Using Tree Density Mapping to Assess Rockfall Hazards at Whitney Portal, California after the June 24, 2020 Lone Pine Earthquake

Abstract:

Understanding the influence of forest cover in mitigating rockfall runout distances is important for assessing hazards posed by damaging rockfall events. Tree removal from slopes beneath rockfall-prone areas reduces potential energy-dissipating impacts to falling rocks and increases the runout distance of future rockfalls. On June 24, 2020, a magnitude 5.8 earthquake near Lone Pine, CA triggered a large rockfall at the Whitney Portal campground. The rockfall originated from at least three distinct source areas on a 400-m-tall granitic cliff and impacted the apex of an existing 200-m-long talus slope that sits above the campground. Falling rocks remobilized talus, destroyed numerous trees, and sent rockfall debris across a narrow creek located at the base of the slope and into parking spaces and campsites. The campground was closed at the time due to COVID-19 restrictions. To determine susceptibility posed by ongoing rockfall hazards at Whitney Portal, we assessed the impacts that tree removal from the June 2020 event had on potential rockfall runout distances. We used pre- and post-event satellite imagery of Whitney Portal to map trees on slopes within the rockfall runout zone and determine changes in tree density before and after the June 2020 rockfall. We also mapped the tree density of adjacent talus slopes that were unaffected by the June 2020 rockfall to compare the rockfall-affected slope to existing background conditions. These comparisons allow for assessment of pre- and post-event rockfall runout conditions for future modeling efforts to determine potential risk of rockfall to the Whitney Portal campground. Our use of tree density data to inform rockfall hazards may provide a useful method for assessing areas beyond Whitney Portal.