SOIL-WATER PROCESSES AND SEDIMENT CONNECTIVITY IN RAINFALL-TRIGGERED LANDSLIDES: A comparative analysis between small catchments in Italy and Brazil

Rainfall-induced landslides are natural processes inherent to the sediment dynamics, fundamentally acting in landscape evolution, while favoring the occurrence of hazardous conditions. Given their potential to mobilize large volumes of sediments from the release zones, they play a major role in sediment production in mountain basins, while affecting sediment connectivity through the deformations produced on the landscape.

These landslides are triggered out by the imbalance of shear strength and shear stress acting on a soil layer, promoted by the presence of water. The soil wetting through infiltration, namely, the vertical transfer of water from the surface layer of the soil to its interior, during intense precipitation events, has thus historically been associated with landslide triggering.

Whereas precipitation intensity and/or volume are recurrently related, the failure of a slope can occasionally be caused by precipitation events with magnitudes that are lower than those that have previously occurred. The triggering is dependent on the overlay of hydrological processes with different spatial and timescales, including the antecedent moisture conditions of the soil and the water storage in the watershed. Thus, different processes that act in the filling, storing, and draining of water in the soil, from seconds to weeks prior to the triggering, as preferential flow or bedrock infiltration and exfiltration are all equally relevant to understand and quantify landslide behavior and sediment connectivity. Furthermore, the soil moisture conditions not only govern landslide triggering but also their potential to fluidize and travel across longer distances, influencing their connectivity.

In this way, we analyzed how soil-water processes affect the filling, storing and draining dynamics during landslide triggering and their effects on sediment connectivity. Two study areas with distinct climatic and geomorphological characteristics located in southern Brazil (Mascarada River Catchment, Rio Grande do Sul) and northern Italy (Gadria River Catchment, Autonomous Province of Bozen-Bolzano) were analyzed. Field collected data was used, together with physical-based models to simulate slope stability as well as saturated and unsaturated flow and water retention in the soil layer. Scenarios including infiltration, preferential flow and bedrock exfiltration were simulated to assess whether this increase in uncertainty and complexity brings better results to the representation of the processes.