Quantitative Rock-Fall Hazard and Risk Assessment for Yosemite Valley, Yosemite National Park, California.

摘要

Rock falls are common in Yosemite Valley, California, posing substantial hazard and risk to the approximately four million annual visitors to Yosemite National Park. Rock falls in Yosemite Valley over the past few decades have damaged structures and caused injuries within developed regions located on or adjacent to talus slopes, highlighting the need for additional investigations into rock-fall hazard and risk. This assessment builds upon previous investigations of rock fall hazard and risk in Yosemite Valley (Wieczorek et al., 1998, 1999; Guzzetti et al., 2003; Wieczorek et al., 2008), and focuses on hazard and risk to structures posed by relatively frequent fragmental-type rock falls (Evans and Hungr, 1999), up to approximately 100,000 m3 in volume. Previous rock-fall hazard assessments delineated two primary hazard zones in Yosemite Valley defined by: (1) a line delineating the base of rock-fall talus and other slope movement debris, and (2) a line delineating the shadow angle limit (Wieczorek et al., 1998, 1999). The base of talus line encapsulates all rock debris on the valley floor and provides an important first approximation of the hazard, as evidence of past rockfalls (such as talus) is a generally a positive predictor of future rock fall deposition areas. However, by encapsulating all rock debris on the valley floor, the base of talus line mapped by Wieczorek et al. (1998, 1999) does not discriminate between active rock fall and debris flow deposits and inactive rock avalanche and debris flow deposits. Furthermore, as a hazard approximation, the base of talus line does not address the expected progradation of the talus edge as future rock falls accumulate on the talus slope. We define a new rock-fall hazard line by integrating the spatial distribution of individual boulders beyond the base of talus with the inferred frequency of boulder deposition in this region. Debris from most rock falls will come to rest on talus slopes, but some rock falls produce boulders that will travel beyond the base of talus and out onto the valley floor, where substantial park infrastructure exists.