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gstock
climber
Yosemite Valley
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Topic Author's Original Post - Jan 29, 2018 - 08:50am PT
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There many large and consequential rockfalls in Yosemite in 2017, with a record 85 events (including rockfalls, rockslides, and debris flows) documented. The cumulative volume of these events was about 36,800 cubic meters. Although this is not the largest annual volume recorded, greater volumes in previous years were typically dominated by one very large event (for example, the 46,700 cubic meter rockfall from Ahwiyah Point in 2009), whereas the cumulative volume for 2017 resulted from several large and medium-sized rockfalls.
The largest event in 2017 almost escaped notice. On the stormy morning of January 12, road crews encountered downed trees and a damaged manhole on the road between Pohono Bridge and the Big Oak Flat Road junction. They also noticed a suspiciously fresh-looking boulder in the Merced River. Subsequent investigation revealed that the boulder was part of a very large rockslide that originated far above the road in an area known (appropriately) as “The Rockslides”. The total volume of this slide was about 20,000 cubic meters (almost 60,000 tons), most of which was scattered throughout the forested slopes above the road. If not for the single boulder that hit the road, this rockslide might have escaped notice for some time.
Much greater road damage occurred on June 12, when about 650 cubic meters (nearly 2,000 tons) of rock fell from “Parkline Slab”, a sloping cliff just east of the park boundary near El Portal. About one-third of the rock debris landed on the El Portal Road, burying a 60 meter (200 foot)-long section of road under tons of rock; fortunately, there were no cars directly under this area, despite the rockfall occurring around noon during the busy summer season. The road was closed for five days as crews cleared debris and repaired the roadbed. Much loose debris remains on the slope above the road, and could continue to slide during intense rainstorms.
The year’s most consequential rockfalls occurred from the southeast face of El Capitan in September. The first of these occurred at 1:52 pm on September 27, when 290 cubic meters (860 tons) of rock fell from the cliff near the path of Horsetail Fall. Two rock climbers were walking along the base of the cliff directly under the area, and, sadly, one of them was killed and the other seriously injured. YOSAR quickly extracted both climbers, as several more rockfalls totaling 163 cubic meters (440 tons) pummeled the base of the cliff over the next few hours. At 3:21 pm the following day (September 28), a much larger rockfall occurred from the same location. This rockfall, totaling 10,324 cubic meters (27,875 tons), buried trees at the base of the cliff and generated a huge dust cloud that fanned out across the valley. A small rock fragment hit a vehicle traveling on Northside Drive, puncturing the sunroof and injuring the driver. Northside Drive was closed for 24 hours as geologists assessed the potential for additional activity. Several smaller rockfalls occurred from this same area in October and November.
Other substantial rockfalls in 2017 occurred at Little Windy Point on the El Portal Road, Ahwiyah Point, Glacier Point, El Capitan, Middle Cathedral Rock, and Hetch Hetchy.
It is very likely that there were additional rockfalls and rockslides in 2017, 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 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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Ed Hartouni
Trad climber
Livermore, CA
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Jan 29, 2018 - 08:57am PT
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thanks for the update Greg!
always good to hear about the dynamic landscape, and be reminded that things naturally fall down.
Be careful out there y'all!
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skywalker1
Trad climber
co
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Jan 29, 2018 - 08:58am PT
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Thanks Greg. Always interesting to read.
S...
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Pete_N
Trad climber
Santa Cruz, CA
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Jan 29, 2018 - 09:21am PT
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Thanks indeed Greg! Clicking on the database link, by the way, includes an extraneous ')'--removing this from the url in your browser will get you to the data.
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mcreel
climber
Barcelona
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Jan 29, 2018 - 10:44am PT
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Thanks! I suppose that seismographs don't help to detect falls like the big one that almost escaped notice? I would have thought so.
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Jan 29, 2018 - 11:43am PT
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Seismometers can certainly detect rockfalls, but in my experience here it works best if the rockfalls are particularly large and energetic (usually by free-falling onto bedrock at the base of the cliff) or if there is a dense array of seismometers on the cliff to record smaller rockfalls. We have had such an array temporarily installed in Yosemite Valley in the past (see this paper), but we don't have one at present.
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Toker Villain
Big Wall climber
Toquerville, Utah
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Jan 29, 2018 - 12:31pm PT
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If you call it "mass wasting" in California too many people misinterpret.
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Jan 29, 2018 - 01:25pm PT
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"You mean you can't just sense a disturbance in the force, young Geo-Jedi?"
One of the biggest rockfalls in Yosemite's history occurred in March of 2009, when 46,700 cubic meters of rock fell from Ahwiyah Point at about 5:30 am. Hundreds, maybe even thousands, of people in the campgrounds and lodges felt the ground shake and heard the rumble. I slept right through it.
That morning happened to be the start of the Yosemite Association's spring gathering, and the Superintendent gave an opening address. As I wandered over I heard him say "So I'm not sure what happened this morning, but here's our Park Geologist and I'm sure he can tell us!" Everyone turned to look at me expectantly, and he beckoned me up to the podium.
I had no idea what he was talking about, so I walked slowly, stalling for time. Just as I reached the podium a friend pushed to the front of the crowd and thrust a piece of paper into my hand. It was a printout from the USGS showing a magnitude 2.4 earthquake centered a mere 500 meters underneath Half Dome. Guessing that it was actually a rockfall masquerading as an earthquake, I stammered out something about a big rockfall up by Half Dome, excused myself, and hurried up to Mirror Lake, where the dust from the rockfall was still hanging in the air.
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SC seagoat
Trad climber
Santa Cruz, Moab, A sailboat, or some time zone
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Jan 29, 2018 - 01:29pm PT
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I love your work Greg.
Whenever I’ve heard you speak I learn so much.
You have a gift for making the very complex understandable.
Thanks!
Susan
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Honu
Big Wall climber
Boulder
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Jan 29, 2018 - 01:45pm PT
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I was on t trip when part of pitch two fell off and on zodiac when pitch five of ZM fell off. I thought I'd share this photo I got of the ZM fall
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Mighty Hiker
climber
Outside the Asylum
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Jan 29, 2018 - 01:51pm PT
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The spring 2009 Ahwiyah Point stealth rockfall, to which Greg refers. Taken a few weeks later.
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looks easy from here
climber
Ben Lomond, CA
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Jan 29, 2018 - 02:10pm PT
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I always appreciate these posts when I'm nice and comfy at home. But I always seem to remember them much less happily when belaying right against a sheer 900' face.
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Clint Cummins
Trad climber
SF Bay area, CA
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Jan 29, 2018 - 04:15pm PT
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Great stuff, Greg - thanks for sharing.
Especially the Ahwiyah Point story!
Got any photos to share of what fell off in "The Rockslides" in 2017?
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Jan 30, 2018 - 08:17am PT
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Here are a couple of photos showing the "The Rockslides" rockslide of 12 January 2017, as well as the new boulder in the Merced River.
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Grippa
Trad climber
Salt Lake City, UT
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Jan 30, 2018 - 11:07am PT
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Excellent write up Dr. Stock! Thanks for the year in review.
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Contractor
Boulder climber
CA
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Jan 30, 2018 - 11:30am PT
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I loaded this video late, after a long involved retreat, dive home and much needed chill period. I shot this footage several seconds into the second fall, viewed from Lay Lady Ledge.
I felt compelled to erase video of the first fall due to the tragic circumstances and distressful audio content. I will always remember the heroics of Andrew.
https://youtu.be/-c-1eikMjh8
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BrassNuts
Trad climber
Save your a_s, reach for the brass...
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Jan 30, 2018 - 06:58pm PT
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Thanks for the data Greg!
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SteveW
Trad climber
The state of confusion
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Jan 30, 2018 - 07:35pm PT
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Thanks for the update, Greg. Always veddy, veddy interesting!
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Clint Cummins
Trad climber
SF Bay area, CA
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Jan 30, 2018 - 09:54pm PT
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Thanks, Greg - I had fun finding this on Xrez.
The "before" shot does not look all that scary to an outsider,
but I can see how a couple of big round boulders could hit the road from there.
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ryankelly
climber
Bhumi
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Jan 30, 2018 - 09:57pm PT
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haha great story Greg
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