Shallow landslides triggered by intense rainfall and land use setting: an example from Cinque Terre (eastern Liguria)

On 25 October 2011 very intense rainfall affected an area of ca. 1000 km2 between eastern Liguria and northern Tuscany (northwest Italy). The storm rainfall caused thousands of shallow landslides, widespread erosive and depositional processes, several floods, 13 casualties, evacuation of thousand of people, interruption of highways and railways, closure of many provincial roads and destruction of some bridges. Along the coast, the western sector of the Cinque Terre was affected by floods at Monterosso and Vernazza, causing four casualties and severe structural and economic damage. Cinque Terre are worldwide known because of their environmental value related to the presence of an historical terraced coastal agricultural landscape. Hence, this area was included in the Cinque Terre National Park and recognized as a world heritage site by UNESCO. Changes of social and economic conditions since the early 1900’s caused a progressive abandonment of cultivation on terraces, with negative consequences for the maintenance of dry stone walls leading to disruption of many sectors of terraced areas. The Vernazza basin, which slopes were terraced for about 50%, was the most affected by shallow landslides triggered by rainfall. A great amount of shallow landsliding materials (soils and debris from colluvial and anthropically reworked deposits and stones derived by dry walls of terraces), reaching the drainage network, increased density and energy of stream flows. A disastrous debris flood originated at the valley floor affecting the Vernazza village with mud and debris heights rising up to 4 meters in the centre of the village and, locally, up to 5-6 m. Geomorphological features of the basin such as small area, very steep slopes and short streams, with considerable erosive power and capacity to transport sediment, played a role in favouring geo-hydrological processes but also other factors are involved. This study deal with the analysis of the distribution of shallow landslides triggered by rainfall in relation to land-use in order to define the influence of this factor in landslide occurrence. The study highlights the high vulnerability of agricultural terraces, which were identified as the main landslide-prone areas. The lack of maintenance of the dry-stone walls could be a primary factor favouring shallow landslides. The study also confirms the positive role of vegetation cover for increasing the rainfall threshold for triggering shallow landslides. The obtained results, providing basic information for landslide susceptibility assessment, could be useful for identifying proper strategies for risk mitigation and land management.