The influences of geological and land use settings on shallow landslides triggered by an intense rainfall event in a coastal terraced environment

Several factors (including slope morphology, geological and structural settings, the mechanical properties of soils, weathering, hydrological and hydrogeological conditions and land use changes) can affect the origin and development of shallow landslides induced by rainfall. On 25 October 2011, an intense rainfall occurred in a wide area of Italy between eastern Liguria and northern Tuscany, triggering thousands of shallow landslides and causing widespread flooding. Floods and landslides caused severe damage to villages and infrastructure and caused 13 fatalities. This study was conducted to investigate the shallow landslides triggered by the rainfall in the Vernazza basin (Cinque Terre in eastern Liguria), where landslides and debris floods caused 3 fatalities and damage of approximately €130 million. The Vernazza basin, which is characterised by steep slopes, geological heterogeneity and unusual land use settings (approximately 50 % of the slopes have been terraced for centuries), was chosen as the focus of this study, conducted to assess the influences of several factors on landslide occurrence. Field surveys and engineering geological investigations, including laboratory and in situ tests, were carried out at various sites to assess the stratigraphic and geotechnical features of the soil slope covers. A basic engineering geological zoning was carried out, taking into account the bedrock lithology, the geotechnical properties of the soils and the land use settings. On the basis of a detailed inventory of the shallow landslides that occurred, the distribution of the landslides was analysed with respect to the slopes, the bedrock lithology, the land uses and the engineering geological setting. Terraced areas on impermeable bedrock were identified as the main landslide-prone areas. The results of the study also showed that the time since abandonment of terraces plays an important role in the occurrence of shallow landslides induced by rainfall.