4 August 2021
A rockfall video from the Couloir du Gypaète in France
Posted by Dave Petley
A rockfall video from the Couloir du Gypaète in France
With thanks to those who spotted this on Youtube, a very surprising rockfall video was posted online last week by Rémi Bourdelle, who was flying above the Couloir du Gypaète in France when a rockfall developed. The video provides an utterly unique perspective on rockfall processes. Take a look:-
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My first reaction to this is of course wow! The video shows boulders bouncing down the gulley and then through the trees below. As noted previously the force of these rocks is extraordinary.
But the most surprising element of this video is the trajectory of the rock seen at the start of the video. Fortunately, this boulder (which appears to have been fragmenting) left a trail of dust that marks its path through the air:
The trajectory of the boulder at Couloir du Gypaète in France. Still from a Youtube video by Rémi Bourdelle.
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This is the sort of trajectory that is normally associated with fly rock from blasting rather than a rockfall. It is quite extraordinary; I have not seen this previously (have any readers?).
Presumably, the boulder struck an inclined surface, whilst travelling at a very high velocity, causing a ricochet that created this ballistic trajectory.
The other obviously possibility would be ejection during fracturing, but this seems less likely to me.
I’m not sure if this is a freak, or something that is quite common. Any views?
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
is a rock, rolling “backward” fast on its shorter axis, could have such a trajectory after hitting something, and then change the rotation to “onward” mode at an impressive speed? (and i don’t know if i’m clear…. i’m thinking of a change of the direction of rotation during the impact…)
Severe internal strain ? Isn’t there a famous rock dome in US that ‘explosively’ de-laminates, ejecting slabs ??
Has this block been laterally squeezed-out between two (or more?) blocks?
When you go to Plaine-joux in Google Earth, he’s flying along the cliff face north of the Guebriant hotel. looking south from the cliff, note the debris flow detention structure to the left left (east) and the triangular diversion wall upslope of the hotel in the 2004 photos.
Then, back yourself up looking north and take a look at the huge crack along the edge of the cliff immediately beyond the source area to the NW.
YIKES!
I have seen something like this before, from the 28 September 2017 El Capitan rockfall. That event was ~9,800 m3 in volume, with all of that material coming to rest on the talus slope below the cliff – with a single exception: a ~0.1 m3 rock fragment that traveled 90 m farther, in what must have been a ballistic trajectory, coming down at a steep angle to smash through the sun roof of a moving vehicle, seriously injuring the driver. This was the only such fragment to travel so far.
10 minutes after seeing your tweet, this video was in my Youtube suggested items sidebar:-
https://www.youtube.com/watch?v=tEe9PuQpB64
As the boulders, (dislodged by a controlled explosion, I think), tumble down the side of the cliff, there are a number which bounce out and away from the slope. Unfortunately the dust tails on these are short lived and none are as impressive as the one in the video from the Couloir du Gypaète.
I saw behaviour similar to this many years ago with a rockfall on the Otira Face of Mt Rolleston in the Southern Alps.
My interpretation, from the velocity when we first see the rock, would be that it it was ejected by compression or as a slab fractured under stress during the fall.
Steep mountains are amazingly beautiful and dangerous!
I’ve seen that weird kind of trajectory for single rocks in rockfalls, but not on that scale.
From the few cases I saw ( a few [ read : 2 or 3 over 30+ years ] in the Alps ) I saw, it’s really a freak event,
Now the rockfalls I witnessed were all on a smaller scale ( except one in Greenland )
Have you ever noticed when a bottle or other glassware is dropped and broken, there always seems to be at least one piece that has bounced 3 or 4 times further then the rest ?
I figure this happens in a fashion similar to croquet or billiard balls colliding with one left still, and the other rolling away with it’s energy.
So, if two boulders falling close together, one behind the other, strike the inclined surface while touching, the front one could receive the bouncing energy of the rear one in addition to it’s own momentum, sending it flying further then others. Leaving the rear boulder floundering behind on the inclined surface.
Found this video (no informations about it), some ballistic rocks flying at 0:44 : https://www.youtube.com/watch?v=0pj0NW4-41M
That’s a massive rockfall bay the way…