Borehole water-level variations and the structure of the subglacial hydrological system of Haut Glacier d’Arolla, Valais, Switzerland

Abstract

Late-summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of Haut Glacier d’Arolla, Switzerland. Interpolated surfaces of minimum diurnal water pressure and diurnal water-pressure variation suggest the presence of a subglacial channel within a more widespread, distributed drainage system. The channel flows along the centre of avariable pressure axis(VPA), some tens of metres wide, that is characterized by low minimum diurnal water pressures (frequently atmospheric) and high diurnal water-pressure variations. These characteristics are transitional over a lateral distance of c. 70 m to higher and more stable subglacial water pressures in the adjacent distributed system. Water-pressure variations recorded in boreholes located close to the centre of the VPA reflect the delivery of surface-derived meltwater to the glacier bed and result in a diurnally reversing, transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight. Subglacial observations suggest that such flow occurs through a vertically confined sediment layer. Borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension. Analysis of the propagation velocity and amplitude attenuation cf the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel, falling from c. 10−4m s−1at the channel boundary to c. 10−7m s−170 m away. These apparent hydraulic conductivities are consistent with Darcian flow through clean sand and typical glacial till, respectively.We suggest that fine material is systematically flushed from basal sediments located adjacent to large, melt-season drainage channels beneath warm-based glaciers. This process may have important implications for patterns of glacier erosion, hydro-chemistry and dynamics.

Hydrogeological implications of glacial landscape evolution at Skeiðarársandur, SE Iceland

“…These values are consistent with hydraulic conductivity values reported for glacial sediments and similar materials (Freeze and Cherry, 1979;Lloyd, 1983;Fountain, 1992;Hubbard et al, 1995). Calculated values of hydraulic conductivity are lower in the morainic relief landsystem (1.1 to 27.4 m d − 1 ) with less variation than in the jökulhlaup and proglacial river channel landsystems (Table 1).…”

Diurnal hydrological – physicochemical controls and sampling methods for minor and trace elements in an Alpine glacial hydrological system

“…3) suggests a predominately hydrological control. Here, at minimum diurnal discharge, bulk discharge is comprised mostly of 'delayed-flow' waters from a relatively inefficient distributed system, that is then passed into the channellised system, due to the high pressure in the distributed system and the low pressure in the channellised system at this time (Hubbard et al, 1995). This promotes protracted, intimate contact with reactive subglacial sediments allowing more concentrated waters to evolve, as has been suggested for diurnal variations of major ion concentrations (Brown et al, 1994(Brown et al, , 1996a.…”

“…Reconstructed hydraulic gradients suggest that this interaction is driven by large diurnal water-pressure variations within the channel. During the day, high pressures in the channel drive water out to the distributed system, while low pressures overnight in the channel drive that water back (Hubbard et al, 1995). The weathering of suspended sediments within these water-filled subglacial channels contributes the majority of major ions present in bulk meltwaters emerging from the snout of Haut Glacier d'Arolla at the height of the ablation season (Brown et al, 1994;Brown et al, 1996a).…”

Morphology of the upper continental slope in the Bellingshausen and Amundsen Seas – Implications for sedimentary processes at the shelf edge of West Antarctica

“…Although a stable channelized meltwater system is unlikely to form in soft substrate, temporary formation of channels to accommodate excess meltwater may occur (Boulton and Hindmarsh, 1987;Alley et al, 1989;Alley, 1993;Clark and Walder, 1994;Tulaczyk et al, 1998). Seasonal reorganization of flow, from distributed systems in winter to channelized flow in summer, is known from land-based glaciers (Hubbard et al, 1995), and the remnants of channels and tunnel valleys suggest that locally higher-energy meltwater streams capable of eroding soft glacial sediments also existed on the mid-to outer shelf within the large cross-shelf troughs ( Fig. 3; Lowe and Anderson, 2003;King et al, 2004;Wellner et al, 2006).…”