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gstock
climber
Yosemite Valley
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As the lead author on the rock-fall hazard and risk assessment report, I'd like to clarify a few things:
First, we assessed both hazard and risk in this study. Hazard is the probability in space and time of a particular event such as a rock fall occurring, as well as information about magnitude if available. Risk is bringing in the human element; the longer someone stays in a high hazard area, the greater their risk. Similarly, a densely occupied area has higher risk than a sparsely occupied area. For this study we calculated risk metrics for each structure, campsite, amphitheater, etc based on the number of people hours per year, and the position of that structure within the hazard zone. We know the people hours per year very accurately because we keep track of cabin and campsite occupancy.
The blue line in the images above represents an approximately 1/500 annualized probability of exceedance. Put more simply, there is about a 10% chance in 50 years that a rock-fall boulder with average diameter of 15 feet will go beyond that line. This information comes from mapping and dating the existing boulders (the white circles in the maps above) to estimate the frequency of past events, and using computer simulations to estimate the frequency of future events. We cannot determine exactly where future boulders will stop, which is why we take a probabilistic approach; such an approach is commonly used to assess other natural hazards such as earthquakes and floods.
Many campsites at Camp 4 are within the new hazard line, but the calculated risk metric varies with the position of the campsite relative to the line. Not surprisingly the highest risk campsites (eight total) are in the back row closest to the cliff. Some SAR cabins are even closer to the cliff but because they are not as densely occupied nor occupied for as long (shorter season) their risk metrics are lower. Throughout the valley (not just at Camp 4) the NPS has identified the highest risk structures and is exploring ways to reduce overall risk by relocating these structures if possible. The NPS intends to have the sites relocated within the greater Camp 4 area by December 1, 2012 or earlier.
Greg Stock
Yosemite Park Geologist
(209) 379-1420
greg_stock@nps.gov
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Mighty Hiker
climber
Vancouver, B.C.
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Looks like site #30 in Camp 4 is pretty much ground zero. I wonder...?
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Toker Villain
Big Wall climber
Toquerville, Utah
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Bwahahaha!
Can't believe I actually got a straight answer!
I wonder how the guys at the SAR cabins feel about their "risk metrics".
Thanks greg.
Due to highly specialized skills it is probably easier to find people to map the boulders as opposed to dating them.
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Jon Beck
Trad climber
Oceanside
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it is probably easier to find people to map the boulders as opposed to dating them.
I have never dated a boulder, but I hear they never say no.
That risk map could serve as the basis for a new three tier pricing structure in Camp 4. After all, it is all about da money
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Peter Haan
Trad climber
San Francisco, CA
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Greg, in your "computer simulations" what type of periodicity are you assuming and using? There are many sorts of periodic behavior in physical sciences...and how did you come to assume or develop this one you apparently use? If it is not the simplest linear behavior and you are using a more sophisticated theorem of periodicity, what is it? "computer simulation" could be anything at all...
Also your risk metric may very well also be arbitrary and set only to produce the results you have wished for...this needs clarification. I am sure these questions don't surprise you.
You see, I also don't trust this work just on the face of it. And it could be tainted; it stands in the long troubled tradition established between the strange culture of Federal Parks people and us real users of the resource there locally.
And lastly, why are we suddenly in the business protecting visitors to this extent? After well more than a hundred years stating otherwise? What has happened to make this policy move, a coherent plan? Even if your metrics are reasonable and valid and your predictability holds some water?
Please advise, thanks.
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gstock
climber
Yosemite Valley
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Peter, those are all good questions, and I do understand your skepticism. You can find all of the details in the report, but here are some simple answers:
(1) The computer simulations are three-dimensional physically based models of rock fall runout. They operate on digital elevation models, which means that we can import the actual topography of the cliffs and then simulate rock falls from them. The model accounts for the actual physics of a falling/bouncing/rolling boulder and tracks the energy dissipation of the boulder in space and time, ultimately indicating its final resting position. Even though these are detailed models there is still a lot we don't know accurately enough to model the details of an individual event (for example the effect of trees, or boulder fragmentation), so the model is better used in a relativistic sense rather than trying to predict exactly where a single boulder will go. So what we did was to simulate rock falls from every slope in the valley > 60 degrees, tracking all of those boulder trajectories. The results show how cliff steepness, bedrock ledges, talus slopes, etc. all affect boulder runout. We can then compare the trajectories to the actual mapped boulders to see how the results relate to the past record of boulder deposition.
(2) The risk metrics are pretty straightforward, combining the position of a structure within the hazard zone (deeper in the zone is higher hazard) with the calculated people hours for that structure. For example, structures with equivalent positions within the hazard zone may have different risk metrics because they have different use; a dormitory on the hazard line will have a higher risk than a storage shed. Conversely structures with similar use but different positions within the hazard zone will also have different risk metrics. Ultimately we rank the structures by their risk metrics and identify the highest risk structures for action (closure, relocation, repurposing, etc.). The actual risk metrics were calculated by an expert in risk assessment from the US Geological Survey (in other words, outside of the NPS).
(3) Regarding the need for this type of action, I would first refer you to this photo:
More than a dozen cabins were damaged by rock fall debris during that rock fall in 2008. Yosemite Institute kids were in these cabins just 20 minutes before the boulders came down, but fortunately most had gotten up early for breakfast. The few that remained there were saved by cowering behind large boulders (previous rock fall boulders by the way, visible in the photo above), suffering only minor injuries. Another boulder went through the wall of a cabin between two men in bed. It was an incredibly close call, and it was not the first - cabins in Curry Village were struck by rock fall debris in 1918, 1998, 1999, 2003, and 2007. The evidence suggests that these kinds of events can also happen in the Camp 4 area, and the boulders are there to prove that it has happened repeatedly in the past. The goal of this study was to help the park move from being "reactive" to rock falls (evacuating cabins or campsites AFTER the rock fall happened) to being more "proactive" and anticipating these events and trying to keep things like cabins and campgrounds out of high hazard areas.
Eliminating all rock-fall risk would mean closing all of Camp 4, among other things. The NPS is NOT doing this, because it recognizes that some risk is always inherent in dynamic natural landscapes. However, this risk can be much reduced with a few "relatively" simple actions, such as relocating eight campsites in Camp 4.
I hope this makes some sense - please feel free to call or email if you want to discuss it further.
Greg
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Toker Villain
Big Wall climber
Toquerville, Utah
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I like that.
Maybe just a few signs;
Warning! You are in a "dynamic natural landscape."
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go-B
climber
2 Timothy 1:9-10
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They are going to rename it to camp 3 5/8's!
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WBraun
climber
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Not one rock ever reached Camp 4.
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Toker Villain
Big Wall climber
Toquerville, Utah
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If there is nobody in C4 and a boulder falls into it does it make a sound?
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gstock
climber
Yosemite Valley
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"Not one rock ever reached Camp 4."
The same could have been said of Curry Village at one point also.
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WBraun
climber
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Those boulders were already there.
When was the last time a rock ever made it into Camp 4?
I lived over there for over 30 years and never saw one enter there.
I was there when all the rock falls were coming down everywhere else but none came close to Camp 4.
The one above the Folly was the closest.
Now if your so called theory is going, the Indians are building their Pow Wow center right in line.
Maybe they know something your theory doesn't?
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stevep
Boulder climber
Salt Lake, UT
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Thanks for the straight up answers Greg. This is an uncomfortable topic as so many here have a strong emotional attachment to Camp 4. And obviously, as climbers, we have probably a higher risk tolerance than your average touron (not really any good way to account for that in this type of calculation however, short of forcing folks to buy insurance).
Interesting to see how you've done the calculations.
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Don Paul
Big Wall climber
Colombia, South America
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Hard to tell how significant this is, moving 8 of the campsites. Maybe its no big deal. It is kind of a big deal, though, that the NPS doesn't take Camp 4's status as a historical site into account.
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gstock
climber
Yosemite Valley
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Climbing will always be much riskier than camping, but campsites clearly hold a different assumption of risk. And it’s not just climbers that stay at Camp 4. I understand that the NPS will get criticized for taking these actions, but the NPS was also criticized for not taking action before the 2008 Curry Village rock falls. A familiar conundrum.
Werner, I really respect your experiences here. There are no historic records of rock falls into Camp 4, but the boulders deposited there in the past strongly suggest that boulders will be deposited there again in the future. We have done our best to estimate what the probability of that might be.
Warbler, cliff susceptibility to rock falls is accounted for to some extent because the hazard line in each area is based on the distribution and frequency of rockfall boulders in that area. We focused on evidence on the ground (talus slopes and boulders) rather than on specific potential for future falls on the cliffs. This was mostly because of the near-impossibility of evaluating every potential unstable rock mass in the valley, but also because we felt that the talus and boulders, which record 15,000 years of rock falls, reasonably represent what future rock-falls might look like (accounting for the fact that talus slopes are always slowly advancing out over the valley floor). I'll be the first to admit that extremely large rock falls unlike anything that have happened in the past are possible (like the face of the Rhombus shearing off) but the probability of that happening in the next 50 years is much lower than the smaller, more frequent rock falls that will enter the hazard zone.
Finally, the NPS consulted extensively with SHPO for the rock-fall-related closures in the historic district of Curry Village in 2008, and will continue to do so.
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Bullwinkle
Boulder climber
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Total and complete NPS bullsh#t.
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JEleazarian
Trad climber
Fresno CA
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Thanks, as always, for your posts, Greg. I particularly appreciate your straightforward responses to our sometimes loaded questions.
The statistical problems in estimating probabilities of rockfall events strike me as a larger intellectual and methodoligical challenge than most people would think, even with years of good data. I'm wading though the details of the reports now, and I'm struck by the thoroughness.
While I, like many others, question the NPS commitment to accommodating demand for camping spots, I most certainly do not question the motivation or rigor of your study, or the very real concern about NPS tort liability if it fails to act on this report.
John
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Wade Icey
Trad climber
www.alohashirtrescue.com
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instead of moving campsites Maybe a sign at the Gate saying something like...Danger. Falling Rock.
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Toker Villain
Big Wall climber
Toquerville, Utah
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NO
"dynamic natural landscape"
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