  Drainage of supraglacial lakes to the ice-sheet bed can occur when a
hydrofracture propagates downward, driven by the weight of the water in the
lake. For supraglacial lakes in the grounding zones of Antarctic glaciers, the
mechanics of drainage is complicated by their proximity to the grounding line.
Recently, a series of supraglacial lake-drainage events through hydrofractures
was observed in the Amery Ice Shelf grounding zone, East Antarctica. The lake
depth at drainage varied considerably between events, raising questions about
the mechanisms that induce hydrofracture, even at low lake depths. Here we use
a modelling approach to investigate the contribution of tidally driven flexure
to hydrofracture propagation. We model the viscoelastic response of a marine
ice sheet to tides, the stresses that are induced, and the contribution of
tidal stresses to hydrofracture propagation. Our results show that ocean tides
and lake-water pressure together control supraglacial lake drainage through
hydrofractures in the grounding zone. We give a model-based criterion that
predicts supraglacial lake drainage as a function of daily maximum tidal
amplitude and lake depth. Our model-based criterion agrees with remotely sensed
data, indicating the importance of tidal flexure to processes associated with
hydrofracturing such as supraglacial lake drainage, rifting and calving.
