WSL-Institut für Schnee und Lawinenforschung SLF

1. WSL-Institut für Schnee und Lawinenforschung SLF Training day for AINEVAavalanche 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 • 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 forbetter traceability • investigation: layering, weak layers, type ofrelease, quality of fracture plane, score • since the 60ties - standard still today (in CH) Föhn, 1987

9. Methods - rutschblock degree 1 - 3unstabledegree 4 - 5 intermediatedegree 6 - 7 stable relation betweenRutschblock 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 lowdegree 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) • 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 05/NP@10 (new ECTN@10) Simenhois et al, 2006

15. Methoden e.g.: ECT 15/PP@42 (new ECTN@42) (new ECTP19@51) 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 anddirection of the sawcut has limited effect(also valid for horizontalterrain). • length of 1m or equalto the slab thickness • critical cut-length:<= 50% of thee columnlength • the weak layer has to be known • interesting in context of recent advances in weak layer collapsemodels 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 • = f(time, space) • = f(precipitation, sublimation, windradiation, 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 scores4 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 alayer of buried surface hoar Wind-slab of small rounded grainsand 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 levelsof 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: 0.1 - 1m (- 10 m) ξ: spatial scale of the variability σ:spatialvariation instrength 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 hasa substantial “knock-down” effect on slope stability and that the effectincreases 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/orresearchers 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%): • grain type: • surface hoar • faceted grains • depth hoar • => „persistant“ (thermodyn. rel. stable) • grain size:  1.5 mm • smooth : 0.5 - 1.75 cm (50%) • soft: 1 or “fist“ • interfaces (45%) • often below or above crusts • transition new snow - old snow • --- as stable the snowpack, as moreimportant 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 • 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 • 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 layersand ... • ... slab structures4 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 • 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 neededless 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 layer4 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 enough4 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