This was the deadliest week of U.S. avalanche accidents in 100 years

dbostedo

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I agree, but geez that doesn’t leave much error room in measuring.
I don't know much, so please take this with a grain of salt. But looking at topo maps for that area, the top hundred or so vert of the slope that slid is showing more like 34-36 degrees.

Here's a line I measured on CalTopo. The left point of the line is at the little stand of trees that's just above the crown in the pics, part way down the whole ridge that slid:

1612988114287.png


That gives this profile:

1612988217492.png


The color bar under the slope profile shows the angle, and hits 36 degrees on this particular line. That obviously mellows out relatively quickly after ~100 feet of vert. I don't know how much impact that has on the avalanche likelihood - clearly the whole area that slid is mostly under 30 degrees, but appears to have a steeper part at the top. Further down the ridge, lines get a max of more like 31-32 degrees.

Presuming that the maps on CalTopo are accurate, any idea how, or how much, that top section affects the avalanche likelihood of the whole slope?
 

HardDaysNight

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To add further complexity to the topic, it’s overly simplistic to suggest that 35 or even 30 degrees is the slope angle that differentiates safety from danger.


To quote:

“While a 28° slope may still be perceived as being too mild to cause an avalanche—based on the often-quoted and implicit rule of thumb that a minimum of 30° is required10—in reality, the critical slope angle can be as low11 as 20° (Fig. 2b), provided the dynamic friction angle (sometimes called crack-face friction11) of the snow is sufficiently low. In fact, field measurements have shown that the dynamic friction angle of snow can be as low as 15°, especially for very low temperatures12.”
 

Tex

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More on the original post.. Amazing they all lived.

 

Mike King

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I don't know much, so please take this with a grain of salt. But looking at topo maps for that area, the top hundred or so vert of the slope that slid is showing more like 34-36 degrees.

Here's a line I measured on CalTopo. The left point of the line is at the little stand of trees that's just above the crown in the pics, part way down the whole ridge that slid:

View attachment 123977

That gives this profile:

View attachment 123978

The color bar under the slope profile shows the angle, and hits 36 degrees on this particular line. That obviously mellows out relatively quickly after ~100 feet of vert. I don't know how much impact that has on the avalanche likelihood - clearly the whole area that slid is mostly under 30 degrees, but appears to have a steeper part at the top. Further down the ridge, lines get a max of more like 31-32 degrees.

Presuming that the maps on CalTopo are accurate, any idea how, or how much, that top section affects the avalanche likelihood of the whole slope?
and the stuff over 31 degrees appears not to have slid. The slope at the crown was 31 degrees.
 

noobski

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and the stuff over 31 degrees appears not to have slid. The slope at the crown was 31 degrees.
The February Utah Avalanche report references research on slopes and slides in the appendix. It's informative if you haven't looked at it yet.
 

noobski

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raisingarizona

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The smartass in me would like to point out that the avy "safe zone" is a LESS than 30 degree slope and Wilson Glade is 31 degree.

Obviously the closer a gentle slope gets to 30 degrees the more the safety factor gets diminished.
It’s overall pitch may be 31 but I’m pretty sure there were convex points of that slide that the crown propagated over that were 34-35.
 
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