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Mechanisms of englacial conduit formation and their implications for subglacial recharge
Abstract
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.
... Much like karst, meltwater drainage into crevasses and permeable seams of englacial debris forms caves within debris-covered glaciers (Gulley & Benn, 2007). Surveys showed that caves often originate or terminate within depressions Gulley, Benn, Screaton, & Martin, 2009). Based on these observations, we hypothesize that depressions form similarly to sinkholes in karst. ...
... Sinkholes also form via cave collapse. Similar to karst bedrock, only preexisting crevasses and permeable seams of englacial debris can route meltwater through glacier ice (Gulley, Benn, Screaton, & Martin, 2009). Human-traversable conduits exist within debris-covered glaciers and develop when meltwater drains through preexisting lines of permeability Gulley & Benn, 2007;Gulley, Benn, Screaton, & Martin, 2009). ...
... Similar to karst bedrock, only preexisting crevasses and permeable seams of englacial debris can route meltwater through glacier ice (Gulley, Benn, Screaton, & Martin, 2009). Human-traversable conduits exist within debris-covered glaciers and develop when meltwater drains through preexisting lines of permeability Gulley & Benn, 2007;Gulley, Benn, Screaton, & Martin, 2009). Surface observations (Benn et al., 2001;Benn et al., 2012;E. ...
Plain Language Summary
As debris‐covered glaciers melt, they develop a complex, “hummocky” surface that is often attributed to variable thickness debris. Thousands of enclosed depressions pit the hummocky topography. An enigmatic quality of the depressions is that most do not contain meltwater ponds. To better understand how depressions form and grow, we examined the size distribution and geometry of depressions on the Ngozumpa glacier, located in the Everest Region of Nepal. These analyses suggest that the depressions undergo positive feedback growth. With a simple model, we demonstrated that positive feedback growth produces distributions of depressions similar to the Ngozumpa Glacier. Variable thickness debris cannot explain positive feedback depression growth because downhill debris movement and pond overflows would inhibit growth. However, englacial drainage is widespread on debris‐covered glaciers, and karst sinkholes undergoing positive feedback growth develop distributions similar to the depressions. Englacial drainage makes positive feedback depression growth possible by focusing incision and debris removal within the depression, limiting the influence of negative feedbacks. Sustained depression growth primes the glacier for the rapid development of glacial lakes, which increase melt rates and can pose outburst flood hazards. Our results show that englacial drainage is an important driver of hummocky topography development on debris‐covered glaciers.
... Recent studies have demonstrated that meltwater can drain efficiently through cold glaciers and their ice margins (e.g. Gulley, Benn, Müller, & Luckman, 2009a;Gulley, Benn, Screaton, & Martin, 2009b;Gulley et al., 2014;Naegeli et al., 2014;Vatne & Irvine-Fynn, 2016). Indeed, in a changing Arctic, the reduced permeability and flow of cold ice may favour an increasing prevalence of drainage dominated by supraglacial and englacial flowpaths. ...
... Speleological mapping has been less common, but has reported reach-scale englacial channel morphologies and been used to infer drainage path genesis and evolution in Arctic glaciers (e.g. Benn, Kristensen, et al., 2009;Gulley, Benn, Müller, & Luckman, 2009a;Gulley, Benn, Screaton, & Martin, 2009b;Holmlund, 1988;Müller, 2007;Myreng, 2015;Naegeli et al., 2014;Pulina & Rehak, 1991;Temminghoff et al., 2019;Vatne, 2001;Vatne & Irvine-Fynn, 2016). Moreover, across a range of thermal regimes, characteristic englacial drainage system entrance and exit crosssectional geometries and planform sinuosities have been described (e.g. ...
... However, while finer-scale morphological features such as grooves and scallops have been described in englacial channels (e.g. Gulley, Benn, Müller, & Luckman, 2009a;Gulley, Benn, Screaton, & Martin, 2009b;Gulley et al., 2014;Vatne, 2001;Vatne & Irvine-Fynn, 2016;Vatne & Refsnes, 2003), their potential to inform our understanding of flow conditions and conduit evolution remains unrealized, despite a longstanding and ongoing (e.g. Bushuk et al., 2019;Gilpin et al., 1980) interest in the formation and energetics of fine-scale morphologies on ice-water interfaces. ...
Meltwater 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 characterised 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.
... Our knowledge of moulin sizes, scales, and time evolution has largely been informed by exploration and mapping of the top 10 to 100 m of a few moulins Covington et al., 2020;Gulley et al., 2009;Holmlund, 1988;Moreau, 2009). These sparse field data indicate that moulin shapes deviate greatly from simple cylinders. ...
... This time range includes hydrofracture during rapid lake drainage (∼ 2 h) and slow lake drainage (<∼ 6 d, e.g., Selmes et al., 2011) and likely also the reactivation of existing moulins in ensuing melt seasons, which, based on the timing difference between surface melt onset and ice acceleration, occurs over multiple days (Andrews et al., 2018;Hoffman et al., 2011). On the other hand, moulin formation by cut-and-closure occurs over years to decades (Gulley et al., 2009), well above the MouSh relaxation time and the Maxwell time for ice, is more likely to create subvertical englacial channels. The interdependence of formation and evolution of these moulins gives us less confidence in applying our model to moulins with cut-and-closure origins. ...
... The interdependence of formation and evolution of these moulins gives us less confidence in applying our model to moulins with cut-and-closure origins. Those moulins primarily occur in temperate near-surface ice within polythermal glaciers (Gulley et al., 2009) and have not been reported on the GrIS. ...
Nearly all meltwater from glaciers and ice sheets is routed englacially through moulins. Therefore, the geometry and evolution of moulins has the potential to influence subglacial water pressure variations, ice motion, and the runoff hydrograph delivered to the ocean. We develop the Moulin Shape (MouSh) model, a time-evolving model of moulin geometry. MouSh models ice deformation around a moulin using both viscous and elastic rheologies and melting within the moulin through heat dissipation from turbulent water flow, both above and below the water line. We force MouSh with idealized and realistic surface melt inputs. Our results show that, under realistic surface melt inputs, variations in surface melt change the geometry of a moulin by approximately 10 % daily and over 100 % seasonally. These size variations cause observable differences in moulin water storage capacity and moulin water levels compared to a static, cylindrical moulin. Our results suggest that moulins are important storage reservoirs for meltwater, with storage capacity and water levels varying over multiple timescales. Implementing realistic moulin geometry within subglacial hydrologic models may therefore improve the representation of subglacial pressures, especially over seasonal periods or in regions where overburden pressures are high.
... Ponds and cliffs are often found close to one another, and it has been hypothesized that ponds contribute to the sustainability of cliffs due to their marginal melt effects, although cliffs can also survive for years without being connected to a pond Miles et al., 2016;Steiner et al., 2019;. Conversely, streams and cliffs may be associated, and it has been suggested that streams meandering across the debris-covered surface can lead to significant melt (Gulley et al., 2009) and to the formation of supraglacial valleys, or cryo-valleys, on the sides of which ice cliffs can form (Mölg et al., 2020;Watson et al., 2016). The development of such valleys has only been studied at one site and the length of time over which they evolve is not clear (Mölg et al., 2020). ...
... However, the development of cryo-valleys enables cliffs to arrange themselves in a very different and denser way than on a purely hummocky surface, and this transition is clearly visible in the case of Urdok (Figure 7). The exact reasons for this transition are unclear but are probably linked to the surge event highlighted by the change in velocity of the debris-covered area, which impacts the glacier hydrological system (Chudley & Willis, 2019;Gulley et al., 2009;. Indeed, the data for Urdok suggests that a surge front migrated through the upper part of the glacier (above the AOI) with little impact on the velocity or strain rates in the AOI but resulted in the routing of more water at the surface of the AOI, thus leading to the development of cryo-valleys. ...
... For example, the development of a supraglacial stream or pond from sub-debris melt and in-debris flow routing (Fyffe et al., 2019;Westoby et al., 2020) has the combined effect of increasing the melt at the base of the slope and removing the debris sliding down it (Benn et al., 2001;Miles et al., 2016;Moore, 2018). The development of supraglacial streams is therefore beneficial to an increase in cliff relative area along cryo-valleys (Mölg et al., 2019) as long as the incision rate does not exceed the sub-debris melt rate (e.g., Reid & Brock, 2010), which would lead the stream to form an englacial conduit via a cut-andclosure mechanism (Gulley et al., 2009;Jarosch & Gudmundsson, 2012). Such a stream could however be interrupted by the opening of a crevasse, which depends on the glacier strain rates, while at the same time such crevasses could initiate ice cliff formation via an increase of the slope angle and the removal of debris (Reid & Brock, 2014). ...
Ice cliffs are common on debris‐covered glaciers and have relatively high melt rates due to their direct exposure to incoming radiation. Previous studies have shown that their number and relative area can change considerably from year to year, but this variability has not been explored, in part because available cliff observations are irregular. Here, we systematically mapped and tracked ice cliffs across four debris‐covered glaciers in High Mountain Asia for every late ablation season from 2009 to 2019 using high‐resolution multi‐spectral satellite imagery. We then quantified the processes occurring at the feature scale to train a stochastic birth‐death model to represent the cliff population dynamics. Our results show that while the cliff relative area can change by up to 20% from year to year, the natural long‐term variability is constrained, thus defining a glacier‐specific cliff carrying capacity. In a subsequent step, the inclusion of external drivers related to climate, glacier dynamics, and hydrology highlights the influence of these variables on the cliff population dynamics, which is usually not a direct one due to the complexity and interdependence of the processes taking place at the glacier surface. In some extreme cases (here, a glacier surge), these external drivers may lead to a reorganization of the cliffs at the glacier surface and a change in the natural variability. These results have implications for the melt of debris‐covered glaciers, in addition to showing the high rate of changes at their surface and highlighting some of the links between cliff population and glacier state.
... Within some glaciers, debris may accumulate in water-filled crevasses from supraglacial sources, or washed in by supraglacial streams, providing an important source for englacial debris entrainment. Debris-filled crevasse traces thus form where a crevasse fills with supraglacial debris and subsequently closes, a process that is, common on debris-mantled glaciers where the debris-filled crevasse can provide a route for englacial water flow Gulley & Benn, 2007;Gulley, Benn, Screaton, & Martin, 2009). ...
... A potentially fruitful, but as yet untapped means of investigating structures at depth, is the exploitation of englacial meltwater conduits for viewing structures in cross-section. At the end of the ablation season, meltwater drainage through englacial channels largely ceases, and conduits have yet to close as a result of ice deformation; therefore, englacial access is possible (Gulley, Benn, Screaton, & Martin, 2009). Glacier speleological investigations have primarily been employed to investigate the formation and evolution of englacial drainage networks to aid our understanding of meltwater routing through ice masses. ...
... The hydrological systems of glaciers are strongly influenced by thermal regime, ice dynamics, and ice structure (Gulley, Benn, Screaton, & Martin, 2009). Temperate glaciers are generally free-draining internally, and water easily finds its way to the bed. ...
The aims of this review are to: (a) describe and interpret structures in valley glaciers in relation to strain history; and (b) to explore how these structures inform our understanding of the kinematics of large ice masses, and a wide range of other aspects of glaciology. Structures in glaciers give insight as to how ice deforms at the macroscopic and larger scale. Structures also provide information concerning the deformation history of ice masses over centuries and millennia. From a geological perspective, glaciers can be considered to be models of rock deformation, but with rates of change that are measurable on a human time‐scale. However, structural assemblages in glaciers are commonly complex, and unraveling them to determine the deformation history is challenging; it thus requires the approach of the structural geologist. A wide range of structures are present in valley glaciers: (a) primary structures include sedimentary stratification and various veins; (b) secondary structures that are the result of brittle and ductile deformation include crevasses, faults, crevasse traces, foliation, folds, and boudinage structures. Some of these structures, notably crevasses, relate well to measured strain‐rates, but to explain ductile structures analysis of cumulative strain is required. Some structures occur in all glaciers irrespective of size, and they are therefore recognizable in ice streams and ice shelves. Structural approaches have wide (but as yet under‐developed potential) application to other sub‐disciplines of glaciology, notably glacier hydrology, debris entrainment and transfer, landform development, microbiological investigations, and in the interpretation of glacier‐like features on Mars.
... Our knowledge of moulin sizes, scales, and time evolution has largely been informed by exploration and mapping of the top tens to hundred meters of a few moulins Covington et al., 2020;Gulley et al., 2009;Holmlund, 1988;Moreau, 2009). These sparse field data indicate that moulin shapes deviate greatly from simple cylinders. ...
... Moulins can form through multiple processes, including cut and closure of supraglacial streams (Gulley et al., 2009) and vertical hydrofracture through cold ice (Das et al., 2008). The formation mechanism dictates the initial geometry, which then evolves rapidly in response to a range of ice and melt processes to reach equilibrium geometry. ...
... and ice acceleration, occurs on a multi-day timescale Hoffman et al., 2011;Zwally et al., 2002). On the 595 other hand, moulin formation by cut-and-closure occurs over years to decades (Gulley et al., 2009), well above the MouSh relaxation time and the Maxwell time for ice. The interdependence of formation and evolution of these moulins gives us less confidence in applying our model to moulins with cut-and-closure origins. ...
Nearly all meltwater from glaciers and ice sheets is routed englacially through moulins, which collectively comprise approximately 10–14 % of the efficient englacial–subglacial hydrologic system. Therefore, the geometry and evolution of moulins has the potential to influence subglacial water pressure variations, ice motion, and the runoff hydrograph delivered to the ocean. We develop the Moulin Shape (MouSh) model, a time-evolving model of moulin geometry. MouSh models ice deformation around a moulin using both viscous and elastic rheologies and models melting within the moulin through heat dissipation from turbulent water flow, both above and below the water line. We force MouSh with idealized and realistic surface melt inputs. Our results show that variations in surface melt change the geometry of a moulin by approximately 30 % daily and by over 100 % seasonally. These size variations cause observable differences in moulin water storage capacity, moulin water levels, and subglacial channel size compared to a static, cylindrical moulin. Our results suggest that moulins are significant storage reservoirs for meltwater, with storage capacity and water levels varying over multiple timescales. Representing moulin geometry within subglacial hydrologic models would therefore improve their accuracy, especially over seasonal periods or in regions where overburden pressures are high.
... The controls on moulin volume and their likelihood to drive systematic differences in storage across the GrIS are currently unknown. Work on valley glaciers indicates that stress regime can exert a critical control on moulin morphologies (Gulley et al., 2009). Phobos moulin shares some similarities with moulins mapped in zones of transverse ice compression, and FOXX moulin is similar to moulins mapped in zones of transverse ice extension (Gulley et al., 2009). ...
... Work on valley glaciers indicates that stress regime can exert a critical control on moulin morphologies (Gulley et al., 2009). Phobos moulin shares some similarities with moulins mapped in zones of transverse ice compression, and FOXX moulin is similar to moulins mapped in zones of transverse ice extension (Gulley et al., 2009). Moulin storage volumes also seem likely to scale with discharge; however, the exact relationship between discharge and moulin volume is unknown. ...
... Moulin lifespans should also impact storage volumes, and, therefore, the extent to which moulins regulate basal pressure is likely to evolve over time. Long-lived moulins, particularly those that are reoccupied over multiple melt seasons (Catania & Neumann, 2010), will have greater time to grow, either through excavation of large entrance chambers, as observed here, or through the propagation of knickpoints that incise and melt ice upstream of the initial site of moulin formation (Gulley et al., 2009). Moulin lifetimes are likely to be longer within areas where ice flow is slower. ...
Plain Language Summary
Each summer season on the Greenland Ice Sheet, meltwater forms stream networks on the ice surface that deliver water to moulins, which are holes in the ice that carry the water to the base of the ice sheet. When water backs up into moulins, and the water pressure beneath the ice increases, glacier sliding accelerates, leading to more rapid loss of ice into the ocean. We directly explored two moulins that contained immense storage volumes that are much larger than previously assumed to exist. Our observations of water levels inside moulins, and a model of water flow through the ice, indicate that storage of water within these large moulins during daily meltwater pulses has a big impact on how much the water pressure beneath the ice changes. Our work suggests that moulin sizes influence the interactions between summer melt and sliding of the Greenland Ice Sheet. Consequently, we need a more complete understanding of how moulin volumes vary in order to better predict how future increases in melt will impact the rate of ice loss from Greenland and to constrain its future contribution to sea level rise.
... Furthermore, the presence of subglacial and englacial conduits plays an important role in the evolution of stagnant, debris-covered glaciers (Benn et al., 2001(Benn et al., , 2012Benn, Thompson, et al., 2017;Gulley et al., 2009;Thompson et al., 2016). First, the collapse of conduit roofs can lead to surface depressions and the formation of surface fractures, creating linear zones of subsidence Gulley & Benn, 2007). ...
... Subglacial conduits are also key in determining how the ice front will recede, especially when considering calving events. Although these are hydrological features of the glacier, they are known to exploit areas of inherent weakness within the ice (Gulley & Benn, 2007;Gulley et al., 2009). ...
... Furthermore, the presence of subglacial and englacial conduits plays an important role in the evolution of stagnant, debris-covered glaciers (Benn et al., 2001(Benn et al., , 2012Benn, Thompson, et al., 2017;Gulley et al., 2009;Thompson et al., 2016). First, the collapse of conduit roofs can lead to surface depressions and the formation of surface fractures, creating linear zones of subsidence Gulley & Benn, 2007). ...
... Subglacial conduits are also key in determining how the ice front will recede, especially when considering calving events. Although these are hydrological features of the glacier, they are known to exploit areas of inherent weakness within the ice (Gulley & Benn, 2007;Gulley et al., 2009). ...
We investigate the role of glacier structures in controlling ice‐front morphology and dynamics of four Himalayan lake‐terminating glaciers over a 20‐year period. At Imja, Trakarding, Lumdin and Dang Pu glaciers, lake area was mapped between 2000 and 2020 using Landsat 5/7/8 and Sentinel‐2 imagery. Discrete glacier flow units were identified, with glacier structures (e.g., open crevasses, transverse structures, longitudinal structures) digitised using the finest resolution panchromatic bands in each year (30, 15, or 10 m). Mapping revealed a distinct pattern of transverse structures towards the terminus of each glacier that influence ice‐front position and morphology and are then exploited via iceberg calving events. Our structural analysis also illustrates the role of subsurface conduits in calving events. During subsurface conduit collapse, glacier recession is enhanced, leading to calving along adjacent transverse structures. Furthermore, our analysis shows that ice‐front morphology influences the pattern of glacier recession. Ice fronts with distinct ice aprons undergo slower periods of recession than ice fronts with ice cliffs. We conclude that glacier structures are important in determining ice‐front morphologies at lake‐terminating Himalayan glaciers, and therefore, structural analysis is vital when assessing future ice‐front positions and behaviour, as well as rates of glacier recession.
... The up-glacier supply of supraglacial water would pour into the englacial hydrological system along the middle sections (sections G and F; Figure 8A), even after these large supraglacial ponds have disappeared, further expanding the englacial conduits. Such a hydrological system with supraglacial water flow into englacial channels has also been observed along Ngozumpa and Khumbu (Gulley et al., 2009; glaciers. A dense englacial conduit network is therefore inferred to exist along the gently sloping middle sections (sections F and G) of Trakarding Glacier. ...
... J. Glaciol. 53,[399][400][401][402][403][404][405][406][407][408][409][410][411][412].3189/ 002214307783258378 Gulley, J. D., Benn, D. I., Screaton, E., and Martin, J. (2009). Mechanisms of Englacial Conduit Formation and Their Implications for Subglacial Recharge. ...
Considerable parts of the ablation zone of many high mountain glaciers are covered
with supraglacial debris. The presence of supraglacial debris complicates the interplay of
processes linking climate change, topography, and glacier dynamics. Debris-covered
glaciers (DCGs) thus differ significantly from their clean-ice counterparts, as they form a
more complex system of forcing factors, couplings, and feedback mechanisms that are yet
to be fully understood. Resolving the uncertain response and evolution of debris-covered
glaciers is vital for devising sustainable management strategies for freshwater availability,
glacier-related hazards, hydro-power generation, and also for more precise estimation of
their contribution to eustatic sea level changes. The articles in this Research Topic cover
conceptual, modelling, and observational approaches to study DCGs at point to glacier
and regional scales.
... Time-consuming geophysical investigation methods, utilizing ground-penetrating radar (GPR) (e.g., Stuart et al., 2003;Baelum and Benn, 2011;Hansen et al., 2020;Schaap et al., 2020;Church et al., 2020Church et al., , 2021 and seismic arrays (Nanni et al., 2021) are used to locate en-and subglacial channels. In wintertime, moulins and meltwater channels are accessible for direct speleological investigations and map-ping of water flow paths in shallow glaciers (e.g., Holmlund, 1988;Vatne, 2001;Gulley et al., 2009;Alexander et al., 2020b;Hansen et al., 2020). Water pressures can be either inferred utilizing seismic observations (Gimbert et al., 2016;Nanni et al., 2020) or measured directly via moulins and boreholes (e.g., Iken, 1972;Iken and Bindschadler, 1986;Engelhardt et al., 1990;Hubbard et al., 1995;Stone and Clarke, 1996;Vieli et al., 2004;Andrews et al., 2014;Rada and Schoof, 2018). ...
... This shows that our method allows for identifying and locating step-pool sequences within glacial channels. This is of relevance, as step-pool sequences feature locally enhanced erosion and are therefore discussed as a mechanism by which supraglacial channels can incise into ice and transform into englacial channels (Gulley et al., 2009;Vatne and Irvine-Fynn, 2016). ...
Information about glacier hydrology is important for understanding glacier and ice sheet dynamics. However, our knowledge about water pathways and pressure remains limited, as in situ observations are sparse and methods for direct area-wide observations are limited due to the extreme and hard-to-access nature of the environment. In this paper, we present a method that allows for in situ data collection in englacial channels using sensing drifters. Furthermore, we demonstrate a model that takes the collected data and reconstructs the planar subsurface water flow paths providing spatial reference to the continuous water pressure measurements. We showcase this method by reconstructing the 2D topology and the water pressure distribution of a free-flowing englacial channel in Austre Brøggerbreen (Svalbard). The approach uses inertial measurements from submersible sensing drifters and reconstructs the water flow path between given start and end coordinates. Validation of the method was done on a separate supraglacial channel, showing an average error of 3.9 m and the total channel length error of 29 m (6.5 %). At the englacial channel, the average error is 12.1 m; the length error is 107 m (11.6 %); and the water pressure standard deviation is 3.4 hPa (0.3 %). Our method allows for mapping of subsurface water flow paths and spatially referencing the pressure distribution within. Further, our method would be extendable to the reconstruction of other, previously underexplored subsurface fluid flow paths such as pipelines or karst caves.
... Some authors, however, have postulated that structural weaknesses in ice, such as fractures, may exert a stronger influence on the development of glacial drainage system than the equipotential gradient (cf. Fountain et al., 2005a;Gulley et al., 2009;Burke et al., 2012). Moreover, the geometry and topography of former ice sheets may be varied as a consequence of varied ice dynamics, which may be manifested by the occurrence of e.g. ...
... This conclusion is supported by studies of the hydrologic systems of modern glaciers, which suggest that englacial distribution of meltwater may be governed by structural weaknesses in the ice rather than by the equipotential gradient (cf. Fountain et al., 2005aFountain et al., , 2005bBenn et al., 2009;Gulley, 2009;Gulley et al., 2009). Evidence for englacial structural control on of the location and development of an esker deposited during jökulhlaup is provided by Burke et al. (2009), based on a study from Skeiðarárjökull, Iceland. ...
We present the study of a subglacial drainage system that developed in the base of the Scandinavian Ice Sheet at the border of the Silesian Lowland and Silesian Upland (southern Poland) during the Drenthian Glaciation as a result of large outburst flood events. This drainage system was characterised by a spatially complex structure, i.e. it was composed of an individual an N-channel (tunnel channel) incised in Mesozoic rocks, i.e. Cretaceous and Triassic marls and limestones, and an R-channel, deposits of which form a set of large esker ridges which also occur on bedrock in the SE prolongation of the tunnel channel. Unlike most known cases of the co-occurrence of such forms, the studied tunnel channel and eskers are directly related, having been formed during the same flow events. The studied forms are unique within the entire southern sector of the Scandinavian Ice Sheet, which, in Central Europe, advanced mainly over soft unconsolidated Quaternary and Neogene sediments. The tunnel channel was formed as a result of enormous erosion of the ice sheet bed. The material eroded from the channel was partially deposited in a subsequent part of the drainage system. The location and orientation of the drainage system did not result directly from water pressure gradients, but was strongly determined by the internal structure of the ice sheet, i.e. it presumably developed along the marginal part of the ice stream supplying the Upper Odra ice lobe, which represented distinct weaknesses within the ice sheet. The development of the drainage system was the effect of the drainage of meltwater stored in the ice sheet system, most probably as a supraglacial lake. Gravelly-sandy rhythms recorded within the esker succession indicate that the process of lake drainage took place in several stages, or that the lake developed and drained several times.
... Under polar frozen bed conditions, it was once thought that little or no transport of sediment occurred other than in the supraglacial SDS. However, evidence now shows that even under polar or subpolar bed conditions, some limited transportation occurs as part of a slow deforming bed (Echelmeyer and Zhongxiang, 1987;Hallet et al., 1996;Alley et al., 1997;Nygård et al., 2007;Gulley et al., 2009;Batchelor and Dowdeswell, 2014;Dowdeswell et al., 2015;Livingstone et al., 2016;Stokes, 2018;Hogan et al., 2019;Reinardy et al., 2019). ...
... Most sediments transported by meltwater within all SDS are subject to reshaping and polishing by fluvial processes, resulting in most particles losing evidence of glacial surface wear. The main characteristics of meltwater transport tend to be the impact of rapid changes in meltwater discharge and hydrostatic pressure either in englacial or in subglacial channel systems (Alley et al., 1997(Alley et al., , 2019Gulley et al., 2009;Delaney et al., 2018). These sediments are all forms of glaciofluvial deposits and making a distinction between supraglacial or englacial origin can occasionally be difficult and highly uncertain. ...
Processes of sediment depositional systems (SDS) and the mechanics of erosion and deposition and related landform development are discussed with reference to glaciers and ice sheets. SDS and the complex, interlinked sediment pathway systems within glacial environments are illustrated. Glacial landforms (drumlins, fluted moraines, MSGL, ribbed moraines, end and lateral moraines, eskers, kame terraces, and outwash fans) are described and their possible origins discussed. Many of these landforms are part of a spectrum of related bedforms. These depositional forms are reviewed within the context of hard and soft sediment bed conditions. Many subglacial bedforms result from instabilities in subglacial glaciodynamic conditions. Hydrological states within glacial environments are also discussed.
... Livingstone et al., 2013;O. V. Sergienko & Hulbe, 2011;Wearing et al., 2024), despite direct observation from boreholes and geophysical surveys providing evidence that it is not universally applicable (Gulley et al., 2009). In this approximation, it is assumed that hydraulic potential is equal to the overburden potential and effective pressure is zero everywhere. ...
Plain Language Summary
Sea level rise is an ongoing threat that requires adaptation and management plans to avoid the worst impacts. A large portion of sea level rise comes from melting of glaciers and ice sheets; how much they will melt, and how quickly, remains very uncertain. It is important to reduce this uncertainty and provide sea level projections that are as accurate as possible. Several physical mechanisms are currently left out of ice flow models, including subglacial hydrology, the flow of water below glaciers, which can have large impacts on ice speed and rates of melting once a glacier reaches the ocean. We use a subglacial hydrology model applied to the full Antarctic Ice Sheet to calculate how fast subglacial water is flowing and where it accumulates below the ice. We compare our results to observations of ice shelf melting and to existing estimates of subglacial water pressure. We find good agreement between our results and observations and find that our results differ significantly from other, simpler estimates of water pressure. We provide our model results for use in ice flow models to better understand the impact of subglacial water on ice flow and to improve estimates of future sea level rise.
... These fractures eventually transform into moulins or conduits, although not all fractures give rise to conduits. Some large conduits form wherever sufficiently stressed ice coincides with a sufficient water supply (Gulley et al., 2009). Long episode of continuous action of melt water through fractures or conduits might have over deepened this whole feature. ...
Surface melting induces hydro-fracturing and deformational changes on the ice surface, which have a substantial impact on its dynamics and stability and therefore, comprehending these processes is crucial for ice sheet mass balance and stability. The present study is an attempt to understand the surface melt characteristics, seasonal ice movement, and calving processes of the Dålk glacier located in the Larsemann Hills of East Antarctica using multi-sensor remote sensing observations from 2017 to 2023. We identified and examined several melt features across the glacier and analysed their evolution during the study period. Frontal hanges, calving events, and seasonal surface ice velocities of the glacier were also investigated during the study period. We found that the evolution and drainage of the melt features over the Dålk glacier is highly dynamic in nature with a significant inter-annual variability. We report substantial morphological changes in the ice doline tructure that may be driven by the surface melt patterns, ice ovement, and ice deformation in the region which also, affect the drainage of the nearby connected melt ponds and features. Several minor to very major calving events observed during the period due o immense disintegration processes during the preceding melting eason that also affected the ice movement. We also highlight the eed of monitoring and studying such melt features over the outlet glaciers that concerns the imminent catastrophic fragmentation and deformational changes over the glacier.
... In addition, the uniformity assumption implies: (a) the glacier ice and subglacial till have an intrinsic permeability that is homogenous and isotropic, and (b) the recharge of water to the glacier bed is spatiotemporally uniform (Gulley et al., 2009(Gulley et al., , 2012. The spatiotemporal heterogeneity of both subglacial water recharge, that is, water entering the subglacial drainage system, and hydraulic conductivity at the glacier bed have both been identified by Gulley et al. (2012) to be important components of estimating hydropotential, and they are not accounted for in the formulation of Equation 15. ...
The Bering‐Bagley Glacier System (BBGS), Alaska, Earth's largest temperate surging glacier, surged in 2008–2013. We use numerical modeling and satellite observations to investigate how surging in a large and complex glacier system differs from surging in smaller glaciers for which our current understanding of the surge phenomenon is based. With numerical simulations of a long quiescent phase and a short surge phase in the BBGS, we show that surging is more spatiotemporally complex in larger glaciers with multiple reservoir areas forming during quiescence which interact in a cascading manner when ice accelerates during the surge phase. For each phase, we analyze the simulated elevation‐change and ice‐velocity pattern, infer information on the evolving basal drainage system through hydropotential analysis, and supplement these findings with observational data such as CryoSat‐2 digital elevation maps. During the quiescent simulation, water drainage paths become increasingly lateral and hydropotential wells form indicating an expanding storage capacity of subglacial water. These results are attributed to local bedrock topography characterized by large subglacial ridges that dam the down‐glacier flow of ice and water. In the surge simulation, we model surge evolution through Bering Glacier's trunk by imposing a basal friction representation that mimics a propagating surge wave. As the surge progresses, drainage efficiency further degrades in the active surging‐zone from its already inefficient, end‐of‐quiescence state. Results from this study improve our knowledge of surging in large and complex systems which generalizes to glacial accelerations observed in outlet glaciers of Greenland, thus reducing uncertainty in modeling sea‐level rise.
... Englacial eskers serve as records of englacial hydrology, which plays a crucial role in delivering supraglacial meltwater to the subglacial environment where it can influence ice dynamics (Catania et al., 2008;Das et al., 2008;Joughin et al., 2008;Zwally et al., 2002). Along with Shreve-type englacial conduit model of formation, other mechanisms include exploitation of fractures (Fountain et al., 2005;Stenborg, 1969), permeable debris-filled structures (Gulley & Benn, 2007), hydrofracturing (Boon & Sharp, 2003;Rothlisberger & Lang, 1987) and incision of supraglacial streams which then close over by the cut and closure mechanism (Fountain & Walder, 1998;Gulley et al., 2009). GPR can image subglacial and englacial drainage systems by penetrating through ice (Church et al., 2020;Church et al., 2021;Gusmeroli et al., 2008;Hansen et al., 2020;Karušs et al., 2022;Matsouka et al., 2007;Murray et al., 2008;Phillips et al., 2013) and provides a valuable tool to investigate the internal architecture of glacial landforms. ...
Directly 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 one-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 in origin. 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 one-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 melt-out 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.
... Slope undercutting and destabilization by streams or ponds has been observed to be one of the main triggers for ice cliff formation (Mölg et al., 2019;Röhl, 2006Röhl, , 2008Sakai & Takeuchi, 2000) and persistence (Benn et al., 2001(Benn et al., , 2012Brun et al., 2016;Kneib et al., 2022;Sato et al., 2021;. Other hypothesized cliff formation mechanisms based on field and remote sensing observations include crevasse opening (Reid & Brock, 2014;Steiner et al., 2019) or the collapse of englacial conduits (Egli et al., 2021;Gulley et al., 2009;Immerzeel et al., 2014;K. E. Miles et al., 2020;Sakai & Takeuchi, 2000), but these hypotheses have never been tested quantitatively. ...
Plain Language Summary
Debris‐covered glaciers are common throughout the world's mountain ranges and are characterized by the presence of steep ice cliffs among the debris‐covered ice. It is well‐known that the cliffs are responsible for a large portion of the melt of these glaciers but the controls on their formation, development and distribution across glaciers remains poorly understood. Novel mapping approaches combined with high‐resolution satellite and drone products enabled us to disentangle some of these controls and to show that the ice cliffs are generally formed and maintained by the surface hydrology (ponds or streams) or by the opening of crevasses. As a result, they depend both at the local and glacier scale on the dynamic state of the glaciers as well as the evolution stage of their debris cover. This provides a pathway to better represent their contribution to glacier melt in predictive glacier models.
... https://doi.org/10.5194/tc-15-3377-405 2021406 Gulley, J. D.,Benn, D. I., Screaton, E., & Martin, J. (2009). Mechanisms of englacial conduit 407 formation and their implications for subglacial recharge. ...
Ice cliff distribution plays a major role in determining the melt of debris-covered glaciers but its controls are largely unknown. We assembled a dataset of 37537 ice cliffs and determined their characteristics across 86 debris-covered glaciers within High Mountain Asia (HMA). We complemented this dataset with the analysis of 202 cliff formation events from multi-temporal UAV observations for a subset of glaciers. We find that 38.9% of the cliffs are stream-influenced, 19.5% pond-influenced and 19.7% are crevasses. Surface velocity is the main predictor of cliff distribution at both local and glacier scale, indicating its dependence on the dynamic state and hence evolution stage of debris-covered glacier tongues. Supraglacial ponds contribute to maintaining cliffs in areas of thicker debris, but this is only possible if water accumulates at the surface. Overall, total cliff density decreases exponentially with debris thickness as soon as debris gets thicker than 10 cm.
... Most subglacial hydrological models treat englacial storage as a spatially uniform and temporally constant model parameter, such as englacial void fraction (Bartholomaus et al., 2011;Hewitt, 2013;Hoffman et al., 2016;Koziol & Arnold, 2018;Sommers et al., 2018;Stevens et al., 2018;Werder et al., 2013). However, limited exploration within moulins in alpine type glaciers (e.g., Gulley et al., 2009;Holmlund, 1988;Vallot, 1898;Vatne & Irvine-Fynn, 2016) and in Greenland (Bourseiller et al., 2002;Covington et al., 2020;Griselin, 1995;Lamberton, 2002;Moreau, 2009) suggests that moulins often have irregular shapes, where storage capacity varies substantially with depth. ...
Subglacial models represent moulins as cylinders or cones, but field observations suggest that the upper part of moulins in the Greenland Ice Sheet has more complex shapes. These more complex shapes should cause englacial water storage within moulins to vary as a function of depth, a relationship not currently accounted for in models. Here, we use a coupled englacial‐subglacial channel model to explore how moulin shape affects depth‐dependent moulin water storage and water pressure dynamics within a subglacial channel. We simulate seven different moulin shapes across a range of moulin sizes. We find that the englacial storage capacity at the water level is the main control over the daily water level oscillation range and that depth‐varying changes in englacial water storage control the temporal shape of this oscillation. Further, the cross‐sectional area of the moulin within the daily oscillation range, but not above or below this range, controls pressures within the connected subglacial channel. Specifically, large cross‐sectional areas can dampen daily to weekly oscillations that occur in the surface meltwater supply. Our findings suggest that further knowledge of the shape of moulins around the equilibrium water level would improve englacial storage parameterization in subglacial hydrological models and aid predictions of hydrodynamic coupling.
... These conduits are usually water filled during the summer months, contained within the glacier by an ice lens or dam located below the exterior debris cover. They are water free during the winter months (Gulley et al 2009;Rounce et al 2017). ...
Glacial lake outburst floods, and specifically those triggered by avalanche-induced seiche waves, have been studied in considerable detail during the past several decades. Less attention has been given to other cryospheric flood phenomena, which include floods sourced primarily from englacial conduits, permafrost-linked rockfall and avalanches, and earthquake-triggered glacial lake floods. The article reviews examples of each phenomenon, based on field sampling and laboratory analyses, that have occurred in the Nepal Himalaya during the past decade, drawing parallels with similar events in other countries throughout the high mountain world. In most cases, the frequency of these events appears to be increasing globally, as is their potential to inflict significant damage downstream. We argue that each type of glacier flood requires more detailed study to develop the most effective prevention, mitigation, and adaptation approaches possible. Such studies will most likely be strengthened if they include a reconnaissance of the event as soon after its occurrence as possible, along with the participation, insights, and experience of local people, in addition to the use of increasingly powerful remote sensing technologies. How scientists can more quickly and effectively share the results of their research with decision-makers, and how decision-makers and governments can deliver more timely mitigation programs, are areas that also require further strengthening.
... Ice cliffs are high-relief, bare-ice areas that are commonly visible as vertical or near-vertical 'scars' on the surface of debris-covered glaciers (Sakai et al., 2002;Steiner et al., 2015;Buri et al., 2016;Buri et al., 2021). Supraglacial ponds are seasonal, i.e., they emerge during the monsoon season (Miles et al., 2018a); some can disappear through intra-glacial conduits (Gulley et al., 2009) or re-emerge in the same location (Taylor et al., 2021). Others persist between seasons and coalesce to form larger supraglacial lakes which may evolve into fully-formed proglacial ice and/or moraine-dammed lakes Thompson et al., 2012) with a potential to create GLOF events (Richardson and Reynolds, 2000;Komori, 2008;Benn et al., 2012;Reynolds, 2014;GAPHAZ, 2017). ...
Glacierized mountain ranges such as the Himalaya comprise a variety of glacier types, including clean and debris-covered glaciers. Monitoring their behaviour over time requires an assessment of changes in area and elevation along with surface features and geomorphology. In this paper we quantify the surface evolution of glacier systems in the Manaslu region of Nepal over the last five decades using 2013/2019 multi-sensor imagery and elevation data constructed from 1970 declassified Corona imagery and 1970 declassified Corona imagery. We investigate area changes, glacier thickness, geodetic glacier mass balance and surface velocity changes at regional scales and focus on the Ponkar Glacier and Thulagi Glacier and Lake for an in-depth assessment of surface geomorphology and surface feature dynamics (ponds, vegetation and ice cliffs). The time series of surface elevation changes for the lower ablation area of Ponkar Glacier is extended using 2019 UAV-based imagery and field-based ablation rates measured over the period 2016–2019. Glaciers in the Manaslu region experienced a mean area loss of −0.26 ± 0.0001% a⁻¹ between 1970 and 2019. The mean surface lowering was −0.20 ± 0.02 ma⁻¹ over the period 1970 to 2013, corresponding to a regional geodetic mass balance of −0.17 ± 0.03 m w. e.a⁻¹. Overall, debris-covered glaciers had slightly higher thinning rates compared to clean ice glaciers; lake-terminating glaciers had double thinning rates compared to land-terminating glaciers. Individual glacier mass balance was negatively controlled by glacier slope and mean glacier elevation. During the period 1970 to 2013, Ponkar Glacier had a geodetic mass balance of −0.06 ± 0.01 m w. e.a⁻¹, inversely correlated with parts of the central trunk thickening. Between 2013 and 2019 there was a nine-fold increase in the thinning rates over the lower parts of the glacier tongue relative to the period 1970–2013. Ice-surface morphology changes between 1970 and 2019 on Ponkar Glacier include a decrease in ogives and open crevasses, an increase in ice cliffs and ponds and the expansion of the supraglacial debris and ice-surface vegetation. These changes point to reduced ice-dynamic activity and are commensurate with the observed recession and negative glacier mass balance over the last five decades.
... This mass loss has been attributed to modified ice dynamics (Vincent et al. 2016;Brun et al. 2018;Anderson et al. 2021;Rounce et al. 2021) and to localized ice ablation rates related to ice cliffs and ponds (Sakai et al. 2000b;Miles et al. 2018b;Buri et al. 2021). The complex surface topography of debris-covered tongues exhibits exposed ice cliffs (Steiner et al. 2015;Buri and Pellicciotti 2018), surface ponds of various sizes A C C E P T E D M A N U S C R I P T (Sakai and Fujita 2010;Watson et al. 2016;Miles et al. 2018b), debris cones/hummocks (Moore 2018;Bartlett et al. 2021), medial moraines (Anderson 2000), lateral and terminal moraines (Hewitt and Shroder 1993;Owen et al. 2003;Benn et al. 2004), supraglacial streams (Fyffe et al. 2019a;Miles et al. 2020), surface depressions Benn et al. 2017;Miles et al. 2017), relict englacial conduits (Gulley and Benn 2007;Gulley et al. 2009b), base-level lakes (terminal, proglacial or supraglacial, and proto-lakes) ) and supraglacial vegetation (Fickert et al. 2007;Tampucci et al. 2016;Anderson et al. 2020) (Fig. 2). Certain supraglacial features act as 'hot spots' for ice melt, particularly ice cliffs (Sakai et al. 2002;Han et al. 2010;Steiner et al. 2015;Buri et al. 2016;Buri et al. 2021) and supraglacial ponds (Sakai et al. 2000b;Miles et al. 2016;Salerno et al. 2017;Miles et al. 2018b). ...
Glaciers respond sensitively to climate variability and change, with associated impacts on meltwater production, sea-level rise and geomorphological hazards. There is a strong societal interest to understand the current response of all types of glacier systems to climate change and how they will continue to evolve in the context of the whole glacierized landscape. In particular, understanding the current and future behaviour of debris-covered glaciers is a ‘hot topic’ in glaciological research because of concerns for eater resources and glacier-related hazards. The state of these glaciers is closely related to various hazardous geomorphological processes which are relatively poorly understood. Understanding the implications of debris-covered glacier evolution requires a systems approach. This includes the interplay of various factors such as local geomorphology, ice ablation patterns, debris characteristics, glacier lake growth and development. Such a broader, contextualized understanding is prerequisite to identifying and monitoring the geohazards and hydrologic implications associated with changes in the debris-covered glacier system under future climate scenarios.
This paper presents a comprehensive review of current knowledge of the debris-covered glacier landsystem. Specifically, we review state-of-the-art field and remote sensing-based methods for monitoring debris-covered glacier characteristics and lakes and their evolution under future climate change. We advocate a holistic process-based framework for assessing hazards associated with moraine-dammed glacio-terminal lakes that are a projected end-member state for many debris-covered glaciers under a warming climate.
... Large meandering supraglacial channels are no longer controlled by layered structures such as longitudinal foliation; however, fractures that intersect channels often divert water-flow or develop as moulins (see hydrofracturing, section 2.5.3) (Stenborg, 1968(Stenborg, , 1969(Stenborg, , 1973Hambrey, 1977b;Benn et al., 2009;Gulley et al., 2009b;Benn and Evans, 2010;Jennings et al., 2014). ...
This study examines how longitudinal foliation develops in glaciers and ice sheets in a
wide range of topographic, climatic, and dynamic settings, at a variety of spatial scales.
Study locations include four valley glaciers in Svalbard (Austre Brøggerbreen, Midtre
Lovénbreen, Austre Lovénbreen, and Pedersenbreen), a valley glacier in Canada
(Sermilik Glacier), and seven outlet glaciers in Antarctica (Hatherton Glacier, Taylor
Glacier, Ferrar Glacier, Lambert Glacier, Recovery Glacier, Byrd Glacier, and Pine Island
Glacier). Detailed structural mapping of the valley glaciers from satellite imagery and
field-based measurements were used to document the formation of longitudinal
foliation in small-scale ice masses. These findings were ‘up-scaled’ and applied to much
larger glaciers and ice streams. Longitudinal foliation develops in concentrated bands
at flow unit boundaries as a result of enhanced simple shear. However, longitudinal
foliation is not directly observable from satellite imagery at the surface of larger-scale
valley glaciers. The longitudinal structures visible at the surface of larger-scale glaciers
form at flow-unit boundaries and are composed of bands of steeply dipping
longitudinal foliation; however, they appear as individual linear features on satellite
imagery as a result of the comparatively low spatial resolution of the imagery. The
persistence of flowlines in the Antarctic Ice Sheet through areas of crevassing and net
ablation (blue-ice areas) suggests that they are the surface representation of a three-dimensional structure. Flowlines are therefore inferred to be the surface expression of
flow-unit boundaries composed of bands of steeply dipping longitudinal foliation. The
survival and deformation of flowlines in areas of ice flow stagnation indicates that
flowlines form in their initiation zones and not along their entire length. Furthermore,
these ice stagnation areas indicate that flowlines record past ice dynamics and
switches in ice flow.
... A GrIS subglacial hydrologic network was found to develop throughout the melt season toward a moulin 41 km up-ice . Initial "slow" water velocity of 0.25 m s -1 in this study is still an order of magnitude faster than water velocities measured from tracer injections into a distributed subglacial drainage system at an alpine glacier (South Cascade Glacier, Washington, USA) in 1987 (Fountain, 1993), suggesting moulins may connect to an already partially channelized network (Gulley et al., 2009). Diurnal variations in borehole hydraulic head reveal variations in subglacial channel pressure, but hydraulic head is correlated with increases in subglacial pressure in some boreholes and anti-correlated in others . ...
The presence, configuration, and efficiency of subglacial hydrologic systems has important implications for both glacial dynamics and the chemistry of meltwaters. These networks may exist in configurations that range from poorly connected and unable to accommodate large volumes of water to fast flowing and highly competent in different regions of the glacial bed, simultaneously or in different seasons. Direct study of the configuration and development of these networks is difficult as they are obscured by ice, yet network configuration is important in glaciological research as it controls the spatial distribution and residence time of subglacial water. Subglacial network efficiency, or the ability of the network to quickly evacuate water, controls under-ice water-rock interaction time affecting chemical weathering reactions and thus solute type and concentration expressed in proglacial meltwaters. Previous research into the configuration of subglacial hydrologic networks is limited in both temporal and spatial resolution, as field research generally occurs during summer months and is limited to more easily accessible glaciers. This dissertation investigates seasonal changes in subglacial hydrologic networks as evidenced by changing meltwater chemistry in late-summer at both a Canadian alpine glacier and an outlet glacier from the Greenland Ice Sheet. I undertook multi-month field campaigns at each location, during which I collected samples and made in situ measurements to correlate changes in chemical constituents carried within melt to changes in seasonality, improving understanding of this understudied time in seasonal glacial development. This dissertation uses laboratory experiments with sediment samples collected at glacial termini to evaluate the use of radon-222 (222Rn) activity concentrations, an intermediary in the uranium-238 (238U) decay chain, as a proxy for subglacial water residence time. These measurements are compared to in field 222Rn activity concentration measurements at sediment collection locations. Results show 222Rn activity concentration serves as a subglacial water residence time proxy but also reflects mineralogical sources of its parent isotope, radium-226 (226Ra). Field measurements of 222Rn activity concentrations as a proxy will be more robust and reliable if supported with laboratory leachate experiments with site-specific sediment samples, addressing likely lithological and sediment-size controls on 226Ra concentrations. I undertook a three-month field study of the alpine Athabasca Glacier in the Canadian Rockies in August through October, 2014. Both in situ and elemental chemistry of pro-glacial meltwaters are investigated relative to water discharge fluxes, air temperatures, and precipitation events to see how the subglacial network responds to climatic and glacial variables during the late summer-early fall. Different chemical weathering rates in response to changes in weather reveal shifts in network configuration, indicating the subglacial environment is dynamic and very responsive to climate conditions. Methods used at the Athabasca Glacier were then applied to Kiattuut Sermiat, an outlet glacier from the Greenland Ice Sheet to investigate possible differences in hydrology between alpine and outlet glaciers. Although environmental conditions are dissimilar between locations, the Kiattuut Sermiat results suggest the possible existence of an interannual subglacial drainage system capable of evacuating waters sourced from significantly further up into the Greenland Ice Sheet concurrent with a well-organized subglacial network configuration. This dissertation presents new measurements of glacial chemistry from an understudied period in seasonal glacial evolution, with interpretations unique to each glacier investigated.
... The up-glacier supply of supraglacial water would pour into the englacial hydrological system along the middle sections (sections G and F; Figure 8A), even after these large supraglacial ponds have disappeared, further expanding the englacial conduits. Such a hydrological system with supraglacial water flow into englacial channels has also been observed along Ngozumpa (Benn et al., 2012;Benn et al., 2017) and Khumbu (Gulley et al., 2009;Miles et al., 2019) glaciers. A dense englacial conduit network is therefore inferred to exist along the gently sloping middle sections (sections F and G) of Trakarding Glacier. ...
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.
... Caves develop mainly in karstic areas, where infiltrating water dissolves soluble rocks such as limestone, dolostone, gypsum, to create extensive networks of voids underground [9]. However, caves can also be found in volcanic areas (lava tubes and evacuated magma chambers) [10], in extremely resistant lithologies such as quartzites [11], in salt deserts [12] and inside glaciers (ice caves due to melt [13]). Although caves can be found in a wide range of environments, they share some general characteristics, such as complete darkness. ...
Caves remain among the most challenging exploration frontiers on planet Earth. They are difficult to access, present a range of unique and unusual environmental characteristics, and can only be mapped through direct human exploration. These challenges and several environmental factors specific to caves mean that speleology shares several analogies with space missions. For humans, cave exploration imposes isolation, confinement, minimal privacy, technical challenges, limited equipment and supplies, a sense of disconnect from the surface and regular life, a lack of diurnal cycles, and the constant presence of risk. As many of the same challenges are imposed on humans during space exploration, in 2005 the European Space Agency (ESA) began examining the possibility of using natural cave systems as a platform for astronaut training. These efforts resulted in a new ESA training programme named CAVES (Cooperative Adventure for Valuing and Exercising human behaviour and performance Skills) being launched in 2011, involving astronauts from partner space agencies. The primary objective of this training is to enhance astronaut individual and team performance and behavioural competencies by exposing them to the challenges of a real mission into an unknown and dangerous environment. To achieve this, the course's training activities are based around a real scientific and technological programme focused on cave science. Many aspects of the location and course content have been designed by a team of behavioural experts, scientists, trainers, operations engineers and speleologists with the support of caving organizations and schools. CAVES training events leverage cave exploration to create situations that are analogues to spaceflight in terms of safety protocols, perception and management of risk, crew composition and role assignments, group living, isolation, and confinement. In addition, these courses provide an opportunity for astronauts to experience spaceflight-like or relevant operations, science, equipment testing, and exploration, in preparation for future planetary endeavours. The scientific, exploration and equipment testing aspects of the course are real (not simulated). This ensures that these activities provide benefits to the speleological and scientific communities, whilst guaranteeing the realism of these activities for training purposes. During six editions of CAVES, from 2011 to 2019, 34 astronauts from 6 different space agencies (ESA, NASA, JAXA, ROSCOSMOS, CSA and CNSA) have taken part in the training. The CAVES training programme has been recognized by all participant astronauts and, in particular, by those who have travelled to space, as one of the best space analogue training opportunities available on Earth. The learning outcomes are applicable to both current and future orbital missions, as well as surface and subsurface missions to other planetary bodies.
... Ground penetrating radar or active seismic imagery is often conducted with wavelengths too large to capture meters-scaled structures and when using shorter wavelengths (i.e. higher frequencies) the signal often does not reach the base of the glacier because of attenuation and scattering (Church et al., 2019). There have been direct investigations through glacial speleology (Gulley et al., 2009), but as they are often conducted in winter it is difficult to assess, for instance, channel's opening rate, water flow velocities or water pressure inside those conduits. Dye tracing experiments (i.e. ...
The way in which water flows in the subglacial environment exerts a major control on ice-bed mechanical coupling, which strongly defines glacier sliding speeds. Today our understanding on the physics of the subglacial hydrology network is limited because of the scarcity of field measurements that yield a partial representation of the heterogeneous subglacial environment. The aim of my PhD work is to use passive seismology to help overcome common observational difficulties and quantify the evolution of the subglacial hydrology network pressure conditions and its configuration. Recent works show that subglacial turbulent water flow generates seismic noise that can be related to the associated hydrodynamics properties. These analyses were conducted over a limited period of time making it unclear whether such approach is appropriate to investigate seasonal and diurnal timescales, I.e. when subglacial water flow influences the most glacier dynamics. In addition, previous studies did not consider spatial changes in the heterogeneous drainage system, and until now, almost no study has located seismic noise sources spatially scattered and temporally varying. In this PhD work I address those seismological-challenges in order to resolve the subglacial hydrology dynamics in time and space.We acquired a 2-year long continuous dataset of subglacial-water-flow-induced seismic power as well as in-situ measured glacier basal sliding speed and subglacial water discharge from the Glacier d'Argentière (French Alps). I show that a careful investigation of the seismic power within [3-7] Hz can characterize the subglacial water flow hydrodynamics from seasonal to hourly timescales and across a wide range of water discharge (from 0.25 to 10 m3/sec). Combining such observations with adequate physical frameworks, I then inverted the associated hydraulic pressure gradient and hydraulic radii. I observed that the seasonal dynamics of subglacial channels is characterized by two distinct regimes. At low discharge, channels behave at equilibrium and accommodate variations in discharge mainly through changes in hydraulic radius. At a high discharge rate and with pronounced diurnal water-supply variability, channels behave out of equilibrium and undergo strong changes in the hydraulic pressure gradient, which may help sustain high water pressure in cavities and favor high glacier sliding speed over the summer.We then conducted a one-month long dense seismic-array experiment supplemented by glacier ice-thickness and surface velocity measurements. Using this unique dataset, I developed a novel methodology to overcome the challenge of locating seismic noise sources spatially scattered and temporally varying. Doing so, I successfully retrieve the first two-dimensional map of the subglacial drainage system as well as its day-to-day evolution. Using this map, I characterize when and where the subglacial drainage system is distributed through connected cavities, which favour rapid glacier flow versus localized through a channelized system that prevents rapid glacier flow. In addition, I also use high frequency seismic ground motion amplitude to study glacier features such as crevasses, thickness or ice anisotropy in a complementary way to what is traditionally done with seismic phase analysis.The first outcome of this cross-boundary PhD work is that one can analyse passive seismic measurements to retrieve the temporal evolution of subglacial channels pressure and geometry conditions over a complete melt-season. The second is that dense seismic array measurements can be used to resolve the subglacial drainage system spatial configuration and observe the switch from distributed to localized subglacial water flow. Such advances open the way for studying similar subglacial process on different sites and in particular in Greenland and Antarctica. This also concerns numerous sub-surface environment that host similar process such as volcanoes, karst, and landslides.
... Lliboutry (1971) argued that englacial conduits have difficulty forming within connected veins as a result of deformation and recrystallisation of the grains closing intergranular channels. Furthermore, field observations by Gulley et al. (2009b) have resulted in the formation mechanisms of englacial conduits within temperate ice being questioned. As within cold ice, englacial conduits seem to form as a result of hydraulically assisted fracture propagation in temperate ice (Gulley, 2009). ...
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 15–20m±6m 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.
... The source of the flood is unknown, but may have been a glacier outburst flood from the Ramdam or other glacier to the northeast, similar to those reported for the Lhotse glacier in Khumbu for the years 2015 [9] and videoed in 2016 [53]. As opposed to GLOFs, which in the Himalayas are usually triggered by catastrophic ice avalanches and resultant moraine dam breaching and collapse [4,54], water sources from glacier outburst floods originate within the glacier itself, i.e., from its inner system of englacial conduits and supraglacial ponds that are often inter-connected [53,[55][56][57][58].The sudden and rapid drainage of a large meltwater pond can trigger the release of water stored within conduits and other surficial ponds located further down the glacier, collectively forming a potentially dangerous flood downstream (see: https://www.youtube.com/watch?v=UM0UnoDGEAc). ...
An interdisciplinary field investigation of historic glacial lake outburst floods (GLOFs) in the Kanchenjunga region of Nepal was conducted between April and May, 2019. Oral history and field measurements suggested that at least six major GLOFs have occurred in the region since 1921. A remote sensing analysis confirmed the occurrence of the six GLOFs mentioned by informants, including two smaller flood events not mentioned that had occurred at some point before 1962. A numerical simulation of the Nangama GLOF suggested that it was triggered by an ice/debris avalanche of some 800,000 m 3 of material, causing a surge wave that breached the terminal moraine and released an estimated 11.2 × 10 6 m 3 ± 1.4 × 10 6 m 3 of water. Debris from the flood dammed the Pabuk Khola river 2 km below the lake to form what is today known as Chheche Pokhari lake. Some concern has been expressed for the possibility of a second GLOF from Nangama as the result of continued and growing landslide activity from its right lateral moraine. Regular monitoring of all lakes and glaciers is recommended to avoid and/or mitigate the occurrence of future GLOF events in the region. Collectively, the paper demonstrates the benefits and utility of interdisciplinary research approaches to achieving a better understanding of past and poorly documented GLOF events in remote, data-scarce high mountain environments.
... The influence of surface meltwater fluxes will be regulated by the englacial hydrological system, but crevasses, conduits and moulins can readily drain surface meltwater to the glacier bed and facilitate 'fast' englacial water transfer (cf. Fountain et al., 2005;Benn et al., 2009;Gulley et al., 2009). Recent instrumental and geophysical studies at Skálafellsjökullan active temperate glacier with a similar landsystem signature to Fjallsjökull (cf. ...
This paper presents detailed geomorphological and sedimentological investigations of small recessional moraines at Fjallsjökull, an active temperate outlet of Öræfajökull, southeast Iceland. The moraines are characterised by striking sawtooth or hairpin planforms, which are locally superimposed, giving rise to a complex spatial pattern. We recognise two distinct populations of moraines, namely a group of relatively prominent moraine ridges (mean height ~1.2 m) and a group of comparatively low-relief moraines (mean height ~0.4 m). These two groups often occur in sets/systems, comprising one pronounced outer ridge and several inset smaller moraines. Using a representative subsample of the moraines, we establish that they form by either (i) submarginal deformation and squeezing of subglacial till or (ii) pushing of extruded tills. Locally, proglacial (glaciofluvial) sediments are also incorporated within the moraines during pushing. For the first time, to our knowledge, we demonstrate categorically that these moraines formed sub-annually using repeat uncrewed aerial vehicle (UAV) imagery. We present a conceptual model for sub-annual moraine formation at Fjallsjökull that proposes the sawtooth moraine sequence comprises (i) sets of small squeeze moraines formed during melt-driven squeeze events and (ii) larger push moraines formed during winter re-advances. We suggest the development of this process-form regime is linked to a combination of elevated temperatures, high surface meltwater fluxes to the bed, and emerging basal topography (a depositional overdeepening). These factors result in highly saturated subglacial sediments and high porewater pressures, which induces submarginal deformation and ice-marginal squeezing during the melt season. Strong glacier recession during the summer, driven by elevated temperatures, allows several squeeze moraines to be emplaced. This process-form regime may be characteristic of active temperate glaciers receding into overdeepenings during phases of elevated temperatures, especially where their englacial drainage systems allow efficient transfer of surface meltwater to the glacier bed near the snout margin.
... Taking into account the seasonal variation of the efficiency of the englacial drainage system appears necessary to simulate the diurnal flow cycle correctly (Hannah and Gurnell, 2001). Therefore, further improvements should be based on studies analyzing the mechanisms of glacier drainage systems in the Khumbu region and their influence on glacier outflows (e.g., Gulley et al., 2009;Benn et al., 2017). These studies show that englacial conduits and supraglacial channels, ponds, and lakes play a key role in the response of glaciers: DHSVM-GDM could thus be upgraded by implementing a parameterization of such systems and delay the response of glacierized areas, as successfully proposed, for instance, in the model developed by Flowers and Clarke (2002). ...
In a context of climate change and water demand growth, understanding the origin of water flows in the Himalayas is a key issue for assessing the current and future water resource availability and planning the future uses of water in downstream regions. Two of the main issues in the hydrology of high-altitude glacierized catchments are (i) the limited representation of cryospheric processes controlling the evolution of ice and snow in distributed hydrological models and (ii) the difficulty in defining and quantifying the hydrological contributions to the river outflow. This study estimates the relative contribution of rainfall, glaciers, and snowmelt to the Khumbu River streamflow (Upper Dudh Koshi, Nepal, 146 km2, 43 % glacierized, elevation range from 4260 to 8848 m a.s.l.) as well as the seasonal, daily, and sub-daily variability during the period 2012–2015 by using the DHSVM-GDM (Distributed Hydrological Soil Vegetation Model – Glaciers Dynamics Model) physically based glacio-hydrological model. The impact of different snow and glacier parameterizations was tested by modifying the snow albedo parameterization, adding an avalanche module, adding a reduction factor for the melt of debris-covered glaciers, and adding a conceptual englacial storage. The representation of snow, glacier, and hydrological processes was evaluated using three types of data (MODIS satellite images, glacier mass balances, and in situ discharge measurements). The relative flow components were estimated using two different definitions based on the water inputs and contributing areas. The simulated hydrological contributions differ not only depending on the used models and implemented processes, but also on different definitions of the estimated flow components. In the presented case study, ice melt and snowmelt contribute each more than 40 % to the annual water inputs and 69 % of the annual stream flow originates from glacierized areas. The analysis of the seasonal contributions highlights that ice melt and snowmelt as well as rain contribute to monsoon flows in similar proportions and that winter outflow is mainly controlled by the release from the englacial water storage. The choice of a given parametrization for snow and glacier processes, as well as their relative parameter values, has a significant impact on the simulated water balance: for instance, the different tested parameterizations led to ice melt contributions ranging from 42 % to 54 %. The sensitivity of the model to the glacier inventory was also tested, demonstrating that the uncertainty related to the glacierized surface leads to an uncertainty of 20 % for the simulated ice melt component.
... Taking into account the seasonal variation of the efficiency of the englacial drainage system appears necessary to simulate the diurnal flow cycle correctly (Hannah and Gurnell, 2001). Therefore, further improvements should be based on studies analyzing the mechanisms of glacier drainage systems in the Khumbu region and their influence on glacier outflows (e.g., Gulley et al., 2009;Benn et al., 2017). These studies show that englacial conduits and supraglacial channels, ponds, and lakes play a key role in the response of glaciers: DHSVM-GDM could thus be upgraded by implementing a parameterization of such systems and delay the response of glacierized areas, as successfully proposed, for instance, in the model developed by Flowers and Clarke (2002). ...
In a context of climate change and water demand growth, understanding the origin of water flows in the Hi-malayas is a key issue for assessing the current and future water resource availability and planning the future uses of water in downstream regions. Two of the main issues in the hydrology of high-altitude glacierized catchments are (i) the limited representation of cryospheric processes controlling the evolution of ice and snow in distributed hydrological models and (ii) the difficulty in defining and quantifying the hydrological contributions to the river outflow. This study estimates the relative contribution of rainfall, glaciers, and snowmelt to the Khumbu River streamflow (Upper Dudh Koshi, Nepal, 146 km 2 , 43 % glacierized, elevation range from 4260 to 8848 m a.s.l.) as well as the seasonal, daily, and sub-daily variability during the period 2012-2015 by using the DHSVM-GDM (Distributed Hydrological Soil Vegetation Model-Glaciers Dynamics Model) physically based glacio-hydrological model. The impact of different snow and glacier parameterizations was tested by modifying the snow albedo parameterization, adding an avalanche module , adding a reduction factor for the melt of debris-covered glaciers, and adding a conceptual englacial storage. The representation of snow, glacier, and hydrological processes was evaluated using three types of data (MODIS satellite images, glacier mass balances, and in situ discharge measurements). The relative flow components were estimated using two different definitions based on the water inputs and contributing areas. The simulated hydrological contributions differ not only depending on the used models and implemented processes, but also on different definitions of the estimated flow components. In the presented case study, ice melt and snowmelt contribute each more than 40 % to the annual water inputs and 69 % of the annual stream flow originates from glacierized areas. The analysis of the seasonal contributions highlights that ice melt and snowmelt as well as rain contribute to monsoon flows in similar proportions and that winter outflow is mainly controlled by the release from the englacial water storage. The choice of a given parametriza-tion for snow and glacier processes, as well as their relative parameter values, has a significant impact on the simulated water balance: for instance, the different tested parameteri-zations led to ice melt contributions ranging from 42 % to 54 %. The sensitivity of the model to the glacier inventory was also tested, demonstrating that the uncertainty related to the glacierized surface leads to an uncertainty of 20 % for the simulated ice melt component.
Fast glacier motion is facilitated by slip at the ice-bed interface: an expression of subglacial physical processes that provide drag to balance gravitational driving stress. Slip over hard beds produces subglacial cavities that play a central role in modulating drag. Classic models of these systems assume cavities evolve through a continuum of steady-states in response to changes in slip and/or effective pressures, but limited field observations, experiments, and emerging models indicate transient forcings violate this assumption. We present the first-ever experimental constraints on slip with cavities under effective pressure transients. We slid an ice ring over a sinusoidal bed and oscillated effective pressures at 24-and 6-hour periods to emulate meltwater forcing cycles observed in nature and investigate the effects of the oscillation period on system mechanics. Using a combination of time-lapse photography and instrumentation, we observed cavity geometries and drag oscillating with systematic lags relative to effective pressure cycles. These lags gave rise to hysteresis inconsistent with classic Cambridge University Press Journal of Glaciology F o r P e e r R e v i e w theories, which we ascribed to a combination of mechanical, thermal, and rheologic processes at ice-bed contacts. Our experiments corroborate earlier studies of transient glacier motion and indicate a transient-induced slip stabilizing feedback not considered in most models that warrants numerical investigation.
Moulins play a pivotal role in delivering surface meltwater and significantly impacting the mass balance of the Greenland ice sheet (GrIS). Unlike crevasses, moulins are difficult to detect from satellite remote sensing imagery due to their significantly small size. Recently, unmanned aerial vehicle (UAV)-based remote sensing has become a prevalent tool for acquiring ultrahigh-resolution (UHR) imagery that facilitates the detailed extraction of small-scale surface features. Nevertheless, distinguishing among various ice surface features formed by ice stress and strain, such as crevasses, desiccated streams, and moulins, remains challenging due to their subtle differences in UAV images. This study proposes a hydrology knowledge-based framework for automatic detection of moulins using UHR (0.06 m) UAV images. By integrating a deep learning (DL) network for identifying supraglacial rivers with terrain data for recognizing significant depressions, this framework introduces multiple geometric and topological constraints to effectively enhance the detection accuracy. Applied to the Sermeq Avannarleq region, the framework achieves a recall of 0.795 and a precision of 0.729 for moulin detection. In contrast to methods relying solely on elevation changes to detect moulins, our approach exhibits a notable improvement of over 20% in
F1
-score accuracy. This enhancement further contributes to increased reliability in stream network modeling when considering the presence of moulins. We also find that this framework exhibits a certain degree of transferability for imagery at a 2-m resolution. These results show that our framework can effectively extract moulin and has the potential to be applied to large-scale moulin surveys using high-resolution (<2 m) satellite images.
Ice cliffs are melt hot spots that contribute disproportionately to melt on debris-covered glaciers. In this study, we investigate the impact of supraglacial stream hydrology on ice cliffs using in situ and remote sensing observations, streamflow measurements, and a conceptual geomorphic model of ice cliff backwasting applied to ice cliffs on Kennicott Glacier, Alaska. We found that 33 % of ice cliffs (accounting for 69 % of the ice cliff area) are actively influenced by streams, while half are nearer than 10 m from the nearest stream. Supraglacial streams contribute to ice cliff formation and maintenance by horizontal meandering, vertical incision, and debris transport. These processes produce an undercut lip at the ice cliff base and transport clasts up to tens of centimeters in diameter, preventing reburial of ice cliffs by debris. Stream meander morphology reminiscent of sedimentary river channel meanders and oxbow lakes produces sinuous and crescent ice cliff shapes. Stream avulsions result in rapid ice cliff collapse and local channel abandonment. Ice cliffs abandoned by streams are observed to be reburied by supraglacial debris, indicating a strong role played by streams in ice cliff persistence. We also report on a localized surge-like event at the glacier's western margin which drove the formation of ice cliffs from crevassing; these cliffs occur in sets with parallel linear morphologies contrasting with the crescent planform shape of stream-driven cliffs. The development of landscape evolution models may assist in quantifying the total net effect of these processes on steady-state ice cliff coverage and mass balance, contextualizing them with other drivers including supraglacial ponds, differential melt, ice dynamics, and collapse of englacial voids.
The systems of internal drainage of glaciers have been studied mainly by indirect methods. In order to reveal the structure of the internal drainage network inside Aldegondabreen, moulins and glacial caves were investigated by speleological methods in 2001–2021, which was accompanied by a semi-instrumental topographic survey in the cavities. This allowed us to see the change in the glacial cavities over time. There are three types of moulins in Aldegondadreen: active, dead and healed ones. We visited active and dead moulins. The depth of the entrance pits in the moulins varies from 52 to 65 m (moulin group No 1), from 70 to 75 (moulin group No 2) and from 45 to 60 m (moulin group No 3). The depth of moulins is equal to the thickness of the cold ice layer. Using the structure of the moulins, we show that the water from moulin group No 1 flows to the right marginal part of the glacier tongue. The water from moulin groups No 2 and No 3 flows to the left margin part of the glacier tongue, which is confirmed by the mapping of healed moulins locations. We find that the number of active and dead moulins has been decreasing since 2001, while the number of healed moulins has increased. We attribute this to a decrease in the thickness of the temperate ice layer at the base of the glacier due to climate change. Many moulins have narrow meanders at the lower part of the entrance pits, which usually finish by siphons. None of the moulins reaches the glacier bed, their lower parts are usually located in clean transparent ice. The lifetime of the moulins usually does not exceed 6 years. Our study of the caves on the glacier tongue revealed that they can be englacial or subglacial, and they originate along sub-horizontal thrusts located in the ice. We assume that the moulins reach the slip planes along thrusts close to the glacier bed. The water from the moulins flows along these slip planes as a film in early summer and turns into channels in mid- or late summer. The presence of thrusts in the ice depths can explain the development of internal drainage systems in glaciers (regardless of their size), outbursts of glacial lakes, surges and the formation of eskers. Clastic material for eskers formation can penetrate into a cave channel from the contact areas of the thrusts with uplifts on the bed. The results obtained can help in the interpretation of the available geophysical data for this glacier.
Glacier motion responds dynamically to changing meltwater inputs, but the multi-decadal response of basal sliding to climate remains poorly constrained due to its sensitivity across multiple timescales. Observational records of glacier motion provide critical benchmarks to decode processes influencing glacier dynamics, but multi-decadal records that precede satellite observation and modern warming are rare. Here we present a record of motion in the ablation zone of Saskatchewan Glacier that spans seven decades. We combine in situ and remote-sensing observations to inform a first-order glacier flow model used to estimate the relative contributions of sliding and internal deformation on dynamics. We find a significant increase in basal sliding rates between melt-seasons in the 1950s and those in the 1990s and 2010s and explore three process-based explanations for this anomalous behavior: (i) the glacier surface steepened over seven decades, maintaining flow-driving stresses despite sustained thinning; (ii) the formation of a proglacial lake after 1955 may support elevated basal water pressures; and (iii) subglacial topography may cause dynamic responses specific to Saskatchewan Glacier. Although further constraints are necessary to ascertain which processes are of greatest importance for Saskatchewan Glacier's dynamic evolution, this record provides a benchmark for studies of multi-decadal glacier dynamics.
This chapter deals with the morphology of the subterranean world accessible to human beings: caves. It also focuses on the gross morphology of dissolution caves, which are the great majority of the caves on Earth. Many geomorphology textbooks adopt a process‐landform‐based approach to describe the different types of geomorphic features, grouping them according to the most common zonal or azonal geomorphic system in which they develop. Caves can form by a variety of processes, which can be categorized into the following groups: dissolution, weathering and erosion, mechanical movement and accumulation, deposition, melting, and solidification. The roofs, walls, and floors of cave passages can be sculpted by medium‐ to small‐scale morphological features formed by solution and mechanical erosion, collectively designated as speleogens. These morphologies carved in the rock can provide valuable information on the processes that were active in the cave passages, especially in the latest (more recent) stages of their development.
Glacier hydrology describes water movement over, through and under glaciers and ice sheets. Water reaching the ice bed influences ice motion and ice dynamical models, therefore requiring a good understanding of glacier hydrology, particularly water pressures and pathways. However, as in situ observations are sparse and methods for direct observations of water pathways and internal pressures are lacking, our understanding of the aforementioned pathways and pressure remains limited. Here, we present a method that allows the reconstruction of planar subsurface water flow paths and spatially reference water pressures. We showcase this method by reconstructing the 2D topology and the water pressure distribution of an englacial channel in Austre Brøggerbreen (Svalbard). The approach uses inertial measurements from submersible sensing drifters and reconstructs the flow path between given start and end coordinates. Validation on a supraglacial channel shows an average length error of 3.9 m (5.3 %). At the englacial channel, the average length error is 107 m (11.6 %) and the average pressure error 3.4 hPa (0.3 %). Our method allows mapping sub- and englacial flow paths and the pressure distribution within, thereby facilitating hydrological model validation. Further, our method also allows the reconstruction of other, previously unexplored, subsurface fluid flow paths.
Understanding glacier drainage system behaviour and its response to increased meltwater production faces several challenges in the High Arctic because many glaciers are transitioning from polythermal to almost entirely cold thermal structures. We, therefore, used ground-penetrating radar data to investigate the thermal structure and drainage system of Waldemarbreen in Svalbard: a small High Arctic glacier believed to be undergoing thermal change. We found that Waldemarbreen retains up to 80 m of temperate ice in its upper reaches, but this thickness most likely is a relict from the Little Ice Age when greater ice volumes were insulated from winter cooling and caused greater driving stresses. Since then, negative mass balance and firn loss have prevented latent heat release and allowed near-surface ice temperatures to cool in winter, thus reducing the thickness of the temperate ice. Numerous reflectors that can be traced up-glacier are interpreted as englacial channels formed by hydrofracturing in the crevassed upper region of the glacier. The alternative cut and closure mechanism of conduit initiation only forms conduits in parts of the lower ablation area. Consequently, Waldemarbreen provides evidence that hydrofracturing at higher elevations can play a major role in englacial water drainage through cold ice.
Glacier hydrology describes water movement around, on, in and under glaciers. It plays an important role for glacier flow and thus ice transport into the sea and the associated sea level rise. It is further relevant for nutrient transport and release into ecosystems, for hydropower, and for drinking water supply in glaciated regions. Glacier related flood events are additionally a frequent geohazard, emphasizing the importance of accurate knowledge of glacier hydrology. This knowledge is, however, very limited due to general inaccessibility of glacial subsurface flows and thus lack of direct observations and technologies that could provide them. The present thesis therefore develops novel technologies, allowing to observe and study water flow inside glacial channels in detail, and applies them on Svalbard glaciers. For this, sensing drifters are proposed and their statistical repeatability tested. The instruments are then used to study water flow over glacier surfaces. Further, a method to reconstruct flow paths from drifter data is proposed and showcased with the example of a channel within a glacier. Additional speleological investigations provide permafrost temperatures under Svalbard glaciers and highlight the importance of meteorological glacier surface conditions for temperature and erosion of the glacier bed.
By regulating the amount, the timing, and the location of meltwater supply to the glacier bed, supraglacial hydrology potentially exerts a major control on the evolution of the subglacial drainage system, which in turn modulates ice velocity. Yet the configuration of the supraglacial hydrological system has received only little attention in numerical models of subglacial hydrology so far. Here we apply the two-dimensional subglacial hydrology model GlaDS (Glacier Drainage System model) to a Svalbard glacier basin with the aim of investigating how the spatial distribution of meltwater recharge affects the characteristics of the basal drainage system. We design four experiments with various degrees of complexity in the way that meltwater is delivered to the subglacial drainage model. Our results show significant differences between experiments in the early summer transition from distributed to channelized drainage, with discrete recharge at moulins favouring channelization at higher elevations and driving overall lower water pressures. Otherwise, we find that water input configuration only poorly influences subglacial hydrology, which instead is controlled primarily by subglacial topography. All experiments fail to develop channels of sufficient efficiency to substantially reduce summertime water pressures, which we attribute to small surface gradients and short melt seasons. The findings of our study are potentially applicable to most Svalbard tidewater glaciers with similar topography and low meltwater recharge. The absence of efficient channelization implies that the dynamics of tidewater glaciers in the Svalbard archipelago may be sensitive to future long-term trends in meltwater supply.
Studying subglacial drainage networks is important for understanding the potential relationship between channel dynamics and rapid glacier recession as well as the role of subglacial channels in subglacial sediment evacuation. In order to delineate the planform geometry of snout marginal subglacial channels, densely spaced ground-penetrating radar (GPR) measurements at a frequency of ~70 MHz were carried out over the snout marginal zones of two temperate glaciers in the southwestern Swiss Alps, the Haut Glacier d'Arolla and the Glacier d'Otemma. Three-dimensional (3-D) data processing and amplitude analysis of the GPR reflection along the glacier bed was used to map the channels. At the Haut Glacier d'Arolla, two relatively straight channels of several meters in width were identified. The positions of these channels correspond well with the locations of channel outlets at the glacier terminus, as well as with fractures appearing on the glacier surface one month after the GPR data acquisition. The latter are believed to represent the beginning of ice collapse above the subglacial channels. At the Glacier d'Otemma, a major subglacial conduit was detected with similar dimensions to those identified at the Haut Glacier d'Arolla, but greater sinuosity. The position of this channel was confirmed by drone-based imagery acquired after glacier margin collapse. Our results confirm that high-density 3-D GPR surveys can be used to map subglacial channels near temperate alpine glacier margins.
The fumarole ice caves of Mount Rainier in the Cascade Volcanic Arc in Washington, USA, provide unique insight into the dynamic equilibrium between thermal flux of fumaroles on volcanic edifices and snow accumulation on summit glaciers. More than 3.5 km of surveyed cave passage nearly circumnavigate the East Crater, reaching within 19 m of the 4,392 m summit and extending to 144 m depth along the glacier‐crater boundary. The large circum‐crater passage connects entrances on the crater rim to steep transverse passages, and cave morphology is maintained by fumarole gas convection and advection. A melt‐ and condensate‐formed lake, Lake Adélie, occupies a portion of the circum‐crater passage. Hourly data were collected between August 2016 and August 2017 and included the measured temperatures at three fumaroles, the cave air temperature and pressure, the lake water temperature and depth, and the outside temperature and snow depth at Paradise Visitors Center. Time‐series analyses of these data reveal complex associations between synoptic to seasonal weather, fumarole activity, and lake level. On seasonal and longer scales, fumarole temperatures follow independent pathways connected to spatial and temporal changes in volcanic heat flux and the circulation of glacial melt. Major snowfall seals the cave entrances, increasing cave air temperature and pressure from fumarole output and causing rising lake levels from increased melt until entrances reopen. Repeating freeze‐thaw cycles observed in the cave monitoring data are a primary cause of crater mass wasting. Despite these transient variations, the scale and morphology of the caves is preserved over decadal or longer scales.
The hydrological characteristics of debris-covered glaciers are known to be fundamentally different from those of clean-ice glaciers, even within the same climatological, geological and geomorphological setting. Understanding how these characteristics influence the timing and magnitude of meltwater discharge is particularly important for regions like High Mountain Asia, where downstream communities rely on this resource for sanitation, irrigation and hydropower. The hydrology of debris-covered glaciers is relatively complex: rugged surface topographies typically route meltwater through compound supraglacial-englacial systems involving both channels and ponds, as well as pathways that remain unknown. Low-gradient tongues that extend several kilometres retard water conveyance and promote englacial storage. Englacial channels are frequently abandoned and reactivated as water supply changes, new lines of permeability are exploited, and drainage is captured due to high rates of surface and subsurface change. Seasonal influences, such as the monsoon, are superimposed on these distinctive characteristics, reorganising surface and subsurface drainage rapidly from one season to the next. Recent advances in understanding have mostly come from studies aimed at quantifying and describing supraglacial processes; little is known about the subsurface hydrology, particularly the nature (or even existence) of subglacial drainage. In this review, we consider in turn the supraglacial, englacial, subglacial, and proglacial hydrological domains of debris-covered glaciers in High Mountain Asia. We summarise different lines of evidence to establish the current state of knowledge and, in doing so, identify major knowledge gaps. Finally, we use this information to suggest priorities for future hydrological research at High Mountain Asian debris-covered glaciers, and how they may influence our ability to be able to make long-term predictions of changes in the water they supply.
Abstract. 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 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations. 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). For all the GPR surveys conducted during the summer, the englacial conduit was 15–20 m wide, ~ 0.4 m thick, ~ 250 m long with a shallow inclination (2°) and having a sinusoidal shape. We speculate that such a geometry is likely the result of extensional hydraulic fracturing. Synthetic GPR waveform modelling using a thin water-filled conduit showed that a conduit thickness larger than 0.4 m (0.3 x minimum wavelength) thick can be correctly identified using 25 MHz GPR data. During the winter periods, the englacial conduit shuts down and either physically closed or becomes 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.
Recent speleological surveys of meltwater drainage systems in cold and polythermal glaciers have documented dynamic englacial and in some cases subglacial conduits formed by the ‘cut-and-closure’ mechanism. Investigations of the spatial distribution of such conduits often require a combination of different methods. Here, we studied the englacial drainage system in the cold glacier Longyearbreen, Svalbard by combining speleological exploration of a 478 m long meltwater conduit with a high-resolution ground penetrating radar (GPR) survey with two different centre-frequencies (25 and 100 MHz). The results yielded a 3-D documentation of the present englacial drainage system. The study shows that the overall form of englacial conduits can be detected from velocity−depth converted GPR data, and that the 3-D model can facilitate a method to pinpoint the reflections in a radargram corresponding with the englacial drainage system, although fine detail cannot be resolved. Visible reflections approximately parallel to the mapped englacial water drainage system likely result from sediment incorporated in the ice or from abandoned parts of the englacial drainage system.
Ground-penetrating radar data acquired in the 2016/17 austral summer on Sørsdal Glacier, East Antarctica, provide evidence for meltwater lenses within porous surface ice that are conceptually similar to firn aquifers observed on the Greenland Ice Sheet and the Arctic and Alpine glaciers. These englacial water bodies are associated with a dry relict surface basin and consistent with perennial drainage into an interconnected englacial drainage system, which may explain a large englacial outburst flood observed in satellite imagery in the early 2016/17 melt season. Our observations indicate the rarely-documented presence of an englacial hydrological system in Antarctica, with implications for the storage and routing of surface meltwater. Future work should ascertain the spatial prevalence of such systems around the Antarctic coastline, and identify the degree of surface runoff redistribution and storage in the near surface, to quantify their impact on surface mass balance.
Proglacial icings (also known as naled or aufeis) are frequently observed in the forefields of polar glaciers. Their formation has been ascribed to the refreezing of upwelling groundwater that has originated from subglacial melt, and thus the presence of icings has been used as evidence of polythermal glacier regime. We provide an updated analysis of icing occurrence in Svalbard and test the utility of icings as an indicator of thermal regime by comparing icing presence with: (1) mean glacier thickness, as a proxy for present thermal regime; and (2) evidence of past surge activity, which is an indicator of past thermal regime. A total of 279 icings were identified from TopoSvalbard imagery covering the period 2008-2012, of which 143 corresponded to icings identified by Bukowska-Jania and Szafraniec (2005) from aerial photographs from 1990. Only 46% of icings observed in 2008-2012 were found to occur at glaciers with thicknesses consistent with a polythermal regime, meaning a large proportion were associated with glaciers predicted to be of a cold or transitional thermal regime. As a result, icing presence alone may be an unsuitable indicator of glacier regime. We further found that, of the 279 glaciers with icings, 63% of cold-based glaciers and 64% of transitional glaciers were associated with evidence of surge activity. We therefore suggest that proglacial icing formation in Svalbard may reflect historical (rather than present) thermal regime, and that icings possibly originate from groundwater effusion from subglacial taliks that persist for decades following glacier thinning and associated regime change.
Debris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system tip the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.l ma−1 of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.
To clear up the changes which had happened at the subglacial catchment of glacier d’Argentière, an extensive study, with 31 borings and a coring down to the bottom (240 m) was performed in 1979/80, just upstream from the catchment, in an overdeepened area. The behaviour of the water level during boring with a hot water jet, and just after, was different from one bore hole to another, mainly because transient leaks appeared in the walls of bore holes. Next, the water level fluctuated slowly, in the same way in most of the deep bore holes, showing that glacier ice below about 100 m deep is slightly pervious. What is so measured is the pore pressure of water in deep ice. The piezometric gradient between bore holes, and the time lag between fluctuations of water level, which increases with distance from the right bank, shows that there is no waterway at the bottom of the overdeepened area, save at its up-stream end. Most of the melt water must flow between ice and rock along the right bank, its free surface rising by about 150 m during the increased discharge in June. No clear-cut correlation between the bottom pore pressure and the air temperature or the discharge at the subglacial catchment down-stream was found.
With the use of a numerical three-dimensional (3-D) model the flow dynamics of the confluence area of Unteraargletscher, Bernese Alps, Switzerland, are studied. Previous predictions, based on conceptual two-dimensional models, about flow characteristics at confluence areas are tested against results from the fully 3-D model. Measured winter velocities are used for model verification. Despite some consistent systematic differences, good overall agreement between measured and calculated surface velocities is obtained. The calculated vertical strain-rate variation with depth is in good agreement with available measurements from boreholes. The ice is found to be almost three times stiffer than standard estimates of rheological parameters for glacier ice would predict. The model predicts a complicated yet realistic pattern of vertical velocity variation along the surface. The most noticeable features of the vertical velocity distribution across the surface are listed, and their relation to topographic surface undulations and the overall dynamics of the confluence discussed. In accordance with previous results from analytical models, a strongly localized surface trough and a concomitant negative (downward orientation) vertical velocity anomaly develop at the junction point. Although depth-integrated strain rates are positive (extension), the basal layer is compressed vertically. The ice-cored medial moraine is formed by differential ablation. The flow mechanics of the confluence area play only an indirect role, by enabling transfer of debris-covered marginal ice towards the confluence center. In the absence of differential ablation, an elongated surface depression would be formed in the down-glacier direction from the junction point instead of an elevated ice-cored medial moraine.
On 4 July 1986, dye was injected at a point slightly above the equilibrium line on Storglaciären, a small valley glacier in northern Sweden. Just below the equilibrium line, the glacier bed is over-deepened. The dye re-appeared in a stream at the glacier terminus over the next 35 d. This stream normally carries relatively little sediment, in constrast to the situation in another nearby stream that also emerges from the glacier. This suggests that the dye traveled in englacial rather than subglacial conduits. Tracer tests utilizing salt in bore holes in the overdeepening support this interpretation, as the bore holes were draining well above the bed. The dye appeared during three distinct events, suggesting that it became divided into at least three separate parcels shortly after injection. This probably occurred in the crevassed area in the vicinity of the injection point.
The englacial location of the drainage may be explained by the fact that, in order to remain at the pressure melting-point, water in subglacial conduits coming out of the overdeepening may have had to warm up faster than would be possible by viscous heating alone. Such conduits would thus tend to freeze closed.
During the 1984 and 1985 melt seasons, flow velocities and dispersive characteristics of the englacial and subglacial hydraulic system on Storglaciären, a small valley glacier in northern Sweden, were studied with the use of dye-trace tests. Similar tests conducted on one of the two principal pro-glacial streams provided a basis for comparison of the combined englacial-subglacial system with the pro-glacial one. Velocities in the two systems were broadly comparable after compensating for the effect of slope differences. However, velocities in the glacial conduits increased almost linearly with discharge. Analysis suggests that this can be explained by an increase in water pressure in the conduits, combined with a decrease in effective sinuosity, as discharge increases. Dispersivity (the ratio of the dispersion coefficient to the water velocity) in the glacial system is high early in the season but decreases progressively during July. This is believed to reflect a change from an extensively braided to a more integrated drainage system. Dispersivity is only slightly lower in the pro-glacial streams than in the late-season glacial conduits, suggesting similar degrees of braiding. However, retardation of dye due to temporary storage is greater in the glacial conduits. This suggests that the glacial streams have a larger number of stable eddies in which dye can be trapped for extended periods of time.
The physics of water flow within and under glacier ice is examined with special reference to the periodic catastrophic outbursts of water (jökulhlaups) from the subglacial lake Grímsvötn, Vatnajökull, Iceland. The lake is sealed until it reaches a critical level which enables it to lift the glacier, helped by a hydrostatic cantilever effect. The differential equations for non-steady water flow in a subglacial tunnel are derived and applied to the 1972 Grímsvötn outburst. The discharge: time relation observed during the growth stage, and the abrupt ending of the flood, are both very well accounted for by a theory which is insensitive to the details of the subglacial tunnel system. The steady state, in which an intergranular vein or tunnel is simultaneously melted open by frictional heat and closed by plastic deformation, may be stable or unstable according to the conditions imposed at the ends. This explains why the flow of water in a vein does not normally increase unstably as in a jökulhlaup. An ice-dammed lake does not drain away through the vein system because the driving force on the vein-water is towards the lake rather than away from it.
With the use of a numerical three-dimensional (3-D) model the flow dynamics of the confluence area of Unteraargletscher, Bernese Alps, Switzerland, are studied. Previous predictions, based on conceptual two-dimensional models, about flow characteristics at confluence areas are tested against results from the fully 3-D model. Measured winter velocities are used for model verification. Despite some consistent systematic differences, good overall agreement between measured and calculated surface velocities is obtained. The calculated vertical strain-rate variation with depth is in good agreement with available measurements from boreholes. The ice is found to be almost three times stiffer than standard estimates of rheological parameters for glacier ice would predict. The model predicts a complicated yet realistic pattern of vertical velocity variation along the surface. The most noticeable features of the vertical velocity distribution across the surface are listed, and their relation to topographic surface undulations and the overall dynamics of the confluence discussed. In accordance with previous results from analytical models, a strongly localized surface trough and a concomitant negative (downward orientation) vertical velocity anomaly develop at the junction point. Although depth-integrated strain rates are positive (extension), the basal layer is compressed vertically. The ice-cored medial moraine is formed by differential ablation. The flow mechanics of the confluence area play only an indirect role, by enabling transfer of debris-covered marginal ice towards the confluence center. In the absence of differential ablation, an elongated surface depression would be formed in the down-glacier direction from the junction point instead of an elevated ice-cored medial moraine.
Debris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.1 m a-1 of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.
On 4 July 1986, dye was injected at a point slightly above the equilibrium line on Storglaciären, a small valley glacier in northern Sweden. Just below the equilibrium line, the glacier bed is over-deepened. The dye re-appeared in a stream at the glacier terminus over the next 35 d. This stream normally carries relatively little sediment, in constrast to the situation in another nearby stream that also emerges from the glacier. This suggests that the dye traveled in englacial rather than subglacial conduits. Tracer tests utilizing salt in bore holes in the overdeepening support this interpretation, as the bore holes were draining well above the bed. The dye appeared during three distinct events, suggesting that it became divided into at least three separate parcels shortly after injection. This probably occurred in the crevassed area in the vicinity of the injection point.
The englacial location of the drainage may be explained by the fact that, in order to remain at the pressure melting-point, water in subglacial conduits coming out of the overdeepening may have had to warm up faster than would be possible by viscous heating alone. Such conduits would thus tend to freeze closed.
Simple theory supports field observations (Lawson and others, 1998) that subglacial water flow out of overdeepenings can cause accretion of layered, debris-bearing ice to the bases of glaciers. The large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal activities or other flow paths. If that flow ascends a sufficiently steep slope out of an overdeepening, the water will supercool as the pressure-melting point rises, and basal-ice accretion will occur. Diurnal, occasional or annual fluctuations in water discharge will cause variations in accretion rate, debris content of accreted ice or subsequent diagenesis, producing layers. Under appropriate conditions, net accretion of debris-bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget.
Cave development in the Madison aquifer of the Black Hills has taken place in several stages. Mississippian carbonates fi rst underwent eogenetic (early diagenetic) reactions with interbedded sulfates to form breccias and solution voids. Later sub-aerial exposure allowed oxygenated meteoric water to replace sulfates with calcite and to form karst and small caves. All were later buried by ~2 km of Pennsylvanian– Cretaceous strata. Groundwater fl ow and speleogenesis in the Madison aquifer were renewed by erosional exposure during Laramide uplift. Post-Laramide speleogenesis enlarged paleokarst voids. Most interpretations of this process in the Black Hills invoke rising thermal water, but they fail to account for the cave patterns. Few passages extend downdip below the present water table or updip to outcrops. None reaches the base of the Madison Limestone, and few reach the top. Major caves underlie a thin cover of basal Pennsylvanian–Permian Minnelusa Formation (interbedded quartzarenite and carbonates). Water infi ltrating through the Minnelusa Formation dissolves car-bonates in a nearly closed system, producing low pCO 2 , while recharge directly into Madison outcrops has a much higher pCO 2. Both are at or near calcite saturation when they enter caves, but their mixture is undersaturated. The caves reveal four phases of calcite deposition: eogenetic ferroan calcite (Mis-sissippian replacement of sulfates); white scalenohedra in paleovoids deposited during deep post-Mississippian burial; palisade crusts formed during blockage of springs by Oligocene–Miocene continental sediments; and laminated crusts from late Pleisto-cene water-table fl uctuations. The caves reveal more than 300 m.y. of geologic history and a close relationship to regional geologic events.
Video observations made in 16 boreholes drilled through a deforming
valley glacier affirm that temperate glacier ice may be reasonably well
represented as homogeneous in glacier flow models, but raise warnings
about the complexities of basal boundary conditions and glacier sliding.
Discrete englacial structures, including clear-ice layers, voids, and
water conduits, compose a total of <3% of the ice mass. Planar
features (clear-ice layers) are oriented near vertical and are not
aligned with the sense of shear strain, meaning that the layers probably
do not influence the homogeneity of the strain. Both direct observations
of the ice and analysis of its light reflectance suggest an increase in
crystal size and decrease in bubble content with depth. However,
previous laboratory work indicates that such changes are unimportant in
terms of the viscosity of the ice. Observations of the basal boundary or
sliding surface indicate that there are areas of both “hard”
bedrock and “soft” deformable till, which should cause
spacial and temporal gradients in sliding rate.
Substrate properties and hydrological conditions at the base of Haut Glacier d'Arolla, Switzerland, are nonuniform. The thickness and grain size of subglacial sediment vary on length scales less than one ice thickness, whereas hydrological conditions vary seasonally and on length scales of about one ice thickness transverse to ice motion. There is a close relationship between the annually averaged velocity field and this nonuniformity of bed conditions. Basal motion dominates in an area with large water-level variations in boreholes, whereas ice deformation contributes significantly to total movement elsewhere. Localized enhanced basal motion occurs primarily in summer, especially during a July ``spring event,'' but this motion is barely discernible in annually averaged surface velocity measurements, because transverse coupling suppresses surface expression of the basal motion discontinuity. These results highlight the need to include representations of bed nonuniformity in models of glacier flow and to consider ice deformation and basal motion as interdependent processes.
Storglaciären is a small valley glacier in northern Sweden. Detailed studies of surface velocity, glacier hydrology and ice dynamics between 1981 and 1995 have revealed unexpected details of dynamic behaviour of the glacier. The glacier accelerates and decelerates, frequently on a diurnal basis, in direct response to subglacial water pressure variations. Furthermore longitudinal coupling effects are evident where the glacier flows across irregularities on the bed. This coupling is caused by a change in the hydraulic system underneath the glacier as the glacier flows across a bedrock ridge. Since the properties of the drainage system can change over small distance, the dynamics of the glacier is also spatially highly variable. Thus, small glaciers, such as Storglaciären, are not as sluggish as may be expected but exhibit behaviour similar to that of larger glaciers. This means that features of large glaciers and ice sheets can be studied on smaller glaciers where logistics and experimental set-ups are easier to establish.
Internal drainage was studied on Mikkaglaciären and Storglaciären in northern Sweden by means of salt injections. The internal drainage of Mikkaglaciären was found to occur in two separate systems in semilateral positions, systems corresponding on the surface of the glacier to areas of crevasses with different strike directions (cf. Stenborg 1968). The relative amounts of ablation for the different areas could be correlated with the discharge in the corresponding frontal streams.
The drainage model devised for Mikkaglaciären is assumed to be valid for other glaciers with the same main characteristics. It appeared to be applicable to Storglaciären, with one exception, caused by a topographical anomaly. In connection with this anomaly, a re-arrangement of the drainage took place between the two years of observation (1960 and 1968).
Summaries of the investigations and conclusions concerning Mikkaglaciären and Storglaciären are given on page 26 and on pages 30 and 34 respectively. Some general views on methods (use of tracers), on the physical possibilities of drainage penetration and the development of internal drainage courses, and on the glaciological and morphological applicability of the drainage model devised for Mikkaglaciären are given in separate sections (pp. 14 f. and pp. 36 ff.).
TV-video observations of four boreholes in Storglaciären, Sweden, revealed that 1.3% of the observed ice column was composed of englacial voids. The form of the voids is presented by photographs from the video. It is suggested that they are openings into englacial channels and cavities which were intersected during drilling. The observations further showed that about 11% of the ice column consisted of air-bubble-poor blue-ice inclusions. The observations show a relation between the blue-ice inclusions and the voids, which indicates that the origin and development of these features are coupled to each other. It is suggested that crevasses in the accumulation area are an important factor in this process.
In temperate glacier ice, in situ, besides water veins, there are water lenses, on grain boundaries more or less perpendicular to the direction of maximum pressure p 1 (at the grain scale). Geometry of veins is developed. Grains are modelled as equal tetrakaidecahedra. The stress and temperature fields around a vein at a smaller, microscopic scale are estimated and the water discharge by a Vein is calculated. The time-derivative of the cross-sectional area S of a vein is governed neither by energy dissipation in the water nor by plasticity, but by capillarity effects and salinity. A “vasodilator threshold” p d for water pressure p w in the veins is defined. Normally, P w P d, then S has a stable value, the same for any orientation of the vein, and the microscopic temperature is uniform. The coefficient of permeability is proportional to (P d-p w)−4, and thus a true Darcy law does not hold. As an application, the percolation of internal meltwater is studied; in an upper boundary layer about 2 m thick this meltwater flows upwards, because in the bulk of the glacier p w is very close to P 1, whereas it is zero at the surface. When, exceptionally, p w > p d, S increases irreversibly. Whether it leads to the formation of “worm-holes” is discussed.
A temperate glacier is defined as a glacier containing liquid inclusions in which the concentration of salts is not too high. Nevertheless these salts suffice to produce a depression in temperature comparable with that due 10 the pressure, and much greater than that due to interfacial energies. Because of this a large part of the liquid water present in the ice is not mobile, contrary to the theory of Nye and Frank. Deformation and recrystallization is bound to close off capillary intergranular channels, for glacier ice is usually impermeable. An explanation is given of why firn, at a depth which the annual cold wave does not reach, is nevertheless transformed relatively suddenly into practically impermeable ice. Saline inclusions will migrate with a velocity inversely proportional to the potential temperature (difference from the melting point of pure ice at the pressure in question) and proportional to the gradient of this potential temperature. This velocity, the salinity, the liquid water content, and the ice temperature, parameters which are all functions of the depth, are calculated for a steady state in a stagnant or moving glacier. Under the action of anisotropic stresses, isolated inclusions perpendicular to the maximum compressive stress will enlarge at the expense of their neighbours. If however the two inclusions are connected by a capillary channel, no enlargement occurs, but instead the salt content decreases, evacuated to the other inclusion. This process would constitute an objection to the theory of glacier sliding by melting and refreezing around small obstacles, unless new subglacial mechanisms were to occur.
The November 1986 survey of the Grand Moulin on the Mer de Glace, Mont Blanc Massif, France - Volume 33 Issue 113 - Louis Reynaud
During the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system. In most of these holes the water levels were at approximately the same depth (around 70 m below surface). The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase. In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h.
The glacier-surface velocity was measured at four lines of stakes several times daily. The velocity variations correlated with variations in subglacial water pressure. The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations. The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed. On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure. The main reason for this disagreement is probably the neglect of friction from debris in the sliding model.
The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed. Possible reasons are considered. The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed.
Water flowing in tubular channels inside a glacier produces frictional heat, which causes melting of the ice walls. However the channels also have a tendency to close under the overburden pressure. Using the equilibrium equation that at every cross-section as much ice is melted as flows in, differential equations are given for steady flow in horizontal, inclined and vertical channels at variable depth and for variable discharge, ice properties and channel roughness. It is shown that the pressure decreases with increasing discharge, which proves that water must flow in main arteries. The same argument is used to show that certain glacier lakes above long flat valley glaciers must form in times of low discharge and empty when the discharge is high, i.e. when the water head in the subglacial drainage system drops below the lake level. Under the conditions of the model an ice mass of uniform thickness does not float, i.e. there is no water layer at the bottom, when the bed is inclined in the down-hill direction, but it can float on a horizontal bed if the exponent n of the law for the ice creep is small. It is further shown that basal streams (bottom conduits) and lateral streams at the hydraulic grade line (gradient conduits) can coexist. Time-dependent flow, local topography, ice motion, and sediment load are not accounted for in the theory, although they may strongly influence the actual course of the water. Computations have been carried out for the Gornergletscher where the bed topography is known and where some data are available on subglacial water pressure.
Experimental studies are reported concerning the thermal behaviour of the water-vein system in ice grown in the laboratory from dilute solutions. The temperature versus vein-size behaviour of these samples is determined. The measurements show that the solute, which is concentrated in the veins, remains in the liquid phase during a temperature change. The mass of the solute per unit length of vein M is found to be of the order of M ≈ 10−8 mol m−1 for samples grown from singly-distilled water. M is seen to vary with temperature only because of the volume expansion (contraction) on freezing (melting) which causes the liquid to flow along the veins. The effect of these flows on the sample is studied. They are found to provide a mechanism for the transport of impurities along the veins and to and from the sample surface. Samples grown from doubly-distilled water doped with small amounts of NaCl or H2SO4 are studied and are found to display the same general behaviour. However, M is an order of magnitude higher in the H2SO4-doped samples than in either the NaCl-doped samples or the samples grown from singly-distilled water. The approach to equilibrium of distortions in the vein-system geometry is studied. It is suggested that these distortions are due to variations in M along the length of the veins and that equilibration is therefore governed by diffusion of the solutes.
In 1970 water pressure was measured in several moulins on the White Glacier. Pressure variations in some moulin channels extended over the full measuring range of the instruments (0–1 and 0–2 bar above atmospheric pressure), even at depths of less than 50 m below the surface. Measurements at different depths showed that total pressure variations were sometimes greater than this. The pressure data are compared with variations in the surface velocity of the glacier.
Microscopic and textural observations were made on ice samples cored from Blue Glacier slightly below the equilibrium line to depths of 60 m. Observations were started within a few minutes after collection. Water was found in veins along three-grain intersections, in lenses on grain boundaries and in irregular shapes. Gas was found in bubbles in the interior of crystals, in bubbles touching veins, and locally in veins. Vein sizes showed some spread; average cross-sectional area was about 7 × 10−4 mm2 with no discernible, trend with texture or depth except within 7 m of the surface. Before the samples were examined they could have experienced a complex relaxation which could have changed them significantly. As a result it is not possible to determine the in situ size of veins, but an upper limit can be determined. Also it is not possible to predict intergranular water flux per unit area, but 1 × 10−1 m a−1 represents an upper limit. In coarse-grained ice the water flux density is likely to be even smaller, because of a low density of veins, and blocking by bubbles. This indicates that only a very small fraction of the melt-water production on a typical summer day can penetrate into the glacier on an intergranular scale except possibly near the surface. The existence of conduit-like features in several cores suggests that much melt water can nevertheless penetrate the ice locally without large-scale lateral movements along the glacier surface. The observed profile of ice temperature indicates that the intergranular water flux may be much smaller than the upper limit determined from the core samples.
To improve our understanding of Svalbard-type polythermal glacier drainage, hydraulic geometry models of the subglacial hydrology of two contrasting glaciers in Svalbard have been constructed. The models are tested against a uniquely long and rich set of field observations spanning 45 years. Digital elevation models (DEMs) were constructed from bedrock data measured with ground penetrating radar and surface data of two medium-sized polythermal glaciers, Hansbreen and Werenskioldbreen, in south-west Spitsbergen. Hansbreen has a low angle bed with over-deepenings and a calving front, while Werenskioldbreen has steeper bed and terminates on land. Together they are representative of many Svalbard glaciers. The DEMs were used to derive maps of hydraulic potential and subglacial drainage networks. Validation of the models was done using field observations including location mapping and speleological exploration of active moulins, positions of main river outflows, dyetracing and water chemistry studies, and observations of water pressure inside moulins. Results suggest that the water pressure is generally close to ice overburden pressure but varies greatly depending on local conditions such as bed location, the thickness of cold ice layer, the thickness of the glacier and seasonal changes in meltwater input.
Water flowing in tubular channels inside a glacier produces frictional heat, which causes melting of the ice walls. However the channels also have a tendency to close under the overburden pressure. Using the equilibrium equation that at every cross-section as much ice is melted as flows in, differential equations are given for steady flow in horizontal, inclined and vertical channels at variable depth and for variable discharge, ice properties and channel roughness. It is shown that the pressure decreases with increasing discharge, which proves that water must flow in main arteries. The same argument is used to show that certain glacier lakes above long flat valley glaciers must form in times of low discharge and empty when the discharge is high, i.e. when the water head in the subglacial drainage system drops below the lake level. Under the conditions of the model an ice mass of uniform thickness does not float, i.e. there is no water layer at the bottom, when the bed is inclined in the down-hill direction, but it can float on a horizontal bed if the exponent n of the law for the ice creep is small. It is further shown that basal streams (bottom conduits) and lateral streams at the hydraulic grade line (gradient conduits) can coexist. Time-dependent flow, local topography, ice motion, and sediment load are not accounted for in the theory, although they may strongly influence the actual course of the water. Computations have been carried out for the Gornergletscher where the bed topography is known and where some data are available on subglacial water pressure.
This book, first published in 2005, provides students and practising glaciologists with the tools they need to understand modern glaciology. Relatively simple concepts are introduced first, followed by mathematically more sophisticated chapters. A knowledge of basic calculus is assumed, but important concepts of physical processes are developed from elementary principles. Emphasis is placed on connections between modern research in glaciology and the origin of features of glacial landscapes. Student exercises are included. This new edition builds on the successful first edition: it has been completely updated, and important new sections and whole chapters have been added. Principles of Glacier Mechanics is designed to be used as a primary textbook in upper division and graduate courses in glaciology, and can be used as either a primary or supplementary text in courses in glacial geology. Practising glacial geologists and glaciologists will also find it useful as a reference book.
The internal geometry of three englacial conduits located on Austre Brøggerbreen, Svalbard, was investigated by tracer experiments and by direct exploration. The tracer tests produced short-lived and highly peaked tracer return curves with high through-flow velocities. The direct inspections revealed the entrances to be of two types. Type I consisted of a moulin with an initial 39 m vertical shaft drained by a semicircular and near-horizontal englacial channel trending diagonally across the direction of ice flow, parallel to tectonic structures observed on the ice surface. The type II entrances sloped gently into the ice in the direction of the ice flow, and became englacial by gradual down-cutting of supraglacial meltwater channels into the glacier surface. No traces of tectonic structures were observed in the ice surface that could have initiated the type II entrances. Englacial conduits with gentle slopes were meandering. With increasing slope, a winding waterfall-plunge-pool geometry was developed, the channels being high (> 3 m) compared to their width (30-150 cm).
