Englacial Conduit Research Articles

The evolution and preservation potential of englacial eskers: An example from Breiðamerkurjökull, SE Iceland

AbstractDirectly observing glacial drainage systems (englacial and subglacial) is challenging. The distribution, morphology and internal structure of eskers can provide valuable information about the glacial drainage system and meltwater processes. This work presents the annual evolution (meltout) and internal structure of an esker emerging from the Breiðamerkurjökull ice margin, southeast Iceland. Changes in esker morphology have been repeatedly mapped over a 1‐year period using high temporal and spatial resolution data acquired by an uncrewed aerial vehicle (UAV). The internal architecture of the esker was investigated using ground‐penetrating radar (GPR) surveys. These data are used to identify the dominant processes driving the formation of this englacial esker and to evaluate the preservation potential. The englacial esker was up to 2.6 m thick and ice‐cored. A large moulin upglacier of the esker, which evolved into an englacial conduit, supplied meltwater to the englacial channel. Upglacier dipping debris‐filled basal hydrofractures, formed by pressurised subglacial meltwater rising up the retrograde bed slope, likely supplied sediment to the englacial conduit. Over the 1‐year period of observation the crest morphology evolved from flat‐ to sharp‐crested and the esker footprint increased by a factor of 5.7 in response to post‐depositional processes. The findings presented here indicate that englacial eskers may have low preservation potential due to post‐depositional reworking such as slumping through ice‐core meltout and erosion by later meltwater flow. As englacial eskers may not be preserved in the landscape, they could represent important glacial drainage system components that are not currently captured in palaeo‐ice sheet reconstructions. This work highlights the value of creating a time series of high‐temporal resolution data to quantify morphological evolution and improve glacial process‐form models.

Morphology, flow dynamics and evolution of englacial conduits in cold ice

AbstractMeltwater routing through ice masses exerts a fundamental control over glacier dynamics and mass balance, and proglacial hydrology. However, despite recent advances in mapping drainage systems in cold, Arctic glaciers, direct observations of englacial channels and their flow conditions remain sparse. Here, using terrestrial laser scanning (TLS) surveys of the main englacial channel of cold‐based Austre Brøggerbreen (Svalbard), we map and compare an entrance moulin reach (122 m long) and exit portal reach (273 m long). Analysis of channel planforms, longitudinal profiles, cross‐sections and morphological features reveals evidence of spatial variations in water flow conditions and channel incision mechanisms, and the presence of vadose, epiphreatic and phreatic conditions. The entrance reach, located at the base of a perennial moulin, was characterized by vadose, uniform, channel lowering at annual timescales, evidenced by longitudinal grooves, whereas the exit portal reach showed both epiphreatic and vadose conditions, along with upstream knickpoint migration at intra‐annual timescales. Fine‐scale features, including grooves and scallops, were readily quantified from the TLS point cloud, highlighting the capacity of the technique to inform palaeoflow conditions, and reveal how pulses of meltwater from rainfall events may adjust englacial conduit behaviour. With forecasts of increasing Arctic precipitation in the coming decades, and a progressively greater proportion of glaciers comprising cold ice, augmenting the current knowledge of englacial channel morphology is essential to constrain future glacier hydrological system change.

Simultaneous drainage events from supraglacial lakes on the southern Inylchek Glacier, Central Asia

AbstractTo understand the mechanism of simultaneous drainage event related to supraglacial lakes on a debris-covered glacier, we investigated water-level variations of supraglacial lakes on the southern Inylchek Glacier in Kyrgyzstan. To examine these variations, we used daily aerial images for 2017–2019 from an uncrewed aerial vehicle that were converted to 15 cm-digital surface models and ortho-images. Our main results are as follows: (1) When one lake drained, the water levels of other lakes simultaneously increased, indicating that drainage water is shared with several lakes through a main englacial conduit. In one drainage event, a branched off englacial conduit clearly connected to a main englacial conduit. (2) Sometimes several lakes discharged simultaneously, indicating that several lakes had connected to a main englacial conduit that had opened. Such cases can cause larger-scale drainage than that from the opening of a branched off englacial conduit. (3) Simultaneous drainage occurred twice in the same year, each time through a different conduit, indicating that the main englacial conduit can be abandoned and reused. (4) In some lakes, the water level on the hydraulic gradient line increased gradually with nearly the same increase rate just before drainage. Such an increase may be an indicator of a possible simultaneous drainage event.

Ice Cliff Dynamics of Debris-Covered Trakarding Glacier in the Rolwaling Region, Nepal Himalaya

Ice cliffs can act as “hot spots” for melt on debris-covered glaciers and promote local glacier mass loss. Repeat high-resolution remote-sensing data are therefore required to monitor the role of ice cliff dynamics in glacier mass loss. Here we analyze high-resolution aerial photogrammetry data acquired during the 2007, 2018, and 2019 post-monsoon seasons to delineate and monitor the morphology, distribution, and temporal changes of the ice cliffs across the debris-covered Trakarding Glacier in the eastern Nepal Himalaya. We generate an ice cliff inventory from the 2018 and 2019 precise terrain data, with ice cliffs accounting for 4.7 and 6.1% of the debris-covered area, respectively. We observe large surface lowering (>2.0 m a−1) where there is a denser distribution of ice cliffs. We also track the survival, formation, and disappearance of ice cliffs from 2018 to 2019, and find that ∼15% of the total ice cliff area is replaced by new ice cliffs. Furthermore, we observe the overall predominance of northwest-facing ice cliffs, although we do observe spatial heterogeneities in the aspect variance of the ice cliffs (ice cliffs face in similar/various directions). Many new ice cliffs formed across the stagnant middle sections of the glacier, coincident with surface water drainage and englacial conduit intake observations. This spatial relationship between ice cliffs and the glacier hydrological system suggests that these englacial and supraglacial hydrological systems play a significant role in ice cliff formation.

Monitoring the seasonal changes of an englacial conduit network using repeated ground-penetrating radar measurements

Abstract. Englacial conduits act as water pathways to feed surface meltwater into the subglacial drainage system. A change of meltwater into the subglacial drainage system can alter the glacier's dynamics. Between 2012 and 2019, repeated 25 MHz ground-penetrating radar (GPR) surveys were carried out over an active englacial conduit network within the ablation area of the temperate Rhonegletscher, Switzerland. In 2012, 2016, and 2017 GPR measurements were carried out only once a year, and an englacial conduit was detected in 2017. In 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations in the detected englacial conduit. The resulting GPR data were processed using an impedance inversion workflow to compute GPR reflection coefficients and layer impedances, which are indicative of the conduit's infill material. The spatial and temporal evolution of the reflection coefficients also provided insights into the morphology of the Rhonegletscher's englacial conduit network. During the summer melt seasons, we observed an active, water-filled, sediment-transporting englacial conduit network that yielded large negative GPR reflection coefficients (<-0.2). The GPR surveys conducted during the summer provided evidence that the englacial conduit was 15–20 m±6 m wide, ∼0.4m±0.35m thick, ∼250m±6m long with a shallow inclination (2∘), and having a sinusoidal shape from the GPR data. We speculate that extensional hydraulic fracturing is responsible for the formation of the conduit as a result of the conduit network geometry observed and from borehole observations. Synthetic GPR waveform modelling using a thin water-filled conduit showed that a conduit thickness larger than 0.4 m (0.3× minimum wavelength) thick can be correctly identified using 25 MHz GPR data. During the winter periods, the englacial conduit no longer transports water and either physically closed or became very thin (<0.1 m), thereby producing small negative reflection coefficients that are caused by either sediments lying within the closed conduit or water within the very thin conduit. Furthermore, the englacial conduit reactivated during the following melt season at an identical position as in the previous year.

Geomorphological evolution of a debris‐covered glacier surface

AbstractThere exists a need to advance our understanding of debris‐covered glacier surfaces over relatively short timescales due to rapid, climatically induced areal expansion of debris cover at the global scale, and the impact debris has on mass balance. We applied unpiloted aerial vehicle structure‐from‐motion (UAV‐SfM) and digital elevation model (DEM) differencing with debris thickness and debris stability modelling to unravel the evolution of a 0.15 km2 region of the debris‐covered Miage Glacier, Italy, between June 2015 and July 2018. DEM differencing revealed widespread surface lowering (mean 4.1 ± 1.0 m a‐1; maximum 13.3 m a‐1). We combined elevation change data with local meteorological data and a sub‐debris melt model, and used these relationships to produce high resolution, spatially distributed maps of debris thickness. These maps were differenced to explore patterns and mechanisms of debris redistribution. Median debris thicknesses ranged from 0.12 to 0.17 m and were spatially variable. We observed localized debris thinning across ice cliff faces, except those which were decaying, where debris thickened. We observed pervasive debris thinning across larger, backwasting slopes, including those bordered by supraglacial streams, as well as ingestion of debris by a newly exposed englacial conduit. Debris stability mapping showed that 18.2–26.4% of the survey area was theoretically subject to debris remobilization. By linking changes in stability to changes in debris thickness, we observed that slopes that remain stable, stabilize, or remain unstable between periods almost exclusively show net debris thickening (mean 0.07 m a‐1) whilst those which become newly unstable exhibit both debris thinning and thickening. We observe a systematic downslope increase in the rate at which debris cover thickens which can be described as a function of the topographic position index and slope gradient. Our data provide quantifiable insights into mechanisms of debris remobilization on glacier surfaces over sub‐decadal timescales, and open avenues for future research to explore glacier‐scale spatiotemporal patterns of debris remobilization. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Detecting and characterising an englacial conduit network within a temperate Swiss glacier using active seismic, ground penetrating radar and borehole analysis

ABSTRACTEnglacial hydrology plays an important role in routing surface water to the glacier's bed and it consequently affects the glacier's dynamics. However, it is often difficult to observe englacial conduit conditions on temperate glaciers because of their short-lived nature. We acquired repeated active surface seismic data over the Rhone Glacier, Switzerland to monitor and characterise englacial conduit conditions. Amplitude-versus-angle analysis suggested that the englacial conduit is water filled and between 0.5 and 4 m thick. A grid of GPR profiles, acquired during the 2018 melt season, showed the englacial conduit network persisting and covering ~ 14,000 m2. In late summer 2018, several boreholes were drilled into the conduit network. We observed generally stable water pressure, but there were also short sudden increases. A borehole camera provided images of a fast flowing englacial stream transporting sediment through the conduit. From these observations, we infer that the englacial conduit network is fed by surface meltwater and morainal streams. The surface and morainal streams merge together, enter the glacier subglacially and flow through subglacial channels along the flank. These subglacial channels flow into highly efficient englacial conduits traversing the up-glacier section of the overdeepening before connecting with the subglacial drainage system.

Pond Dynamics and Supraglacial-Englacial Connectivity on Debris-Covered Lirung Glacier, Nepal

The hydrological systems of heavily-downwasted debris-covered glaciers differ from clean-ice glaciers due to the hummocky surface and debris mantle of such glaciers, leading to a relatively limited understanding of drainage pathways. Supraglacial ponds represent sinks within the discontinuous supraglacial drainage system, and have been documented to sporadically drain englacially. To assess pond dynamics, pond water level measurements were made on Lirung Glacier during May and October of 2013 and 2014. The four field seasons coincided with aerial, satellite, and terrestrial orthomosaic images and digital elevation models, which provided snapshots of the ponds and their surroundings. We analysed the glacier's closed surface catchments to identify surface drainage pathways and englacial drainage points, and compared this to field observations of surface discahrge. The ponded area was higher in the pre-monsoon than post-monsoon, with individual ponds filling and draining seasonally associated with the surface exposure of englacial conduit segments. We recorded four pond drainage events, all of which occurred gradually (duration of weeks), observed diurnal fluctuations indicative of varying supply and discharge, and we documented instances of interaction between distant ponds. The DEM drainage analysis identified numerous sinks >3m across the glacier surface, few of which exhibited ponds (23%), while the field survey highlighted surface discharge only explicable via englacial routes. Taken together our observations provide evidence for widespread supraglacial-englacial connectivity for meltwater drainage paths. Results suggest that progressive englacial conduit collapse events, themselves likely driven by supraglacial pond drainage, enable the glacier surface to evolve into a configuration following relict englacial conduit systems. Within this system, ponds form in depressions of reduced drainage efficiency and link the supraglacial and englacial drainage networks.

Lemthang Tsho glacial Lake outburst flood (GLOF) in Bhutan: cause and impact

BackgroundThe Hindu Kush Himalayan (HKH) region being seismically active and sensitive to climate change is prone to glacial lake outburst flood (GLOF). The Lemthang Tsho GLOF breached in the evening of 28 July 2015 innorth-western Bhutan is reminds of the looming threat, and stresses the need to have good risk management plan. The need to understand the physical processes in generating GLOF to is therefore imperative in order to effectively manage the associated risk. The paper therefore assesses the cause and impact of the Lemthang Tsho GLOF event using field and remote sensing data.ResultsThe collapse of near vertical wall of supraglacial lake triggered by 2 days of incessant rainfall, opened up the englacial conduit resulting in emptying of interconnected supraglacial lakes into Lemthang Tsho. The5.1 magnitude earthquake epicentered 187 km to southeast in the Indian state of Assam in the morning (7:10 am Bhutan Standard Time) of the same day is unlikely to have played any role in triggering the event. The estimated volume of water unleased is 0.37 million m3, with peak discharge estimated to be ranging from 1253 to 1562 m3/s, and velocity of 7.14–7.57 m/s. The impact was minimal and confined up to 30 km downstream from the lake. The flood took lives of 4 horses, washed away 4 timber cantilever bridges, 148 pieces of timber, damaged 1 acre of land, and washed away 1 km of trail. The team also monitored 3 out of 25 identified critical glacial lakes and downgraded the risk of all 3 critical glacial lakes based on the finding. This brings the number of critical glacial lakes in Bhutan to 22.ConclusionThe threat of GLOF still looms large in the Himalaya, particularly in view of impact of climate change and frequent seismic activities. There is a need for good risk management practices which starts fromidentification of critical glacial lakes, to prioritize in-depth investigation. As the present list of critical glacial lakes are largely based on inventory done over a decade based on topographic maps some of which datedback to 1960s, we need to revisit the critical glacial lakes and assess the risk considering recent changes. The new assessment needs to consider supraglacial lakes as one of the criteria in evaluating the GLOF risk, as highlighted by the Lemthang Tsho GLOF.

Quantifying ice loss in the eastern Himalayas since 1974 using declassified spy satellite imagery

Abstract. Himalayan glaciers are important natural resources and climate indicators for densely populated regions in Asia. Remote sensing methods are vital for evaluating glacier response to changing climate over the vast and rugged Himalayan region, yet many platforms capable of glacier mass balance quantification are somewhat temporally limited due to typical glacier response times. We here rely on declassified spy satellite imagery and ASTER data to quantify surface lowering, ice volume change, and geodetic mass balance during 1974–2006 for glaciers in the eastern Himalayas, centered on the Bhutan–China border. The wide range of glacier types allows for the first mass balance comparison between clean, debris, and lake-terminating (calving) glaciers in the region. Measured glaciers show significant ice loss, with an estimated mean annual geodetic mass balance of −0.13 ± 0.06 m w.e. yr−1 (meters of water equivalent per year) for 10 clean-ice glaciers, −0.19 ± 0.11 m w.e. yr−1 for 5 debris-covered glaciers, −0.28 ± 0.10 m w.e. yr−1 for 6 calving glaciers, and −0.17 ± 0.05 m w.e. yr−1 for all glaciers combined. Contrasting hypsometries along with melt pond, ice cliff, and englacial conduit mechanisms result in statistically similar mass balance values for both clean-ice and debris-covered glacier groups. Calving glaciers comprise 18 % (66 km2) of the glacierized area yet have contributed 30 % (−0.7 km3) to the total ice volume loss, highlighting the growing relevance of proglacial lake formation and associated calving for the future ice mass budget of the Himalayas as the number and size of glacial lakes increase.

Morphological dynamics of an englacial channel

Abstract. Despite an interest in the hydraulic functioning of supraglacial and englacial channels over the last 4 decades, the processes and forms of such ice-bounded streams have remained poorly documented. Recent glaciological research has demonstrated the potential significance of so-called "cut-and-closure" streams, where englacial or subglacial flow paths are created from the long-term incision of supraglacial channels. These flow paths are reported to exhibit step-pool morphology, comprising knickpoints and/or knickzones, exaggerated in dimensions in comparison to supraglacial channels. However, little is known of the development of such channels' morphology. Here, we examine the spatial organisation of step pools and the upstream migration of steps, many of which form knickzones, with repeated surveys over a 10-year period in an englacial conduit in cold-based Austre Brøggerbreen, Svalbard. The observations show upstream step recession to be the dominant process for channel evolution. This is paralleled by an increase in average step height and conduit gradient over time. Characteristic channel-reach types and step-riser forms are consistently observed in each of the morphological surveys reported. We suggest that the formation of steps has a hydrodynamic origin, where step-pool geometry is more efficient for energy dissipation than meanders. The englacial channel system is one in rapid transition towards a quasi-equilibrium form within a decadal timescale. The evolution and recession of knickzones reported here result in the formation of a 37 m deep moulin shaft, suggesting that over time an incising supraglacial channel may evolve towards an englacial channel form exhibiting a stable end-point characterised by a singular vertical descent, which potentially can reach the glacier bed. This challenges the prevailing notions that crevasses or hydrofractures are needed to form deep moulins. Our observations highlight the need to further examine the adjustment processes in cut-and-closure channels to better understand their coupling to supraglacial meltwater sources and potential significance in cold-based glacier hydrology and ice dynamics.

A numerical model for meltwater channel evolution in glaciers

Abstract. Meltwater channels form an integral part of the hydrological system of a glacier. Better understanding of how meltwater channels develop and evolve is required to fully comprehend supraglacial and englacial meltwater drainage. Incision of supraglacial stream channels and subsequent roof closure by ice deformation has been proposed in recent literature as a possible englacial conduit formation process. Field evidence for supraglacial stream incision has been found in Svalbard and Nepal. In Iceland, where volcanic activity provides meltwater with temperatures above 0 °C, rapid enlargement of supraglacial channels has been observed. Supraglacial channels provide meltwater through englacial passages to the subglacial hydrological systems of big ice sheets, which in turn affects ice sheet motion and their contribution to eustatic sea level change. By coupling, for the first time, a numerical ice dynamic model to a hydraulic model which includes heat transfer, we investigate the evolution of meltwater channels and their incision behaviour. We present results for different, constant meltwater fluxes, different channel slopes, different meltwater temperatures, different melt rate distributions in the channel as well as temporal variations in meltwater flux. The key parameters governing incision rate and depth are channel slope, meltwater temperature loss to the ice and meltwater flux. Channel width and geometry are controlled by melt rate distribution along the channel wall. Calculated Nusselt numbers suggest that turbulent mixing is the main heat transfer mechanism in the meltwater channels studied.

Processes of annual moraine formation at a temperate alpine valley glacier: insights into glacier dynamics and climatic controls

This paper presents the first detailed sedimentological study of annual moraines formed by an alpine valley glacier. The moraines have been forming since at least AD 1980 by a subsidiary lobe of Gornergletscher, Switzerland that advances up a reverse bedrock slope. They reach heights of 0.5–1.5 m, widths of up to 6 m and lengths of up to several hundreds of metres. Sediments in these moraines are composed of proglacial outwash and debris flow units; subglacial traction till is absent entirely. Based on four representative sections, three genetic process combinations have been identified: (i) inefficient bulldozing of a gently sloping ice margin transfers proglacial sediments onto the ice, causing differential ablation and dead‐ice incorporation upon retreat; (ii) terrestrial ice‐contact fans are formed by the dumping of englacial and supraglacial material from point sources such as englacial conduit fills; debris flows and associated fluvial sediments are stacked against a temporarily stationary margin at the start, and deformed during glacier advance in the remainder, of the accumulation season; (iii) a steep ice margin without supraglacial input leads to efficient bulldozing and deformation of pre‐existing foreland sediments by wholesale folding. Ice‐surface slope appears to be a key control on the type of process responsible for moraine formation in any given place and year. The second and third modes result in stable and higher moraines that have a higher preservation potential than those containing dead ice. Analysis of the spacing and climatic records at Gornergletscher reveals that winter temperature controls marginal retreat and hence moraine formation. However, any climatic signal is complicated by other factors, most notably the presence of a reverse bedrock slope, so that the extraction of a clear climatic signal is not straightforward. This study highlights the complexity of annual moraine formation in high‐mountain environments and suggests avenues for further research.

A rapidly growing moraine-dammed glacial lake on Ngozumpa Glacier, Nepal

Moraine-dammed glacial lakes are becoming increasingly common in the Himalaya as a result of glacier mass loss, causing concern about glacier lake outburst flood risk. In addition to extant lakes, the potential exists for many more to form, as more glaciers ablate down to the level of potential moraine dams. In this paper, we document the recent rapid growth of, a moraine-dammed lake on Ngozumpa Glacier, Nepal. Using a combination of ground-based mapping and sonar surveys, aerial photographs (<1m resolution), and ASTER imagery (15m resolution), processes and rates of lake expansion have been determined. The lake first formed between 1984 and 1992 when collapse of an englacial conduit allowed water to accumulate at the level of a gap in the lateral moraine, ~km from the glacier terminus. Lake growth was initially slow, but since 2001 it has undergone exponential growth at an average rate of 10%y−1. In 2009, the lake area was 300,000m2, and its volume was at least 2.2million m3. Calving, subaqueous melting, and melting of subaerial ice faces all contribute to the expansion of the lake; but large-scale, full-height slab calving is now the dominant contributor to growth. Comparison with other lakes in the region indicate that lake growth will likely continue unchecked whilst the spillway remains at its current level and may attain a volume of hundreds of millions of cubic metres within the next few decades.

Mechanisms of englacial conduit formation and their implications for subglacial recharge

Ideas about the character and evolution of englacial drainage systems have been deeply influenced by the theoretical model developed by Shreve [1972. Movement of water in glaciers. Journal of Glaciology 11(62), 205–214]. The Shreve model is based on three main assumptions: (1) englacial drainage is in steady state; (2) englacial water will flow along the steepest hydraulic gradient within the glacier; and (3) pressure head equals the pressure of the surrounding ice minus a small component due to melting of the walls. The Shreve model has been widely adopted as a fundamental component of englacial drainage theory. There is no evidence, however, that the model provides a realistic picture of actual glacial drainage systems.To evaluate Shreve's theory, we used speleological techniques to directly survey englacial conduits. We mapped a total of 8.25km of passage in 27 distinct englacial conduits in temperate, polythermal, cold-based and debris-covered glaciers between 2005 and 2008. New information reported here is supplemented by published data on 40 other englacial conduits located worldwide and surveyed to ice depths of 176m using speleological techniques. In all cases, englacial drainage systems consisted of a single unbranching conduit. Englacial conduit morphologies were found to be intimately linked to the orientation of a glacier's principal stresses or the presence of pre-existing lines of high hydraulic conductivity. If a sufficient supply of water is available, hydrofracturing forms vertical conduits in zones of longitudinal extension and subhorizontal conduits where longitudinal stresses are compressive. On unfractured glacier surfaces, relatively shallow subhorizontal conduits with migrating nickpoints form by cut-and-closure provided channel incision is significantly faster than surface lowering. Conduits can also form along permeable debris-filled crevasse traces that connect supraglacial lake basins of different potential. Our results suggest that Shreve-type englacial drainage systems do not exist and implies that englacial conduits can only penetrate through thick ice to recharge the bed where supraglacial water bodies either intersect, or are advected through, zones of acceleration.

A cut-and-closure origin for englacial conduits in uncrevassed regions of polythermal glaciers

AbstractOn uncrevassed regions of polythermal glaciers, englacial conduits can form by incision of supraglacial stream channels followed by roof closure. The origin and evolution of examples in Longyearbreen, Svalbard, and Khumbu Glacier, Nepal, were determined by speleological survey. The development of perennial incised channels requires that incision is significantly faster than glacier surface ablation, and thus will be favoured by high meltwater discharges in combination with cool climatic conditions or thick debris cover. Incised canyons can become blocked by drifted winter snow, refrozen meltwater, ice rafting from non-local sources (allochthonous breccias) and roof collapses (autochthonous breccias). Conduit closure can also occur in response to ice creep, particularly at depth. Following isolation from the surface, englacial conduits continue to evolve by vadose incision down to local base level. In the case of Longyearbreen, incision allowed the channel to reach the glacier bed, but on Khumbu Glacier deep incision is prevented because an effectively impermeable terminal moraine provides a high base level for the glacier drainage system. During our period of observations, deeper parts of the Longyearbreen conduit became blocked by a combination of ice accumulation and creep, causing the stream course to be re-routed to higher levels. On that glacier, incision, blockage and upward re-routing are cyclic. We conclude that ‘cut and closure’ is the dominant mechanism of englacial conduit formation on uncrevassed regions of polythermal glaciers.

Controls on the sedimentary architecture of a single event englacial esker: Skeiðarárjökull, Iceland

Eskers have been used to infer the dynamics and palaeo-hydrology of large ice sheets yet there are few suitable contemporary analogues for esker sedimentation during Quaternary glaciations. Consequently, models of esker sedimentation have been derived from morpho-sedimentary analyses of Quaternary eskers, but these lack rigorous constraints on their depositional timescales. We identify the controls on the large-scale sedimentary architecture of a large (>700 m long, <30 m high) proto-esker deposited during a single, high-magnitude jökulhlaup at Skeiðarárjökull, Iceland. Over 3.8 km of ground-penetrating radar (GPR) lines were collected (in 2006 and 2007) as grids on all workable areas of the proto-esker and associated supraglacial ice-walled canyon-fill sediment. Six main radar facies were identified: (1) discontinuous, undular reflections, interpreted as trough-cross strata, associated with the development of antidune or dune sequences; (2) sub-horizontal reflections (dip <10°), associated with deposition of upper-stage gravel plane beds; (3) up-flow dipping moderate to high-angle (>10°) inclined reflections, interpreted as backset accretion associated with large-scale bedform development; (4) down-flow dipping high-angle (>15°) inclined reflections, interpreted as foreset accretion associated with macroform progradation; (5) quasi-hyperbolic reflections, interpreted as zones of boulder clustering; and (6) zones of enhanced noise and EM signal attenuation, bounded on the upper surface by a continuous reflection sub-parallel to the ground surface, interpreted as buried ice associated with the conduit/channel base. These data suggest that deposition took place within a non-uniform englacial conduit, which has resulted in major up-flow to down-flow variations in sedimentary architecture. The complexities observed are a direct consequence of variations in sediment supply and the within-event evolution of conduit geometry. This study provides the first detailed insight into the large-scale sedimentary architecture of a single event jökulhlaup proto-esker.

Observations of englacial water passages:a fracture-dominated system

AbstractTo test models of the hydraulics and geometry of englacial conduits, 48 holes (3900m of ice) were drilled into Storglaciaren, Sweden, in search of conduits. About 79% of the holes intersected a hydraulically connected englacial feature. A video camera was used to examine the features and measure local water-flow rates. Because of the extremely clear ice that surrounded most features, their geometry could not be discerned. Of the remaining features, 80% (36) were fracture-like, 16%(6) were of complex geometry, and 4%(2) exhibited a conduit-like geometry. The fracture-like features exhibited steep plunges (∼70˚), narrow openings (∼40mm) and slow water-flow speeds (∼10mms–1). We argue that these fracture-like features are indeed englacial fractures of unknown origin. The depth to fractures intersection varied from near the glacier surface to 96% of local ice depth, with a maximum depth of 131 m. Few hydraulically connected fractures exhibited water motion, indicating some preferential flow pathways exist. We found one ‘traditional’ englacial conduit after an intentional search in a field of moulins. These results suggest that englacial water flow is conveyed through a ubiquitous network of fractures and that conduits are relatively rare.

Stability of quasi-steady flow in an englacial conduit

The stability of water flow in an englacial conduit is examined with particular reference to catastrophic outbursts of water. Quasi-steady flow of water in a conduit is considered, the conduit being simultaneously enlarged by frictional heating and compressed by plastic deformation in response to the pressure difference across the tunnel wall. The conduit is fed by an ice-dammed reservoir. With the aid of simplifying assumptions, we have devised a mathematical model consisting of two time-dependent, non-linear, dimensionless ordinary differential equations, which describe the time evolution of the conduit cross-section and the water depth in the reservoir. The conditions leading to different types of time-dependent flow behaviour are examined. Regions of the parameter space where the water flow is stable and unstable have been identified. In the unstable regime, the process of emptying the reservoir has either an oscillatory or an exponential character. In the stable regime, the system's return to equilibrium, following a perturbation, also exhibits an oscillatory or exponential character. Examples of this time-dependent behaviour are presented. The model has also been used to study the influence of the glacier, conduit and reservoir geometries on the system's stability. The results show that an increase in the horizontal area of the water reservoir or an increase in the slope of the conduit enhance the likelihood of a sudden outburst. However, an increase in the glacier thickness or the conduit length stabilizes the equilibria.

Temporal Switching between Englacial and Subglacial Drainage Pathways: Dye Tracer Evidence from the Haut Glacier d'Arolla, Switzerland

Thirty-four dye tracing experiments conducted from a moulin (m3Cf), located 1.5 km from the snout of the Haut Glacier d'Arolla, Switzerland, during the summers of 1989–1991 reveal two distinct drainage paths between the moulin and the glacier snout. In July, drainage appears to be predominantly via a hydraulically efficient channelised drainage system which results in dye breakthrough curves with velocities of >0.3 m s-1 and dispersivities of < 10 m. In August, drainage is via a more hydraulically inefficient distributed system which results either in velocities of <0.2 m s-1 and dispersivities > 10 m or in zero dye recovery. One injection conducted in mid July 1991 produced a double peaked breakthrough curve, indicating simultaneous drainage through both systems. The observed behaviour is opposite to that displayed by injections made at over 50 other moulins on the glacier, in which channelised flow replaces flow through a distributed system as the melt season progresses. It can be explained if moulin m3Cf intersects an englacial conduit which conveys water rapidly to a major subglacial channel in July when the distributed system is poorly developed and water pressures are high, but which is abandoned later in the year when increased glacier bed separation has expanded the drainage capacity of the distributed system. Dilution and retardation of dye within this enlarged distributed system can account for the frequency of zero recoveries from the August dye injections.