Geography Reference
In-Depth Information
comprehensive definition, which helps to
distinguish landslides from the other
geomorphological processes, is: 'the downward
or outward movement of a mass of slope-
forming material under the influence of gravity,
occurring on discrete boundaries and taking
place initially without the aid of water as a
transportational agent.' As this second
definition indicates, landslides are more than
just a simple downslope movement of material.
The three most widely used classifications
involving landslides (Sharpe 1938; Varnes 1958;
1978; Hutchinson 1988) separate 'mass
movements' (Fairbridge 1968) into two
categories: 'subsidence' (which is the vertical
sinking of material) and those movements that
occur on slopes. These 'slope movements' are
then usually divided first into 'landslides', as
defined above, and second into the slower, more
widespread and ill-defined movements such as
'creep', 'sagging' and 'rebound'. Of all the types
of slope movement, it is landslides that have the
potential to undergo rapid movement, making
them a potentially dangerous form of natural
hazard.
Of the many different landslide classifications
in existence (Hansen 1984a) the system devised
by Varnes (1978) is often preferred because it is
simple and easy to apply in the field (Table 6.1).
The criteria used to define the landslide types are
mechanism of movement, shape of the failure
surface, degree of disruption and type of material.
These are all characteristics that reflect an aspect
of hazard or which need to be known in order to
carry out stability analysis. In applying this
classification, it is important to remember that the
material criterion refers to the original slope
material, not to what may subsequently appear in
the deposit.
In order to choose the most appropriate
method of reducing risk from the landslides
(whether prevention, control, avoidance or
compensation for loss) it is important to know
something about the range of factors that lead to
slope failure and how they operate. For example,
it may be found that risk can be reduced more
cheaply and more effectively by draining
groundwater from a slope than by zoning it as
unsuitable for use. Comprehensive lists of causative
factors are available (Varnes 1958; Cooke and
Doornkamp 1990; Crozier 1995), but it is useful
to simplify these into categories. One way of
doing this is to consider the function that various
factors have in changing the conditions of slope
stability.
Figure 6.1 introduces some concepts in slope
stability. It shows that a slope may pass from a
'stable' condition to a 'marginally stable'
condition and finally to the 'actively unstable'
condition, where the slope actually fails as a
Table 6.1
Landslide classification
Source:
Varnes 1978
