure occurred along a surface-parallel vertically oriented sheeting joint in a classic example of exfoliation. Stress concentration at crack tips likely propagated fractures through the partially attached slab, leading to failure. Our results demonstrate the utility of high-resolution gigapixel imaging techniques for quantifying rock falls from the far-range (>1 km), largely inaccessible, vertical rock faces of Yosemite Valley and providing highly accurate and precise data for hazard assessment.