Dear Friends,
I greet you from Yosemite
Yosemite rockfall activity for the year 2015, was average as reported by the Park, although the exfoliation of the Northwest Face of Half Dome made history by taking with it two pitches of one of the world’s most famous climbing routes.
“Despite a fourth consecutive year of drought, rockfall activity in 2015 was about average, with 66 documented events (rockfalls, rockslides, and debris flows).
The cumulative volume of all events was about 8,700 cubic meters (roughly 25,000 tons).
Surprisingly, the largest and most notable rockfall of 2015 was not directly observed.
On July 5, two rock climbers attempting the Northwest Face of Half Dome found themselves stymied by a new expanse of blank rock. Sometime in the previous days a rock slab totaling some 1,800 cubic meters (about 5,200 tons) parted from the cliff in a classic case of exfoliation, taking with it two pitches of one of the world’s most famous climbing routes. Although the rockfall happened at the height of the summer tourist season – and also the Half Dome climbing season – the stormy weather that apparently triggered the rockfall ensured that no one was in the immediate vicinity to witness the event.
Another rockfall from Half Dome on July 15 originated from a different location near “The Visor” and was apparently unrelated to the earlier event.
Other substantial rockfalls in 2015 occurred from Middle Brother west of Camp Four, Washington Column, Clouds Rest, Glacier Point, and the north wall of Hetch Hetchy Valley.
In a notable departure from past years, more than half (54%) of the cumulative volume for 2015 was related to debris flows triggered by intense rainstorms.
In particular, two thunderstorms in July and October – the former a remnant of Hurricane Dolores – generated substantial runoff and debris from within the burned areas of the Dog Rock and El Portal fires. The El Portal Road was closed for three days as debris from the July event was cleared from the road. Although burned areas proved susceptible to debris flows, unburned areas also experienced large debris flows, indicating that localized weather plays the primary role in triggering these events.
It is very likely that there were additional rockfalls, rockslides, and debris flows in 2015, but these events either were not witnessed or went unreported.
If you witness a rockfall of any size, encounter fresh rock debris, or hear cracking or popping sounds emanating from the cliffs, please contact park geologist Greg Stock at 209/379-1420 or by email at greg_stock@nps.gov, or contact Park Dispatch by dialing 911 within the park. Documented rockfalls are added to the park database, enabling long-term evaluation of rockfall activity to improve public safety.”
Yosemite rockfall activity for the year 2015, was average as reported by the Park, although the exfoliation of the Northwest Face of Half Dome made history by taking with it two pitches of one of the world’s most famous climbing routes.
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