Rockfall Kinematics from Massive Rock Cliffs: Outlier Boulders and Flyrock from Whitney Portal, California, Rockfalls

Abstract

Geologic conditions and topographic setting are among the most critical factors for assessing rockfall hazards. However, other subtle features of rockfall motion may also govern the runout of rockfall debris, particularly for those sourced from massive cliffs where debris can have substantial momentum during transport. Rocks may 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 debris. Collectively, these intricacies of rockfall kinematics may substantially govern the hazards expected from rockfall to both persons and infrastructure 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, CA, a heavily used outdoor recreation gateway to the Sierra Nevada mountains. Our results, obtained in part by rockfall runout model simulations, indicate that outlier boulder trajectories resulted from opportunities 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 (>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 in the use of a single rockfall shadow angle for estimating future rockfall hazards at the site.

Highly energetic rockfalls: back analysis of the 2015 event from the Mel de la Niva, Switzerland

“…Indeed, the slightly longer trajectory composed of two vaulted parabolas between p28, p29, and p31 compared to a shorter trajectory built with three or four flatter parabolas requires a marginally higher velocity to connect the bounding timed impact points p28 and p31. Additionally, the effect of the forest is neglected since its effect is marginal for such a large block (Jonsson 2007;Rickli et al 2004;Collins et al 2022).…”

The formation mechanism and failure mode of a talus slope induced by rockfalls in Nayong County, Southwest China

“…In addition, a large boulder on the talus slope reduced the angle of the slope (Evans and Hungr, 1993). Moreover, slope microtopography affected the movement characteristics of the rockfall (Imaizumi et al, 2020;Collins et al, 2022). The relatively flat terrace (Figure 7B) in the middle section of the talus slope increases the jump height of the boulders.…”

“…The evaluation of the talus slope is one of the most important slope surface processes on the exposed hard rocks in mountainous regions (Statham, 1976;Evans and Hungr, 1993;De Blasio and Saeter, 2015). Recently, research on rockfall activity in outcrop-talus slope systems has been conducted mainly for the European Alps (Jomelli and Francou, 2000;McCarroll et al, 2001;Otto and Sass, 2005;Sanders, 2010;Sanders et al, 2014;Colucci et al, 2016;Vehling et al, 2017;Messenzehl et al, 2018;Hendrickx et al, 2020), the Japanese Alps (Imaizumi et al, 2020), and some mountains in the UK (Curry and Morris, 2004), North America (Veilleux et al, 2020;Collins et al, 2022), and the Tibetan Plateau (Wan et al, 2021).…”