DISCRETE-ELEMENT SIMULATION OF SUBGLACIAL SEDIMENTS: GROUNDING-LINE PROXIMATE TILL MECHANICS AND SOFT-BED CHANNEL DYNAMICS (Invited Presentation)

The mechanics of subglacial sediments and subglacial hydrology are important controls on ice-sheet dynamics, particularly in fast-moving outlet glaciers and ice streams. Ice-flow models including soft-bed sliding or deformation are typically based on a priori assumptions regarding till rheology. In this work we use a grain-scale numerical formulation for granular dynamics coupled to a diffusive parameterization of pore-water dynamics, with a bulk rheology resultant of self-organizing complexity fundamental to sediments.

We demonstrate that perturbations in water pressure in a subglacial bed can drive highly non-linear stick-slip dynamics, similar to observations of tidally perturbed flow of marine-terminating glaciers and ice streams. We also apply the model to investigate the structural stability of channels incised into soft beds, and observe that an upper bound for channel cross-sectional size is linearly linked to the effective pressure on the channel flanks. While smaller conduits are stable for a given effective stress, the modeled channel rapidly collapses as soon as the channel grows and the yield strength is exceeded. The water-transport capacity increases with channel size, providing a negative feedback which limits channel dimensions and hydraulic transmissivity. We suggest that this can cause branching of channels into several parallel elements, which may be consistent with landforms from the ice-marginal areas of the past Scandinavian Ice Sheet and geophysical observations from Thwaites Glacier in West Antarctica.