Seasonal reorganization of subglacial drainage inferred from measurements in boreholes

Abstract

The effect of the formation of a major subglacial drainage channel on the behaviour of the subglacial drainage system of Haut Glacier d'Arolla, Switzerland, was investigated using measurements of borehole water level and the electrical conductivity and turbidity of basal meltwaters. Electrical conductivity profiles were also measured within borehole water columns to identify the water sources driving water level changes, and to determine patterns of water circulation in boreholes. Prior to channel formation, boreholes showed idiosyncratic and poorly coordinated behaviour. Diurnal water level fluctuations were small and driven by supraglacial/englacial water inputs, even when boreholes were connected to a subglacial drainage system. This system appeared to consist of hydraulically impermeable patches interspersed with storage spaces, and transmitted a very low water flux. Drainage reorganization, which occurred around 31 July, 1993, in response to rapidly rising meltwater and rainfall inputs, seems to have involved the creation of a connection between an incipient channel and a well‐established channelized system located further down‐glacier. Once a major channel existed within the area of the borehole array, borehole water level fluctuations were forced by discharge‐related changes in channel water pressure, although a diversity of responses was observed. These included (i) synchronous, (ii) damped and lagged, (iii) inverse, and (iv) alternating inverse/lagged responses. Synchronous responses occurred in boreholes connected directly to the channel, while damped and lagged responses occurred in boreholes connected to it by a more resistive drainage system. Pressure variations within the channel resulted in diurnal transfer of mechanical support for the ice overburden between connected and unconnected areas of the bed, producing inverse and alternating patterns of water level response. © 1998 John Wiley & Sons, Ltd.

Evolution of glacial flow and drainage during the ablation season

“…By always using the same law, which is dependent on a single b-coefficient, it is possible to monitor the growth of the drainage network during the ablation period. The results obtained using this approach is consistent with the conclusions derived from other studies of subglacial drainage systems (Hock and Hooke, 1993;Gordon et al, 1998;Nienow et al, 1998).…”

“…Glacial sediment evacuation (Swift et al, 2002), dye tracer tests (Hasnain et al, 2001) and borehole water pressure measurements (Hubbard and Nienow, 1997) have confirmed the existence of more or less efficient drainage networks. One of the main characteristics of glacial drainage systems is the rapidity of their reorganisation during the ablation season (Gordon et al, 1998).…”

Greenland subglacial drainage evolution regulated by weakly connected regions of the bed

“…On almost all days, modelled hydraulic head in the weakly connected system is out of phase with channel pressure, with diurnal amplitudes of a few percent of overburden, matching borehole observations. This out of phase behaviour, observed for isolated or weakly connected boreholes on both the GrIS and mountain glaciers42034353637, has been explained as the transfer of normal stress from hydraulically well-connected regions of the bed20343637, which at our site occurs over kilometers20. Because our model does not include diurnal variations in normal stress transfer, our results indicate that diurnal variations in cavity opening rate associated with changes in ice sliding are a possible alternative or additional mechanism for inducing pressure variations that are out of phase with the well-connected drainage system41337.…”

Particle size characteristics of suspended sediment transported in meltwater from the Gangotri Glacier, central Himalaya — An indicator of subglacial sediment evacuation

“…The drainage system continuously evolves with reduced seasonal snow cover from the ablation zone, effective ice melting, and increasingly peaked supraglacial runoff (Willis et al, 2002;Haritashya, 2005) resulting in the generation of hydraulically efficient channels (Gordon et al, 1998) as the season progresses. This dynamic change in channel behavior allows high sediment and meltwater flux during the peak melt period.…”