Geography Reference
In-Depth Information
eventually initiate a landslide, in which case they
become 'triggering factors'. The most common
triggering factors include rainstorms, earthquake
shaking and removal of support from the toe of
the slope. Finally, once a landslide begins to
occur, 'movement-controlling factors' take over.
These may determine, for example, how
disrupted the slide becomes, or how far it runs
out. All these factors ultimately influence the
stress conditions within the slope either by
reducing the shear strength or increasing the
shear stress (Selby 1993).
Figure 6.1
Stability factors classified by function.
landslide. Whether a slope is likely to move
through these stages depends on factors referred
to as 'preconditions' (or inherent factors). These
may include features of the locality such as
susceptible rock structure, weak material and
slope form. 'Preparatory factors', on the other
hand, are more active and produce changes that
make the slope more vulnerable to failure
without actually initiating movement. They
change the slope from a stable to a marginally
stable condition. Some of the most common
preparatory factors include deforestation (Plate
6.1), removal of the toe of the slope (either
naturally or artificially) and alteration of slope
drainage. Some preparatory factors may
IDENTIFYING THE HAZARD
The ultimate goal in landslide hazard assessment is
the successful prediction of the place of
occurrence of an event, its impact characteristics
and its relationship with time (Hansen 1984b). In
other words, identifying the threat from landslide
hazard means finding out what, where, when and
how dangerous? Indeed, this goal is common to
the assessment of all hazards. In the conventional
definition adopted by United Nations Disaster
Relief Organisation, 'time' and 'character' are
Plate 6.1
Rainfall-triggered
soil landslides in a part of
New Zealand that has been
deforested within the last
100 years. These landslides
seriously deplete the soil
resource and reduce
pasture productivity
(photograph: Hawkes Bay
Regional Council).
