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

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

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

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

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

Methods - shear frame measuring of shear strength

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

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

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?

Rutschblock - quality of fracture plane clean partly clean rough

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

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

Methoden - ECT (extended column test) e.g.: ECT 05/NP@10 (new ECTN@10) Simenhois et al, 2006

Methoden --> Film instable: 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

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

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

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

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)

Variability scales - slope - region - kleiner als Hang

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)

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

Variability Snowpack stability with apparent patterns. Campbell, 2004

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

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)

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

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

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: 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

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

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

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

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

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

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

Overview parameters grain type

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 : 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 more important are interfaces

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

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)

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)

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

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

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

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

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?

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)

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

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

Interpretation rutschblocktest

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

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

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

Data - tools - products (CH)

Data - tools - products

Data - tools - products

Exercise estimation of 9 snowprofiles

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

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

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

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

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

Comparison ECT - RB - CT

Vergleich ECT - RB - CT classification stable / unstable

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

Comparison ECT - RB - CT Reproducibility of weak layers RB; ECT 51% the same weak layer CT; RB 42 - 48% the same weak layer CT; ECT 42 - 48% the same weak layer threshold sum: 13 - 33% the same weak layer

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)