Observation of surface features on an active landslide, and implications for understanding its history of movement

Abstract

Abstract. 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.