A hybrid approach for estimating hydro-geomorphological rainfall thresholds to shallow landslides

The societal risk related to rainfall-triggered rapid debris flows is commonly managed in urbanized areas by means of early warning systems based on monitoring of hydrological parameters (such as rainfall or soil moisture) and thresholds values. In Alpine catchments, this type of landslides is recurrent and represent one of the major geohazards. Debris flows are typically initiated by high-intensity rainstorms, prolonged rainfall with moderate intensity or snow melting. They frequently happen in situations of temporary infiltration into soils that are initially unsaturated. During significant rainfall events, the rise in pore water pressure can become crucial for the stability of slopes in particular areas. This phenomenon relies on hydraulic and geotechnical characteristics, along with the thickness of the involved soils. This procedure can result in a local drop in shear strength, as both apparent cohesion and effective stress decline, while driving forces rise because of the increase in unit weight. Accordingly, this study estimates Intensity-Duration (I-D) rainfall thresholds at the site-specific and distributed scales by combining empirical and physics-based approaches and modeling of soil coverings involved in soil slips or debris slides inducing debris flows. The approach was tested for mountain slopes of the Valtellina valley (Lombardia region, northern Italy), which suffered several catastrophic landslide events in the last decades. The empirical approach was adopted to reconstruct physics-based slope models of representative source areas of past debris flows events. To such a scope, non-punctual but distributed data of hydro-mechanical soil properties and thicknesses were considered. Thus, to reconstruct the unsaturated/saturated critical conditions leading the slope instability, a combined hydrological modeling and infinite-slope stability analysis was adopted. This combined hydro-mechanical numerical model was used to attempt to determine a three-dimensional Intensity-Duration threshold for landslide initiation considering plausible rainfall for the Valtellina valley. Due to the lack of reliable records of past landslide hindering a thorough empirical analysis, the presented approach can be considered as a feasible approach for establishing a warning standard in urbanized areas at risk of shallow landslides triggered by rainfall. Moreover, findings highlight the importance of having access to spatially distributed soil characteristics to define and enhance input data for physics-based modelling. Finally, the proposed approach can aid an early warning system for the onset of shallow landslides by utilizing real-time rainfall monitoring or now-casting through a meteorological radar technique.