Geography Reference
In-Depth Information
given magnitude; and
total risk
is the expected loss
for the time period and place under consideration.
The concepts embodied in this equation
acknowledge that risk, in any given place, is a
function of the relationship between the physical
and human environment.
One of the most important activities in both
risk assessment and risk mitigation is the analysis
of previous events. The East Abbotsford landslide
disaster (see Box 6.1 on p. 86) provides an example
of event analysis.
At the point of failure, FoS=1.0. In many
engineering studies, stability analysis is the only
way in which the probability of occurrence is
determined, and hazard is implicitly assumed to
be inversely proportional to the factor of safety.
This form of analysis requires detailed information
on shear strength, slope hydrology, slope geometry,
and the shape and position of the potential failure
surface. Such information may require expensive
subsurface investigation, as well as field and
laboratory testing.
Until recently, either assumptions or detailed
measurements on pore water pressure were
required before stability analysis could be
performed. However, computer models are now
available that simulate changes in slope hydrology
in response to rainfall while continuously
analysing the factor of safety on numerous
potential shear planes throughout the slope. One
of the most successful of these models (the
CHASM model) has been developed through the
extensive fieldwork carried out largely by
geographers (Anderson
et al
. 1988). With such
techniques, it is possible to determine the
magnitude of rainfall required to produce failure
on a given slope. The return period of this
threshold value can then be determined from the
climatic record to provide a measure of the
probability of occurrence of landsliding.
Site assessment
The question of where a landslide may occur may
not always be a major factor in landslide hazard
analysis. This is because the site of concern may
have already been identified by the potential risk
or by the fact that a landslide already exists and
presents a serious threat. Such situations can occur
along highways or reservoirs, or near population
centres or valuable assets.
The assessment of hazard at an important site
usually takes the form of stability analysis. In the
first instance, this may be done by qualitative
observations. For example, if landslide features are
present, an attempt may be made to determine
whether the slide is still active, or when the last
movement took place. There are many features
that can be used to determine the state of activity
(Crozier 1984). If the site has no evidence of
previous movement, then stability may be
determined by the 'precedence' approach. This
involves comparing both stable and unstable slopes
in the same terrain in order to identify the
threshold conditions (e.g. slope angle and height)
that have been associated with landsliding in the
past. The site of concern is then compared with
these conditions, and if it is found to have
similarities to failed slopes, a more detailed
quantitative analysis may be required.
Quantitative stability analysis compares the
magnitude of resisting forces to the magnitude of
shearing forces, expressed as a factor of safety (FoS):
Regional assessment
From a planning and management perspective,
territorial authorities in many countries are
required to make an assessment of the landslide
hazard within their jurisdictions. The scale of this
requirement generally precludes the use of
detailed geotechnical investigations and stability
analysis. Instead, other techniques are employed
that require careful analysis of the terrain and use
of existing information sources. Geographers have
been at the forefront of regional landslide hazard
assessment employing techniques such as terrain
analysis, geomorphic mapping and geographical
information systems (GIS) (Dikau 1989). Many
different regional hazard assessment methods are
currently in use (Varnes 1984; Crozier 1995), and
