013-neel.txt (4761B)
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 [7] https://adamsgaard.dk/video/neel.mp4