Quantitative rock-fall hazard and risk assessment for Yosemite Valley, Yosemite National Park, California

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 pastfew 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 posedby relatively frequent fragmental-type rock
falls (Evans and Hungr, 1999), up to approximately 100,000 m
3
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 rockavalanche 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.