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 Fig.5: Hard slab stopped by steel bridges, Val Giandains-Pontresina
smallslabshavetobepreventedaswell,thedistance
between the lines of structures should be reducedand the
supporting plane should be more dense.
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SAFETY ANALYSIS OF AREAS CONTROLLED WITH
SUPPORTING STRUCTURES
Afterthe building of supporting structures the protective
effect has to be taken into account forthe revision of haz-
ard mapswithrespect toland-use planning.Supporting
structuresreducethe probability of avalanche fractureand
theirextent.Thefollowingquestionsarecrucial:which
degreeofsafetycanbeattained belowcontrolledareas,
how much smaller arethe hazard zones? According to our
experiencethereareno generalsolutions. Every singlecase
has its own characteristics. InSwitzerland theresponsibles
are
verycautiousinreducing hazardzones.Changing a
red zone (high hazard) into a white one (no hazard) after
the
realization ofsupportingstructures isanexception,
often a blue zone (low hazard)is introduced. Todetermine
the
residualriskbelowacontrolledareathefollowing
criterias should be checked:
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Extent of the starting zone
Often the terrain of starting zones is very complex.Be-
side orbelow the main starting zone which is control-
led there are secondary ones which are then decisive
foravalanche dynamic calculations.The frequency of
these avalanchesis often smallercompared with those
of the main path.
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Arrangement of structures
Thearrangement ofthe structuresshould correspond
to the Swiss Guidelines(1990). Important points are the
upper and lower boundaries of the controlled area, the
slope distance between the lines of structures and the
lengthof the lines. In every starting zonethereareplaces
like narrow gullies or steep rockcliffs that are more dif-
ficult to control. Avalanchesreleasedincontrolledareas
are calculated with reduced flow rates due to smaller
velocities or lower turbulent friction parameter x . Either
the speedis calculatedafter adistancethat corresponds
to the slopeparalleldistancebetweentwo structurelines
orthe turbulent friction parameterxis reduced to 280
m/s2 .
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Extreme snow depth
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Fig.6: Snow net andsteel bridges in a springtime situation, Oberalp
According totheSwissGuidelines (1990)supporting
structures are designed for the 100-yearextreme snow
depthexpected atthe site.Howeverforhazard map-
ping events up to300years have to be considered in
Switzerland. Thereis a risk that structures are filled up
beforeanimportantsnowfalloccursandaslabis
releasedover the structures. At Weissfluhjoch (2540 m
a.s.l.) maximal snow depths are recorded normally be-
tween March and May and the most frequent snowfall
periodsare inaverage between Decemberand Febru-
ary. There are no approved methodsyet todetermine
realistic fracture depthfor avalanche dynamic calcula-
tionsasafunctionofextremesnowdepthand fora
return period T of 300 years.In practice the following
rough estimates could be used:
I.It is assumed that the difference DH between the struc-
ture height H kand the 300 years extreme snowdepth
H
meanex texpected inthe starting zonecorrespondstothe
fracture depth. The necessary data originate from
observationseriesoverup to60yearsofthe SFISAR
network.With the mean slopeythe slab thickness d
can be computed fromthe snow depth differenceDH0
according to Salm et al.(1990).
Example forWeissfluhjoch (2540m):
Hex t =456cm (T~300years)
Hk =411cm (T~100years)
[!]H =456-411=45cm
II.It is assumed that theprobability P
ekthat thesnow height
equals the structurheight Hktimes the probability P
fora certain increase of snow depth within 3 consecu-D
tive days(H3,is equal to1/300,corresponding tothe
probabilityused in hazard mapping:
Example forWeissfluhjoch (2540m):
Hk =411cm - D(T~100years- Pk=1/100
1/300=1/100 - PD- P
D=1/3(T D=3years
An increase of snow depth within 3 consecutive days
DH3of79cmcorrespondstoa returnperiodTDof3
years.Thisisnearlythe doublecomparedtoI.Prob-
ably itisoverestimated because of lessintense snow-
fall during the time of maximum snow depth. Withthe
meanslopeythe slabthicknessd
0can becomputed
fromDH3 according to Salm et al. (1990).
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