S n o w

C o v e r

S t a b i l i t y,

A v a l a n c h e

I n i t ia t i o n

a n d

F o r e c a s t i n g

Fig.4showstheresultsoftheexperimentsperformed
during the last two winters(1994-95 and 95-96)in soft
snow(partlynewsnow/decomposingpar ticles/
rounded grains;hand hardness index:fist or4 fingers).
Themeannormalforcesforstandingatop,weighting
and fifth jump are given together with calculated values
forastatic line load (500 N/m) on a homogeneousone
layersnow cover(Schweizer,1993).The dynamic load
stepweightingseemsbesttocorrespondtothecalcu-
lated static load,whereas standing atop and fifthjump
are smallerand greaterrespectively.

The skier stability index has to be supplemented with slab
properties (sublayering). Toinclude ski penetration as pro-
posed by Jamieson and Johnston (1995) isa first step to-
wards an improved skierstabilityindex.
Forcertain snow conditions the measured skier's im-
pact is of the same order of magnitude as the strength of
weak layer obtained by shear framemeasurements, found
to be typically 500 to 1000 Pa (Föhn, 1993). The dynamic
loads are applied within fractions of seconds (0.03to 0.3
s).Comparing ourfieldmeasurements to laboratory test
results (e.g. Narita, 1980) it seems most likely that a skier
will cause (dependingon snow conditions and slab depth)
a deformation in a potential weak layer that is both, large
and fast enough to start brittle failure.
Fig. 5 shows that theimpact increases strongly, but non-
linearly with rutschblock load steps.The substantialin-
crease fromsteptostepisreasonablesinceitgivesthe
test more sensibility in the lower stability ranges. The dif-
ferences arisingfromdifferentsnow coverlayering and
different weak layersdepths explain much of the varia-
tion observed with stability tests and support the sugges-
tions that the rather rough rutschblock scale is by far good
enough and that the rutschblock score alone as a stability
test result farfromcomplete.As important is additional
information on the depth, age and type of weak layer,on
the type of release of the block (whole block or only a part
of it) and on the snow cover(slab)characteristicsat the
test site. Only while considering all these information an
extrapolation should be tried.

REFERENCES

Camponovo. C. 1995. (unpublished). Measurements of the ski-
er's impact in the snow cover. Diploma thesis, Eidgenössische
Technische Hochschule, Zürich, Switzerland, 135 pp.

Föhn, P.M.B.1987. The stabilityindexandvarious triggering
mechanism. IAHS Publication, 162, 195-214.

Föhn, P.M.B. 1993. Characteristics of weaksnow layers or in-
terfaces. In:Proc.Int. SnowScience Workshop,Breckenridge,
Colorado, U.S.A., October 4-8, 1992, 160-170.

Jamieson, J.B. and Johnston, C.D. 1995. Monitoring a shear frame
stabilityindexandskiertriggeredavalanchesinvolvingper-
sistentsnow-packweaknesses.In:Proc.Int.SnowScience
Workshop, Snowbird, Utah,U.S.A., October 30- November 3,
1994, 14-21.

Narita,H.1980.Mechanicalbehaviorandstructureofsnow
under uniaxial tensile stress. Journal of Glaciology, 26 (94), 275-
282.

Schweizer,J. 1993. The influenceof thelayeredcharacterof
the snow cover on the triggering of slab avalanches. Annals of
Glaciology, 18, 193-198.

Schweizer, J., Schneebeli, M., Fierz, C. and Föhn, P.M.B. 1995a.
Field experiments on the dynamic response of the snow cover.
Surveys in Geophysics, 16, 621-633.

Schweizer, J., Camponovo, C., Fierz, C. and Föhn, P.M.B. 1995b.
Skier triggered slab avalanche release - some practical implica-
tions. In: Proc. Int. Symposium:Sciences and mountain - The
contribution of scientific research tosnow, ice and avalanche
safety, ANENA, Chamonix, May 30 - June 3, 1995, 309-315.

77

static normal force for a line load (500 N/m). Assuming normal and
uniform loading of theload cell (area 0.25 m²) the normal stress
would correspond to four times the normal force.

DISCUSSION

The measurements show the importance of the weak layer
depth. The impact substantially decreases with increasing
depth, explainingwhy triggering points are often observed
near rocks or to the marginsof a slope, wheresnow depthis
smaller and additionallythe snow cover weaker in general.
In particular for the load stepsstandingatop andweight-
ing thetype of sublayeringof the snow cover is more impor-
tant, or at least as important, as the weak layer depth. Hard
layers causea bridging effect, which distributes the skier's
impactoveralargerarea,butlessefficient in depth.Ski
penetration is directly relatedto layering (surfacehardness)
and is important as well,because the effective weak layer
depthdecreases with increasing skipenetration,thereby
increasingstress. However,the large variety of snow cover
conditions makes it very difficult to derive simple rules.
Measurements done by the ski manufacturing industry
and forbiomechanical studies indicate that the dynamic
load of skiing (snowplough and parallel turning) is most
comparable to the impact measured on the snow surface
for the load step weighting. For short turns and fast skiing
the impact is greaterand similarto the measured one for
jumping. As shown in Fig. 4 the calculated static load due
toalineloadcorrespondtothemeasuredimpactfor
weighting and can therefore simulate the impact forski-
ing,however it does not account forlayering.