# NEW PAPER OUT ON SUBGLACIAL SHEAR SIMULATION 

Today, [1]Indraneel Kasmalkar had his paper published in [2]Journal of
Geophysical Research: Earth Surface. Congratulations Neel! He used my software
[3]sphere, and sheared a granular assembledge with a non-trivial forcing in
order to learn more about subglacial sediment behavior.

Abstract

    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.

    Abstract:
    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.

Links and references:

  • [4]Publication on journal webpage (closed access)
  • [5]Preprint PDF
  • [6]Simulation software
  • [7]Visualization of example simulation


References:

[1] mailto:ineel@alumni.stanford.edu
[2] https://agupubs.onlinelibrary.wiley.com/journal/19422466
[3] https://src.adamsgaard.dk/sphere
[4] https://doi.org/10.1029/2021JF006460
[5] https://adamsgaard.dk/papers/Kasmalkar%20et%20al%202021%20Shear%20variation%20at%20the%20ice-till%20interface%20changes%20the%20spatial%20distribution%20of%20till%20porosity%20and%20meltwater%20drainage.pdf
[6] https://src.adamsgaard.dk/sphere
[7] https://adamsgaard.dk/video/neel.mp4