Mechanics of Glacier Hydrology

Abstract

Liquid water exists throughout the glacier system. At the surface of glaciers and ice sheets, liquid water is produced when the seasonal surface energy forcing warms the ice above its melting temperature. This meltwater percolates through the porous snow matrix and potentially refreezes, thereby warming the surrounding ice by the release of latent heat. Here I describe a continuum model for the Darcy flow of meltwater through porous snow to help constrain the role that meltwater percolation and refreezing will have on ice-sheet mass balance and hence sea level. Another focus of my research on glacier hydrology is the role of subglacial hydrology in ice stream shear margins. These regions of relatively fast flow drain over 90\% of the ice sheet and generate significant amounts of frictional heat at the ice stream margins where there is a transition to slow flow in the ridge. This heat can lead to temperate ice along the margins, where meltwater is produced and drains to the bed. The hydrology sets the basal effective pressure, defined as the difference between ice overburden and water pressure, and therefore the basal sliding speed. My model results indicate an abrupt transition from a thin-film hydrologic system to a R\"{o}thlisberger (R-) channel melted into the ice. In an R-channel the effective pressure increases and can strengthen the bed, reducing glacier sliding. Variation in downstream ice velocity along the channel axis decreases the effective viscosity, which softens the ice, increasing channel closure velocities. In this way, shear allows channels of a fixed radius to operate at lower effective pressure. Additionally, I show that the effect of shear on the closure of R-channels can be succinctly captured using the conserved M Integral. The deformation of ice subglacially can also be dictated by topography and I show that when ice flows across valleys, viscous overturnings called Moffatt eddies can form. Using a finite-element model, I simulate 100 meters tall Moffatt eddies in the Gamburtsev Subglacial Mountains. These results affect our understanding of the total glacial ice flux and how ice sheets will respond to a changing climate.