Scary stuff. No matter how knowledgeable and careful, stuff will happen. That diagram of different stability all up and down the slope is a scary one. How can you possibly gather enough info to truly make a safe call? That’s why Ski resorts, where they learn the mountain in detail, are safest. Even Heli/Snowcat operations that use the same areas over and over for decades sometimes get it wrong. Scary stuff.
5:55 “If we consider what we talked about in this video, we can see why the crown walls typically end up on the convexity roll.” The slope normal crown “walls” are where the slope parallel crack ends. This is the avalanche perimeter in stable snow.
From transcript “Let's rewind to when the slab avalanche was triggered. Below the skier, we see the first crack through the slab. The crack in the weak layer was likely triggered a second or less before this frame…” Skier dropped off the ridge traveled 10m then skis submerge, skis emerge at critical crack length (minimal failure size) described @6:39 slope normal flank appears at the same time @0:32… skier triggered mixed mode anti-crack.
@2:00 Bruce explained diagram "However, on top of the convexity, more solar radiation will reach the snow surface, so surface hoar crystals are often smaller or sometimes absent.” Saltation, ablation and sublimation explain the higher more stable rutchblock scores near the ridge in snow where no surface hoar is found.
@1:10 “This dry slab avalanche released on a layer of surface hoar. Noticed that it released on steeper slope below the convexity.” Bruce points at Crown and traditional “start zone … “ No tracks can be seen at or near the crown. Avalanche likely triggered elsewhere.
For simple shear cracking, we see that the computed critical load to trigger fracture is several tenfold the static skier load in the 30° to 45° window, and that the critical load rapidly decreases with increasing slope angle. From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
“Anticracks, as opposed to simple shear cracks, allow for mechanical work to be done by the compressive component of the load since the collapse of the weak layer provides a small but sufficient room for slope normal displacements in the crack region (Fletcher and Pollard, 1981; Heierli et al., 2008a, Heierli et al., 2008b, Heierli et al., 2010). As we shall see, about 80% or more of the mechanical work that drives fracture can be attributed to the transient decompression in the anticracked area, and only about 20% or less to the transient reduction in shear stress.” From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
“Thus we infer from the model that weak layer fracture is not more difficult to trigger on gentle slopes than on steep slopes. Since this proposition relates to safe travel in avalanche terrain, we tested it in field experiments using the extended column test(ECT) method to trigger fracture (Simenhois and Birkeland, 2006)” From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
@8:27 Since these works study the traditional "starting zone" no valid information may be found since this feature is not the origin of skier triggered avalanches. It is only one of many slope normal features including flanks and stauchwall. "This work focused on analyzing the terrain characteristics of human triggered avalanches and their starting zones… It was possible to digitalize 142 starting zones and to analyze them…" International Snow Science Workshop Grenoble - Chamonix Mont-Blanc - 2013 Terrain analysis of skier-triggered avalanche starting zones Irene Vontobel et al
6:12 “However, there are also mechanical reasons why avalanches are more likely to be triggered from a steep slope.” This statement is contradicted by hundreds of deadly accidents involving snowshoers, hikers, some climbers, roof avalanches, dog walkers and children playing in snow regardless of slope.
On the contrary, for mixed-mode anticracking the computed critical load to trigger fracture is much lower (two to three times the static load) and virtually independent of slope angle up to about 60°. From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
@9:01 The use of the terms "release area", "starting zones" and "fracture depth" indicate crowns are mapped and NOT skier trigger areas “…First, terrain characteristics of 5200 mapped avalanche starting zones observed in the region of Davos were analyzed…“ "This model simulates the runout of an avalanche taking into account the terrain for a defined release area (polygon) including the extent and predefined input variables, e.g. fracture depth…” AVALANCHE TERRAIN MAPS FOR BACKCOUNTRY SKIING IN SWITZERLAND Stephan Harvey Proceedings: International Snow Science Workshop Proceedings 2018, Innsbruck, Austria
9:20 again focusing attention on crown ( convexities) and not the human trigger ("Remote Trigger") “…But convexities were assumed to have a role in remote triggering. For remote triggering from below release zones, propagation upwards was assumed to stop at convexities. Want to see that cheesy animation again?...”
Since traditionally only shear stresses were taken into account in the formulation of skier triggering, and because the shear stress increases with increasing slope angle while the strength remains a constant or decreases, these approaches imply that fracture should be easier to trigger on steeper slopes, a view that is now fundamentally questioned (Heierli et al., submitted). From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHESJoachim Heierli et al
Great material, please for more
Scary stuff. No matter how knowledgeable and careful, stuff will happen. That diagram of different stability all up and down the slope is a scary one. How can you possibly gather enough info to truly make a safe call? That’s why Ski resorts, where they learn the mountain in detail, are safest. Even Heli/Snowcat operations that use the same areas over and over for decades sometimes get it wrong. Scary stuff.
5:55 “If we consider what we talked about in this video, we can see why the crown walls typically end up on the convexity roll.” The slope normal crown “walls” are where the slope parallel crack ends. This is the avalanche perimeter in stable snow.
From transcript “Let's rewind to when the slab avalanche was triggered. Below the skier, we see the first crack through the slab. The crack in the weak layer was likely triggered a second or less before this frame…” Skier dropped off the ridge traveled 10m then skis submerge, skis emerge at critical crack length (minimal failure size) described @6:39 slope normal flank appears at the same time @0:32… skier triggered mixed mode anti-crack.
@2:00 Bruce explained diagram "However, on top of the convexity, more solar radiation will reach the snow surface, so surface hoar crystals are often smaller or sometimes absent.” Saltation, ablation and sublimation explain the higher more stable rutchblock scores near the ridge in snow where no surface hoar is found.
@1:10 “This dry slab avalanche released on a layer of surface hoar. Noticed that it released on steeper slope below the convexity.” Bruce points at Crown and traditional “start zone … “ No tracks can be seen at or near the crown. Avalanche likely triggered elsewhere.
For simple shear cracking, we see that the computed critical load to trigger fracture is several tenfold the static skier load in the 30° to 45° window, and that the critical load rapidly decreases with increasing slope angle. From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
“Anticracks, as opposed to simple shear cracks, allow for mechanical work to be done by the compressive component of the load since the collapse of the weak layer provides a small but sufficient room for slope normal displacements in the crack region (Fletcher and Pollard, 1981; Heierli et al., 2008a, Heierli et al., 2008b, Heierli et al., 2010). As we shall see, about 80% or more of the mechanical work that drives fracture can be attributed to the transient decompression in the anticracked area, and only about 20% or less to the transient reduction in shear stress.” From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
“Thus we infer from the model that weak layer fracture is not more difficult to trigger on gentle slopes than on steep slopes. Since this proposition relates to safe travel in avalanche terrain, we tested it in field experiments using the extended column test(ECT) method to trigger fracture (Simenhois and Birkeland, 2006)” From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
@8:27 Since these works study the traditional "starting zone" no valid information may be found since this feature is not the origin of skier triggered avalanches. It is only one of many slope normal features including flanks and stauchwall. "This work focused on analyzing the terrain characteristics of human triggered avalanches and their starting zones… It was possible to digitalize 142 starting zones and to analyze them…" International Snow Science Workshop Grenoble - Chamonix Mont-Blanc - 2013 Terrain analysis of skier-triggered avalanche starting zones Irene Vontobel et al
6:12
“However, there are also mechanical reasons why avalanches are more likely to be triggered from a steep slope.” This statement is contradicted by hundreds of deadly accidents involving snowshoers, hikers, some climbers, roof avalanches, dog walkers and children playing in snow regardless of slope.
On the contrary, for mixed-mode anticracking the computed critical load to trigger fracture is much lower (two to three times the static load) and virtually independent of slope angle up to about 60°. From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHES Joachim Heierli et al
@9:01 The use of the terms "release area", "starting zones" and "fracture depth" indicate crowns are mapped and NOT skier trigger areas “…First, terrain characteristics of 5200 mapped avalanche starting zones observed in the region of Davos were analyzed…“ "This model simulates the runout of an avalanche taking into account the terrain for a defined release area (polygon) including the extent and predefined input variables, e.g. fracture depth…” AVALANCHE TERRAIN MAPS FOR BACKCOUNTRY SKIING IN SWITZERLAND Stephan Harvey Proceedings: International Snow Science Workshop Proceedings 2018, Innsbruck, Austria
9:20 again focusing attention on crown ( convexities) and not the human trigger ("Remote Trigger") “…But convexities were assumed to have a role in remote triggering. For remote triggering from below release zones, propagation upwards was assumed to stop at convexities. Want to see that cheesy animation again?...”
Since traditionally only shear stresses were taken into account in the formulation of skier triggering, and because the shear stress increases with increasing slope angle while the strength remains a constant or decreases, these approaches imply that fracture should be easier to trigger on steeper slopes, a view that is now fundamentally questioned (Heierli et al., submitted).
From “2010 International Snow Science Workshop NEW INSIGHTS INTO SKIER-TRIGGERING OF SLAB AVALANCHESJoachim Heierli et al