Simulation of scree‐slope dynamics: investigating the distribution of debris avalanche events in an idealized two‐dimensional model

Abstract

We present a two-dimensional model of the development of scree slopes using the discrete-element method. We concentrate on the dynamics of the accumulating debris at the cliff foot rather than on the failure modes of the cliff-face or shape of the underlying rock surface. The evolution of this unconsolidated material is intermittent and systematically changing over time, with an early high disturbance regime, dominated by a characteristic event size (where 65% of particles in the debris are in motion to some extent), replaced at later times by many shallow slides interspersed with infrequent large events that involve motion through almost the full scree depth. These large slides lead to a stratigraphy in which the layers of material are stretched almost horizontal near the base of the slope. The scree surface thus shows a gradient in age with most recent rock-fall close to the cliff and the oldest rock-fall debris outcropping at the foot. The final surface slope tends to show little curvature, and the final mean slope is well correlated with the angle of internal friction of the particles, although the change is very small over a wide range of friction angles [final slope (in degrees relative to horizontal)~0.043 × internal friction angle + 17.49, with a correlation coefficient of 0.89, p-value 0.0001]. Some weak size-segregation of the debris is found, but this seems to have little to do with individual particles bounding down the slope. The shape of the rock core agrees largely with the analytic forms given by Fisher-Lehmann and Bakker-Le Heux expressions, but the original simple Fisher quadratic can give the best fit. Overall the evolution shows a remarkable insensitivity to the model parameters, suggesting that the controls on dry scree-slope evolution are primarily geometric in character.

Dynamics of grains falling on a sloping granular medium: application to the evolution of a talus

“…Note, however, that the numerical simulations of Bithell et al . () reproduce avalanching even without the formation of a weak layer, indicating that the mechanics of avalanching in the talus may be even more general than suggested by De Blasio and Sæter ().…”

“…This numerical technique was extended to the case of particles of various sizes by Bithell et al . (). While a size effect and long spread of large particles becomes in this case evident, even more interesting is the spontaneous organization of the talus toward quasi‐periodical avalanching, with consequent redistribution of the material.…”

“…Figure shows schematically a tentative model for the development of stratification in a talus as deduced jointly from the experiments of Kirkby and Statham (), of this work and those of De Blasio and Sæter (), and the numerical work of Bithell et al . (). In the first phase (Figure (1)) the talus is dominated by ‘normal’ single‐block behaviour, in which the largest blocks reach the distal part of the talus (Figure (1,2)).…”

“…Cliff retreat, however, adds another time‐dependent component to the problem of talus evolution, not included in the conceptual model of Figure . This is because, as clarified by numerical simulations (Utili and Crosta, ; Bithell et al ., ), the foot of the talus grows to a higher level as time progresses, and thus the height of fall and the input energy of blocks diminishes.…”

Weathering Processes on Martian Craters: Implications on Recurring Slope Lineae and the Location of Liquid Water

“…However, liquid flows on Earth, more specifically liquid debris flows (which is the most similar liquid process associated with rock weathering on steep rocks similar to Martian craters) get more concentrated as the flow travels down the slope, ending up in a semi-circular, tongue-shaped form [33], not being the case for RSL, in which the bottom ends get wider and in different directions, a behavior associated with dry rock flows on Earth, or "scree avalanches", which are loose rock sliding down the slope as they break apart by the action of gravity, weathering processes, and occasional liquid flows, as shown in Figure 3 [34]. Scree slopes are characteristic of Andean or Alpine rock weathering processes on steep slopes, among other sites on Earth [35] [36] [37], similar to those experiencing RSL in Mars [7].…”