Methodological Approach in the Analysis of Two Landslides in a Geologically Complex Area: The Case of Varenna Valley (Ligury)

This paper provides a brief study of Varenna basin (Ligury) through the analysis of its instability features and mass movement proneness; this area, during the five years from 1991–1995, has gone through frequent episodes of flood damages and landslides. The quick slide of topsoil debris and its displacement into the stream has dramatically increased the over-alluvion along the riverbed reaches with a strong dissipation of energy, and has made the river more liable to an esondation; besides, in correspondence of a restricted hydraulic section it reaches, several new bank erosions, being themselves responsible for the triggering mechanism in the slopes. For mountain basins, as well as the present one, it is necessary to define geomorphologic and geotechnical characteristics of the landslides, rainfall triggering threshold and thickness critical rate of the materials involved in the mass movements, in order to set out new activities for the reduction of the flood and landslides integrated hazards. These criteria have been used in the analysis of two mass-movements which occurred in Varenna valley during the autumnal rains in 1993–1994; they are still representative of the geological characteristics of most of the examined basin: the figures derived from the study show that these typical landslides always occur in presence of serpentine-schists strongly influenced by tectonic movements and widely spread along the whole basin. The geomorphologic and geotechnical data have been compared to those derived from the survey for the assessment of the slope stability, using the “back analysis” method suggested by Jambu. The results allow to define several degrees of proneness to the instability and, as well as, the degree of geomorphologic hazards in all the above-mentioned area.