1. Training day for AINEVA avalanche forecasters groupe
WSL-Institut für Schnee und Lawinenforschung SLF
Training day for AINEVA avalanche forecasters groupe
Thomas Stucki

2. Content
snowpack stability - methods for measurement stability tests
variability - some comments
estimation of snowpack stability - as used in the operational avalanche warning service in CH
comparison ECT - RB - CT

3. Content
snowpack stability - methods for measurement stability tests
variability - some comments
estimation of snowpack stability - as used in the operational avalanche warning service in CH
comparison ECT - RB - CT

4. Snowpack stability strength stability = load/stess
for one snow layer or interface
snowpack stability = index for the whole snowpack (minimum) additionally: depth of the instability
stability = measure for the loading capacity (shear force) (backcounty skier, new snow, wind loaded snow)  probability for a slab release?  prospektive!
strength
stability =
load/stess

5. Snowpack stability
As lower the snowpack stabiliy, the higher the degree of danger
Schweizer, ????

6. Methods - shear frame
measuring of shear strength

7. Methods - rutschblock isolated block, 3m2
snow profile byside the Rutschblock for better traceability
investigation: layering, weak layers, type of release, quality of fracture plane, score
since the 60ties - standard still today (in CH)
Föhn, 1987

9. Methods - rutschblock degree 1 - 3 unstable degree 4 - 5
intermediate degree 6 - 7
stable
relation between Rutschblock degree
and slab avalanche
frequency
Föhn, 1987

10. Methods - rutschblock
type of release (whole block, below the skis, only an edge)
quality of the fracture plane (clean, partly clean, rough)
limitations - not for near-surface layers - deep instabilities: take into account the cohaesion of the overlaying layers - always along with a snow profile
not the only, but very important information (specially by low degree of danger)
representativity?

11. Rutschblock - quality of fracture plane
clean
partly clean
rough

12. Methods - CT (compression test)
30 x 30 cm
since the 70ties
less operating expense
locates „too many weak layers“
Jamieson, 1999

13. Methoden - ECT (extended column test)
crack initiation and crack propagation
differentiation stable / unstable
Simenhois et al, 2006

14. Methoden - ECT (extended column test)
e.g.: ECT (new
Simenhois et al, 2006

15. Methoden --> Film instable:
e.g.: ECT (new
(new
instable:
difference between taps for crack initiation and
crack propagation <= 2 taps
--> Film
Simenhois et al, 2006

16. Methoden - PST (propagation saw test)
new test
tests crack propagation
slope angle and direction of the saw cut has limited effect (also valid for horizontal terrain).
length of 1m or equal to the slab thickness
critical cut-length: <= 50% of thee column length
the weak layer has to be known
interesting in context of recent advances in weak layer collapse models for failure initiation and propagation on horizontal terrain
D. Gauthier et al, 2008

17. Content
snowpack stability - methods for measurement stability tests
variability - some comments
estimation of snowpack stability - as used in the operational avalanche warning service in CH
comparison ECT - RB - CT

18. Variability topograpy = f(time, space)
= f(precipitation, sublimation, wind radiation, temperature, wind / snow metamorphism)
= mechanical properties of layers within the snowpack and the relationship between layers
essential for the evaluation of the slope stability / avalanche formation
uncertainty for forecasts - what is the present variability and its influence on avalanche formation - is the tomorrow variability (+) or (-) for the snowpack stability - ...
topograpy

19. Variability
concept of fracture mechanics = snow is not a perfect material
variation in weak layer strength : numerical models suggest that a slope becomes unstable long before the load has reached the average strength („knock-down“ effect)

20. Variability
scales - slope - region - kleiner als Hang

21. Variability slope scale weak layers are „continuous“ on this scale
layer properties are more continuous than stability scores 4 RB/CT release type more repeatable than RB/CT scores
representativity of the RB? 97% of the cases found to be within ±1° of the slope median (rather sheltered slope) 70–80% for avalanche start zones
each snowpack layer has a unique spatial structure (depositional pattern / the subsequent changes)

22. Variability
No pattern could be found for stability for this investigation.
Jamieson, 1995

23. Variability
Snowpack stability with apparent patterns.
Campbell, 2004

24. Variability
Penetration resistance (SMP) of a layer of buried surface hoar
Wind-slab of small rounded grains and some facets.
Kronholm, 2004

25. Variability regional scale
weak layers were consistently found (in certain aspects and elevations) even over hundreds of kilometers
small patterns (local wind regime, valley clouds, ...)
terrain (Höhenlage, Exposition, Schneeklima) --> variability
a reliable prediction from a single point observation is not possible, but if locations are selected by experts the variability and representativity is expected to be higher ... considering several predictors (related to the fracture process) will result in a more robust estimation (see later)

26. Variability
Characteristic point stability distributions (regional scale) for the three lower danger levels of Low, Moderate and Considerable.
Schweizer et al., 2003b

27. Variability sub-slope-scale
radiation, wind, terrain roughness (are trees present ?) or quality (grassland, talus,...)
water-infiltration
... very high variability

28. Variability spatial variability and avalanche formation:
„knock-down“ effect
l: critical length of the initial failure: m (- 10 m)
ξ: spatial scale of the variability
σ: spatial variation in strength
m: mean snow stability
p: probability of snow slab avalanche release
> l
< l
ξ / l < 1: stabilizing effect
Numericalmodels suggest that spatial variation of strength properties has a substantial “knock-down” effect on slope stability and that the effect increases with increasing length of spatial correlation.
Kronholm et al., 2004c

29. Content
snowpack stability - methods for measurement stability tests
variability - some comments
estimation of snowpack stability - as used in the operational avalanche warning service in CH
comparison ECT - RB - CT

30. Snowpack investigations
Without digging ...
... apparent informations lack!
One of different sources of information for evaluating avalanche danger.
Very good informations for one point.
The variation of snowpack characteristics is less than the variation of snowpack stability.
4 combination of various predictors (structural properties, type of release, quality of the fracture plane)
4 & RB score

31. Procedure
to seek for signs of instability (are easier to interpret and extrapolate, clear indication for caution)
multi factorial estimation
not each criterion has to be fulfilled
are two criterions for two classes fulfilled 4 important criterions get more weight (RB score > profile type)
RB: only if „whole block“ and „clean“, otherwise next more stable class
only for dry snow, with skier as trigger

32. Survey Relative importance of parameters for profile interpretation
Schweizer und Wiesinger, 2001

33. Survey Relative importance of parameters for RB interpretation
Schweizer und Wiesinger, 2001

34. Survey
78% rated within half a level
Deviation of the stability rating of the 10 forecasters and/or researchers compared to the verified stability rating for the
14 profiles evaluated (N=140).
Schweizer und Wiesinger, 2001

35. Overview parameters
grain type

36. Grain types persistant weak layers (55%): smooth : 0.5 - 1.75 cm (50%)
surface hoar
faceted grains
depth hoar
=> „persistant“ (thermodyn. rel. stable)
grain size:  1.5 mm
smooth : cm (50%)
soft: 1 or “fist“
interfaces (45%)
often below or above crusts
transition new snow - old snow
--- as stable the snowpack, as more important are interfaces

37. weak layer weak interface
We are looking for:
- faceted grains, e.g. depth or surface hoar

38. Melt-freeze crusts and ice lenses
tend to stabilize the snowpack provided they are thick enough
gliding surfaces as long as the bonding of new snow to the crust is insufficient - interface failures frequently involve a crust
vapour barrier (faceting below the crust)
wetting of these impermeable layers may cause a reduction of friction (spring)

39. Cracks within new snow
during a snow storm at the interface between new and drifted snow of different caracteristics
rime and graupel are rarely observed to form weak layers (shortly after deposition on a smooth crust)

40. Grain size grains > 1mm
large grains < number of bonds per unit < smooth grains
significant differences in grain size from one layer to the other ==> usually unfavourable
grains > 1mm
significant differences (> 1mm) in grain size

41. Hand hardness weak layers 4 „fist“ difference > 2 steps
rather subjectively estimated
weak layers mostly 1 or 1-2
differeces >2 steps 4 instability
exclusion: thick layers of low strength (even with a prominent weak layer directly below) 4 no slab structure
weak layers 4 „fist“
difference > 2 steps

42. Snow temperature in the evaluation subordinated
no statistically significant difference between stability and snow temperature (excluded: (short term) snow temperature differences!!)
it is used to assess the stability trend given a certain temperature gradient
isothermal snowpack 4 snow temperature becomes more important again for evaluating wet snow instability

43. Ram profile       Schweizer and Wiesinger, 2001
DeQuervain and Meister, 1987)    

44. Ram profile measurement
only weak layers from 5 to 10 cm thickness can be detected
detection of e.g. (basal) depth hoar layers and ...
... slab structures 4 how far does a avalanche break in deeper layers?

45. Density
measurements (of distinct thin layers) are usually not available
density does not directly show instability
density is used to calculate the load on a weak layer, but unless there is no strength measurement this is again of limite value
dense (warm) snow on loose (cold) snow is unfavourable (see hardness or grain size difference)

46. Layer thickness less deep than 1m
a snowpack with many thin layers is in general rather more unstable than a snowpack that only consists of a few, relatively thick layers
weak layer 4 usually less than a few centimetres, sometimes very thin (mm)
the entire snowpack can be weak
most favourable range in view of skier instability: 15 bis 75 cm
The thicker and harder the slab overlying the weak layer, the more unlikely is skier triggering, but ...
... a thick slab on a weak layer may produce a spontaneous avalanche as the slab increases due to loading (snowfall, snowdrift).
less deep than 1m

47. Rutschblock / estimation of snowpack stability
weak layer toughness
Nieten
= highly significant variables in classifying - skier-triggered
- skier-tested (not released)
Release type most robust predictor: a whole block release = unambiguous indication of instability
Schweizer et al., 2008

48. Interpretation rutschblocktest

49. Rutschblock - general remarks
valid for „ whole block“ and „ clean shear“
between 30° and 40° no correction is needed less steep than 30° or steeper than 40° a correction of 1 step of RB score is needed
failer in a deep weak layer covered by a thick strong slab layer 4 triggering a slab on a similar slope is still rather unlikely, except maybe at a shallow spot
layers close to the surface cannot be tested (shallower than about ski penetration) but need to be considered as well
after a snowfall: the slab might not yet be cohesive enough 4 the RB score tends to underestimate the situation in the near future
crack initiation, crack propagation

50. Interpretation „threshold sum“
Failure layer characteristics
1. grain size (>=1mm)
2. hardness („fist“)
3. grain type: persistant
Caracteristics of the interface
4. difference in grain size (about 1mm)
5. difference in hardness (2 steps)
6. interface less then 1m below the surface
Interpretation
5 or 6 critical variables: probably critical weak layer
3 or 4 critical variables: possibly critical weak layer
1 or 2 critical variables: no pronounced weak layer, favorable

51. Interpretation rutschblocktest + threshold sum
general estimation
RB release type: not whole block
RB score >= 4
threshold sum < 5
stable, if non of the variables are in a critical range
intermediate, if one of the variables is in a critical range
RB release type: whole block
RB score < 4
threshold sum >= 5
instable, if at least two of the variables are in a critical range

52. Data - tools - products (CH)

53. Data - tools - products

54. Data - tools - products

55. Exercise
estimation of 9 snowprofiles

57. Content
snowpack stability - methods for measurement stability tests
variability - some comments
estimation of snowpack stability - as used in the operational avalanche warning service in CH
comparison ECT - RB - CT

58. Comparison ECT - RB - CT
Winkler et al., 2008

59. Comparison ECT - RB - CT ECT: stable / instable:
- crack propagation in one layer
- crack propagation within max. two tapps
RB: stable / instable:
- RB score
- type of release
- threshold sum
CT: stable / instable:
- CT score
- type of release
- threshold sum

60. Comparison ECT - RB - CT estimation of the slope „instable“:
one of the following criteria fulfilled:
signs of instability (wumm, crack formation)
recent avalanches on nearby slopes (less then one day old - spontaneous or human triggered)
estimation of snowpack stability by interpreting the snow profiles
Data:
146 profiles (CH alps, mainly GR)
between 1936 and 3184 m
mainly on shadowed slopes
Winkler et al., 2008

61. Comparison ECT - RB - CT estimation of the slope „instable“:
one of the following criteria fulfilled:
signs of instability (wumm, crack formation)
recent avalanches on nearby slopes (less then one day old - spontaneous or human triggered)
estimation of snowpack stability by interpreting the snow profiles
Data:
146 profiles (CH alps, mainly GR)
between 1936 and 3184 m
mainly on shadowed slopes
Winkler et al., 2008

62. Comparison ECT - RB - CT

63. Vergleich ECT - RB - CT
classification stable / unstable

64. Comparison ECT - RB - CT Reproducibility of weak layers
2 ECT‘s 83% the same layer, if unstable (mean 65%) 58% if stable
2 CT‘s score: 81% the same layer, if unstable (score: 61% mean)
2 CT‘s type of release: 78% the same layer, if unstable (type of release : 57% mean)
2 RB‘s not possible, not two adjacent tests

65. Comparison ECT - RB - CT Reproducibility of weak layers
RB; ECT 51% the same weak layer
CT; RB % the same weak layer
CT; ECT % the same weak layer
threshold sum: % the same weak layer

66. Comparison ECT - PST conclusions:
ECT: differentiate well stable from unstable slopes, similar false alarms and false stable prediction
2 adjacent ECT‘s: 87% of the slopes were classified with accuracy of about 90%
ECT: intermediate stability class would be useful for operational use
ECT is done faster as the RB test (1 RB test = 2 ECT‘s ??)
CT: low specificity (correct stables) - high sensitiviy (correct unstables)
two different types of stability tests adjacent to each other: 50% of the test identified the same critical failure layer (higher for rather unstable conditions)