Zoning and mapping landslide hazard in the Castelnuovo di Garfagnana region (Tuscany, Italy)

ISPRA (Italian Institute for Environmental Protection and Research) and Tuscany Region Administration recently promoted a research project, aimed at assessing and zoning landslide hazard. The project considered the region of the map n. 250 «Castelnuovo di Garfagnana» (Geological Map of Italy at 1:50,000 scale), mainly including the middle and upper Serchio River basin (Italy). The selected area, about 630 km2 wide, exhibits peculiar geological and geomorphological features, severe climatic conditions (1500-2000 mm of rainfall/year, on average) and high seismicity, which cause widespread landslide hazard for population and facilities. Following a multidisciplinary approach, the data coming from geological, geomorphological and geo-engineering characterization were transformed into GIS-oriented layers and matched with the actual landslide distribution, which was mainly concerned with the most representative landslide types (slide and complex slide-flow movements). Conditional analysis was firstly applied, aiming at evaluating the importance of the considered instability factors. Then an advanced statistical method (Logistic Regression) was applied, to evaluate the effectiveness of the predisposing factors and to provide the hazard ranking of the mapping units. So, the methodology proceeded step by step, as follows: - on site, 1:10,000 scale geomorphological survey, aerial view interpretation and performing of the landslide inventory map; - geo-engineering investigation and in situ and laboratory tests, to analyse physical and mechanical properties of rocks (discontinuity characterization, compression strength, rock mass classification) and soils (grain size, consistency); - organization of the spatial distribution of the considered factors in different layers, each related to a specific factor. The spatial overlay of the layers and their matching with the landslide distribution lead to connections between different instability factors and landslide occurrence; - GIS supported statistical analysis (spatial analysis, conditional and multivariate analyses, neural network technique), so allowing to supply hypothetical connections with an objective and quantitative response; - construction of a final landslide hazard map at 1:50,000 scale. In this map, basing on the stability probability, the landslide hazard is ranked into five classes (very low, low, middle, high and very high hazard). At present, the final map depicts spatially defined landslide susceptibility areas, and no estimate is given about the time of occurrence. The next steps of the research will confront these results with the critical rainfall thresholds for triggering landslides and with the rainfall infiltration models, in order to realise early warning systems and protect population, villages and activities.