Pierre-Marie Lefeuvre

• Lefeuvre, Pierre-Marie; Zwinger, Thomas; Jackson, Miriam; Gagliardini, Olivier; Lappegard, Gaute & Hagen, Jon Ove Methlie (2018). Stress redistribution explains anti-correlated subglacial pressure variations. Frontiers in Earth Science.  ISSN 2296-6463.  5 . doi: 10.3389/feart.2017.00110 Full text in Research Archive. Show summary

  We used a finite element model to interpret anti-correlated pressure variations at the base of a glacier to show the importance of stress redistribution in the basal ice. Two pairs of load cells are installed 20 m apart at the base of the 210 m thick Engabreen glacier in Northern Norway. Pressurisation of a subglacial channel located over one pair led to anti-correlation in pressure between them. A full Stokes 3D model of a 210 m thick and 25-200 m wide glacier with a pressurised subglacial channel represented as a pressure boundary condition was used to investigate the anti-correlated response at the bed. The model reproduced the anti-correlated pressure response at the glacier bed and variations in pressure of the same order of magnitude as the load cell observations. The anti-correlation pattern was shown to depend on the bed/surface slope such that the anti-correlated pressure variations were reproduced at a distance greater than 10-20 m from the channel when the bed slope was zero, whereas anti-correlation occurred within 10 m of the channel when the bed was inclined by 5 degrees. Pressurisation of the channel led to lateral or vertical ice flow away from the channel, support of the overlying ice and reduction of the normal stress on the bed. If the modelled cross-section was laterally constrained and the bed flat, the resulting bridging effect diverted some of the normal forces acting on the bed to the sides. In contrast, if the bed was inclined, then channel support was vertical only. The model showed that the effect of stress redistribution depends on the slope as well as the geometry of the subglacial channel and glacier, and can lead to an opposite response in pressure at the same distance from the channel.

• Lefeuvre, Pierre-Marie Benoit Émile; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2015). Interannual variability of glacier basal pressure from a 20 year record. Annals of Glaciology.  ISSN 0260-3055.  56(70), s 33- 44 . doi: 10.3189/2015AoG70A019

View all works in Cristin

• Lefeuvre, Pierre-Marie & Kääb, Andreas (2017). Surging glaciers in the Karakoram.
• Lefeuvre, Pierre-Marie; Nuth, Christopher & Köhler, Andreas (2017). CalvingSeis cocktail: combining time-lapse pictures, seismic monitoring and terrestrial radar.
• Lefeuvre, Pierre-Marie; Nuth, Christopher & Köhler, Andreas (2017). Characterising size and frequency of calving events based on high temporal time-lapse and automatic image processing.
• Lefeuvre, Pierre-Marie; Nuth, Christopher; Köhler, Andreas; Lauknes, Tom Rune; Rouyet, Line; Petlicki, Michal; Noormets, Riko; Buscaino, Giuseppa; Kohler, Jack; How, Penelope; Papale, Elena; Grammauta, Rosario & Strozzi, Tazio (2017). Distribution and characteristic of calving events at Kronebreen.
• Kääb, Andreas; Treichler, Désirée Silvana; Girod, Luc Maurice Ramuntcho; Lefeuvre, Pierre-Marie; Schellenberger, Thomas; Altena, Bas; Jonassen, Vetle; Nuth, Christopher; McNabb, Robert Whitfield & Gilbert, Adrien (2016). On the Karakoram anomaly.
• Lefeuvre, Pierre-Marie Benoit Émile (2016). Subglacial Processes and Subglacial Hydrology. Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo.. 1719. Full text in Research Archive. Show summary

  The hydrology of glaciers is known to have a significant influence on glacier dynamics. Rapid variations in dynamics can occur due to changes in the glacial and subglacial hydrology. However, our understanding of the subglacial drainage system is based on few direct and usually short-term observations. In this thesis, two decades of pressure measurements at the glacier bed are investigated. The measurements were carried out at the Svartisen Subglacial Laboratory in Northern Norway where load cells installed at the ice-rock interface under ≥200 m of glacier ice record normal stress. A statistical study of the records for the period 1992-2014 demon- strates that the glacier bed response depends strongly on surface melt and the routing of subglacial water. Changes in subglacial hydrology cause several types of mechanical response at the glacier base. Water increases basal connectivity in summer whereas local processes, such as local ice flow, dominate the pressure signal in winter. The comparison of two summers with high, but contrasting connectivity (correlated vs anti-correlated pressure signal) demonstrates that fluctuations in meltwater input force a load transfer from the connected hydrological system to the unconnected part of the drainage system. Modelling of load transfer near a pressurised channel simulates the observed anti-correlated response in normal stress at the glacier bed. Daily pressure events are also investigated and characterise the response in pressure of the unconnected system, which differs from daily fluctuations in water pressure as measured in boreholes. The peak and decay in pressure that comprise these events are inferred to be a result of the stress bridging effect that occurs during the contraction of the drainage system. The peak in pressure is reproduced with an experiment where an artificial cavity contracts over the load cell and with a stress bridging model that incorporates shear stress transfer near a subglacial cavity. Observations from load cells and boreholes are not contradictory; instead they com- plement each other and help us characterise the mechanical and hydrological dynamics occurring at the glacier bed.

• Lefeuvre, Pierre-Marie; Jackson, Miriam; Zwinger, Thomas; Lappegard, Gaute; Hagen, Jon Ove Methlie & Gagliardini, Olivier (2016). Subglacial Processes and Hydrology.
• Lefeuvre, Pierre-Marie; Nuth, Christopher & How, Penelope (2016). Automatic detection of calving events using high temporal time lapse.
• Rouyet, Line; Lauknes, Tom Rune; Solbø, Stian; Sivertsen, Agnar Holten; Storvold, Rune; Strozzi, Tazio; Petlicki, Michal; Nuth, Christopher & Lefeuvre, Pierre-Marie (2016). Unmanned Aircraft System imagery and Ground-Based Radar measurements for the invetigation og calving glaciers (Kronebreen & Hansbreen, Svalbard).
• Lefeuvre, Pierre-Marie (2015). Mapping Bedrock and Measuring Creep Closure.
• Lefeuvre, Pierre-Marie (2015). The Glacier Bed.
• Lefeuvre, Pierre-Marie (2015). The glacier bed - does it know it's raining: Combining a statistical study of two decades of subglacial data with modelling stress redistribution near a channel.. Show summary

  The glacier bed - does it know it's raining: Combining a statistical study of two decades of subglacial data with modelling stress redistribution near a channel Basal pressure has been recorded at the Svartisen Subglacial Laboratory (Northern Norway) for 20 years, and is measured by pressure sensors installed at the ice rock interface under ~200 m of glacier ice. A running correlation is used to study the temporal variation in the response of several sensors. By studying the nature of this correlation as well as the correlation between sensor pairs, it is possible to investigate evolution of the degree of basal connectivity at the glacier bed. Persistent seasonal variations associated with the melt season are observed over the entire measurement period, indicating dependence on surface hydrological forcing. Overlying this pattern, specific years with longer periods of positive and negative correlation of pressure between sensors are presented to show contrasting inter-annual variability in basal pressure. An anti-correlated connectivity is associated with an increase locally in the rate of daily subglacial discharge, and is caused by load transfer or passive cavity opening. Load transfer is investigated with a simple full Stoke model of a subglacial channel to quantify the impact of channel pressurisation on the glacier bed response. An anti-correlated signal between the channel and its surrounding is reproduced over a distance greater than 45 m. The changes in pressure gradient have implications for water flow in the vicinity of channels.

• Lefeuvre, Pierre-Marie; Zwinger, Thomas & Jackson, Miriam (2015). Stress Bridging near a Contracting Subglacial Channel: Experiment and Modelling. Show summary

  Observations from load cells installed at the glacier bed of Engabreen show that normal stress on the bedrock can temporally reach values exceeding the local overburden pressure. This peak in pressure is assumed to be caused by stress bridging effect that occurs near subglacial channels or cavities. Stress bridging enables the load of the ice overlying the roof of a subglacial channel to be supported by the sidewalls of the channel. This work investigates the effect of stress bridging on the temporal variation in normal stress by combining a field experiment conducted at the Svartisen Subglacial Laboratory with a model study. The experiment aims to reproduce the peak in pressure. It consists of melting out an artificial cave over load cells and monitor pressure as the cavity contracts and its wall migrates over the load cells. The experiment confirms that the migration of a cave wall causes a peak in pressure at the glacier bed, with the shape of the peak varying with the cave geometry. The experiment results are compared to a stress bridging model around a contracting subglacial channel. The modeled normal stress distribution at the bed is shown to peak at the edge of the channel and then to decay exponentially further away from it. The simulated value for the maximum pressure varies with channel size, geometry, sliding velocities and ice rheology, but reaches the same order of magnitude than observed values. We demonstrate that stress bridging exists at the glacier bed and cause the peak in pressure observed at the load cell records. We identify the different shapes of subglacial channels to cause the variations observed in the pressure signal.

• Lefeuvre, Pierre-Marie; Zwinger, Thomas; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2015). Characterisation and Cause of Subglacial Perturbations Directly Measured at the Glacier bed. Show summary

  Direct measurements of pressure at the glacier bed are crucial in understanding temporal variation in basal friction and its effect on sliding. We investigate a 20-year pressure record from sensors installed at the ice-rock interface (under 200 m of glacier ice) part of the instrumentation at the Svartisen Subglacial Laboratory. We characterise a regularly occurring subglacial perturbations called “pressure events” in order to identify the involved mechanisms. These events are assumed to depend on the intensity of the glacier response to surface melting and changes in the capacity of the drainage system. They occur on a daily basis during periods of stable weather in the summer and show a drop in pressure followed by a pressure peak and a slow decline until pressure returns to local overburden pressure. Pressurisation of the hydrological system triggers the observed drop in pressure, although the exact process that causes this change locally remains uncertain. The pressure peak and subsequent exponential decay, however, are well understood and due to stress redistribution at the glacier bed. The presence of channels and cavities at the glacier bed transfers the load to areas where the ice rests directly on bedrock, which significantly increases pressure and friction. After the peak, the shape of the decrease in pressure is dependent on ice flow and the form of the contraction of nearby channels and cavities. Sometimes, pressure events do not show the contraction of the hydrological system and we demonstrate that stress redistribution between a pressurised channel and a hydrologically isolated part of the bed can also explain this feature based on a full Stokes model.

• Lefeuvre, Pierre-Marie; Zwinger, Thomas; Messerli, Alexandra; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2015). Direct monitoring of the subglacial system: how does it differ from borehole measurements?. Show summary

  Measurements of pressure at the glacier bed are made from sensors installed either directly at the glacier bed (load cells) or in boreholes. Results from both kinds of instrumentation show a seasonal evolution of the subglacial drainage system. However, the characteristics of the response of each to surface melting and rainfall are significantly different. This study summarizes results from time series analysis of basal pressure and modelling work conducted at the Svartisen Subglacial Laboratory in order to compare the outcome with the understanding of the subglacial system from borehole campaigns. Installed under 200 m of glacier ice, the load cell network shows periods with spatial anti-correlation in summer, which is caused by stress redistribution between the efficient drainage system and isolated parts of the glacier bed. A full stress ice flow model reveals the importance of this mechanical stress transfer around subglacial channels that is responsible for causing daily pressure events at the glacier bed during stable weather. A characteristic event recorded at a load cell is a sudden drop-and-peak in pressure, usually corresponding with the peak in subglacial water pressure. These typical events are not observed in borehole measurements, they instead measure sinusoidal fluctuations in water pressure. Different response characteristics are also observed during spring speed-up events. During these events, pressure sensors in boreholes generally become connected to an efficient drainage system and then exhibit daily pressure fluctuations for the rest of the melt season. Instead of presenting an increase in connection and activity, the load cells show a damped response after the subglacial drainage system has accommodated the first input of meltwater. The observations from load cells and boreholes are not contradictory, instead they complement each other and help characterize the mechanical and hydrological dynamics occurring at the glacier bed.

• Engelhardt, Markus & Lefeuvre, Pierre-Marie (2014). Modeling the runoff components of Engabreen, northern Norway. Show summary

  In highly glacierized catchments, snow and ice melt are the most important contributors to the magnitude and variations in streamflow. Changes in discharge are connected to both, changes in temperature and precipitation and can be amplified o r balanced by the presence of a glacier in the catchment. Therefore, variations in annual glacier mass balances alter the streamflow regime and with ongoing climate change, it is expected to see major changes in timing and magnitude of future water availab ility. Daily discharge rates are available for the catchment of Engabreen (38 km²) at three different locations comprising a) 33 km² (100 % glacier cover), b) 40 km² (95 % glacier cover), and c) 53 km² (73 % glacier cover). These measurements are compared with simulated discharge rates calculated from a melt model for both, the glacierized and, if applicable, the non - glacierized parts of the catchment. The model uses gridded temperature and precipitation from seNorge (http://senorge.no) as input and runs on a daily time step from 1957 to present. It accounts for evaporation, retention of surface water, refreezing processes and transformation of snow to firn and ice. The simulated discharge data can be split up into their water sources rain, glacier melt and snowmelt both on and outside the glacier area. For calibration and validation of the model, both measured seasonal and annual mass - balance measurements of the glacier are used. Additionally, the runoff components are compared with basal pressure records fr om the Svartisen Subglacial Laboratory situated 200 m below the glacier surface.

• Hillerup Larsen, Signe; Ahlstrøm, Andreas Peter & Lefeuvre, Pierre-Marie (2014). Isskolen - Ice School. [Diasserie ].
• Lefeuvre, Pierre-Marie (2014). Subglacial Hydrology of The Greenland Ice-Sheet.
• Lefeuvre, Pierre-Marie; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2014). Effect of diurnal surface melt and capacity of the subglacial drainage system on glacier bed perturbations. Show summary

  We investigate the relationship between glacier bed response and successions of days with melt driven diurnal discharge as an attempt to qu antify the temporal evolution of the capacity of the subglacial drainage system, or in other words its history. The study focuses on successive diurnal discharge that identifies when the drainage capacity is greater than/or in balance with surface melt i nput. We quantify how the bed responds during these periods based on a 20 year record of direct observations of basal pressure from the Svartisen Subglacial Laboratory (under ~200m thick ice). Results show an intermittent connection between series of daily discharge and daily pressure events. The bed response is maximal at first, but the diurnal signal dampens non - linearly over the next days to finally reach local ice overburden pressure. Decrease in pressure events amplitude is assumed to indicate a reduct ion of the area affected by pressurised channels through load transfer mechanisms. A full Stokes model with simple mesh geometry reproduces these basic amplitude variations. Our observations contrasts with the perception from surface boreholes that subgl acial water pressure can vary from atmospheric to ice overburden on diurnal time scale. We discuss the differences.

• Lefeuvre, Pierre-Marie; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2014). Statistical study of the subglacial connectivity as revealed by pressure records. Show summary

  At the Svartisen Subglacial Laboratory, pressure cells installed at the ice–rock interface under ~200 m of glacier ice have been recording basal pressure for 20 years. Four sensors in particular are studied, two pairs of sensors located 0.45 and 1 m apart that generally show correlation within the pair in the sensor response to changes in the subglacial hydrology. By studying the nature of this correlation as well as the correlation (or anti-correlation) between sensors in different pairs, it is possible to investigate evolution of the degree of connectivity of the hydrological system at the glacier bed. We compare the response of the first pair of sensors installed on an exposed gentle bump to a second pair, which are installed at the base and top (facing down) of an overhanging cliff. The sensitivity of the exposed pair is more pronounced than the latter, indicating the importance of their location and giving us vital information on the extent of the area affected by these events. A statistical overview of the nature of the response shows that certain signals are seen repeatedly in the load cell records and reflect the importance of the different kinds of perturbations in the hydrological system. It is also a firm confirmation that the connectivity of the subglacial hydrological system is highly dependent on the season, with the connectivity showing a monotonic increase in connectivity from early spring to early autumn (September). Significant changes in connectivity appear to be triggered by an increase in the rate of subglacial discharge measured nearby, and may be related to cavity opening at the glacier bed.

• Lefeuvre, Pierre-Marie; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2014). Two summers with contrasting spatial variability in glacier basal pressure. Show summary

  At the Svartisen Subglacial Laboratory, load cells installed at the ice rock interface under ~200 m of glacier ice have been recording basal pressure for 20 years. Synchronous pressure variations between load cells are investigated as evidence of hydrological bed connectiv ity and stress redistribution. A running Pearson correlation is used to gain insights into the temporal variation in response within a sensor array. By studying the nature of this correlation as well as the correlation between sensor pairs, it is possibl e to investigate evolution of the degree of synchronous response and to some extent basal connectivity at the glacier bed. Persistent seasonal variations associated with the melt season are observed over the entire measurement period, indicating dependenc e on surface hydrological forcing. Overlying this trend, particular years with longer periods of positive and negative correlation are presented to show contrasting inter - annual variability in basal pressure. Stable weather appears to enhance connectivity of the sensors, which may be attributed to the development of a persistent drainage system. An anti - correlated connectivity is associated with an increase in the rate of daily subglacial discharge measured nearby, and may be related to channel migration or cavity opening at the glacier bed.

• Lefeuvre, Pierre-Marie; Jackson, Miriam; Lappegard, Gaute; Hagen, Jon Ove Methlie & Engelhardt, Markus (2014). The Glacier Bed. Does it know it's raining?. Show summary

  At the Svartisen Subglacial Laboratory, pressure cells installed at the ice rock interface under ~200 m of glacier ice have been recording basal pressure for 20 years. Five sensors in particular are studied, covering a 22m zone, that generally show correlation in the sensor response to changes in the subglacial hydrology. By studying the nature of this correlation as well as the correlation (or anti correlation) between sensors, it is possible to investigate the evolution of the degree of connectivity at the glacier bed. Significant changes in connectivity on an annual basis are reported and its potential impact on glacier flow is discussed.

• Lefeuvre, Pierre-Marie (2013). Remote Sensing Under a Glacier: Applications of Laser and Structure-From-Motion. Show summary

  The Svartisen Subglacial Laboratory (SSL) located in Northern Norway gives us the unique opportunity to crawl under 210m of ice to understand how water flows at the glacier bed. The first step has been to map the glacier bed surrounding the SSL, which is a key element in controlling small-scale water flow paths. Recent field campaigns have tested two novel remote sensing techniques, which are the XBOX Kinect©, a small cheap laser scanner and the structure from motion technique, which produces 3D models from classic camera images. Technical details and current issues will be shortly presented. In a second step, the Kinect© is used to measure rate of ice deformation in 2D, primary motor of closure of subglacial channels. Nice pictures and movies will punctuate the talk and will hopefully make you want to travel up North.

• Lefeuvre, Pierre-Marie; Jackson, Miriam & Lappegard, Gaute (2013). 20 years of basal pressure measurements at Engabreen, Norway and insights into the hydrological system. Show summary

  The subglacial environment is one of the most unknown domains in glaciology and yet processes at the glacier bed are crucial for behaviour of the glacier . At the Svartisen Subglacial Laboratory (SSL), pressure sensors installed in bedrock have recorded the interaction between water, ice and the bed since 1993. Vertical basal pressure varies daily, seasonally and annually, and can be related to changes in melt or rainfall input, development of the hydrological system and general glacier evolution, respectively. Furthermore, the six analysed load cell records show a non-uniform response to different hydrological forcing even for those that are less than 0.5 m apart. These variations suggest a strong dependency of pressure on local topography as well as changes in flow pathways. We also identify short-term pressure events exhibiting pressure more than twice the overburden pressure (around 1.8-1.9 MPa at the SSL). Their number and frequency increase with the seasonal development of the hydrological system. They are inferred to be a result of a phenomenon called stress bridging or ice beam, which is caused by an ice-bed contact problem. Plausible explanations for this stress concentration are channel or cavity formation. By using the unique access to the glacier bed offered by the subglacial laboratory we are able to directly investigate channel closure and migration at the glacier bed, and use this to assess the significant processes of a dynamical drainage system.

• Lefeuvre, Pierre-Marie; Jackson, Miriam & Lappegard, Gaute (2013). Basal Pressure measurements at Engabreen.
• Lefeuvre, Pierre-Marie; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2013). Delay in Basal Pressure Response Caused by Changes in Bed Conditions. Show summary

  At the Svartisen Subglacial Laboratory, pressure cells have been recording basal pressure discontinuously for 20 years. Two pairs of sensors installed in bedrock under ~200m of ice located 0.45 and 1 meter apart offer the possibility to investigate time delay of pressure variation between sensors. Based on a wavelet transform analysis, we suggest that this delay is not constant in time, but its distribution characterises a particular group of pressure events. We compare the results for the first pair of sensor installed on an exposed gentle bump to the second, which is placed at the base and top (facing down) of an overhanging cliff. The sensitivity of the former is much more pronounced than the latter stressing the importance of their location and the globality of the detected events. The latter appears to be triggered by increasing rate in subglacial discharge measured nearby (~600m a.s.l.), suggesting either a relation with cavity opening at the glacier bed or a rapid change in ice dynamics causing variation in stress components. This variation depends on the initial condition of the cavity, basal sliding and ice rheology. We try to identify from our observations, which one is likely to be the main control despite the limited range of data.

• Lefeuvre, Pierre-Marie; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2013). Impact of small-scale distribution of basal pressure for subglacial hydrology. Show summary

  With the ongoing development of 2D subglacial hydrological models, new challenges to simulate the geometry of the drainage system arise notably as their resolution becomes finer. Indeed, the influence of small-scale ice mechanics and small-scale bed topography on subglacial water and ice flow is more likely to take over large-scale forcing. At the Svartisen Subglacial Laboratory (SSL - 200 m below the ice surface), we investigate the importance of basal normal pressure on constraining water flow path over different time scale and hypothesise its importance for water transfer from a channelized drainage system to a distributed one. Twenty years of field observations of normal basal pressure reveal short-lived pressure events, which are assumed to be related to the seasonal development of the hydrological system. We analyse their frequency and amplitude in relation to subglacial runoff measured close to a subglacial tunnel intake located 600-500m above the glacier snout. These events are characterised by a drop in pressure of as much as 1 MPa in a matter of several hours and a sudden rise reaching more than twice the overburden pressure. The cause of this phenomenon seems to be the melting/migration of subglacial channels. Their occurrence is linked to periods of melt and rainfall, when the high capacity drainage system is pressurized above the ice overburden pressure. Then, as water pressure becomes lower than the ice flotation level, the centre part of channels is decoupled from the glacier bed. This decrease in contact area between the ice and the bed produces stresses greater than the mean ice overburden pressure around the channel sides. As a consequence, the flooded area surrounding the high capacity drainage system is sealed off by this large increase in pressure, creating local storage as mentioned in other studies. This water retention can be a highly significant feedback for ice dynamics creating large areas of high pressure that can be reactivated during speed-up events or a surge. In this study, we provide a long-term analysis of the occurrence of those events and infer their significance for maintaining areas of high water pressure over different time-scales (days, months and years) as well as for the basal drag. Finally, we discuss the importance of small-scale observations in the light of measurements showing very strong spatial disparities in normal pressure for two pairs of sensors located less than a meter apart. Discharge measurements provide further insights into the overall behaviour of the subglacial system. Strong correlation with subglacial normal pressure demonstrates the existence of rare global events. In contrast, locally independent events are more frequent suggesting very variable water flow paths and a more distributed drainage system. These last observations suggest periods of water flow controlled mainly by small-scale pressure distribution and small-scale topography. These results highlight the importance of the transition between high and low pressurised drainage system regarding local water storage and its possible impact on ice dynamics. As the resolution of models increases, new small-scale processes will have to be included.

• Lefeuvre, Pierre-Marie; Jackson, Miriam; Lappegard, Gaute & Hagen, Jon Ove Methlie (2013). Small-Scale Variation in Basal Pressure and their Interpretation.
• Serail, Paul; Lefeuvre, Pierre-Marie & Jackson, Miriam (2013, 01. februar). Onder Glad Ijs. [Fagblad].  "Quest,braintainment", Holland.
• Benn, Doug; Lefeuvre, Pierre-Marie; Ng, Felix & Nicholson, Lindsey (2012). Response of Himalayan debris-covered glaciers to climate warming: from observations to predictive modeling. Show summary

  Field observations and remote-sensing studies have shown that Himalayan debris-covered glaciers tend to follow distinctive evolutionary pathways during periods of negative mass balance. Initially, debris-covered glacier tongues downwaste rather than retreat, resulting in thinning and a reduction of ice surface gradient. Reduced driving stresses lead to lower velocities and eventual stagnation of the tongue. These geometrical and dynamic changes reduce the efficiency of the hydrological system, leading to increased retention of meltwater and the formation of ephemeral supraglacial lakes. High ablation rates around lakes and internal ablation in association with englacial conduits serve to accelerate mass loss and downwasting. In some cases, this evolutionary cascade results in the formation of moraine-dammed lakes, which can present significant outburst flood risks if large lake volumes coincide with weak moraine dams . While this evolutionary sequence has been observed on numerous glaciers, numerical prediction of future glacier behavior requires quantification or parameterization of several complex processes. In addition, system behavior is highly non-linear with multiple process thresholds, creating considerable modeling challenges. An essential first step is to develop robust mass-balance models, including patterns of snow accumulation in extreme terrain and the effects of both debris and climate on melting. Accumulation models need to incorporate vertical variations in precipitation as well as redistribution by wind and avalanching. Newly available precipitation estimates from satellite data can provide important model input. Ablation modeling can be done using a range of approaches, including degree-day and full energy balance models. Mass balance gradients calculated using the latter approach indicate ablation maxima some distance above the glacier termini, where debris cover is relatively thin. Mass balance modeling also indicates that in monsoonal regions, temperature increases have a strong impact on glacier mass balance, increasing melt and the altitude of the rain-snow transition in the summer accumulation season. At a more detailed level, melt models can simulate evolving surface topography and backwasting in response to variations in debris thickness. Recent simulations using coupled mass-balance - flow models can mimic key aspects of the observed behavior of debris-covered glaciers, including debris thickness - melt rate feedbacks, asymmetric advance - retreat cycles, as well as detachment and stagnation of glacier tongues in response to ELA rise. Future modeling challenges include parameterization of lake expansion and deepening processes, developing criteria for switching between ablation regimes, and upscaling the effects of spatially and temporally variable local processes. In the foreseeable future, advanced modeling efforts informed closely by observational studies should enable quantitative predictions to be made about the fate of debris-covered glaciers in the Himalaya.

• Booth, Adam M. & Lefeuvre, Pierre-Marie (2012, 03. januar). Ice on top of the world! Breakthroughs in mountain glacier research. [Internett].  Blog EGU, GeoLog. Show summary

  "[...] A major boost in our understanding is provided by Pierre-Marie Lefeuvre, formerly an MSc student at the University of Sheffield (UK), now a PhD student at the University of Oslo (Norway). In collaboration with Dr Felix Ng, Pierre-Marie has developed a computational method that offers new understandings of the coupling between debris-cover and glacier-flow model. The model predicts that, as a debris-covered glacier starts to melt, the relative area of its debris cover becomes larger; as the glacier wastes away, previously-buried rockfalls become exposed and linger on the surface. Differences in debris cover can even cause a glacier to split, with a lower section stranded from its higher-altitude counterpart. Future predictive models will undoubtedly be indebted to the pioneering steps of Pierre-Marie’s work, and he advises that we keep an eye out for an imminent publication. [...]"

• Lefeuvre, Pierre-Marie (2012). Basal and Water Pressure: History, Evolution and Recent Observations.
• Lefeuvre, Pierre-Marie; Clark, Chris D. & Ng, Felix (2012). Topography Analysis in the Sikkim Himalayas: Debris-covered vs Debris-free Glaciers.
• Lefeuvre, Pierre-Marie & Ng, Felix (2012). Uncovering glacier dynamics beneath a debris mantle.
• Lefeuvre, Pierre-Marie & Ng, Felix (2012). Uncovering glacier dynamics beneath a debris mantle. Show summary

  Debris-covered glaciers (DCGs) have an extensive sediment mantle whose low albedo influences their surface energy balance to cause a buffering effect that could enhance or reduce ablation rates depending on the sediment thickness. The last effect suggests that some DCGs may be less sensitive to climate change and survive for longer than debris-free (or 'clean') glaciers under sustained climatic warming. However, the origin of DCGs is debated and the precise impact of the debris mantle on their flow dynamics and surface geometry has not been quantified. Here we investigate these issues with a numerical model that encapsulates ice-flow physics and surface debris evolution and transport along a glacier flow-line, as well as couples these with glacier mass balance. We model the impact of surface debris on ablation rates by a mathematical function based on published empirical data (including Ostrem's curve). A key interest is potential positive feedback of ablation on debris thickening and lowering of surface albedo. Model simulations show that when DCGs evolve to attain steady-state profiles, they reach lower elevations than clean glaciers do for the same initial and climatic conditions. Their mass-balance profile at steady state displays an inversion near the snout (where the debris cover is thickest) that is not observed in the clean-glacier simulations. In these cases, where the mantle causes complete buffering to inhibit ablation, the DCG does not reach a steady-state profile, and the sediment thickness evolves to a steady value that depends sensitively on the glacier surface velocities. Variation in the assumed englacial debris concentration in our simulations also determines glacier behaviour. With low englacial debris concentration, the DCG retreats initially while its mass-balance gradient steepens, but the glacier re-advances if it subsequently builds up a thick enough debris cover to cause complete buffering. We identify possible ways and challenges of testing this model with field observations of DCGs, given the inherent difficulty that such glaciers may not be in steady state.

• Lefeuvre, Pierre-Marie; Zwinger, Thomas; Jackson, Miriam & Lappegard, Gaute (2012). Ice dynamics around a closing channel: insights on stress distribution and its time evolution. Show summary

  For the past 20 years, load cell sensors located at the Svartisen Subglacial Laboratory have monitored regular local pressure events at the base of Engabreen glacier, Norway. Those micro-scale events were assumed to be related to subglacial channel mechanics, notably stress bridging. This is illustrated by stress transfer of the overburden on top of a void toward its supports, involving shear stress components. This e ect could be of major importance for the connectivity of water in channels and their surroundings as well as water pressure. Here, we investigate parameters that a ect vertical pressure on the sides of closing channels in synthetic setups, using a nite element model (Elmer/Ice) that solves the full- Stokes equation. With our simulation results, we attempt to mimic temporal evolution of the load cell signal as being measured at the Subglacial Laboratory. Sliding and transversal slope a ect the shape of these signals. High pressure peaks similar to our measurements are observed at the channel edges, which decrease in intensity and migrate over time as the channel closes. Shear stress appeared to be a major component in that process that cannot be resolved by lower order approximation of the Stokes equations.

• Lefeuvre, Pierre-Marie & Ng, Felix (2011). Uncovering glacier dynamics beneath a debris mantle: its control on surface profile shape.. Show summary

  Unlike normal glaciers, debris-covered glaciers (DCG) have an extensive sediment mantle, whose albedo and buffering effect significantly enhances and reduces ice melt. This last phenomenon suggests that DCGs are less sensitive to climate change and might persist longer than debris free glaciers. Despite the considerable implication of this insulating layer, the origin of DCG and the extent to which the debris mantle impacts on glacier dynamics is relatively unknown. Although the DCG front seems motionless, the debris-mantled zone shows surface lowering and often low surface velocities compared to the clean ice zone. Supra-glacial sediments appear to be an effect and a cause of such an evolution. Research on DCG also aims to assess geo-hazards linked to the sudden release of water stored within supra-glacial lakes. As glacier melts down, those lakes generally form at the surface of DCGs within troughs. Their occurrence appears closely linked to the debris-covered zone dynamics. This project investigated debris cover impact on glacier dynamics and surface geometry through a numerical analysis based on classic glaciological laws and a steady-state flow-line model. The latter couples ice dynamics, mass balance and debris cover feedback on ablation rates. The model simulates indefinitely flowing debris-covered glacier that shows an inverted mass balance. In the model, the sediment thickness is highly dependent on the glacier turn over around the ELA. Moreover, model insights indicate a bi-modal glacier behaviour. For thin debris cover, glaciers retreat as mass balance gradient steepens whereas glaciers continuously advance when they have enough time to build up a sediment buffering effect. In the light of the model results, the location of the debris cover and the ice sediment concentration strongly affects ablation zone dynamics. This analysis produces a theoretical framework to interpret modern glacier profile.

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