Yue Meng , Riley Culberg, Ching-Yao Lai

On the Greenland Ice Sheet, hydrofracture connects the supraglacial and subglacial hydrologic
systems, coupling surface runoff dynamics and ice velocity. Over the last two decades, the growth
of low-permeability ice slabs in the firn above the equilibrium line has expanded Greenland’s runoff
zone, but the vulnerability of these regions to hydrofracture is still poorly understood. Observations
from Northwest Greenland suggest that when meltwater drains through crevasses in ice slabs, it
is stored in the underlying relict firn layer and does not reach the ice sheet bed. Here, we use
poromechanics to investigate whether water-filled crevasses in ice slabs can propagate vertically
through a firn layer. Based on numerical simulations, we develop an analytical estimate of the water
injection-induced effective stress in the firn given the water level in the crevasse, ice slab thickness,
and firn properties. We find that the firn layer reduces the system’s vulnerability to hydrofracture
because much of the hydrostatic stress is accommodated by a change in pore pressure, rather than
being transmitted to the solid skeleton. This result suggests that surface-to-bed hydrofracture will
not occur in ice slab regions until all pore space proximal to the initial flaw has been filled with solid
ice.