The Effects of Upstream Flexible Barrier on the Debris Flow Entrainment and Impact Dynamics on a Terminal Barrier

The destructive nature of debris flows is mainly caused by flow bulking from entrainment of an erodible channel bed. To arrest these flows, multiple flexible barriers are commonly installed along the predicted flow path. Despite the importance of an erodible bed, its effects are generally ignored when designing barriers. In this study, three unique experiments were carried out in a 28 m-long flume to investigate the impact of a debris flow on both single and dual flexible barriers installed in a channel with a 6 m-long erodible soil bed. Initial debris volumes of 2.5 m³ and 6 m³ were modelled. For the test setting adopted, a small upstream flexible barrier before the erodible bed separates the flow into several surges via overflow. The smaller surges reduce bed entrainment by 70% and impact force on the terminal barrier by 94% compared to the case without an upstream flexible barrier. However, debris overflowing the deformed flexible upstream barrier induces a centrifugal force that results in a dynamic pressure coefficient that is up to 2.2 times higher than those recommended in guidelines. This suggests that although compact upstream flexible barriers can be effective for controlling bed entrainment, they should be carefully designed to withstand higher impact forces.

Erosion effect in the movement process of rapid loess landslide using an improved two-layer model

<p>Erosion effect plays a significant role in the run-out process of a rapid loess landslide. This effect is manifested in bed entrainment and frontal plowing on terraced material during movement, leading to the volume amplification. Therefore, an improved two-layer model is proposed to describe the frontal plowing and bed entrainment in this paper. In addition, the bed entrainment rate is further calculated by introducing the bed entrainment physical model. The sliding mass and plowed material are assumed to be immiscible in this model, and the mechanical behaviour between the materials is simulated by considering the interaction force between the two layers. Furthermore, the governing equations are deduced from the mass and momentum conservation. It is then applied to analyze a typical rapid loess landslide, Dongfeng landslide. The results indicate that the bed entrainment and frontal plowing have a significant impact on the mobility of the landslide, which is mainly shown in the following two aspects: 1) the bed entrainment effect significantly increases the speed and volume of the landslide; 2) The frontal plowing effect will impede the motion of the frontal sliding mass, and there is a clear separation between the sliding mass and the plowed material, which is more consistent with the field observations. The improved two-layer model proposed in this paper can provide more reliable assessment to describe the rapid loess landslides with erosion.</p>

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

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.