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This paper investigates the relationships between hillslope stability and fabric anisotropy of brittle rock materials and the implications for landscape shaping. We use discrete element models to study the stability and failure modes of slopes made of transverse isotropic rock materials, focusing more particularly on the influence of the material orientation relative to the topographic slope. After validating the numerical approach with a limit equilibrium analytical solution in the case of an isotropic material, we modify our numerical slope models to simulate the rheological features of anisotropic gneissic rocks. Systematic exploration of the transverse isotropy plane's orientation in two dimensions (dip angle) reveals that slope collapse requires strength values that are highly dependent on the orientation of the material relative to the slope. For a 1000 m high escarpment, the stability of a slope with a fixed gradient requires strength that is one order of magnitude greater in a configuration where the isotropy plane is slightly less inclined than the topographic slope (i.e., cataclinal overdip configuration) than in a configuration where the isotropy plane is perpendicular to the slope (i.e., anaclinal configuration). Mirroring this highly variable stability according to the relative orientation of the material, four modes of deformation or gravitational instability are observed: in order of appearance, when the transverse isotropy plane orientation goes from 0 to 180° with respect to the horizontal (going from cataclinal to anaclinal configurations), the slope collapses respectively by sliding, buckling, toppling and crumbling. The crumbling mode corresponds to a very stable configuration for which the preferred ground movements will be rock falls from the cliff compared to the structurally controlled, deep-seated deformation modes leading to sliding and toppling. Despite the simplifications inherent to the numerical approach, our study highlights the essential characteristics of landslides occurring along slopes cut in transverse isotropic materials and reproduces the various instability modes observed in natural slopes. It also enables assessing their respective kinetics as well as the volumes of material they mobilize. Finally, by comparing our findings on the azimuthal variations in hillslope gradients observed along the central Himalaya (Nepal), in an area characterized by the relatively uniform orientation of the anisotropy in gneissic and mica-schist formations, we show that, even though multiple environmental factors come into play, landscape shaping is indeed strongly controlled by material anisotropy. • Anisotropic rock strength affects slope stability by foliation dip angle. • Stability and failure modes are investigated by means of discrete element models. • Sliding and buckling failures occur in cataclinal slopes. • Toppling and crumbling failures occur in anaclinal slopes. • Himalayan slopes confirm how anisotropy controls landscape evolution. [ABSTRACT FROM AUTHOR]

Rock block characteristics are the controlling factors that define the potential rockfall volume, associated hazard and design of preventive practices. The present study defines a methodology to demonstrate the impact of block characteristics on rockfall hazard and its implications on the design of mitigative measures by analysing a recurring rockfall site along a critical highway corridor in NW Himalaya, India. The field data were quantitatively characterised for statistical distributions of size and volume of in situ as well as rockfall blocks. Possible block failures and rockfall initiation zones were predicted using modified kinematic analysis and distinct element method. The size and volume distributions as well as shapes were simulated against these predicted initiation zones to determine the cumulative impact of block characteristics on rockfall hazard by analysing the block kinetics and trajectories. It is found that the block initiation zones majorly govern the relative bounce height, kinetic energy, run-out distance and impact down the slope respective of block sizes and associated mass coupled with slope geometry. However, the effect of block shapes is observed to be minor compared to the blocks greater than a particular size range. Therefore, the design of rockfall protection such as barrier can be more effective when considering the kinetic energies of such block size range assisted by initiation zones. This study proposed that the knowledge of block size frequencies and rockfall initiation zones could provide a rational threshold for which a protection strategy can be designed and optimised to prevent rockfall hazard along the Himalayan highway corridors. [ABSTRACT FROM AUTHOR]

Debris-covered glaciers are increasingly studied because it is assumed that debris cover extent and thickness could increase in a warming climate, with more regular rockfalls from the surrounding slopes and more englacial melt-out material. Debris energy-balance models have been developed to account for the melt rate enhancement/reduction due to a thin/thick debris layer, respectively. However, such models require a large amount of input data that are not often available, especially in remote mountain areas such as the Himalaya, and can be difficult to extrapolate. Due to their lower data requirements, empirical models have been used extensively in clean glacier melt modelling. For debris-covered glaciers, however, they generally simplify the debris effect by using a single melt-reduction factor which does not account for the influence of varying debris thickness on melt and prescribe a constant reduction for the entire melt across a glacier. In this paper, we present a new temperature-index model that accounts for debris thickness in the computation of melt rates at the debris-ice interface. The model empirical parameters are optimized at the point scale for varying debris thicknesses against melt rates simulated by a physically-based debris energy balance model. The latter is validated against ablation stake readings and surface temperature measurements. Each parameter is then related to a plausible set of debris thickness values to provide a general and transferable parameterization. We develop the model on Miage Glacier, Italy, and then test its transferability on Haut Glacier d’Arolla, Switzerland. The performance of the new debris temperature-index (DETI) model in simulating the glacier melt rate at the point scale is comparable to the one of the physically based approach, and the definition of model parameters as a function of debris thickness allows the simulation of the nonlinear relationship of melt rate to debris thickness, summarised by the Østrem curve. Its large number of parameters might be a limitation, but we show that the model is transferable in time and space to a second glacier with little loss of performance. We thus suggest that the new DETI model can be included in continuous mass balance models of debris-covered glaciers, because of its limited data requirements. As such, we expect its application to lead to an improvement in simulations of the debris-covered glacier response to climate in comparison with models that simply recalibrate empirical parameters to prescribe a constant across glacier reduction in melt. [ABSTRACT FROM AUTHOR]