The evolution of isolated cavities and hydraulic connection at the glacier bed. Part 2: a dynamic viscoelastic model

Abstract. Many large-scale subglacial drainage models assume implicitly or explicitly that the distributed part of the drainage system consists of subglacial cavities. Few of these models however consider the possibility of hydraulic disconnection, where cavities exist but are not numerous or large enough to be pervasively connected with one another so that water can flow. Here I use a process-scale model for subglacial cavities to explore their evolution, focusing on the dynamics of connections that are made between cavities. The model uses a viscoelastic representation of ice, and computes the pressure gradients that are necessary to move water around basal cavities as they grow or shrink. The latter model component sets the work here apart from previous studies of subglacial cavities, and permits the model to represent the behaviour of isolated cavities, and of uncavitated parts of the bed at low normal stress. I show that connections between cavities are made dynamically when a cavitation ratio (the fraction of the bed occupied by cavities) reaches a critical value due to decreases in effective pressure. I also show that existing simple models for cavitation ratio and for water sheet thickness (defined as mean water depth) fail to capture even qualitatively the behaviour predicted by the present model.