Interplay of rheology and entrainment in debris avalanches: a numerical study

Abstract

Flow-type landslides are a major global hazard. They occur worldwide, and are responsible for a large number of casualties, significant structural damage to property and infrastructure, and economic losses. The features of debris avalanches are particularly important, as they involve open slopes and affect triangular source areas when initial slides turn into avalanches through further failures or eventual soil entrainment. In this paper, the propagation stage of debris avalanches is numerically modelled to provide information such as the propagation pattern of the mobilized material and its velocity, thickness, and run-out distance. The use of a “depth-integrated” model has the following advantages: (i) it adequately accommodates the irregular topography of real slopes, which greatly affects the evolution of the propagation stage; and (ii) it is less time consuming than full three-dimensional approaches. The model is named “GeoFlow_SPH” and has previously been applied to theoretical, experimental, and real case histories. The behaviour of debris avalanches is analysed with particular attention to the apical angle, one of the main features of this type of landslide, in relation to soil rheology, hillslope geometry, and the geometric aspect ratio of the triggering area. The role of bed entrainment is also investigated with reference to differences in steepness of the uppermost parts of open slopes. First, simplified benchmark slopes are analysed using both water-like materials (with negligible shear strength) and debris-type materials (saturated frictional soil). Next, the paper addresses three important case studies from the Campania region of southern Italy (Cervinara, Nocera Inferiore, and Sarno), where debris avalanches occur in pyroclastic soils that originated from the eruptive products of the Mount Vesuvius volcano. In all of the cases analysed, the effects of erosion rate are compared with those of simulated soil propagation height, run-out distance, and velocity. In a novel contribution to the existing research, the results obtained from analysis of both the benchmark slopes and the real case histories indicate that landslide propagation depends on the interplay of rheology and bed entrainment. In particular, increased erosion growth rates correspond to shorter run-out distances, lower velocities, and larger propagation depths. It is further shown that erosion depth increases with either friction angle or the consolidation coefficient of pore-water pressure; the latter reduces bed entrainment but does not significantly affect the apical angle of debris avalanches. Globally, the results are particularly satisfactory because they indicate that the GeoFlow_SPH model is a suitable tool for the analysis and forecasting of debris avalanches.