Vulnerability of Firn to Hydrofracture: Poromechanics Modeling

On the Greenland Ice Sheet, hydrofracture connects the supraglacial and subglacial hydrologicsystems, coupling surface runoff dynamics and ice velocity. Over the last two decades, the growthof low-permeability ice slabs in the firn above the equilibrium line has expanded Greenland’s runoffzone, but the vulnerability of these regions to hydrofracture is still poorly understood. Observationsfrom Northwest Greenland suggest that when meltwater drains through crevasses in ice slabs, itis stored in the underlying relict firn layer and does not reach the ice sheet bed. Here, we useporomechanics to investigate whether water-filled crevasses in ice slabs can propagate verticallythrough a firn layer. Based on numerical simulations, we develop an analytical estimate of the waterinjection-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 hydrofracturebecause much of the hydrostatic stress is accommodated by a change in pore pressure, rather thanbeing transmitted to the solid skeleton. This result suggests that surface-to-bed hydrofracture willnot occur in ice slab regions until all pore space proximal to the initial flaw has been filled with solidice.