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human lives and property from damage caused
by water, mass movements, snow avalanches
and forest fires. It is now a requirement that
canton administrative units produce registers of
hazard types and maps of hazard areas for use in
land-use planning. Raetzo et al. (2002) describe
a three-step procedure for the hazard assess-
ment of mass movements. This involves hazard
identification, which consists of classification of
landslides, mapping of landslide phenomena and
a register of slope instability events. The second
step involves hazard assessment, which requires
the determination of the magnitude or intensity
of an event over time and the expression of this
on a hazard map. The hazard maps are designed
to show degree of danger and by convention use
colour coded danger zones (red - high danger;
blue - moderate danger; yellow - low danger;
and white no danger). These categories are closely
related to the intensity and probability of the
event occurring. The third step is risk manage-
ment and land-use planning. This utilizes the
hazard map, which is the basic document for
land-use planning. This feeds directly into the
local authority planning process. For example,
standard colour coded hazard zones indicate
the degree of development that can occur, for
example: in red zones construction is prohibited;
in blue zones construction is allowed only if
certain conditions are met; and in yellow zones
construction is permitted without additional
restrictions.
In Austria a similar scheme for the classifica-
tion and mapping of hazardous mountain events
has been produced for torrents, avalanches and
floods (Aulitzky 1994). This is based on work
undertaken by the Austrian Forest Technical
Service for Torrent and Avalanche Control
and is legally bound by the 1975 Forest Law. It
was realized that following a series of major
mountain disasters in the 1960s in Tyrol that
two-thirds of losses were related to some form of
human influence. Because most of these hazards
occur on alluvial cones and fans a formula was
developed which delimits different degrees of
risk across the fan surface in relation to the
amount of debris deposited ( G ) (Hampel 1980).
This is based on an assessment of the gradient of
the alluvial fan ( J ) and the mean particle size of
material of the sediment transported ( d m ):
55 d m 1.65 ) 1/(0.42−0.4 d m )
3.6
( J %
G (%)
=
This simple formula is very sensitive to the
measured fan slope (Aulitzky 1994). Additional
criteria are used for hazards in torrents (Torrent
Index method) and landslides. This informa-
tion is used to produce hazard maps, which
show hazard zones for events with an estimated
return period of less than 150 years. These include
a red zone (permanent settlement and traffic
prohibited); yellow zone (damage certain - some
development allowed but with restrictions) and
a white zone (no recognized hazard).
2.5.2 Monitoring
Hand-in-hand with hazard assessment and
mapping goes hazard monitoring. In areas that
have been established to be at high risk hazard,
monitoring will be undertaken. Usual hazard
monitoring involves direct measurements of
the behaviour of a river, slope or glacier, which
can be relayed in real-time or over short time-
scales to an observer who can plan a response,
for example activate a warning and evacuation
system.
The Randa rock slide discussed earlier (sec-
tion 2.3.3) provides an excellent example of
how both short-term (emergency) monitoring
and longer term monitoring provide important
information. The initial rock slide at Randa in
April 1991 prompted the authorities to install a
seismographic and geodetic warning system. As a
result the second event in May was forecast from
field evidence and geodetic and seismic surveys
and the area was successfully evacuated. Prior to
the second slide geodetic displacement measure-
ments indicated an accelerated motion of the rock
mass (Gotz & Zimmermann 1993) (Fig. 2.12b).
Approximately 2.5
10 6 m 3 of active block are
still being monitored. The rock mass is still mov-
ing towards the south-east at a maximum rate of
15 mm yr −1 (Ornstein et al. 2001). The sliding
rock mass is bounded by shallow dipping joints
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