Abstract

Debris flows are high-speed and unpredictable phenomena, considered among the main sources of hazard worldwide, since they can affect structures, the economy, and human lives. Rainfall typically triggers these events, causing the flowing of the unconsolidated soil downslope. This work focuses on debris-flow events characterized by multiple triggering areas, which are extremely complex since they involve a spatial sequence of numerous triggers in a relatively small portion of the slope. Numerical modelling of this type of phenomenon can contribute to hazard and risk assessment, which is key to designing effective mitigation structures. In this article, two different models are applied for triggering and propagation, respectively. The former computes the transient pore-pressure changes and the consequent factor of safety variation caused by rainfall infiltration, inducing the triggering of the event. The latter is a depth-averaged numerical model that simulates the event runout, and whose parameters are calibrated through back-analysis. The applicability of the two combined approaches is tested through modelling of an historical event in Southern Italy, which was characterized by large mass releases from multiple triggering zones. Residential areas were hit, suffering serious consequences. Two rheologies are compared to individuate the most suitable propagation model for the study case and obtained results are commented.

Introduction

Debris flows [1] are dangerous events characterized by high kinetic energy, often causing casualties and serious damage to the economy. They consist of three main phases: triggering, propagation and deposition. The most common triggering cause is rainfall. Water infiltrates inside the susceptible soil, thus increasing the pore water pressure, and reducing the shear strength. As a consequence, the soil can slide downstream [2]. In this context, the numerical modeling of the phenomenon can help in risk assessment, and in designing more effective protective structures.

The article describes the triggering and runout phases through numerical modelling. The former is approached through a program for the grid-based slope stability determination, by calculating the changing pore water pressure and consequent factor of safety variation caused by rainfall (TRIGRS [3]). The latter is carried out with a depth-averaged model, based on a continuum mechanics approach (Rash3D [4]). The mass behavior is simulated comparing two rheologies, whose parameters are calibrated through back-analysis. This modelling approach is tested on an event happened in 2009 in Southern Italy [5]. The phenomenon was characterized by multiple triggering areas on the slope, and the involved mass was large. A settlement was hit, causing extensive damages and numerous causalities.

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