Experimental determination of a double-valued drag relationship for glacier sliding

The contribution of glaciers to sea-level rise and their effects on landscape evolution depend on the poorly known relationship between sliding speed and drag at the ice/bed interface. Results from experiments with a new rotary laboratory device demonstrate empirically for the first time a double-valued drag relationship like that suggested by some sliding theories: steady drag on a rigid, sinusoidal bed increases, peaks and declines at progressively higher sliding speeds due to growth of cavities in the lee sides of bed undulations. Drag decreases with increased sliding speed if cavities extend beyond the inflection points of up-glacier facing surfaces, so that adverse bed slopes in contact with ice diminish with further cavity growth. These results indicate that shear tractions on glacier beds can potentially decrease due to increases in sliding speed driven by weather or climate variability, promoting even more rapid glacier motion by requiring greater strain rates to produce resistive stresses. Although a double-valued drag relationship has not yet been demonstrated for the complicated geometries of real glacier beds, both its potential major implications and the characteristically convex stoss surfaces of bumps on real glacier beds provide stimulus for exploring the effects of this relationship in ice-sheet models

Transient evolution of basal drag during glacier slip

Glacier slip is usually described using steady-state sliding laws that relate drag, slip velocity and effective pressure, but where subglacial conditions vary rapidly transient effects may influence slip dynamics. Here we use results from a set of laboratory experiments to examine the transient response of glacier slip over a hard bed to velocity perturbations. The drag and cavity evolution from lab experiments are used to parameterize a rate-and-state drag model that is applied to observations of surface velocity and ice-bed separation from the Greenland ice sheet. The drag model successfully predicts observed lags between changes in ice-bed separation and sliding speed. These lags result from the time (or displacement) required for cavities to evolve from one steady-state condition to another. In comparing drag estimates resulting from applying rate-and-state and steady-state slip laws to transient data, we find the peaks in drag are out of phase. This suggests that in locations where subglacial conditions vary on timescales shorter than those needed for cavity adjustment transient slip processes control basal drag.This article is published as Zoet, L., Iverson, N., Andrews, L., & Helanow, C. (2022). Transient evolution of basal drag during glacier slip. Journal of Glaciology, 68(270), 741-750. doi:10.1017/jog.2021.131. Posted with permission.This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited

Inferring forms of glacier slip laws from estimates of ice-bed separation during glacier slip

Sea-level projections depend sensitively on the parameterization used for basal slip in glacier flow models. During slip over rock-beds, ice-bed separation increases with slip velocity and basal water pressure. We present a method for using these variables and measured bed topography to estimate the average bed slope in contact with ice, . Three-dimensional numerical modeling of slip over small areas of former beds has shown that changes in with increasing slip velocity and water pressure mimic changes in basal drag. Computed values of can thus provide the form of the slip law that relates drag to velocity and water pressure, avoiding computationally expensive numerical modeling. The method is applied to 618 sections from four former glacier beds. Results generally show an increase in , and hence inferred basal drag, with slip velocity up to a limiting value, consistent with a regularized Coulomb slip law.This article is published as Woodard, J., Zoet, L., Iverson, N., & Helanow, C. (2022). Inferring forms of glacier slip laws from estimates of ice-bed separation during glacier slip. Journal of Glaciology, 1-9. doi:10.1017/jog.2022.63. Posted with permission. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited

Sliding Relations for Glacier Slip With Cavities Over Three-Dimensional Beds

International audienceResults of glacier flow models and associated estimates of future sea level rise depend sensitively on the prescribed relation between shear stress and slip velocity at the glacier bed. Using a fully three-dimensional numerical model of ice flow, we compute steady-state sliding relations for where ice slips over a rock bed with three-dimensional, periodic topography. In agreement with studies of two-dimensional beds, water-filled cavities that form down-glacier from bedforms cause basal shear stress to peak at a threshold slip velocity and decrease at higher velocities (i.e., rate-weakening drag). However, the shear stress magnitude and extent of rate-weakening drag depend systematically on lateral topographic variations not considered previously. Moreover, steep up-glacier-facing slopes of bedforms can result in shear stress that increases monotonically over a wide range of slip velocity, helping to stabilize slip. These results highlight the potential variability of sliding relations and their likely sensitivity to the morphological diversity of glacier beds

Wiens (2012), Motion of an Antarctic glacier by repeated tidally modulated earthquakes, Nat

Between debris-laden glacial ice and bedrock, basal seismicity can develop that yields information about bed properties 1,2 , stress distribution 3 , outburst flooding 4 , and crevassing and calvin