El Capitan rockfall 11 June 2014

cliff does not show obvious signs of instability

Or, alternatively, we may have no idea what's going on up there.

Rockfalls are common natural events in Yosemite National Park. A rockfall inventory database for Yosemite [Stock et al., 2013] documents 925 rockfalls between 1857 and 2011, resulting in 15 fatalities; 85 injuries; and extensive damage to buildings, roads, and trails. Although virtually all of Yosemite Valley is subject to rockfalls, the developed area of Curry Village, nestled among talus and large boulders, has proven particularly vulnerable. Established in 1899, Curry Village is a complex of visitor and employee accommodations, consisting mainly of rustic wooden and tent cabins. Rockfalls affected Curry Village almost from its inception, but the direct consequences became more prevalent in the past few decades [Stock et al., 2013].

A rockfall in July 1996 devastated an area just east of Curry Village when the impact of about 30,000 cubic meters of rock generated an air blast that felled 1000 trees, causing one fatality and several injuries. Rockfalls from above Curry Village in 1998–1999 caused another fatality and destroyed several tent cabins. A rockfall in December 2003 caused minor injuries and damaged 14 wooden cabins, and another rockfall from the same location in June 2007 caused additional damage. These impacts culminated in October 2008, when a roughly 5700-cubic-meter rockfall damaged or destroyed 25 wooden and tent cabins (Figure 2a). Three people sustained minor injuries, and many more narrowly avoided injury or death.

The most effective method for mitigating rockfall risk in Yosemite is to reduce exposure by removing structures from hazardous areas and by re-purposing buildings to low-occupancy uses. About one-third of all structures in Curry Village were located within the newly established hazard zone, and many were among the highest risk identified. Accordingly, the NPS removed more than 200 buildings from Curry Village in 2013 (Figure 2b). Three other buildings were repurposed from residences to storage. Elsewhere in Yosemite Valley, campsites were relocated, and other buildings were assigned reduced use levels.

The success of these mitigation actions was quickly realized. In the early morning of February 2014, a rockfall sent boulders into Curry Village, but this time, there were no lon- ger buildings there to be affected. A boulder with a volume of about 1 cubic meter impacted within the footprint of a former wooden cabin and then came to rest within the foundation of another (Figure 2c). Had these cabins been standing, they would have been extensively damaged, and had they been occupied, there almost certainly would have been injuries and perhaps even fatalities....

It is not possible to completely eliminate risk from rockfalls in Yosemite; indeed, the soaring cliffs are precisely why so many peo- ple visit the park....