The distribution and drainage of meltwater at the base of glaciers
sensitively affects fast ice flow. Classical studies suggest that thin
meltwater films between the overlying ice and a hard-rock bed channelize
into efficient drainage elements by melting the overlying ice. However,
these studies do not account for the presence of soft deformable
sediment observed underneath many West Antarctic ice streams, and the
inextricable coupling that sediment exhibits with meltwater drainage.
Our work presents an alternate channel initiation mechanism where
meltwater films grow by eroding the sediment beneath. We conduct a
linearized stability analysis on a meltwater film flowing over an
erodible bed. We solve the Navier Stokes equations for the film flow,
and we compute bed evolution with the Exner equation. We identify a
regime where the coupled dynamics of hydrology and sediment transport
generate a morphological instability that would indicate channel
initiation. We show that this instability operates at time scales much
faster than ice dynamics, thus occurring prior to the classical
channelization instabilities. We discuss the physics of the instability
using the framework of ripple formation on erodible beds.