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Abstract. Hazard assessment of shallow landslides represents an important aspect of land management in mountainous areas. Among all the methods proposed in the literature, physically based methods are the only ones that explicitly includes the dynamic factors that control landslide triggering (rainfall pattern, land-use). For this reason, they allow forecasting both the temporal and the spatial distribution of shallow landslides. Physically based methods for shallow landslides are based on the coupling of the infinite slope stability analysis with hydrological models. Three different gridbased distributed hydrological models are presented in this paper: a steady state model, a transient "piston-flow" wetting front model, and a transient diffusive model. A comparative test of these models was performed to simulate landslide occurred during a rainfall event (27-28 June 1997) that triggered hundreds of shallow landslides within Lecco province (central Southern Alps, Italy). In order to test the potential for a completely distributed model for rainfall-triggered landslides, radar detected rainfall intensity has been used. A new procedure for quantitative evaluation of distributed model performance is presented and used in this paper. The diffusive model results in the best model for the simulation of shallow landslide triggering after a rainfall event like the one that we have analysed. Finally, radar

[1] Grain size data from the deposit of the 1987 Val Pola rock avalanche (central Italian Alps) are compared with data concerning rock avalanching, rock fragmentation, and comminution. The Weibull distribution fits a small part of the entire particle-size distribution of debris samples, with a mean value of the curve shape factor of 0.54 ± 0.28. This is typical of multiple comminution, or fragmentation with much shearing. A fractal distribution fits over a greater size range. Computed fractal dimensions range between 1.3 and 3.2 within the deposit, with average values of about 2.6-2.7. These values cover the range between the theoretical values of the plane-of-weakness model (1.97) and the pillar-of-strength model (2.84) and are close to the theoretical value for the constrained comminution model (2.58). These suggest that both texturally mature and immature deposits are present and that more than a single comminution process acted during the rock avalanche motion. Variation of the grain size distribution within the deposit and grain size segregation show as trends in the fractal dimension and arise from variation in the fragmentation process. A variety of different physical and empirical laws suggest that 1-30% of the energy expended in the rock avalanche was consumed in fragmentation.

Forecasting the failure of large rock slides is difficult because of nonlinear time dependency and seasonal effects, which affect the displacements. Starting from the accelerating creep theory proposed by Voight, a method is suggested to forecast slope failures and to assess alert velocity thresholds using monitoring data. The 20 Mm3 Ruinon rock slide (Valfurva, Central Alps, Italy), susceptible to evolve into a rock avalanche, is studied. Three different evolutionary patterns of displacements have been recognized through the analysis of the monitoring data for a 5 year period. Data representing the surface-based large-scale behaviour of the rock mass were fitted by power-law curves, according to the "accelerating creep" model. Voight's equation has been expressed in terms of displacement and used to fit the data by nonlinear estimation techniques. Values for the controlling parameters (A, α, tf), representative of the mechanical behaviour of the rock mass approaching failure, have been determined both for single and multiple accelerating phases. "Characteristic velocity curves" have been computed by assuming these parameters are representative of the rock mass behaviour. Velocity threshold values for pre-alert, alert, and emergency phases have been computed. The method has been validated by collecting and analysing literature data for historical rock slope failures.Key words: slope stability, rock slide, accelerating creep, monitoring, failure forecasting, velocity thresholds.

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