Structure and stability of the former subglacial drainage system of the glacier De Tsanfleuron,Switzerland

A well-developed subglacial drainage system consisting of large cavities developed in the lee of bedrock steps connected together by a network of Nye channels is exposed on an area of recently deglaciated limestone bedrock in front of Glacier de Tsanfleuron, Switzerland. This system covers some 51 per cent of the bedrock surface area, and is believed to have transported the bulk of supraglacially-derived meltwaters through the glacier. Using the cavity hydraulics model of Kamb (1987), it is shown that the geometry of the system rendered it stable against collapse by meltback of channel roofs into a tunnel-dominated system. For likely combinations of glacier geometry and meltwater discharge, the steady state water pressure in this system would have been only a small fraction of that required for flotation, and for discharges of less than about 0·5–5 m³ s^?1 water would have flowed at atmospheric pressure. The system appears to have adjusted to varying discharges by a combination of varying water pressure and changing the total cross-sectional area of flow by altering the number of active channels connecting cavities. Glacier sliding velocity would have been independent of meltwater discharge for discharges at which water flowed at atmospheric pressure, but would have risen with increasing discharge for higher flows. Velocities on the order of 0·1 m d^?1 are predicted for a realistic range of discharges and effective pressures, and these are believed to be plausible. Episodes of enhanced sliding in glaciers with similar drainage systems could be triggered by a rise in meltwater discharge across the threshold between flows at atmospheric pressure and flow under pressure from the glacier.     

Seasonal changes in the morphology of the subglacial drainage system,Haut Glacier d'Arolla,Switzerland

Dye tracing techniques were used to investigate the glacier-wide pattern of change in the englacial/subglacial drainage system of Haut Glacier d'Arolla during the ablation seasons of 1990 and 1991. Analysis of breakthrough curve characteristics indicate that over the course of a melt season, a system of major channels developed by headward growth at the expense of a hydraulically inefficient distributed system. By the end of the melt season, this channel system extended at least 3·3 km from the snout of the 4 km long glacier and drained the bulk of supraglacially derived meltwater passing through the glacier. The upper limit of the channel system closely followed the retreating snowline up-glacier. Rates of headward channel growth reached c. 65 m d⁻¹, although these rates decreased in the upper 1 km of the glacier where snowline retreat exposed a patchy firn aquifer. It appears that the removal of snow (with its high albedo and significant water storage capacity) from the glacier surface resulted in a dramatic increase in the volume of runoff into moulins, and in the peakedness of daily runoff cycles. This induced transient high water pressures within the distributed drainage system, which caused it to evolve rapidly into a channelised system. It is therefore likely that, at a local scale, channel growth occurred down-glacier from moulins, and that the overall up-glacier-directed pattern of channel formation was caused by the retreating snowline exposing new moulins and crevasses to inputs of ice-derived meltwater. Damping of diurnal melt inputs by storage in the firm aquifer accounts for the slowing of channel growth in the upper glacier. © 1998 John Wiley & Sons, Ltd.     

Borehole drainage and its implications for the investigation of glacier hydrology: experiences from Haut Glacier d'Arolla,Switzerland

Studies of glacier hydrology rely increasingly on measurements made in boreholes as a basis for reconstructing the character and behaviour of subglacial drainage systems. In temperate glaciers, in which boreholes remain open to the atmosphere following drilling, the interpretation of such data may be complicated by supraglacial or englacial water flows to and from boreholes. We report on a suite of techniques used to identify borehole water sources and to reconstruct patterns of water circulation within boreholes at Haut Glacier d'Arolla, Switzerland. Results are used to define a number of borehole ‘drainage’ types. Examples of each drainage type are presented, along with the manner in which they influence interpretations of borehole water‐levels, borehole water‐quality data, and borehole dye traces. The analysis indicates that a full understanding of possible borehole drainage modes is required for the correct interpretation of many borehole observations, and that those observations provide an accurate indication of subglacial conditions only under relatively restricted circumstances. Copyright © 2001 John Wiley & Sons, Ltd.     

Diurnal fluctuations in glacier ice deformation: Haut Glacier d'Arolla,Switzerland

Measurements of surface velocity, ice deformation (at 42 and 89% ice depth) and proglacial stream discharge were made at Haut Glacier d'Arolla, Switzerland, to determine diurnal patterns of variation in each. Data are analysed in order to understand better the relationship between hydraulically induced basal motion and glacier ice deformation over short timescales. The data suggest that hydraulically induced localized basal ‘slippery’ spots are created over diurnal cycles, causing enhanced basal motion and spatially variable glacier speed‐up. Our data indicate that daily glacier speed‐up is associated with reduced internal deformation over areas previously identified as slippery spots and increased deformation in areas located adjacent to or down‐glacier from slippery spots. We interpret this pattern in terms of a transfer of mechanical support for basal shear stress away from slippery spots to adjacent sticky areas, where the resulting stronger ice–bed coupling causes increased ice deformation near the bed. These patterns indicate that basal ice is subjected to stress regimes that are variable at a high spatial and temporal resolution. Such variations may be central to the creation of anomalous vertical velocity profiles measured above and down‐glacier of basal slippery zones, which have shown evidence for ‘plug flow’ and extrusion flow over annual timescales. Copyright © 2007 John Wiley & Sons, Ltd.     

Subglacial drainage system structure and morphology of Brewster Glacier,New Zealand

A global positioning system and ground penetrating radar surveys is used to produce digital elevation models of the surface and bed of Brewster Glacier. These are used to derive maps of subglacial hydraulic potential and drainage system structure using three different assumptions about the subglacial water pressure (P_w): (i) P_w = ice overburden; (ii) P_w = half ice overburden; (iii) P_w = atmospheric. Additionally, 16 dye‐tracing experiments at 12 locations were performed through a summer melt season. Dye return curve shape, together with calculations of transit velocity, dispersivity and storage, are used to infer the likely morphology of the subglacial drainage system. Taken together, the data indicate that the glacier is underlain by a channelised but hydraulically inefficient drainage system in the early summer in which water pressures are close to ice overburden. By mid‐summer, water pressures are closer to half‐ice overburden and the channelised drainage system is more hydraulically efficient. Surface streams that enter the glacier close to the location of major subglacial drainage pathways are routed quickly to the channels and then to the glacier snout. Streams that enter the glacier further away from the drainage pathways are routed slowly to the channels and then to the snout because they first flow through a distributed drainage system. Copyright © 2008 John Wiley & Sons, Ltd.     

Impact of post-mixing chemical reactions on the major ion chemistry of bulk meltwaters draining the haut glacier d'arolla,valais, Switzerland

Until now, alpine glacial meltwaters have been assumed to consist of two components, dilute quickflow and concentrated delayed flow, the mixing of which has been regarded as chemically conservative for the major dissolved ions and electrical conductivity. Dye tracing results suggest that this two-component model adequately represents the sub-glacial hydrology of the Haut Glacier d'Arolla, Switzerland. However, laboratory dissolution experiments in which various concentrations of glacial rock flour are placed in dilute solutions show that this rock flour is highly reactive and suggest that bulk meltwaters may acquire significant amounts of solute through rapid chemical reactions with suspended sediment which occur after mixing of the two components. This view is supported by detailed analysis of variations in the hydrochemistry of meltwaters draining from the Haut Glacier d'Arolla over three diurnal cycles during the 1989 melt season. Variations in the composition of bulk meltwaters are controlled by two main factors: dilution of the delayed flow component by quickflow, and the extent of post-mixing reactions. The latter depends on the suspended sediment concentration in bulk meltwaters and on the duration of contact between these waters and suspended sediment. Seasonal changes in the magnitude of these factors result in changes in the character and causes of diurnal variations in meltwater chemistry. In June, these variations reflect discharge-related variations in residence time within a distributed subglacial drainage system; in July, when a channelized drainage system exists beneath the lower glacier, they primarily reflect the dilution of delayed flow by quickflow; in August, when suspended sediment concentrations are particularly high, they reflect varying degrees of solute acquisition by post-mixing reactions with suspended sediment that take place in arterial channels at the glacier bed.     

Periodic drainage of ice-dammed lakes as a result of variations in glacier velocity

Previous discussions of the catastrophic drainage of ice-dammed lakes have centred on mechanisms where characteristics of the lake are crucial to drainage initiation, for example dam flotation or tunnel formation at a critical lake depth. This paper describes a mechanism for lake drainage where drainage initiation depends on the characteristics of the glacier and is independent of the characteristics of the lake. Prediction of this mechanism must be based on glacier dynamics, whereas the mechanisms most commonly discussed previously are best predicted primarily on the basis of lake evolution. An ice-dammed lake at the margin of the glacier Solheimajokull, in southern Iceland, was observed to drain rapidly into the sub- or englacial drainage system, supplying water and debris to the bed or interior of the glacier. Geomorphological evidence suggests that the lake drains and refills periodically, discharging up to 13300 m³ of water into the glacier-hydrological system. The depth of the maximum lake is insufficient to cause either flotation of the ice margin or tunnel opening by plastic deformation of the ice, and we suggest that sudden drainage is related to ice-bed separations associated with specific glacier flow states rather than to a critical lake depth threshold. This mechanism of lake drainage has implications for conditions at the glacier bed, for the development of basal ice and for the entrainment of debris into the glacier, as well as for the prediction of potentially hazardous catastrophic drainage events and jokulhlaups from ice-dammed lakes.     

Basal sediment evacuation by subglacial meltwater: suspended sediment transport from Haut Glacier d'Arolla,Switzerland

Proglacial suspended sediment transport was monitored at Haut Glacier d'Arolla, Switzerland, during the 1998 melt season to investigate the mechanisms of basal sediment evacuation by subglacial meltwater. Sub‐seasonal changes in relationships between suspended sediment transport and discharge demonstrate that the structure and hydraulics of the subglacial drainage system critically influenced how basal sediment was accessed and entrained. Under hydraulically inefficient subglacial drainage at the start of the melt season, sediment availability was generally high but sediment transport increased relatively slowly with discharge. Later in the melt season, sediment transport increased more rapidly with discharge as subglacial meltwater became confined to a spatially limited network of channels following removal of the seasonal snowpack from the ablation area. Flow capacity is inferred to have increased more rapidly with discharge within subglacial channels because rapid changes in discharge during highly peaked diurnal runoff cycles are likely to have been accommodated largely by changes in flow velocity. Basal sediment availability declined during channelization but increased throughout the remainder of the monitored period, resulting in very efficient basal sediment evacuation over the peak of the melt season. Increased basal sediment availability during the summer appears to have been linked to high diurnal water pressure variation within subglacial channels inferred from the strong increase in flow velocity with discharge. Basal sediment availability therefore appears likely to have been increased by (1) enhanced local ice‐bed separation leading to extra‐channel flow excursions and[sol ]or (2) the deformation of basal sediment towards low‐pressure channels due to a strong diurnally reversing hydraulic gradient between channels and areas of hydraulically less‐efficient drainage. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis and interpretation of patterns within and between hydroclimatological time series in an alpine glacier basin

This paper explores patterns within and between climatological and hydrological time series from an alpine glacier basin. Time series recorded in the basin of the Haut Glacier d'Arolla over the 1989 ablation season are subdivided into five subperiods. Box-Jenkins ARIMA (AutoRegressive Integrated Moving Average) and TFN (Transfer Function-Noise) models are estimated for each of the five subperiods and differences between the models are interpreted in the context of changing glacier hydrology, particularly the changing nature and extent of the glacier drainage network.     

Sulphide oxidation under partially anoxic conditions at the bed of the Haut Glacier d'Arolla,Switzerland

Oxygen isotopic data are presented for bulk glacial meltwaters draining the Haut Glacier d'Arolla, Valais, Switzerland and for the sulphate contained within them in an attempt to assess the redox status of the subglacial chemical weathering environment. The sulphate derived from subglacial chemical weathering is so depleted in ¹⁸O that it must have formed, at least partially, in an anoxic environment. Under these conditions, Fe³⁺ can act as an oxidizing agent and oxygen atoms incorporated into sulphate are derived from ¹⁸O‐depleted water molecules (by contrast, dissolved O₂ is strongly enriched in ¹⁸O). These data therefore support the hypothesis that sectors of the glacier bed are anoxic and that Fe³⁺ may act as a significant oxidizing agent under these conditions. Copyright © 2002 John Wiley & Sons, Ltd.     

Initial results from a distributed,physically based model of glacier hydrology

This paper describes the development and testing of a distributed, physically based model of glacier hydrology. The model is used to investigate the behaviour of the hydrological system of Haut Glacier d'Arolla, Valais, Switzerland. The model has an hourly time-step and three main components: a surface energy balance submodel, a surface flow routing submodel and a subglacial hydrology submodel. The energy balance submodel is used to calculate meltwater production over the entire glacier surface. The surface routing submodel routes meltwater over the glacier surface from where it is produced to where it either enters the subglacial hydrological system via moulins or runs off the glacier surface. The subglacial hydrology submodel calculates water flow in a network of conduits, which can evolve over the course of a melt season simulation in response to changing meltwater inputs. The main model inputs are a digital elevation model of the glacier surface and its surrounding topography, start-of-season snow depth distribution data and meteorological data. Model performance is evaluated by comparing predictions with field measurements of proglacial stream discharge, subglacial water pressure (measured in a borehole drilled to the glacier bed) and water velocities inferred from dye tracer tests. The model performs best in comparison with the measured proglacial stream discharges, but some of the substantial features of the other two records are also reproduced. In particular, the model results show the high amplitude water pressure cycles observed in the borehole in the mid-melt season and the complex velocity/discharge hysteresis cycles observed in dye tracer tests. The results show that to model outflow hydrographs from glacierized catchments effectively, it is necessary to simulate spatial and temporal variations in surface melt rates, the delaying effect of the surface snowpack and the configuration of the subglacial drainage system itself. The model's ability to predict detailed spatial and temporal patterns of subglacial water pressures and velocities should make it a valuable tool for aiding the understanding of glacier dynamics and hydrochemistry. © 1998 John Wiley & Sons, Ltd.     

Structure,morphology and water flux of a subglacial drainage system,Midtdalsbreen, Norway

Digital elevation models of the surface and bed of Midtdalsbreen, Norway are used to calculate subglacial hydraulic potential and infer drainage system structure for a series of subglacial water pressure assumptions ranging from atmospheric to ice overburden. A distributed degree‐day model is used to calculate the spatial distribution of melt on the glacier surface throughout a typical summer, which is accumulated along the various drainage system structures to calculate water fluxes beneath the glacier and exiting the portals for the different water pressure assumptions. In addition, 78 dye‐tracing tests were performed from 33 injection sites and numerous measurements of water discharge were made on the main proglacial streams over several summer melt seasons. Comparison of the calculated drainage system structures and water fluxes with dye tracing results and measured proglacial stream discharges suggests that the temporally and spatially averaged steady‐state water pressures beneath the glacier are ~70% of ice overburden. Analysis of the dye return curves, together with the calculated subglacial water fluxes shows that the main drainage network on the eastern half of the glacier consists of a hydraulically efficient system of broad, low channels (average width/height ratio ≈ 75). The smaller drainage network on the west consists of a hydraulically inefficient distributed system, dominated by channels that are exceptionally broad and very low (average width/height ratio ≈ 350). The even smaller central drainage network also consists of a hydraulically inefficient distributed system, dominated by channels that are very broad and exceptionally low (average width/height ratio ≈ 450). The channels beneath the western and central glacier must be so broad and low that they can essentially be thought of as a linked cavity system. Copyright © 2011 John Wiley & Sons, Ltd.     

Subglacial drainage system changes of the Gulkana Glacier,Alaska: discharge and sediment load observations and modelling

Hydrological characteristics of englacial and subglacial drainage systems in Gulkana Glacier, Alaska, were examined by analysing temporal variations of discharge and sediment load in the proglacial Phelan Creek in 2001. From data plots on semi‐log paper, it appeared appropriate to separate both discharge and sediment load into fast and slow components. The two components were possibly produced by two different drainage systems: an englacial and subglacial, ‘channellized’ system in the ablation zone, and a subglacial, ‘distributed’ system in the accumulation zone. The data indicate the occurrence of an event during which part of the ‘distributed’ drainage system changed into the ‘channellized’ drainage system. The daily time‐series of discharge and sediment load were represented using a tank model. In the model, the drainage from an additional tank was added, supposing that a subglacial reservoir full of water and sediment collapsed slowly when the subglacial drainage system changed from distributed to channellized. The simulation with the collapsed tank gave much more reasonable results than those with no collapsed tank. The contribution of the collapsed tank to total sediment load is 24%, which is much larger than 9% to total discharge. Copyright © 2006 John Wiley & Sons, Ltd.     

Evolution of glacial flow and drainage during the ablation season

Proglacial discharge is controlled by the geometry of the glacial drainage network and by meltwater processes. The glacial drainage systems of some alpine glaciers have been characterised using a model based on a proglacial discharge analysis. In this paper, we apply high frequency systemic analysis to data collected from the Baounet Glacier (French Alps) during two successive ablation seasons. Our approach is based on an analysis of the evolution of daily recession curves during the ablation period. The observed data are fitted by a single β-coefficient dependant recession law. Changes in β are compared to variations in the daily discharge amplitude and the time lag between air temperature and proglacial discharge. The changes in the β values do not appear to be related to changes in the time lag and the amplitude of the daily discharge. There were significant variations in the β-coefficient during the two ablation periods studied here even if there was no time lag or the daily discharge amplitude change. High values of β correspond to high drainage velocities; therefore increases in β values can be used as an indicator of the evolution of the glacier’s internal drainage network.     

Ice flow‐unit influence on glacier structure,debris entrainment and transport

The links between structural glaciology, glacial debris entrainment and transport have been established in a number of different glacier settings. Here we document the structural evolution of a temperate Alpine valley glacier (Vadrec del Forno, Switzerland) and demonstrate that individual flow units within the glacier have very different structural and debris characteristics. The glacier consists of a broad accumulation area with multiple basins feeding a relatively narrow tongue and is formed from six distinct flow units. Each flow unit has its own characteristic structural assemblage. Flow units that narrow rapidly down‐glacier are dominated by primary stratification that has evolved into longitudinal foliation. In contrast, wider flow units preferentially develop an axial planar foliation. Glacier structure plays a limited role in the entrainment of debris, which is more strongly influenced by ice‐marginal rockfall and avalanche inputs onto the glacier surface. However, once entrained, glacier structure controls the reorientation and redistribution of debris within the ice mass. By taking a whole‐glacier approach to describing glacier structure and debris transport, we conclude that individual flow units are unique with regard to structure and debris transfer. Copyright © 2013 John Wiley & Sons, Ltd.     

Subglacial topography and internal structure of central and western Dronning Maud Land, Antarctica, determined from airborne radio echo sounding

During five austral summers, from 1994/1995 until 1998/1999, the Alfred Wegener Institute (AWI) carried out a large airborne radio echo sounding (RES) survey in Dronning Maud Land (DML), Antarctica. These ice thickness measurements are part of the AWI contribution to the pre-site survey for a deep ice core drill site in DML within the European Project for Ice Coring in Antarctica (EPICA). The survey encompasses more than 90,000 km of RES profiles over DML and the adjacent coastal area, covering more than 1 million km². The lower boundary of the ice sheet could be determined area-wide. Internal horizons occurring in the upper two-thirds of the ice column can also be traced for several hundred kilometers. This work presents the latest maps of the subglacial topography of the investigated area as well as of an internal horizon.     

Impact of artificial subsurface drainage on groundwater travel times and baseflow discharge in an agricultural watershed,Iowa (USA)

Pollutant delivery through artificial subsurface drainage networks to streams is an important transport mechanism, yet the impact of drainage tiles on groundwater hydrology at the watershed scale has not been well documented. In this study, we developed a two‐dimensional, steady‐state groundwater flow model for a representative Iowa agricultural watershed to simulate the impact of tile drainage density and incision depth on groundwater travel times and proportion of baseflow contributed by tile drains. Varying tile drainage density from 0 to 0.0038 m^?1, while maintaining a constant tile incision depth at 1.2 m, resulted in the mean groundwater travel time to decrease exponentially from 40 years to 19 years and increased the tile contribution to baseflow from 0% to an upper bound of 37%. In contrast, varying tile depths from 0.3 to 2.7 m, while maintaining a constant tile drainage density of 0.0038 m^?1, caused mean travel times to decrease linearly from 22 to 18 years and increased the tile contribution to baseflow from 30% to 54% in a near‐linear manner. The decrease in the mean travel time was attributed to decrease in the saturated thickness of the aquifer with increasing drainage density and incision depth. Study results indicate that tile drainage affects fundamental watershed characteristics and should be taken into consideration when evaluating water and nitrate export from agricultural regions. Copyright © 2011 John Wiley & Sons, Ltd.     

Using fluidized bed and flume experiments to quantify cohesion development from aging and drainage

Temporal variations in soil erosion resistance are often the result of decreased soil cohesion due to physical disruption followed by a regain of soil strength through a process called aging, stabilization or consolidation. The goal of this study was to quantify changes in soil cohesion due to aging and subsurface hydrologic condition using a fluidized bed method. A flume experiment was also used to verify that findings from the fluidized bed experiment translated into measurable changes in soil erodibility. Tests were performed on three different soils (a Miami soil, a Cecil soil and Crosby–Miami soil complex). Changes in soil cohesion due to aging and drainage state were successfully detected by the fluidized bed technique. For all soils tested, cohesion developed in a two‐stage process where an increase in cohesion with aging duration immediately after the soil was rewetted, was followed by a decrease in cohesion which often started after 24 h of aging. When soils were aged at field capacity, the resulting cohesion measured by the fluidized bed method was on average 3.13 times higher than that measured when aging was performed at saturation. Trends in soil rill erodibility K_r with aging duration measured in the flume experiment were consistent with the two‐stage pattern observed in soil cohesion estimates but the legacy effect of suction applied at field capacity faded after 72 h of aging. Copyright © 2013 John Wiley & Sons, Ltd.