The mechanics of landslide mobility with erosion.

Erosion can significantly increase the destructive power of a landslide by amplifying its volume, mobility and impact force. The threat posed by an erosive landslide is linked to its mobility. No mechanical condition has yet been presented for when, how and how much energy erosive landslides gain or lose. Here, we pioneer a mechanical model for the energy budget of erosive landslides that controls enhanced or reduced mobility. Inertia is related to an entrainment velocity, is a fundamentally new understanding. This ascertains the true inertia of erosive landslides, making a breakthrough in correctly determining the landslide mobility. Erosion velocity, which regulates the energy budget, determines the enhanced or reduced mobility. Newly developed energy generator offers the first-ever mechanical quantification of erosional energy and a precise description of mobility. This addresses the long-standing question of why many erosive landslides generate higher mobility, while others reduce mobility. We demonstrate that erosion and entrainment are different processes. Landslides gain energy and enhance mobility if the erosion velocity exceeds the entrainment velocity. Energy velocity delineates distinct excess energy regimes. Newly introduced mobility scaling and erosion number deliver the explicit measure of mobility. Presented dynamical equations correctly include erosion induced net momentum production.

Cratering response during droplet impacts on granular beds.

This experimental work focuses on the cratering response of granular layers induced by liquid droplet impacts. A droplet impact results in severe granular layer deformation, crater formation and deposits in the vicinity of the impact center. High-precision three-dimensional imaging of the granular layer surface revealed important characteristics of liquid impacts on granular matter, such as singular asymmetric deformations of the layer. Our analysis also demonstrated that the impact energy and the granular packing, and its inherent compressibility, are not the unique parameters controlling the bed response, for which granular fraction heterogeneities may induce strong variations. Such heterogeneous conditions primarily influence the magnitude but not the dynamics of liquid impacts on granular layers. Finally, a general equation can be used to relate the enery released during cratering to both the impact energy and the compressibility of the granular matter. However, our results do not support any transition triggered by the compaction-dilation regime. Hence, higly detailed numerical simulations could provide considerable insights regarding the remaining questions related to heterogeneous packing conditions and its influence over the bulk compressibility and the compaction-dilation phase transition.

How well can we simulate complex hydro-geomorphic process chains? The 2012 multi-lake outburst flood in the Santa Cruz Valley (Cordillera Blanca, Perú).

Changing high-mountain environments are characterized by destabilizing ice, rock or debris slopes connected to evolving glacial lakes. Such configurations may lead to potentially devastating sequences of mass movements (process chains or cascades). Computer simulations are supposed to assist in anticipating the possible consequences of such phenomena in order to reduce the losses. The present study explores the potential of the novel computational tool r.avaflow for simulating complex process chains. r.avaflow employs an enhanced version of the Pudasaini (2012) general two-phase mass flow model, allowing consideration of the interactions between solid and fluid components of the flow. We back-calculate an event that occurred in 2012 when a landslide from a moraine slope triggered a multi-lake outburst flood in the Artizón and Santa Cruz valleys, Cordillera Blanca, Peru, involving four lakes and a substantial amount of entrained debris along the path. The documented and reconstructed flow patterns are reproduced in a largely satisfactory way in the sense of empirical adequacy. However, small variations in the uncertain parameters can fundamentally influence the behaviour of the process chain through threshold effects and positive feedbacks. Forward simulations of possible future cascading events will rely on more comprehensive case and parameter studies, but particularly on the development of appropriate strategies for decision-making based on uncertain simulation results. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Shock waves in rapid flows of dense granular materials: theoretical predictions and experimental results.

Strong shocks in rapid dense granular flows are studied theoretically and analyzed in detail to compare with benchmark experimental data. The experimental data includes particle image velocimetry measurements of dry granular flow following its continuous release from a silo. The rapidly moving material down the chute impinges on an obstruction wall erected perpendicular at the end of a long and steep channel. Impact leads to a sudden change in the flow regime from a fast moving supercritical thin layer to a stagnant thick heap with variable thickness. This flow configuration is particularly interesting because it is analogous to some hydraulic and aerodynamic situations. We present results about the depth and the velocity evolution and their comparisons with theoretical predictions associated with frictional granular flow equations incorporating anisotropic pressure conditions. These flow equations are integrated by implementing high-resolution nonoscillatory central differencing total variation diminishing schemes. The dynamical and geometrical effects of the flow will be discussed in detail. These include geometry evolution and depositions at supercritical and subcritical flows, the impact velocity, shock speed, its position and evolution, choice of numerical limiters, and the influence of friction angles on the dynamics and depositions. An excellent agreement between theoretical predictions and experimental observations will be demonstrated. These results can be applied to estimate impact pressures exerted by avalanches on defense structures or infrastructure along the channel and in the run-out zones, and to study the complex flow dynamics around the obstacles and in depositions when the mass comes suddenly to a standstill. Importantly, these results can form a basis for calibration of numerical simulations when strong shocks occur in granular flows.

The Savage-Hutter avalanche model: how far can it be pushed?

The Savage-Hutter (SH) avalanche model is a depth-averaged dynamical model of a fluid-like continuum implementing the following simplifying assumptions: (i) density preserving, (ii) shallowness of the avalanche piles and small topographic curvatures, (iii) Coulomb-type sliding with bed friction angle delta and (iv) Mohr-Coulomb behaviour in the interior with internal angle of friction phi> or =delta and an ad hoc assumption reducing the number of Mohr's circles in three-dimensional stress states to one. We scrutinize the available literature on information regarding these assumptions and thus delineate the ranges of validity of the proposed model equations. The discussion is limited to relatively large snow avalanches with negligible powder snow component and laboratory sand avalanches starting on steep slopes. The conclusion of the analysis is that the SH model is a valid model for sand avalanches, but its Mohr-Coulomb sliding law may have to be complemented for snow avalanches by a second velocity-dependent contribution. For very small snow avalanches and for laboratory avalanches starting on moderately steep and bumpy slopes it may not be adequate.

Rapid motions of free-surface avalanches down curved and twisted channels and their numerical simulation.

This paper presents a new model and discussions about the motion of avalanches from initiation to run-out over moderately curved and twisted channels of complicated topography and its numerical simulations. The model is a generalization of a well established and widely used depth-averaged avalanche model of Savage & Hutter and is published with all its details in Pudasaini & Hutter (Pudasaini & Hutter 2003 J. Fluid Mech. 495, 193-208). The intention was to be able to describe the flow of a finite mass of snow, gravel, debris or mud, down a curved and twisted corrie of nearly arbitrary cross-sectional profile. The governing equations for the distribution of the avalanche thickness and the topography-parallel depth-averaged velocity components are a set of hyperbolic partial differential equations. They are solved for different topographic configurations, from simple to complex, by applying a high-resolution non-oscillatory central differencing scheme with total variation diminishing limiter. Here we apply the model to a channel with circular cross-section and helical talweg that merges into a horizontal channel which may or may not become flat in cross-section. We show that run-out position and geometry depend strongly on the curvature and twist of the talweg and cross-sectional geometry of the channel, and how the topography is shaped close to run-out zones.