Tuning characteristic scales of creep on soil-mantled hillslopes

Abstract:

Hillslope soil creep is not a Culling-like "diffusive" process, as currently envisioned and modeled in geomorphology. Quasi-static motion of grains stressed below their yield is fundamentally modulated by granular-friction, and shares much with dynamics of other soft, fragile and glassy materials that are sensitive to disturbances. New experiments have shown that prescribed perturbations may excite aging (slowing down in creep rates) or rejuvenation (increase in creep rates). But how other agents of disturbance that operate on natural hillsides contribute to this behavior is an open question. Here we observe sub-yield creep deformation within an experimental hillslope, where we introduce three elements of `natural' hillslopes: i) sprouting vegetation in-situ; ii) cohesion and grain friction; iii) changing the roughness and geometry of the boundaries. A speckle-imaging method illuminates how dynamical length scales of grain rearrangement arise and evolve under gravity-driven creep. All three treatments modulate the mixing length ($\lambda$) of the velocity profiles (akin to Prandalt's mixing length), the size of the plastic avalanches, and correlation length scales in the system. Rough and periodic boundaries truncate length scales to a discrete value. In cohesive clay, avalanches are larger, cluster more tightly together, and are long-lived. Roots locally compact grains and develop a yield-type surface, reminiscent of aging dynamics in gently tapped sand piles. Bio-physical disturbances - omnipresent on natural hillslopes - likely play a role akin to temperature in glasses in facilitating creep phenomenology in the field.