New paper out on subglacial shear simulation

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Today, Indraneel Kasmalkar had his paper published in Journal of Geophysical Research: Earth Surface. Congratulations Neel! He used my software sphere, and sheared a granular assembledge with a non-trivial forcing in order to learn more about subglacial sediment behavior.

Here's an example visualization from the study:


Shear Variation at the Ice-Till Interface Changes the Spatial Distribution of Till Porosity and Meltwater Drainage

Indraneel Kasmalkar(1), Anders Damsgaard(2), Liran Goren(3), Jenny Suckale(1,4,5)

1: Department of Computational and Mathematical Engineering, Stanford University, CA, USA
2: Department of Geoscience, Aarhus University, Denmark
3: Department of Earth and Environmental Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel
4: Department of Geophysics, Stanford University, CA, USA
5: Department of Civil and Environmental Engineering, Stanford University, CA, USA

Plain-language summary:
The ice at the base of certain glaciers moves over soft sediments that route meltwater through the pore spaces in between the sediment grains. The ice shears the sediment, but it is not clear if this slow shearing is capable of changing the structure or volume of the pore space, or the path of the meltwater that flows through the sediment. To study the relations between the shearing of the sediment and the changes in its pore space, we use computer simulations that portray the sediment as a collection of closely packed spherical grains, where the pores are filled with meltwater. To shear the simulated sediment, the grains at the top are pushed with fixed speeds in the horizontal direction. Despite the slow shear, which is generally thought of as having no effect on pore space, our results show that shearing changes the sizes of the pores in between the grains, where large pores are formed near the top of the sediment layer. If the grains at the top are pushed with uneven speeds, then the largest pores are formed in the areas where grain speeds vary the most. We show that the exchange of meltwater between neighboring pores is faster than the movement of the grains, indicating that the meltwater can adjust quickly to changing pore space.

Many subglacial environments consist of a fine-grained, deformable sediment bed, known as till, hosting an active hydrological system that routes meltwater. Observations show that the till undergoes substantial shear deformation as a result of the motion of the overlying ice. The deformation of the till, coupled with the dynamics of the hydrological system, is further affected by the substantial strain rate variability in subglacial conditions resulting from spatial heterogeneity at the bed. However, it is not clear if the relatively low magnitudes of strain rates affect the bed structure or its hydrology. We study how laterally varying shear along the ice-bed interface alters sediment porosity and affects the flux of meltwater through the pore spaces. We use a discrete element model consisting of a collection of spherical, elasto-frictional grains with water-saturated pore spaces to simulate the deformation of the granular bed. Our results show that a deforming granular layer exhibits substantial spatial variability in porosity in the pseudo-static shear regime, where shear strain rates are relatively low. In particular, laterally varying shear at the shearing interface creates a narrow zone of elevated porosity which has increased susceptibility to plastic failure. Despite the changes in porosity, our analysis suggests that the pore pressure equilibrates near-instantaneously relative to the deformation at critical state, inhibiting potential strain rate dependence of the deformation caused by bed hardening or weakening resulting from pore pressure changes. We relate shear variation to porosity evolution and drainage element formation in actively deforming subglacial tills.

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