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Supplements of Geografia Fisica e Dinamica Quaternaria
Volume VII - 2005

 

Proceedings of the Conferences “Mountains and Plains”

BRANDOLINI P., FACCINI F., NICCHIA P. & TERRANOVA R.

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)

Pages 69-77

Abstract

Erosion and inst ability triggered by brief, but heavy rainfall are processes that represent one of the most frequent ly occurring environment al 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 land slide took place on the left side of the Rio del Culeio drainage area. It involved a volume of about 28,000 m’ 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 det achment. The central part of the landslide skimmed past several buildings located on the adjacent ridge and also destroyed rwo 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 rwo inte rpretative geologic al sections. This work was followed by seismic refraction surveys and geotechnical labor atory testin g. Geoph ysical pr ospectin g 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 berween 35 and 46. The unit weight soil proved to be in the 17.5-18 kN/m J range; permeability was very low, as the coefficient was equal to 2.73 xlO- 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 stru ctural nature, hum an activities carried out over the last few decades are held to be a determining factor. These activiti es led to the almost total abandonment of the area , which, in turn, led to th e degradation of the dry-stone walls, above and underground, connected with the ancient system of terraced slope cultivation. The landslide involved the loose soil, which is characterized by a silty-clayey particle size with minor coarse-grained component, and also the altered and fractured surface portion of the underlying slates. The stratum joint ‘s landslide position certainly contributed to the planar sliding of the debris cover. Th e land slide started up by means of a mechanism characterized by residual strenght state and then the movement develop ed into a complex type, which is partly att ribut able to tr anslation and partly to earthflow.

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