T234 Engineering Geology for the Betterment of Society: Honoring the Legacy of Jerome V. De Graff

Wednesday, 13 October 2021
1:30 PM - 5:30 PM
Portland Ballroom 254 (Hybrid Room)

ROCKFALL KINEMATICS FROM MASSIVE ROCK CLIFFS: OUTLIER BOULDERS AND FLYROCK FROM WHITNEY PORTAL, CALIFORNIA ROCKFALLS

Paper number: T234-8
Program: Technical Sessions
Time: 3:05 PM – 3:25 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-366602

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.