The perpetual fragility of creeping hillslopes

Abstract

Soil-mantled hillslopes owe their smooth, convex shape to creep; the slow and persistent, gravity-driven motion of grains on slopes below the angle of repose. Existing models presume that soil creep occurs via mechanical displacement of grains by (bio)physical disturbances. Recent simulations, however, suggest that soil can creep without these disturbances, due to internal relaxation dynamics characteristic of disordered and fragile solids such as glass. Here we report experimental observations of creeping motion in an undisturbed sandpile, at micron resolution over timescales of 10^0-10^6 s, for a variety of natural and synthetic granular materials. We observe two behaviors typically associated with creeping glass: strain occurs as localized and spatially-heterogeneous grain motions; and creep rates decay as a power-law function of time. Further, creep can be accelerated or suppressed by thermal cycles and shaking, respectively. Averaged strain profiles decay exponentially with depth, in agreement with field observations of creeping hillslope soils. Our findings demonstrate that soil is fragile in terms of sensitivity to disturbances, but that creep dynamics are robust across grains and glasses. Mapping soil creep to the more generic glass problem provides a new framework for modeling hillslope sediment transport, and new insights on the nature of yield and failure.

Arctic soil patterns analogous to fluid instabilities

“…In essence, µ(I ) is very similar to fluid rheology, but allows for the role of changing confining pressure with depth. However, the extremely low solifluction velocities observed in the field indicate that solifluction occurs not as a granular flow but well within the granular creep regime (63) that has been shown to describe soil transport velocities on temperate hillslopes (64,65). Granular creep rheology is still at the forefront of granular physics research.…”

Hilltop Curvature Increases with the Square Root of Erosion Rate

“…( 8), particle size is a factor in the transport coefficient because it controls the mean free path of particles in a soil creeping by dilational processes [Furbish et al, 2009]. Although field data from Neeley et al [2019] suggest that coarser soils have a higher transport coefficient, laboratory experiments have demonstrated that, for the same input of energy, coarse-grained soils will creep faster than fine-grained soils [Supplement to Deshpande et al, 2020]. In addition, of the various factors that could affect the rate of soil creep, particle size is the one with the most potential to vary by multiple orders-of-magnitude between watersheds eroding at different rates [Marshall and Sklar, 2012].…”

Rarefied particle motions on hillslopes – Part 1: Theory

“…Herein we focus on rarefied motions of particles which, once entrained, travel downslope over the land surface. This notably includes the dry ravel of particles down hillslopes following disturbances (Roering and Gerber, 2005;Doane, 2018;Doane et al, 2019;Roth et al, 2020) or upon their release from obstacles (e.g., vegetation) following failure of the obstacles (Lamb et al, 2011(Lamb et al, , 2013DiBiase and Lamb, 2013;DiBiase et al, 2017;Doane et al, 2018Doane et al, , 2019, and the motions of rockfall material over the surfaces of talus and scree slopes (Gerber and Scheidegger, 1974;Kirkby and Statham, 1975;Statham, 1976;Dorren, 2003;Luckman, 2013) (Fig. 1).…”