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gstock
climber
Yosemite Valley
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Topic Author's Original Post - Jan 15, 2016 - 09:20am PT
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Yosemite Rockfall Year in Review 2015
Despite a fourth consecutive year of drought, rockfall activity in Yosemite National Park 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 (http://pubs.usgs.gov/ds/746/ ), enabling long-term evaluation of rockfall activity to improve public safety.
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JEleazarian
Trad climber
Fresno CA
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Jan 15, 2016 - 09:24am PT
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Thanks, Greg. I greatly appreciate what you contribute on this forum.
John
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Rhodo-Router
Gym climber
sawatch choss
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Jan 15, 2016 - 09:38am PT
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Our tax dollars at work! Thanks Greg!
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Jan
Mountain climber
Colorado, Nepal & Okinawa
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Jan 15, 2016 - 11:18am PT
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Thanks! Interesting as always.
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johntp
Trad climber
socal
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Jan 19, 2016 - 04:44pm PT
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Any input yet on the I140 rockfall?
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Jan 20, 2016 - 01:05pm PT
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I didn't include the recent rockslide onto the El Portal Road (park extension of Highway 140) in the 2015 Rockfall Year in Review because it happened in 2016. But here's some info on it:
The rockslide occurred at around 5:45 am on January 7. The total volume of material was about 60 cubic meters, which slid from a point about 15 meters above the road. The road was closed for 2.5 days for hazard assessment and subsequent debris clearing. Although the slide occurred within the area burned by the 2014 Dog Rock Fire, I doubt that this relatively deep-seated bedrock failure was directly related to the fire. Rather, it was fairly typical of the kinds of rockfalls that occur in the Merced River Gorge every winter and spring.
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johntp
Trad climber
socal
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Jan 20, 2016 - 01:47pm PT
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Thanks Greg. Mine was an OT post but seemed relevant.
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c wilmot
climber
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Jan 20, 2016 - 02:01pm PT
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I always thought that plant growth,rooting, and rain was part of the cause for rockfalls. perhaps the drought helps by slowing such things down?
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thebravecowboy
climber
The Good Places
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Jan 20, 2016 - 02:02pm PT
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geologists rock!
thanks gstock!
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ryankelly
Trad climber
Bhumi
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Jan 20, 2016 - 03:38pm PT
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Thanks for adding great content to this forum Greg
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pyro
Big Wall climber
Calabasas
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Jan 20, 2016 - 04:00pm PT
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Our tax dollars at work
+1
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Sula
Trad climber
Pennsylvania
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Jan 20, 2016 - 04:03pm PT
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Slightly OT, but ... What's the current status of the Half Dome RNWF route?
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Jan 20, 2016 - 06:07pm PT
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Other threads have information about the state of the route from a climbing perspective, but from a geological perspective there have been no new rockfalls there, at least that I know of.
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Reilly
Mountain climber
The Other Monrovia- CA
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Mar 29, 2016 - 08:47am PT
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It may not be 2015 but here is a good article in the LA Times, which non-subscribers may not
be able to access:
Yosemite Cliffs Feel The Heat
A study finds that ‘spontaneous’ summertime granite rockfalls can be traced to hot weather
http://eeditionmobile.latimes.com/Olive/Tablet/LATimes/Default.aspx
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Clint Cummins
Trad climber
SF Bay area, CA
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Mar 29, 2016 - 12:10pm PT
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Yosemite's granite cliffs are 'breathing,' and heat can make them fall
by Sean Greene
March 28, 2016
[Geologists studied a slab of rock slowly peeling from Yosemite's Royal Arches to see how temperature affects rockfalls.] (Brian Collins and Greg Stock)
The domes and arches etched into Yosemite's famed granite cliffs may seem frozen in time, but in reality they're constantly moving.
The dramatic rock formations were formed as layers of rock peeled away from the mountainside, like an onion. The flakes remain attached at a few points but are completely hollow in the middle. If you were to pound on one with your fist, you’d hear an echo.
In Yosemite, these precarious attachments – geologists call them “exfoliations” – fall at a rate of one a week, on average. Most often, they collapse because water repeatedly freezes and thaws in the cracks, destabilizing the cliffs. Sometimes they fall apart during an earthquake.
Other times though, rockfalls happen on sunny days with no sign of rain or seismic activity. Now geologists from the U.S. Geological Survey and the National Park Service have found a potential cause for the seemingly spontaneous rockfalls: heat.
[Studying Yosemite's cliffs]
[Yosemite National Park geologist Greg Stock and USGS civil engineer Brian Collins download data from instruments measuring how much granitic exfoliation sheets move.] (Valerie Zimmer / National Park Service)
As the temperature rises from morning to afternoon, the thin outer layer of rock moves ever so slightly away from the cliff, then returns as the evening cools.
A pair of geologists collected evidence for this idea in the park's Royal Arches, a cliff overlooking Yosemite Valley not far from the Majestic Yosemite Hotel (formerly known as the Ahwahnee Hotel). For 3.5 years, Brian Collins of USGS’s landslide hazards program and Greg Stock of the park service monitored a 19-meter-tall exfoliation that clings to a near-vertical cliff.
Collins and Stock climbed alongside the flaking rock and installed three “crackmeters” behind the slab to see how much it moved throughout the day. The sensors, wedged in place by scissor jacks, measured the strain on a taut wire and were able to detect movements as slight as 0.001 centimeters.
The scientists also installed a variety of temperature and other weather gauges.
Their measurements revealed that a 20-metric ton wall of granite can move about one centimeter a day.
“We look around the landscape and we see thousands and thousands of these flakes and we have to assume they’re all moving,” Collins said. “They’re kind of breathing.”
As the cliffs inhale and exhale, the tips of the cracks weaken. Over time, the cracks slowly opens wider and the stress from the heat can prompt the rock to fall.
Heat not only moves the rock, it deforms it. Using lasers, the geologists observed a phenomenon known as thermal bowing, in which the center of the slab bulges outward as the exterior heats up while the interior remains cool. As the cracks get longer, the stress on the points where the slab is still attached grows larger. This further separates the slab from the cliff.
The effect is significant enough for climbers to notice, Collins said. Climbers will scale a particular route in the morning, leaving their hardware for securing ropes embedded in the wall. When they return in the afternoon, they notice their gear has shifted in some way.
The results, published Monday in the journal Nature Geoscience, offer a potential explanation for the “spontaneous” summertime rockfalls that occur not only in Yosemite, but also in mountain ranges in Japan, France, Brazil and Switzerland, the authors wrote.
Park officials maintain a database of all rockfalls in Yosemite since 1857. Usually weather and earthquakes cause the rocks to fall, but there have also been cases when tree roots jostled the rocks loose or lightning struck the cliffs. A few years ago, the park service had to move a campground because of the risk of rockfalls.
For many of the 925 incidents, too little information was recorded at the time to determine a cause. In about 200 other cases, the rockfall was well documented but the cause remained unclear.
The geologists found that 15% of all rockfalls with no recognizable cause occurred in the hottest hours of the day, between 1 p.m. and 6 p.m., and during the hottest months, July, August and September. It may not sound like much, but if these rockfalls had occurred randomly, only 6% of them would have happened during these times.
“All of a sudden we can say, ‘Well, maybe the thermal stress factor had something to do with it,’” Collins said.
[Yosemite rockfall]
[Rocks fall from the side of a cliff in Yosemite Valley on a clear day in October 2010.] (Tom Evans)
Even normal swings in temperature over the course of a day or season are enough to trigger rockfalls. That realization should inform the way officials view the risks associated with living or camping near such formations.
“Rockfalls do not always need an extreme event like an earthquake or rainstorm to occur,” wrote Valentin S. Gischig, a geologist at the Swiss Competence Center for Energy Research, in a commentary accompanying the study.
“Rockfalls induced by temperature fluctuations should be considered in rockfall hazard assessments — and not just for the famous granitic cliffs of Yosemite, but for inhabited mountain regions throughout the world," he added. ”Possibly, as the climate warms in the coming decades, thermally induced rockfalls may become even more important to hazard assessment and cliff erosion.”
Although the study authors didn't examine long-term data, Collins said it’s reasonable to expect that rising temperatures would affect Yosemite’s cliffs.
“If you continue to heat things up, these sheets could expand even further,” he said.
Mostly, Collins said the study offers a new perspective on the park’s beloved mountains.
“The sun and the temperature are enough to make the rock move up there,” he said. “It’s just kind of a neat thing that I think people don’t think about.”
Follow me on Twitter @seangreene89 and "like" Los Angeles Times Science & Health on Facebook.
Copyright © 2016, Los Angeles Times
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