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WSL-Institut für Schnee und Lawinenforschung SLF
Training day for AINEVA
avalanche forecasters groupe
Thomas Stucki
c Stucki/SLF
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!
Snowpack stability
As lower the snowpack stabiliy, the higher the
degree of danger
Degree of danger
6
5
4
3
2
1
0
0 1 2 3 4 5 6
Index of snowpack stability
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)
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
e.g.: ECT 15/PP@42 (new ECTN@42)
instable:
difference between taps for crack initiation and
crack propagation <= 2 taps
(new ECTP19@51)
--> 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
=
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., 2003
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:
>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
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
liquid water content
hand hardness
snow temperature
grain type and size
Rutschblock
weak layers
ram hardness profile
0
0.2
0.4
0.6
0.8
1
Schweizer und Wiesinger, 2001
Survey
Relative importance of parameters for RB interpretation
s lab har dne s s
s lab thick ne s s
s now de pth
s lope angle
RB s cor e
type of failur e
pype of fr actur e plane
0
0.2
0.4
0.6
0.8
1
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
persistant
Grain types
• 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 more
important are interfaces
weak layer
We are looking for:
- faceted grains, e.g. depth or surface hoar
weak interface
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
•
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
•
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
how far does a avalanche break in deeper
layers?
4
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
•
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
Release type most robust predictor:
a whole block release = unambiguous indication of instability
- skier-triggered
- skier-tested (not released)
Schweizer et al., 2008
1 digging or sawing
2 gently approaching,
standing atop the block
3 weighting of skis
4 1. jump with skis
5 2. and 3. jump with skis
6 jump without skis
7 no clean failure planes
producted
stability, avalanche probability
Interpretation rutschblocktest
unstable /
probable
intermediate /
possible
stable /
low probability
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:
RB score
threshold sum
not whole block
>= 4
<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:
RB score
threshold sum
whole block
<4
>= 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:
1. signs of instability (wumm, crack formation)
2. recent avalanches on nearby slopes (less then one
day old - spontaneous or human triggered)
3. 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:
1. signs of instability (wumm, crack formation)
2. recent avalanches on nearby slopes (less then one
day old - spontaneous or human triggered)
3. 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
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
(mean 65%)
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)
c Stucki/SLF