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Response of debris-covered glaciers in the Mount Everest region to recent warming, and implications for outburst flood hazards

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In areas of high relief, many glaciers have extensive covers of supraglacial debris in their ablation zones, which alters both rates and spatial patterns of melting, with important consequences for glacier response to climate change. Wastage of debris-covered glaciers can be associated with the formation of large moraine-dammed lakes, posing risk of glacier lake outburst floods (GLOFs). In this paper, we use observations of glaciers in the Mount Everest region to present an integrated view of debris-covered glacier response to climate change, which helps provide a long-term perspective on evolving GLOF risks.
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... Glaciers with a thick debris cover are limited in terms of their use as climatic indicators since the debris cover influences the energy balance, delays the dynamic response, and influences the ablation rate as well as the discharge of meltwater (Nakawo et al., 2000;Reid and Brock, 2010;Ragettli et al., 2015;Ayala et al., 2016). When melting rapidly, glaciers waste down or back to where they have a clean surface, while changes in the debris-covered part typically are significantly smaller and the processes more complex (Benn et al., 2012;Mölg et al., 2020). Corresponding process differences can, for instance, enhance the potential of lake formation in the contact zone of clean and debris-covered ice and may cause glacier lake outburst floods Benn et al., 2012). ...
... When melting rapidly, glaciers waste down or back to where they have a clean surface, while changes in the debris-covered part typically are significantly smaller and the processes more complex (Benn et al., 2012;Mölg et al., 2020). Corresponding process differences can, for instance, enhance the potential of lake formation in the contact zone of clean and debris-covered ice and may cause glacier lake outburst floods Benn et al., 2012). ...
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Digitized aerial images were used to monitor the evolution of perennially frozen debris and polythermal glacier ice at the intensely investigated Gruben site in the Swiss Alps over a period of about 50 years. The photogrammetric analysis allowed for a compilation of detailed spatio-temporal information on flow velocities and thickness changes. In addition, high-resolution GNSS (global navigation satellite system) and ground surface temperature measurements were included in the analysis to provide insight into short-term changes. Over time, extremely contrasting developments and landform responses are documented. Viscous flow within the warming and already near-temperate rock glacier permafrost continued at a constant average but seasonally variable speed of typically decimetres per year, with average surface lowering limited to centimetres to a few decimetres per year. This constant flow causes the continued advance of the characteristic convex, lava-stream-like rock glacier with its oversteepened fronts. Thawing rates of ice-rich perennially frozen ground to strong climate forcing are very low (centimetres per year) and the dynamic response strongly delayed (timescale of decades to centuries). The adjacent cold debris-covered glacier tongue remained an essentially concave landform with diffuse margins, predominantly chaotic surface structure, intermediate thickness losses (decimetres per year), and clear signs of down-wasting and decreasing flow velocity. The former contact zone between the cold glacier margin and the upper part of the rock glacier with disappearing remains of buried glacier ice embedded on top of frozen debris exhibits complex phenomena of thermokarst in massive ice and backflow towards the topographic depression produced by the retreating glacier tongue. As is typical for glaciers in the Alps, the largely debris-free glacier part shows a rapid response (timescale of years) to strong climatic forcing with spectacular retreat (>10 m a−1) and mass loss (up to >1 m w.e. specific mass loss per year). The system of periglacial lakes shows a correspondingly dynamic evolution and had to be controlled by engineering work for hazard protection.
... Future research in the region must target less-studied landscape responses to climate change 91 , including paraglacial adjustments, slope instability, hazard cascades and glacial/permafrost erosion and related sediment yields rather than focusing solely on cryosphere reduction and changes to freshwater supply 1,2,7,8,[10][11][12]15 . Recently, glacier status and glacial lakes in HMA have been mapped [34][35][36][79][80][81][108][109][110] , and knowledge regarding glacial lake evolution in relation to glacier changes has improved 79,111 . Predictions of future glacial lake development and GLOF risks are also being produced 70,86,112 . ...
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Global warming-induced melting and thawing of the cryosphere are severely altering the volume and timing of water supplied from High Mountain Asia, adversely affecting downstream food and energy systems that are relied on by billions of people. The construction of more reservoirs designed to regulate streamflow and produce hydropower is a critical part of strategies for adapting to these changes. However, these projects are vulnerable to a complex set of interacting processes that are destabilizing landscapes throughout the region. Ranging in severity and the pace of change, these processes include glacial retreat and detachments, permafrost thaw and associated landslides, rock–ice avalanches, debris flows and outburst floods from glacial lakes and landslide-dammed lakes. The result is large amounts of sediment being mobilized that can fill up reservoirs, cause dam failure and degrade power turbines. Here we recommend forward-looking design and maintenance measures and sustainable sediment management solutions that can help transition towards climate change-resilient dams and reservoirs in High Mountain Asia, in large part based on improved monitoring and prediction of compound and cascading hazards.
... In recent decades, nearly worldwide glacier shrinkage and mass loss have been observed (Gardner et al., 2013;Hugonnet et al., 2021). Glacier changes can induce glacier hazards such as landslides, glacier lake outburst floods, and debris flows, which affect the security of the downstream areas (Benn et al., 2012;Rankl et al., 2014;Brun et al., 2017;Yao et al., 2019). In the context of climate fluctuation, mountain glaciers have received extensive attention, and timely investigation and study of glacier changes are necessary. ...
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Mountain glaciers are an important component of the global hydrological cycle. Existing research about glacier changes in the Altai focused on limited regions. Study about recent glacier changes in the entire Altai Mountains is still lacking. We presented a consistent method for identifying glacier margins. The two new glacier inventories in 2000 and 2020 were derived from Landsat satellite imagery. Glacier surface elevation change and mass balance were obtained by comparing the 2000 Shuttle Radar Topography Mission (SRTM) and 2020 Digital Elevation Models (DEMs) generated from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images. The spatial pattern of glacier changes was discussed in conjunction with climate trends. We mapped a total area of 1,096.06 ± 53.32 km2 around 2020, which amounts to 1,927 glaciers in the Altai Mountains. That was 12.02 ± 3.01% (or 0.60 ± 0.15%·a−1) less than the 1,245.75 ± 58.52 km2 around 2000. The geodetic mass balance of the monitoring glaciers in the Aktru basin for the period 2000–2011 was used to validate the geodetic survey. The average geodetic mass balance of -0.32 ± 0.09 m w. e.·a−1 on monitoring glaciers was slightly exaggerated than the observed mass balance of -0.26 m w. e.·a−1, but it was proved that the geodetic mass balance could reflect glacier changes in the Altai Mountains. An average mass loss of 14.55 ± 1.32 m w. e. (or 0.74 ± 0.07 m w. e.·a−1) was found during 2000–2020 in the Altai Mountains. Although the glacier area changes and mass balance were characterized by spatial heterogeneity, the glaciers in the Altai had experienced an accelerated shrinkage from 2000 to 2020 compared to the 20th century. The rising temperature is the foremost reason for glacier area shrinkage and mass loss according to the Climatic Research Unit (CRU) reanalysis data.
... The snow avalanche also causes considerable accumulation due to very steep and rugged terrain above the glaciers. The Himalayan glaciers are characterized by the presence of debris in their lower ablation zones and the thickness of debris varies from a few centimetres to meters (Benn et al., 2012;Chand et al., 2015;Chand and Kayastha, 2018;Fujii and Higuchi, 1977;Mattson and E., 1993;Östrem, 1959). The source of debris for these glaciers is due to the presence of large valleys and steep headwalls (Kraaijenbrink et al., 2017), which alter the surface energy balance and work as a barrier between the atmosphere and ice (Nicholson and Benn, 2013). ...
... Debris-covered glaciers can also be related to the formation of moraine-dammed lakes, creating a high potential for the hazard of glacier lake outburst floods (GLOFs) (Benn et al., 2012) (Konrad, 1998). In recent years, there have been more fluctuations in DCG due to climate change effects which resulted in a frequent number of GLOFs, debris flows, and debris-covered glacier flows (Zaginaev et al., 2019;Y. ...
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In recent years, deep learning (DL) methods have proven their efficiency for various computer vision (CV) tasks such as image classification, natural language processing, and object detection. However, training a DL model is expensive in terms of both complexities of the network structure and the amount of labeled data needed. In addition, the imbalance among available labeled data for different classes of interest may also adversely affect the model accuracy. This paper addresses these issues using a new convolutional neural network (CNN) based architecture. The proposed network incorporates both spatial and spectral information that combines two sub-networks: spatial-CNN and spectral-CNN. The spectral-CNN extracts spectral information, while spatial-CNN captures spatial information. Moreover, to make the features more robust, a multiscale spatial CNN architecture is introduced using different kernels. The final feature vector is formed by concatenating the outputs obtained from both spatial-CNN and spectral-CNN. To address the data imbalance problem, a generative adversarial network (GAN) was used to generate data for the underrepresented class. Finally, relatively a shallower network architecture was used to reduce the number of parameters in the network and improve the processing speed. The proposed model was trained and tested on Senitel-2 images for the classification of the debris-covered glacier. The results showed that the proposed method is well-suited for mapping and monitoring debris-covered glaciers at a large scale with high classification accuracy. In addition, we compared the proposed method with conventional machine learning approaches, support vector machine (SVM), random forest (RF) and multilayer perceptron (MLP).
... Under the ongoing atmospheric warming, Himalayan glaciers are experiencing significant thinning (mass loss) as well as reductions in length and area (Bolch et al. 2012(Bolch et al. , 2019. As a result of this, unstable moraine-dammed lakes are formed near the snouts of glaciers (Benn et al. 2012). These moraine-dammed lakes are unstable, thus causing GLOF in downstream areas (Allen et al. 2015). ...
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On 7th February 2021, a catastrophic flash flood occurred in Raunthi Gad, Rishi Ganga catchment of Dhualiganga Basin. It caused the death of around 200 people and devastated the hydropower projects and other associated infrastructure in the downstream areas of the basin. While the extent of damage and devastation in the downstream region around Rini and Tapovan has been extensively reported, the reconstruction of the event has still not been definitively established. Based on an analysis of the data reported in previous papers and our field and remote-sensing data, we present a detailed reconstruction of the events that occurred in Raunthi Gad that morning. Our analysis supports previous reports that the basic cause was that a portion of the hanging glacier located at Raunthi peak (5600 m asl) along with a large amount of rock fell and hit the Raunthi valley at about 1.5 km downstream of the current snout of Raunthi glacier at an elevation of around 3800 m asl. We present evidence, supported by previous data of transient ponding in the region between the impact zone and the confluence of Raunthi Gad and Rishi Ganga. We estimate the flood volume at Rini to be around 10 MCM and the volume of water available in the valley in the form of ice and snow to be around 6 MCM. We argue that this deficit can be accounted for by the debris volume. The material gained around 8 × 10 14 J of energy during the initial slide whereas around 1.5 × 10 14 J is required to melt the ice and snow.
... Moraine soils are geomorphological phenomena formed during glacier retreat and can be found in mountainous regions at high latitudes or altitudes, such as the Qinghai-Tibet Plateau. In the last several decades, a number of major geohazards related to moraine soils have been documented all around the world, including landslides (Cui et al., 2015;Emmer et al., 2020), the moraine-dammed lake outbursts (Clague and Evans, 2000;Benn et al., 2012;Westoby et al., 2014), and glacial debris flows (Shaun and John, 2000;Chiarle et al., 2007;Evans et al., 2009;Legg et al., 2014;Deng et al., 2017;Wei et al., 2018;Wang et al., 2018). The sensitivity of glaciers and permafrost to climate change is exacerbating these hazards (Shugar et al., 2021). ...
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Moraine soils are widely distributed in the Qinghai-Tibet Plateau. With increased global warming, moraine soil-related geohazards have become increasingly common, posing a serious threat to the infrastructure (e.g., the Sichuan–Tibet Railway) and inhabitants of this region. This paper aims to investigate the mechanical behavior of ice-rich moraine soil in the Tianmo Valley by conducting a series of triaxial constant strain rate (CSR) and coupled thermomechanical (CTM) tests on artificially moraine soil containing different ice forms (crushed ice and block ice). The results show that in general, compared to moraine soil containing crushed ice, moraine soil with block ice has a higher peak strength, a similar internal frictional angle and a considerably larger cohesive strength. The stress–strain curve of the soil containing crushed ice shows a strain-hardening form, while that of the soil containing block ice is represented by a strain-softening model similar to that of dense soil. In the CTM tests, it is revealed that the rising temperature could cause a sharp increase in strain and lead to sample failure, even when the axial load is far below the material's peak strength. A comparison between samples with different ice forms reveal that the soil containing crushed ice is more sensitive to temperature change. The tests demonstrate that the ice form has a significant influence on the mechanical behavior of moraine soil, and the temperature rise can result in a dramatic decrease in soil strength. Therefore, efforts should be made to detect the occurrence form of buried ice and the changes in the environmental temperature as well as the stress state of moraine soil slopes in situ.
... The thick supraglacial debris can inhibit the glacier mass balance, while the thin supraglacial debris can promote. The positive or negative correlation between supraglacial debris and mass balance may depend on the supraglacial debris thickness (Scherler et al., 2011;Benn et al., 2012;Fyffe et al., 2014). If the surface features of the debris are considered, such as ice cliffs and supraglacial lake, the influence of debris will be more complicated (Huang et al., 2018;Steiner et al., 2019;Stefaniak et al., 2021). ...
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High Mountain Asia (HMA) region contains the world's highest peaks and the largest concentration of glaciers except for the polar regions, making it sensitive to global climate change. In the context of global warming, most glaciers in the HMA show various degrees of negative mass balance, while some show positive or near-neutral balance. Many studies have reported that spatial heterogeneity in glacier mass balance is strongly related to a combination of climate parameters. However, this spatial heterogeneity may vary according to the dynamic patterns of climate change at regional or continental scale. The reasons for this may be related to non-climatic factors. To understand the mechanisms by which spatial heterogeneity forms, it is necessary to establish the relationships between glacier mass balance and environmental factors related to topography and morphology. In this study, climate, topography, morphology, and other environmental factors are investigated. Geodetector and linear regression analysis were used to explore the driving factors of spatial variability of glacier mass balance in the HMA by using elevation change data during 2000–2016. The results show that the coverage of supraglacial debris is an essential factor affecting the spatial heterogeneity of glacier mass balance, followed by climatic factors and topographic factors, especially the median elevation and slope in the HMA. There are some differences among mountain regions and the explanatory power of climatic factors on the spatial differentiation of glacier mass balance in each mountain region is weak, indicating that climatic background of each mountain region is similar. Therefore, under similar climatic backgrounds, the median elevation and slope are most correlated with glacier mass balance. The interaction of various factors is enhanced, but no unified interaction factor plays a primary role. Topographic and morphological factors also control the spatial heterogeneity of glacier mass balance by influencing its sensitivity to climate change. In conclusion, geodetector method provides an objective framework for revealing the factors controlling glacier mass balance.
... Ragettli et al., 2016) rather than to a retreat of the terminus (e.g. Benn et al., 2012). This causality is confirmed when the evolution of Langtang Glacier is re-computed using the same climatic conditions but when re-calibrating the model parameters to match the observed volume changes without activating the debris-cover module (Fig. 9b). ...
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Currently, about 12 %–13 % of High Mountain Asia’s glacier area is debris-covered, which alters its surface mass balance. However, in regional-scale modelling approaches, debris-covered glaciers are typically treated as clean-ice glaciers, leading to a bias when modelling their future evolution. Here, we present a new approach for modelling debris area and thickness evolution, applicable from single glaciers to the global scale. We derive a parameterization and implement it as a module into the Global Glacier Evolution Model (GloGEMflow), a combined mass-balance ice-flow model. The module is initialized with both glacier-specific observations of the debris' spatial distribution and estimates of debris thickness. These data sets account for the fact that debris can either enhance or reduce surface melt depending on thickness. Our model approach also enables representing the spatiotemporal evolution of debris extent and thickness. We calibrate and evaluate the module on a selected subset of glaciers and apply GloGEMflow using different climate scenarios to project the future evolution of all glaciers in High Mountain Asia until 2100. Explicitly accounting for debris cover has only a minor effect on the projected mass loss, which is in line with previous projections. Despite this small effect, we argue that the improved process representation is of added value when aiming at capturing intra-glacier scales, i.e. spatial mass-balance distribution. Depending on the climate scenario, the mean debris-cover fraction is expected to increase, while mean debris thickness is projected to show only minor changes, although large local thickening is expected. To isolate the influence of explicitly accounting for supraglacial debris cover, we re-compute glacier evolution without the debris-cover module. We show that glacier geometry, area, volume, and flow velocity evolve differently, especially at the level of individual glaciers. This highlights the importance of accounting for debris cover and its spatiotemporal evolution when projecting future glacier changes.
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There have been rapid increases in both the number and expansion of the proglacial lakes across High Mountain Asia. However, the relationship between proglacial lakes and glacier dynamics remains unclear in the Himalayan region. Here we present the surface elevation, flow-velocity changes, and proglacial lake expansion of Thorthormi and Lugge glaciers in the Lunana region, Bhutanese Himalaya, during the 2000–2018 period using photogrammetry and GPS survey data. The lake expansion and surface lowering rates and flow-velocity field of Lugge Glacier, a lake-terminating glacier, have remained approximately constant since 2000. Conversely, there have been accelerated proglacial lake expansion and a 2-fold increase in the thinning rate of Thorthormi Glacier since 2011, as well as a considerable speed-up in the flow-velocity field (>150 m a−1). We reveal that the lake formation and transition of Thorthormi Glacier from a land- to lake-terminating glacier have triggered glacier speed-up and rapid thinning via a positive (compressive) to negative (extensional) change in the emergence velocities. This study provides the first evidence of dynamic glacier changes that are associated with proglacial lake formation across the Himalayan region.
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Patterns of annual variation of air temperature in the world provide two types of the patterns of ablation rate of the glacier, namely the “summer-maximum” and the “non-maximum” through a year, while those of precipitation and air temperature provide three types of accumulation rate, namely, the above two and the “winter-maximum”. In six combinations of these types, annual variation of balance rate can be classified into the types of the winter-maximum, the non-maximum and the summer-maximum. The “summer-accumulation type glaciers” in the Nepal Himalaya, which have more accumulation in summer than winter in the whole area of a glacier, belong to the non-maximum type of balance rate. In the case of this type glacier, direct observations of accumulation and ablation are quite difficult, since accumulation and ablation mainly occur simultaneously in summer. Therefore, the methods of estimation of accumulation and ablation are discussed. Accumulation can be estimated on the basis of the linear relation between surface air temperature and the probability of occurrence of solid precipitation in all cases of precipitation. Local characteristics of melting process of precipitation elements which control such relation are described. For the estimation of ablation, the effect of high albedo of new snow is important for the summer-accumulation type. The variation of mass balance through the balance year in the case of the summer-accumulation type is compared with that of the winter-accumulation type.
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In 1978, the first detailed glaciological studies were carried out on the small glacier AX010 in a glacier inventory of Dudh Kosi region, in the Nepal Himalaya. Its length, area and highest and lowest elevations in 1978 were 1.7 km, 0.57 km2, 5360 and 4950 m a.s.l., respectively. Two resurveys of its terminus position and/or surface elevations were carried out in 1989 and 1991. The glacier retreated by about 30 and 28 m during the periods 1978–89 and 1989–91, respectively. In 1995, annual monitoring of this glacier by means of ground survey, stake method and topographical mapping began, in order to obtain its mass balance, surface flow velocity and extent, and link them with climatic conditions. The results obtained in 1995 are summarized as follows: (1) During the period 1991–95, the glacier retreated by 12 m. (2) Associated with the ice-thickness loss in the lower part of the glacier, the horizontal surface velocities along the center line in 1995 (June–October) showed a remarkable decrease on the glacier tongue, to about 50% of those in 1978. Shrinkage of the glacier in the near future is predicted from a simple model calculation for the case that climatic conditions remain unchanged from 1995. The results show that the present shrinkage should continue and accelerate.
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The thickness of supraglacial debris on the Khumbu Glacier, Nepal Himalaya, has been mapped by a combination of direct measurements and morphological and lithological studies. All three processes, englacial, supraglacial, and subglacial, must be considered in establishing the distribution of debris. Taking advantage of the lithological characteristics of the debris and their bedrock source, the denudation rate of the schistose bedrock was estimated to be about 0.02 mm a−1. A rough estimate of the production rate of supraglacial debris indicated that most of the present debris has formed since the last advance of the glacier, which took place a few hundred years B.P.
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The debris-covered area of Khumbu Glacier was topographically mapped in 1995 and the morphological evolution was determined by comparing the 1995 maps with those made in 1978. There had been significant changes in the surface morphology during this 17-year period: The area with a rough uneven surface with large relative relief had extended both upglacier and downglacier, and area of high ablation had increased. The glacier shrinkage in the ablation area where there was a thick debris cover was associated with an increase in surface relief and relative height, mainly caused by rapid ablation on exposed ice and lateral erosion at streams and ponds.
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The Tsho Rolpa Glacier Lake stores about 100x 106 m3 of water behind an unconsolidated moraine dam in the Roiwaling Valley of central Nepal. The 150 m high terminal moraine has been deteriorating rapidly since the last 4-5 years, as buried dead-ice is being exposed. Displacement waves formed by the calving of the Trakarding Glacier terminus into the Tsho Rolpa are increasing in magnitude. Seepage from springs on the distal flank of the end moraine some 50 m below the dam crest and shallow slumps were also observed, and all of these phenomena indicate deterioration of the moraine dam. The majority of past studies suggested lowering the lake level to prevent a glacier lake outburst flood. Engineering work to lower the lake level down to 3 m by constructing an open channel on the terminal moraine began in May 1999 and ended successfully in July 2000. Several studies, including the ground penetrating radar survey, were conducted in association with the construction work. They provided the detailed information on subsurface conditions of the lake area. The construction of the open channel reduced the volume of water available to form a potential glacier lake outburst flood by about 20%. This work demonstrated for the first time in Nepal that such remediation of a glacier lake outburst is quite effective.
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The Neoglacial evolution of the Tasman Glacier is reconstructed from the distribution of ice-marginal moraines and from the subglacial topography. The glacier has overridden its margins, creating two shelves of thin ice by c. 3700 years before present (BP) and c. 2000 years BP. The proglacial foreland is dominated by outwash aggradation and lacks pre-nineteenth century terminal moraines. The glacier has experienced successively larger expansions over the Neoglacial period (c. 5000 years), prior to drastic twentieth-century thinning and retreat. Over the same period, uncovered glaciers have shown progressively smaller re-advances. The expansionary tendency of the debris-covered glacier is interpreted as a response to long-term (millennial) accumulation of both subglacial and supraglacial debris. Subglacial aggradation has probably raised the bed of the glacier, promoting debris cover growth and reducing ablation even as less favourable balance regimes developed. Comparison with other glaciers shows that the expansionary tendency is widespread but may be manifest in a variety of sediment-landform associations.
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ABSTRACT: 15.1 INTRODUCTION The concept of the glaciated valley landsystem was introduced by Boulton and Eyles (1979) and Eyles (1983b), to describe the characteristic sediments and landforms associated with valley glaciers in upland and mountain environments. By focusing on the scale of the whole depositional basin, the glaciated valley landsystem has a broader compass than most of the other landsystems explored in this book, which are specific to particular depositional environments. Indeed, glaciated valley landsystems may incorporate ice-marginal, supraglacial, subglacial, proglacial, periglacial and paraglacial landsystems, recording the juxtaposition and migration of very different depositional environments. Additionally, because glaciated valleys occur in every latitudinal environment from equatorial to polar regions, the dimensions of climate and glacial thermal regime add even more variability. Thus the 'glaciated valley landsystem' should be regarded as a family of landsystems, which exhibits considerably more variety than suggested by the original Boulton and Eyles model (Fig. 15.1). Despite this variability, landsystems in glaciated valleys tend to have certain recurrent features, as a result of two main factors: 1. the strong influence of topography on glacier morphology, sediment transport paths and depositional basins 2. the importance of debris from supraglacial sources in the glacial sediment budget. In this chapter, we emphasise the contrasts between glaciers with limited supraglacial debris ('clean glaciers') and glaciers with substantial debris covers in their ablation zones ('debris-covered glaciers'), although it should be recognized that intermediate forms occur between these end members. Before examining the landsystems of glaciated valleys, we begin by considering debris sources and transport pathways through valley glaciers, and the ways in which debris cover influences glacier dynamics. 15-Evans-Glacial-15-ppp 5/27/03 2:38 PM Page 372 Full-text · Article · Jan 2004
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Field and geophysical studies have allowed us to identify processes leading to ice-cored moraine degradation for three natural dams investigated in Peru and Nepal. As potentially hazardous lakes form on the snouts of debris-covered glaciers they may separate a stagnant ice body from the upper reaches of the glacier to form an ice-cored end-moraine complex. The ice-cored moraines appear to degrade through ablation beneath the debris cover, by localized thermokarst development, and by associated mass movement. Relict glacier structures serve as a focal point for the onset of accelerated thermokarst degradation. Once exposed, the ice core then undergoes accelerated wastage through the combined affects of solar radiation and mechanical failure due to the rheological response of the ice to deepening kettle forms. Continuing degradation reduces the lake freeboard, weakens the moraine dam, and can lead to its catastrophic failure.
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Analyses of satellite imagery obtained in 1989-1990 and topographic maps from 1966 have been undertaken for the glacierized areas of Bhutan. It has been found that where supraglacial lakes have formed since 1966, surface gradients of the glaciers concerned were in all cases less than 2°. At gradients of 2-10°supraglacial ponds can form but tend to be transient due to the opening and closing of crevasses. By identifying the conditions under which large supraglacial lakes form it is possible to use these criteria to predict where such lakes may develop in the future. This will allow suitable monitoring programmes to be introduced and, where necessary, suitable engineering remediation works to be undertaken in order to prevent the collection of large volumes of water that may be liable to form glacier lake outburst floods in the future.