Dissesti geomorfologici, eventi alluvionali ed insediamenti antropici: il caso di Tribogna in val Fontanabuona (Appennino Ligure)

Geomorphological instability, floods and communities: the case of Tribogna in Fontanabuona valley (Ligurian Apennines)

Abstract

Erosion and instability triggered by brief, but heavy rainfall are processes that represent one of the most frequently occurring environmental problems of very recent decades. These processes have serious repercussions on land management, often jeopardizing infrastructures and communities. Considering the 21 landslides that affected the middle section of Fontanabuona valley (eastern Liguria) during the November 6th flood in 2000, this study focused on the analysis of the Aveno-Geppotti slope in the village of Tribogna. The landslide took place on the left side of the Rio del Culeio drainage area. It involved a volume of about 28,000 m³ of soil and rock into motion, was 240 m long and had an average width of 40 m. The lower part of the town of Aveno lies at the edge of the point of detachment. The central part of the landslide skimmed past several buildings located on the adjacent ridge and also destroyed two sections of the roadway. In the lower part, the landslide material passed over a third section of the road and reached the bed of the Rio del Culeio. The investigation first consisted of a detailed geological and geomorphological survey, which allowed to draw up a geomorphological map and two interpretative geological sections. This work was followed by seismic refraction surveys and geotechnical laboratory testing. Geophysical prospecting revealed a variable thickness of the debris up to 8 m thick covering a very altered bedrock of slate. Laboratory tests allowed us to classify the topsoil as inorganic silt characterized by medium compressibility (ML according to the unified system procedures) and a liquid limit between 35 and 46. The unit weight soil proved to be in the 17.5-18 kN/m³ range; permeability was very low, as the coefficient was equal to 2.73x10^-7 cm/s. An analysis of the pluviometric data available for the study area and surroundings was carried out. Rainfall in November 2000 was more than two times higher than the average for that month, with a maximum peak of almost 100 mm in less than 12 hours on the day of the landslide. Among the predisposing factors, in addition to those of geomorphological and structural nature, human activities carried out over the last few decades are held to be a determining factor.