GSA Connects 2021 in Portland, Oregon

Paper No. 234-7
Presentation Time: 3:05 PM


COLLINS, Brian1, CORBETT, Skye C.1, HORTON, Elizabeth J.2 and GALLEGOS, Alan3, (1)U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, P.O. Box 158, Moffett Field, CA 94035, (2)Department of Earth Sciences, University of California, Santa Barbara, 1006 Webb Hall, Santa Barbara, CA 93106, (3)USDA Forest Service, 1600 Tollhouse Road, Clovis, CA 93611

Geologic conditions and topographic setting are among the most critical factors for assessing rockfall hazards. However, other more subtle features of rockfall motion may also govern the runout of rockfalls, particularly for those sourced from massive cliffs containing few or widely spaced joints and/or fractures and that result in launching of large, competent, rockfall boulders. Boulders such as these can have substantial momentum during transport and may travel down the entire length of a talus slope, sometimes resulting in so-called outlier boulders that stop some distance beyond the talus toe. Mobilized rock debris may also undergo collisions with trees and talus boulders, with the latter potentially generating flyrock – launched rock pieces resulting from boulder collisions that follow distinctively different paths than the majority of rockfall debris. Collectively, these intricacies of rockfall kinematics may substantially govern the hazards expected from rockfall to both life and property located beneath steep cliffs. Here we investigate the kinematics, including outlier boulder and flyrock trajectories, of seismically-triggered rockfalls on 24 June 2020 that damaged campground facilities near Whitney Portal, California, USA – a heavily utilized outdoor recreation gateway to the Sierra Nevada mountains. In a joint partnership carried out by the U.S. Geological Survey and the U.S. Forest Service that exemplifies the call to public service and collaborative spirit of Jerry DeGraff, we undertook field and modeling studies to identify the immediate ongoing and expected future hazards from these rockfalls. Our results indicate that outlier boulder trajectories resulted from conditions provided by less steep terrain beyond the talus edge. The influence of trees – initially thought to have served a protective capacity in attenuating rockfall energy – appears to have been negligible for the large boulder volumes (in excess of 50 m3) mobilized, although they did potentially deflect the trajectory of flyrock debris. Rockfall outlier boulders from the event were comparable in volume and runout distance to prehistoric boulders located beyond the talus slope, thereby providing some level of confidence to the use of a single rockfall shadow angle for estimating future rockfall hazards.