The increasing availability of digital elevation models (DEM) makes
it possible to perform quick slope hazard assessments using semi-automatic
procedures. The main morphological and structural features of a landscape
can, for example, be identified using a DEM, taking into account
its mesh size.
The geomorphological concept of base level, which is defined by the
lowest level that can be eroded by a stream, can be useful for landslide
identification and hazard assessment. Assuming that erosion by landsliding
can affect only a limited thickness of the slope ? i.e. from 0 to
approximately 50 m ? during a period of 1,000 to 10,000 years, a
short-term local base level is thus defined. This concept means that
all slope volumes that are not supported at their bottom can slide
rapidly or slowly down towards the valley.
The computation of the base level uses the streams as invariant levels
of the topography. The base level is thus anchored to the lines defined
by the streams. Depending on the position of the upper part of the
streams compared to the crest, the highest crests of the mountain
range can also be considered as invariant. At the term scale of landslide
activity, only rock-falls can affect the flanks of the crest. Large
landslides occur rarely, because of the low force involved: only
large landslides affecting the entire slope can affect crests. If
necessary, an erosion function can be used to undercut the slope
in order to cause its destabilization.
The calculation of the short-term local base level is derived from
the procedure used to trace the background of a physical signal such
as an X-Ray diffraction spectrum. Different possibilities which can
be divided into two categories exist: 1) static procedures, which
converge to a base limiting short term local base level, and 2) dynamic
approaches that take erosion processes leading to a dynamic base
level depending on the duration of the process of ?base level definition?
into account.
Let us consider that all the slope volume located above the short-term
local base level can slide on this base level surface. Different
procedures allow to compute the ?weight? of the pixels (volume in
excess above the base level) that potentially bring additional stress
directly or indirectly on a pixel situated below. This leads to a
first, simple approximation of the force acting on each pixel. The
procedure for computing the involved pixels is based on a routine
similar to the one for watershed analysis. The streams can be determined
artificially by a routine, in order to avoid the problem of the sedimentation
and/or to take into account the erosion by the streams.
Results indicate a good agreement between highly stressed zones and
observed active rockfalls areas. In order to refine the hazard assessment,
the highly stressed zones can be matched with other parameters such
as fracturing or estimated water table level. The rock-fall activity
indicated by active scree deposits must also be crossed with the
results of the above method that indicates the high likelihood of
large rock-falls.