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In this work, we apply a physically-based model, namely the HIRESSS (High REsolution Stability Simulator) model, to forecast the occurrence of shallow landslides at regional scale. The final aim is the set-up of an early warning system at regional scale for shallow landslides. HIRESSS is a physically based distributed slope stability simulator for analysing shallow landslide triggering conditions in real time and in large areas using parallel computational techniques. The software can run in real-time by assimilating weather data and uses Monte Carlo simulation techniques to manage the geotechnical and hydrological input parameters. The test area is a portion of the Valle d'Aosta region, located in North-West Alpine mountain chain. The geomorphology of the region is characterized by steep slopes with elevations ranging from 400ma.s.l. of Dora Baltea's river floodplain to 4810ma.s.l. of Mont Blanc. In the study area, the mean annual precipitation is about 800-900mm. These features lead to a high hydrogeological hazard in the whole territory, as mass movements interest the 70% of the municipality areas (mainly shallow rapid landslides and rock falls). In order to apply the model and to increase its reliability, an in-depth study of the geotechnical and hydrological properties of hillslopes controlling shallow landslides formation was conducted. In particular, two campaigns of on site measurements and laboratory experiments were performed with 12 survey points. The data collected contributes to generate input map of parameters for HIRESSS model. In order to take into account the effect of vegetation on slope stability, the contribution of the root cohesion has been also taken into account based on the vegetation map and literature values. The model was applied in back analysis on two past events that have affected Valle d'Aosta region between 2008 and 2009, triggering several fast shallow landslides. The validation of the results, carried out using a database of past landslides, has provided good results and a good prediction accuracy of the HIRESSS model both from temporal and spatial point of view. A statistical analysis of the HIRESSS outputs in terms of failure probability has been carried out in order to define reliable alert levels for regional landslide early warning systems. [ABSTRACT FROM AUTHOR]

Vegetation plays a crucial role in slope stability affecting soil behaviour through many hydrological and mechanical processes. At the catchment scale, the hydrological effects of interception, suction, evapotranspiration and infiltration strongly affect runoff processes, whilst at the local scale the mechanical effect of root reinforcement is the leading factor for slope stability. Root systems of plants increase the shear strength of soils through a combined action by the large and the small roots: large woody roots can anchor the superficial soil layers to more stable substrates crossing potential planes of weakness; small roots strengthen the bounds with the soil particles increasing the overall cohesion of the matrix soil-roots. The belowground conditions that determine the root reinforcement are extremely variable, so that field measurements of the additional cohesion provided by roots can vary by an order of magnitude even for plants of same species and age. The extreme variability of this parameter coupled to the practical difficulties in assessing its spatial variations, especially for large areas, represent a significant limit in including proper values of root cohesion in slope stability models.In this study we analysed the effect of the roots cohesion on slope stability simulations obtained using the HIRESSS model. HIRESSS is a physically based distributed slope stability simulator developed to provide results in near real time and for large areas. For the latest version, the simulator was modified to insert the root reinforcement among the geotechnical parameters that considers to compute the factor of safety in probabilistic terms. A comparison of the results between two different simulations was performed: considering a large area (900 km2) and 30 days of precipitations, in one case the root cohesion was set to zero for all the area, in the other case different values of the parameter based on the plant species was instead inserted in the model.To build a map of the root cohesion for the study area, the following solution was adopted to solve the problem of the evaluation of the parameter: the distribution of plant species in the area was obtained from CORINE land cover 2012 4th level map, then a value of root cohesion and a range of variation was defined for each plant species based on the most recent literature in this field, finally, to reproduce the natural variability, the root reinforcement was treated as variable in Monte Carlo simulations, as well as the other geotechnical parameters.The results of the simulations for the study area were processed and analysed in order to evaluate the effect of root cohesion on failure probabilities. The results were also analysed considering landslide actual events to assess the contribution of the parameter to the HIRESSS forecasting capabilities. [ABSTRACT FROM AUTHOR]