Using artificial moulins to characterise englacial R-channels

The englacial and subglacial drainage system exerts key controls on glacier dynamics.  However, due to its inaccessibility, it is still only poorly understood and more detailed observations are important, particularly to validate and tune physical models describing its dynamics.

By creating artificial glacier moulins - boreholes connected to the subglacial drainage system and supplied with water from surface streams - we present a novel method to monitor the evolution of englacial hydrological systems with high temporal resolution.  Here, we use artificial moulins as representations for vertical, pressurised, englacial R-channels.  From tracer and pressure measurements we derive time series of the hydraulic gradient, discharge, flow speed and channel cross-sectional area.  Using these, we compute the Darcy-Weisbach friction factor, obtaining values which increase from 0.1 to 13 within five days of channel evolution (corresponding to a Manning friction factor of 0.03 to 0.3 s m^-1/3).

Furthermore, we simulate the growth of the channel cross-sectional area using different temperature gradients.  The comparison to our measurements largely supports the common assumption that the temperature follows the pressure melting point.  The deviations from this behaviour are analysed using various heat transfer parameterisations to assess their applicability.

Finally, we discuss how artificial moulins could be combined with glacier-wide tracer experiments to constrain parameters of subglacial drainage more precisely. The presented approach allows to accurately quantify the englacial transit time of the tracer and thus, in turn, to quantify the subglacial transit time; something which has not been achieved to date.