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on the snow quality, the height of the natural snowpack,
the fracture heightandthe densityof the supporting plane.
The efficiency of steel bridges with a distance between the
crossbeamsofabout25cmisgenerally satisfactory.For
cohesive or wind-depositedsnow whichis typical of major
avalanchesituations the catchingcapabilityis up to 100%.
Very loose snow in combination with small fracture depth
is more problematic.Then the catching capability can be
much less than 50%.
In two areas controlled with steel bridgesand snow nets
the differences between the two types of structures were
studied:
*At the Oberalp site the catchingand retarding effect of
snow nets with a mesh width of 25 cm and a density of
about 7% was negligible (Fig.6). During the same situa-
tions the steelsnow bridgescaughtanimportant amount
of the avalanchingsnow.
*To ameliorate the catching effect in the Duchli area the
nets were covered with dense meshrope stripes fixed
diagonally on the net in distances of 50 cm.A density
of 50% could be achieved. Observation shows that even
withthesestripesthe catching effectistoolow.The
distance of50cmseemstobetoolarge tostoploose
sluffs.Alsointhissituationthelowersteelbridges
caught most of the snow flowing through the nets.
Toimprove the catchingcapability of the nets the support-
ing plane should be protected with a wire net with a mesh
width of 5 cm or dense meshrope stripes diagonally fixed
indistancesof25cm.Noprematurefillingupofthe
structuresbecauseof the higher densitycould be observed.
Mostlythesesmallslabsflowingthroughthestructures
are not destructive to the objects to beprotected. If such
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