The Coupled Dynamics of Meltwater Percolation and Granular Deformation in the Sediment Layer Underlying Parts of the Big Ice Sheets

Ice streams are corridors of fast flowing ice that drain the interiors of the large Antarctic and Greenlandic ice sheets. The abrupt transition from the fast flowing ice of the streams to the almost stagnant ice to their side occurs along narrow shear margins, which experience intense internal deformation. This deformation can lead to warming and weakening of the ice, which makes shear margins prone to migration. Previous studies have suggested that efficient drainage of meltwater at the interface between the ice and the underlying weak sediment layer might control the stability of shear margins. However, weak sediment deformation is expected to be tightly coupled to meltwater drainage, and therefore the feedbacks between them are likely of great importance for ice stream stability, while they remain little understood. Here, we use a 3D model to capture the dynamic interactions between meltwater percolation and granular deformation.We solve for the granular mechanics using a discrete element algorithm, and compute the flow in the deforming porous media on a superimposed Eulerian grid. The model intends to represent the upper portion of the weak and unconsolidated sediment layer directly beneath the ice in the shear margin. We use it to study (1) how spatial variations in the shearing velocity affect meltwater percolation, and (2) to what degree the till grains are stable in the presence of horizontal gradients in water pressure. Our simulation results show that horizontal variations in shear velocity enhance the local porosity and permeability, leading to a potentially significant increase in the horizontal water transmissivity. Results further demonstrate that grain stability depends on the magnitude of the horizontal pressure gradient, and cascading events of grain mobilization precede bulk instability flows.