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m 3/s to 2547 m 3 /s (Table. 1). Because of the smaller flow
rate thecalculatedrunoutinthesituationof1994is
about 210 mshorter.
Seewis - Vilan, 12.1.1995
The snowstorm from8th to 12th of January 1995 brought
large amountsof new snow to the easternSwiss Alps, with
cold temperatures and strong NW-winds. The increase of
snow depth within 3days wasabout 100cm.The mean
density of the fresh snow was 80 kg/m3 . The snow fell on a
thin and loose snowpack of only 0.2 to 0.5 m.The degree
of dangerin theSwiss avalanchebulletinwas high.Pictures
taken the 13th of January show that a big slab was released
about50mbelowand 100mbesidethe controlledarea
(Fig.3).The slabthicknesswasestimatedtobe 1m,the
width250mandthevolume20'000-30'000m 3 .The
avalanchestoppedaftera trackof1.5kmandaheight
differenceofmorethan700m.Smalldamagesto
afforestations were recorded.
Inthecontrolledarea between thelines ofstructures
No.10and11asmallslabwithawidth of28manda
thickness between 0.5 m and 1.7 m was observed too. Pro-
files of the snowpack (Fig.4) before and after the avalanche
made it possible to estimate the catching efficiency of the
structures. About 45% of the fractured snow was stopped
by the next and 30% bythe afternext line ofstructures.
The inclination of the surface of the banked-up snow was
about 7º.Only a few snowballs left the controlled area.
Between the next 4 lines of structures above line No. 10
long tensile cracks in thesnow pack were observed. Mostly
the cracks occurredalongthe supports becauseof the stress
concentration due to the perforation of the snowpack. The
propagation of the shear fracture and the release of a slab
seemedtobeimpededbytheadditionalcompressive
stresses dueto the structures. In the uncontrolledareathese
artificialzoneswithahighersnowpackstabilityare
missing.
Without detailed snow stabilitymeasurements it is not
possible toanswerthe question ifa big slab would have
beenreleased,ifthearea hadnot beencontrolledwith
structures.
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ANALYSIS OF THE EFFECTIVENESS
The analysis of considerable avalanchecycles in the win-
ters 1984,1994and 1995showgenerally satisfactory re-
sults.Nodestructiveavalancheswerereleased inareas
controlled by supporting structuresaccordingto the Swiss
Guidelines (1990).Only small slabs and loose snow ava-
lanches occurredbetween the structures.Outsideofthe
controlledareaslargeavalanches couldbeobserved.It
appeared thatone ofthemostproblematicpointsisthe
extent of the controlled area:
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Some slabs have been released above the controlled ar-
eas.Itiscrucialthat thefracture linelieswithinthe
back-pressure zoneof the topmost structures. Neverthe-
less in some cases the flowing avalancheswere retarded
by the lines of structures toa harmless degree (Munt,
February1984,Giandains,January1994).Butifthe
avalanches are too fast the structures canbe damaged.
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Some big avalanches were released belowand beside
controlled areas (Grindelwald First,Rietstöckli,Janu-
ary1995;Clavaniev,Crap Stagias,February1984).In
some cases avalanches spread into the controlled area.
There were areas only partly controlled either because
theconstructions were notyetfinished orbyinsuffi-
cientfinancial means.To prevent therelease ofcata-
strophic avalanches the whole starting zone has to be
controlled by structures.
It
is
ratherdifficulttoquantifythestabilizingeffectof
supporting structuresby means of field investigations. The
observed avalanches between the structures were mostly
releasedeither after very loose or heavy new snow falls or
duringspring time situations.Soft and wet slabs seem to
be more problematic than hard slabs. After theinitial shear
fracture and thesecondarycrowntension fracture,hard
slabs were often stopped by the next line of structure after
downslope displacement in the centimeter to meter range
without breaking into blocks (Fig.5). Soft and wet slabs are
breakingupimmediately. Furtherthe propagationof a shear
fracture is limited in a controlled area because the weak
layerisinterrupted bythelinesofstructuresand hasa
smallerextent due to the additional compressive stresses
behind the structures.
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