QUANTIFYING 40 YEARS OF ROCKFALL ACTIVITY IN YOSEMITE VALLEY WITH STRUCTURE-FROM-MOTION AND TERRESTRIAL LIDAR ANALYSES

Historical rockfalls in Yosemite National Park are documented in a database that spans 160 years (1857-2016) and contains more than 1,200 events. However, the database is prone to bias because it relies primarily on human observation. We employed structure-from-motion (SfM) and repeat terrestrial lidar scans since 2010 to document rockfalls and assess the observation bias. We used 37 photographs, taken from a helicopter by Yosemite’s Search and Rescue team in 1976, to generate high resolution SfM terrain models (>5 million points) of El Capitan and Middle Brother in Yosemite Valley. By comparing the SfM models with recent lidar scans, we are able to quantify 40 years of rockfall activity from these cliffs. Change detection indicates that 57 rockfalls occurred from the southeast face of El Capitan between 1976 and 2016, compared to 30 rockfalls reported in the database over the same time period. 116 rockfalls occurred from Middle Brother between 1976 and 2016, compared to 62 rockfalls reported in the database. Both cases suggest that slightly more than half of all rockfalls were reported. Although most large (>200 m³) rockfalls were reported in the database, the accuracy of their reported volumes varies widely; reported volumes generally underestimated actual volumes by up to a factor of 2, but in one case (the 10 March 1987 Middle Brother rockfall) overestimated the actual volume by a factor of 25. Larger rockfalls tend to have less accurate estimated volumes. Our analyses also show that some rockfall source areas were only active once in the past 40 years, whereas others continue to fail progressively, with one area of Middle Brother producing at least 11 rockfalls since 2000. Given that Yosemite Valley has long been one of the world’s most photographed landscapes, there may be opportunities to use SfM with historical photographs to extend the rockfall record back many more decades.