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t013-neel.txt (4761B)
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1 Today, [1]Indraneel Kasmalkar had his paper published in [2]Journal of
2 Geophysical Research: Earth Surface. Congratulations Neel! He used my software
3 [3]sphere, and sheared a granular assembledge with a non-trivial forcing in
4 order to learn more about subglacial sediment behavior.
5
6 Abstract
7
8 Shear Variation at the Ice-Till Interface Changes the Spatial Distribution
9 of Till Porosity and Meltwater Drainage
10
11 Indraneel Kasmalkar(1), Anders Damsgaard(2), Liran Goren(3), Jenny Suckale
12 (1,4,5)
13
14 1: Department of Computational and Mathematical Engineering, Stanford
15 University, CA, USA
16 2: Department of Geoscience, Aarhus University, Denmark
17 3: Department of Earth and Environmental Sciences, Ben-Gurion University of
18 the Negev, Beer-Sheva, Israel
19 4: Department of Geophysics, Stanford University, CA, USA
20 5: Department of Civil and Environmental Engineering, Stanford University,
21 CA, USA
22
23 Plain-language summary:
24 The ice at the base of certain glaciers moves over soft sediments that
25 route meltwater through the pore spaces in between the sediment grains. The
26 ice shears the sediment, but it is not clear if this slow shearing is
27 capable of changing the structure or volume of the pore space, or the path
28 of the meltwater that flows through the sediment. To study the relations
29 between the shearing of the sediment and the changes in its pore space, we
30 use computer simulations that portray the sediment as a collection of
31 closely packed spherical grains, where the pores are filled with meltwater.
32 To shear the simulated sediment, the grains at the top are pushed with
33 fixed speeds in the horizontal direction. Despite the slow shear, which is
34 generally thought of as having no effect on pore space, our results show
35 that shearing changes the sizes of the pores in between the grains, where
36 large pores are formed near the top of the sediment layer. If the grains at
37 the top are pushed with uneven speeds, then the largest pores are formed in
38 the areas where grain speeds vary the most. We show that the exchange of
39 meltwater between neighboring pores is faster than the movement of the
40 grains, indicating that the meltwater can adjust quickly to changing pore
41 space.
42
43 Abstract:
44 Many subglacial environments consist of a fine-grained, deformable sediment
45 bed, known as till, hosting an active hydrological system that routes
46 meltwater. Observations show that the till undergoes substantial shear
47 deformation as a result of the motion of the overlying ice. The deformation
48 of the till, coupled with the dynamics of the hydrological system, is
49 further affected by the substantial strain rate variability in subglacial
50 conditions resulting from spatial heterogeneity at the bed. However, it is
51 not clear if the relatively low magnitudes of strain rates affect the bed
52 structure or its hydrology. We study how laterally varying shear along the
53 ice-bed interface alters sediment porosity and affects the flux of
54 meltwater through the pore spaces. We use a discrete element model
55 consisting of a collection of spherical, elasto-frictional grains with
56 water-saturated pore spaces to simulate the deformation of the granular
57 bed. Our results show that a deforming granular layer exhibits substantial
58 spatial variability in porosity in the pseudo-static shear regime, where
59 shear strain rates are relatively low. In particular, laterally varying
60 shear at the shearing interface creates a narrow zone of elevated porosity
61 which has increased susceptibility to plastic failure. Despite the changes
62 in porosity, our analysis suggests that the pore pressure equilibrates
63 near-instantaneously relative to the deformation at critical state,
64 inhibiting potential strain rate dependence of the deformation caused by
65 bed hardening or weakening resulting from pore pressure changes. We relate
66 shear variation to porosity evolution and drainage element formation in
67 actively deforming subglacial tills.
68
69 Links and references:
70
71 • [4]Publication on journal webpage (closed access)
72 • [5]Preprint PDF
73 • [6]Simulation software
74 • [7]Visualization of example simulation
75
76
77 References:
78
79 [1] mailto:ineel@alumni.stanford.edu
80 [2] https://agupubs.onlinelibrary.wiley.com/journal/19422466
81 [3] https://src.adamsgaard.dk/sphere
82 [4] https://doi.org/10.1029/2021JF006460
83 [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
84 [6] https://src.adamsgaard.dk/sphere
85