| Surface features are produced as a
result of internal deformation of active landslides, and are continuously
created and destroyed by the movement. Observation of their presence and
distribution, and surveying of their evolution may provide insights for
the zonation of the mass movement in sectors characterized by different
behaviour. The present study analyses and describes some example of
surface features observed on an active mass movement, the Slumgullion
earthflow, in the San Juan Mountains of southwestern Colorado. The
Slumgullion earthflow is one of the most famous and spectacular landslides
in the world; it consists of a younger, active part which moves on and
over an older, much larger, inactive part. Total length of the earthflow
is 6.8 km, with an estimated volume of 170 × 10 6 m 3
. Its nearly constant rate of movement (ranging from about 2 m per year at
the head, to a maximum of 6–7 m per year at its narrow and central part,
to values between 1.3 and 2 m per year at the active toe), and the
geological properties of moving material, are well suited for the
observation of the development and evolution of surface features.
In the last 11 years, repeated surveying at the
Slumgullion site has been performed through recognition of surface
features, measurements of their main characteristics, and detailed
mapping. In this study, two sectors of the Slumgullion earthflow are
analysed through comparison of the features observed in this time span,
and evaluation of the changes occurred: they are the active toe and an
area located at the left flank of the landslide. Choice of the sectors was
dictated in the first case, by particular activity of movement and the
nearby presence of elements at risk (highway located only 250 m downhill
from the toe); and in the second case, by the presence of many surface
features, mostly consisting of several generations of flank ridges.
The active toe of the landslide is characterized by
continuous movement which determines frequent variations in the presence
and distribution of surface features, as evidenced by the multi-year
observations there performed. In addition, monitoring of the inactive
material just ahead of the active toe showed that this sector is
experiencing deformation caused by the advancing toe. Mapping and
interpretation of the different generations of flank ridges at the
narrowest and central part of the active Slumgullion landslide evidenced,
on the other hand, the gradual narrowing of the mass movement, which was
accompanied by a reduction in the thickness of the material involved in
landsliding.
Multi-time observation of the surface features at the
Slumgullion earthflow allowed to reconstruct the evolution of specific
sectors of the mass movement. This low-cost approach, whose only
requirements are the availability of a detailed topographic map, and
repeated surveying, is therefore particularly useful to better understand
the kinematics of active mass movements, also in order to design the more
appropriate stabilization works. |