Article

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

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Abstract

Moraine-dammed glacial lakes are becoming increasingly common in the Himalaya as a result of glacier mass loss, causing concern about glacier lake outburst flood risk. In addition to extant lakes, the potential exists for many more to form, as more glaciers ablate down to the level of potential moraine dams. In this paper, we document the recent rapid growth of, a moraine-dammed lake on Ngozumpa Glacier, Nepal. Using a combination of ground-based mapping and sonar surveys, aerial photographs (<1 m resolution), and ASTER imagery (15 m resolution), processes and rates of lake expansion have been determined. The lake first formed between 1984 and 1992 when collapse of an englacial conduit allowed water to accumulate at the level of a gap in the lateral moraine, similar to km from the glacier terminus. Lake growth was initially slow, but since 2001 it has undergone exponential growth at an average rate of 10%y(-1). In 2009, the lake area was 300,000 m(2), and its volume was at least 2.2 million m(3). Calving, subaqueous melting, and melting of subaerial ice faces all contribute to the expansion of the lake; but large-scale, full-height slab calving is now the dominant contributor to growth. Comparison with other lakes in the region indicate that lake growth will likely continue unchecked whilst the spillway remains at its current level and may attain a volume of hundreds of millions of cubic metres within the next few decades.

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... These catchments have experienced atmospheric warming since the mid-1970mid- 's (Shrestha et al., 1999Shrestha and Aryal, 2011) and weakened summer monsoons, which has reduced glacier accumulation (Salerno et al., 2015). Consequently, glacial mass balance in the region has been strongly negative since the 1970s (Bolch et al., 2008;Benn et al., 2012;Kӓӓb et al., 2012). ...
... These ice surface lakes can then coalesce to form a larger proglacial lake, which carries the risk of producing GLOFs (e.g. Richardson and Reynolds, 2000;Quincey et al., 2007;Thompson et al., 2012;Mertes et al., 2017). ...
... Supraglacial ponds are highly variable in character (e.g. in shape, size, turbidity and ice-cliffs), dynamic in nature, and are 55 expected to become increasingly prevalent in a warming climate (Thompson et al., 2016;Miles et al., 2017;Watson et al., 2016;2017b).Understanding the spatial and temporal patterns of pond growth is therefore vital for accurately forecasting proglacial lake growth and the associated hazard of GLOFs (Richardson and Reynolds, 2000;Quincey et al., 2007;Benn et al., 2012). ...
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Supraglacial ponds are characteristic of debris-covered glaciers and greatly enhance local melt rates. They can grow rapidly and coalesce to form proglacial lakes, which represent a major hazard. Here, we use Sentinel-2A satellite imagery (10 m) to quantify the spatiotemporal changes of 6,425 supraglacial ponds for 10 glaciers in the Everest region, Nepal, between 2015 and 2018. During the study period, ponded area increased on all glaciers, but showed substantial temporal and spatial variation, and the rate of pond growth increased substantially relative to 2000–2015 (Watson et al., 2016). Both Imja and Spillway Lake expanded and Khumbu Glacier developed a chain of connected ponds. 54 % of ponds were associated with an ice-cliff, but the proportion of ponds with cliffs decreased during the study period. Pond location generally corresponded to lower surface velocity, but this relationship was not ubiquitous. Ponds are now predominantly found at mid-elevations on our study glaciers, suggesting that conditions conducive to pond formation have advanced up-glacier compared to general theory. Results demonstrate the need to utilize high-resolution imagery (
... Insights into the surface characteristics of debris-covered glaciers and the enhanced local ablation rates gave rise to another concern relevant to both the scientific community and local communities, namely the accelerated growth of supraglacial and proglacial lakes associated with glacier thinning and recession documented around the world (Paul et al. 2007;Komori 2008;Gardelle et al. 2011;Thompson et al. 2012;Wang et al. 2014;Nie et al. 2017;Shukla et al. 2018;Chen et al. 2020;Shugar et al. 2020). Proglacial lakes exhibit an effect on glacier ice dynamics through enhanced ablation at the glacier margins via mechanical and thermal stresses; they modify meltwater routing and sediment fluxes through sedimentation (Carrivick and Tweed 2013). ...
... The evolution of moraine-dammed lakes associated with debris-covered glaciers has been addressed in few studies (e.g. Bolch et al. 2008;Thompson et al. 2012;Thompson et al. 2017;Harrison et al. 2018). Assessing the hazard potential of these lakes presents significant challenges both in the field due the highly dynamic environment and in remote sensing due to limited multi-temporal, high resolution data needed to estimate glacier surface evolution in some areas (Wang et al. 2020). ...
... Benn et al. (2012) is one example of a holistic study that links climate, mass balance, ice dynamics, topographic evolution and hydrology in the Everest region of Nepal and explores how observed behaviour (and hazard potential) emerges from interactions between these process domains. Lake hazard assessments are often conducted from a remote-sensing or geophysical perspective (Pant and Reynolds 2000;Rana 2000;Richardson and Reynolds 2000b;Reynolds 2006;Hambrey et al. 2008;Thompson et al. 2012;Thompson et al. 2017). A systems approach combines various perspectives to provide a more complete picture of the debris-cover-lake system, i.e., the interplay between the glacier surface topography, the lake dynamics and the ablation patterns related to supraglacial debris cover and its characteristics. ...
Article
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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.
... The recent history of Spillway Lake was discussed in detail by Thompson et al. (2012Thompson et al. ( , 2016 and is briefly reviewed here. The present spillway through the SW side of the terminal moraine has been in existence since at least 1965, when water emerged from the glacier and entered a small pond behind the lateral moraine (1; Fig. 13). ...
... On the Survey of Nepal (1996) map based on aerial photographs taken in 1992, the lake has a ribbon-like form, extending NE for ∼ 600 m from the spillway. The lake had essentially the same outline at the time of our first field survey in 1998, when water was observed to enter the lake via a subaerial portal and an upwelling point ( Fig. 13; Benn et al., 2001;Thompson et al., 2012). Between 1998 and 1999, several chasms and holes opened up on the glacier surface north of the western portal, and by 2001 these had evolved into linear ponds and lakes (2; Fig. 13). ...
Article
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We provide the first synoptic view of the drainage system of a Himalayan debris-covered glacier and its evolution through time, based on speleological exploration and satellite image analysis of Ngozumpa Glacier, Nepal. The drainage system has several linked components: (1) a seasonal subglacial drainage system below the upper ablation zone; (2) supraglacial channels, allowing efficient meltwater transport across parts of the upper ablation zone; (3) sub-marginal channels, allowing long-distance transport of meltwater; (4) perched ponds, which intermittently store meltwater prior to evacuation via the englacial drainage system; (5) englacial cut-and-closure conduits, which may undergo repeated cycles of abandonment and reactivation; and (6) a "base-level" lake system (Spillway Lake) dammed behind the terminal moraine. The distribution and relative importance of these elements has evolved through time, in response to sustained negative mass balance. The area occupied by perched ponds has expanded upglacier at the expense of supraglacial channels, and Spillway Lake has grown as more of the glacier surface ablates to base level. Subsurface processes play a governing role in creating, maintaining, and shutting down exposures of ice at the glacier surface, with a major impact on spatial patterns and rates of surface mass loss. Comparison of our results with observations on other glaciers indicate that englacial drainage systems play a key role in the response of debris-covered glaciers to sustained periods of negative mass balance.
... Most of these studies demonstrated the development of glacial lakes usually on a decadal basis [8,41,42], and were regionally aggregated [11]. Furthermore, these studies were glacier or lake specific [13,24,43,44] or used one time satellite imagery [45]. Remote sensing techniques are also used for monitoring supraglacial ponds [16,19,26]. ...
... Significant surface lowering of glaciers in many parts of the Himalayas has been observed [2,3,5] with increase in temperature which resulted in lowering surface gradient and glacier velocity [14], which favors the development of supraglacial ponds and glacial lakes [24,67,68]. Wastage of glacier in recent period provides the sufficient melt water to develop the supraglacial ponds and helps in expanding their size [10,17,43], which may likely grow monotonically if glaciers continue losing their mass [24] . The expansion in ponded area contributes substantially to ablation of the glacier due to undercutting, calving, and melting imposed by ponded water. ...
Article
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Several supraglacial ponds are developing and increasing in size and number in the Himalayan region. They are the precursors of large glacial lakes and may become potential for glacial lake outburst floods (GLOFs). Recently, GLOF events originating from supraglacial ponds were recorded; however, the spatial, temporal, and seasonal distributions of these ponds are not well documented. We chose 23 debris-covered glaciers in the Everest region, Nepal, to study the development of supraglacial ponds. We used historical Landsat images (30-m resolution) from 1989 to 2017, and Sentinel-2 (10-m resolution) images from 2016 to 2018 to understand the long-term development and seasonal variations of these ponds. We also used fine-resolution (0.5-2 m) WorldView and GeoEye imageries to reveal the high-resolution inventory of these features and these images were also used as references for accuracy assessments. We observed a continuous increase in the area and number of ponds from 1989-2017, with minor fluctuations. Similarly, seasonal variations were observed at the highest ponded area in the pre-and postmonsoon seasons, and lowest ponded area in the winter season. Substantial variations of the ponds were also observed among glaciers corresponding to their size, slope, width, moraine height, and elevation. The persistency and densities of the ponds with sizes >0.005 sq.km were found near the glacier terminuses. Furthermore, spillway lakes on the Ngozompa, Bhote Koshi, Khumbu, and Lumsamba glaciers were expanding at a faster rate, indicating a trajectory towards large lake development. Our analysis also found that Sentinel-2 (10-m resolution) has good potential to study the seasonal changes of supraglacial ponds, while fine-resolution (<2 m) imagery is able to map the supraglacial ponds with high accuracy and can help in understanding the surrounding morphology of the glacier.
... This glacier retreat has been accompanied by a growth in the population living close to the high mountains and potentially dangerous glacial lakes are emerging in the glaciated regions. Given the right trigger produced by an avalanche, rockslide or calving, the natural moraine or ice dams that impound these glacial lakes can be breached, resulting in catastrophic glacial lake outburst floods (GLOFs) [10][11][12]. In the Having an ongoing and continuously updated inventory of glacial lakes remains important for the investigation of the latest distribution of the lakes and their spatialtemporal heterogeneities, and the assessment of glacier and climate change as well as GLOF hazard risks. ...
... The melting and retreat of the source glaciers is the main reason for the lake area expansion. Other factors such as glacier morphology, calving, subaqueous melting, and ice face ablation all contribute to the development of the glacial lakes [12,59]. ...
Article
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The current glacial lake datasets in the High Mountain Asia (HMA) region still need to be improved because their boundary divisions in the land–water transition zone are not precisely delineate, and also some very small glacial lakes have been lost due to their mixed reflectance with backgrounds. In addition, most studies have only focused on the changes in the area of a glacial lake as a whole, but do not involve the actual changes of per pixel on its boundary and the potential controlling factors. In this research, we produced more accurate and complete maps of glacial lake extent in the HMA in 2008, 2012, and 2016 with consistent time intervals using Landsat satellite images and the Google Earth Engine (GEE) cloud computing platform, and further studied the formation, distribution, and dynamics of the glacial lakes. In total, 17,016 and 21,249 glacial lakes were detected in 2008 and 2016, respectively, covering an area of 1420.15 ± 232.76 km2 and 1577.38 ± 288.82 km2; the lakes were mainly located at altitudes between 4400 m and 5600 m. The annual areal expansion rate was approximately 1.38% from 2008 to 2016. To explore the cause of the rapid expansion of individual glacial lakes, we investigated their long-term expansion rates by measuring changes in shoreline positions. The results show that glacial lakes are expanding rapidly in areas close to glaciers and had a high expansion rate of larger than 20 m/yr from 2008 to 2016. Glacial lakes in the Himalayas showed the highest expansion rate of more than 2 m/yr, followed by the Karakoram Mountains (1.61 m/yr) and the Tianshan Mountains (1.52 m/yr). The accelerating rate of glacier ice and snow melting caused by global warming is the primary contributor to glacial lake growth. These results may provide information that will help in the understanding of detailed lake dynamics and the mechanism, and also facilitate the scientific recognition of the potential hazards associated with glacial lakes in this region.
... Furthermore, supraglacial lakes may transform into proglacial lakes lacking any ice core (full-depth lakes) through melting of lakebottom ice. However, this is a slow process in which energy is conducted from the overlying water and cannot account for some observed instances of fast lake-bottom lowering with rates exceeding 10 m yr −1 (Thompson et al., 2012). It has been argued that fast lake-bottom lowering could occur by buoyant calving (Dykes et al., 2010;Thompson et al., 2012), but the rare and episodic nature of such events means that little is known about how buoyant calving might contribute to the transformation of supraglacial lakes into full-depth lakes. ...
... However, this is a slow process in which energy is conducted from the overlying water and cannot account for some observed instances of fast lake-bottom lowering with rates exceeding 10 m yr −1 (Thompson et al., 2012). It has been argued that fast lake-bottom lowering could occur by buoyant calving (Dykes et al., 2010;Thompson et al., 2012), but the rare and episodic nature of such events means that little is known about how buoyant calving might contribute to the transformation of supraglacial lakes into full-depth lakes. ...
Article
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Rapid growth of proglacial lakes in the current warming climate can pose significant outburst flood hazards, increase rates of ice mass loss, and alter the dynamic state of glaciers. We studied the nature and rate of proglacial lake evolution at Pasterze Glacier (Austria) in the period 1998–2019 using different remote-sensing (photogrammetry, laser scanning) and fieldwork-based (global navigation satellite system – GNSS, time-lapse photography, geoelectrical resistivity tomography – ERT, and bathymetry) data. Glacier thinning below the spillway level and glacier recession caused flooding of the glacier, initially forming a glacier-lateral to supraglacial lake with subaerial and subaquatic debris-covered dead-ice bodies. The observed lake size increase in 1998–2019 followed an exponential curve (1998 – 1900 m2, 2019 – 304 000 m2). ERT data from 2015 to 2019 revealed widespread existence of massive dead-ice bodies exceeding 25 m in thickness near the lake shore. Several large-scale and rapidly occurring buoyant calving events were detected in the 48 m deep basin by time-lapse photography, indicating that buoyant calving is a crucial process for the fast lake expansion. Estimations of the ice volume losses by buoyant calving and by subaerial ablation at a 0.35 km2 large lake-proximal section of the glacier reveal comparable values for both processes (ca. 1×106 m3) for the period August 2018 to August 2019. We identified a sequence of processes: glacier recession into a basin and glacier thinning below the spillway level; glacio-fluvial sedimentation in the glacial–proglacial transition zone covering dead ice; initial formation and accelerating enlargement of a glacier-lateral to supraglacial lake by ablation of glacier ice and debris-covered dead ice forming thermokarst features; increase in hydrostatic disequilibrium leading to destabilization of ice at the lake bottom or at the near-shore causing fracturing, tilting, disintegration, or emergence of new icebergs due to buoyant calving; and gradual melting of icebergs along with iceberg capsizing events. We conclude that buoyant calving, previously not reported from the European Alps, might play an important role at alpine glaciers in the future as many glaciers are expected to recede into valley or cirque overdeepenings.
... Surface lowering causes these debris-covered glaciers to develop very low or even reversed long-profile topographical gradients through their ablation areas, which promotes the formation of supraglacial water bodies. These ponds and lakes influence the seasonal transport of water through the glacial system, and can expand and coalesce to form substantial supraglacial or proglacial, moraine-dammed lakes that may eventually pose a potential flood hazard ( Thompson et al. 2012;Watson et al. 2016). Such features are commonly bordered by steep, debris-free ice cliffs, which progressively backwaste and, if connected to a supraglacial pond or lake, may undergo thermoerosional notch development at the ice-water interface, promoting the onset of calving processes ( Fig. 2) ( Hambrey et al. 2008;Thompson et al. 2016). ...
Article
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High Mountain Asia contains the largest volume of glacier ice outside the polar regions, and contain the headwaters of some of the largest rivers in central Asia. These glaciers are losing mass at a mean rate of between -0.18 and -0.5 m water equivalent per year. While glaciers in the Himalaya are generally shrinking, those in the Karakoram have experienced a slight mass gain. Both changes have occurred in response to rising air temperatures due to Northern Hemisphere climate change. In the westerly influenced Indus catchment, glacier meltwater makes up a large proportion of the hydrological budget, and loss of glacier mass will ultimately lead to a decrease in water supplies. In the monsoon-influenced Ganges and Brahmaputra catchments, the contribution of glacial meltwater is relatively small compared to the Indus, and the decrease in annual water supplies will be less dramatic. Therefore, enhanced glacier melt will increase river flows until the middle of the twenty-first century, but in the longer term, into the latter part of this century, river flows will decline as glaciers shrink. Declining meltwater supplies may be compensated by increases in precipitation, but this could exacerbate the risk of flooding.
... This is despite the supraglacial debris layer acting to insulate shallow ice temperatures from atmospheric warming. However, the presence of supraglacial ponds 35,36 hosting bare-ice cliffs, which are subject to thermal erosion 37 , appears to at least counteract the insulating effect of the surface debris layer at Khumbu Glacier. Our measurements of the thermal regime of Khumbu Glacier have important implications for the future of Himalayan glaciers. ...
Article
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Runoff from high-elevation debris-covered glaciers represents a crucial water supply for millions of people in the Hindu Kush-Himalaya region, where peak water has already passed in places. Knowledge of glacier thermal regime is essential for predicting dynamic and geometric responses to mass balance change and determining subsurface drainage pathways, which ultimately influence proglacial discharge and hence downstream water availability. Yet, deep internal ice temperatures of these glaciers are unknown, making projections of their future response to climate change highly uncertain. Here, we show that the lower part of the ablation area of Khumbu Glacier, a high-elevation debris-covered glacier in Nepal, may contain ~56% temperate ice, with much of the colder shallow ice near to the melting-point temperature (within 0.8?C). From boreholes drilled in the glacier?s ablation area, we measured a minimum ice temperature of -3.3?C, and even the coldest ice we measured was 2?C warmer than the mean annual air temperature. Our results indicate that high-elevation Himalayan glaciers are vulnerable to even minor atmospheric warming.
... Compared with land-terminating glaciers, lake-terminating glaciers shrink faster in Sikkim (Basnett et al., 2013), have more negative rates of elevation changes in Bhutan, Everest region, and West Nepal (Gardelle et al., 2013), and have more negative mass balances in the Everest region (King et al., 2017). Terminal lakes develop when supraglacial ponds coalesce into a single lake dammed by a terminal moraine (e.g., Benn et al., 2012;Thompson et al., 2012). Eventually, the terminal lake enhances the frontal ablation by dynamic thinning, calving, and thermo-erosion (e.g., Benn et al., 2012). ...
Article
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We investigate the control of the morphological variables on the 2000-2016 glacier-wide mass balances of 6,470 individual glaciers of High Mountain Asia. We separate the data set into 12 regions assumed to be climatically homogeneous. We find that the slope of the glacier tongue, mean glacier elevation, percentage of supraglacial debris cover, and avalanche contributing area all together explain a maximum of 48% and a minimum of 8% of the glacier-wide mass balance variability, within a given region. The best predictors of the glacier-wide mass balance are the slope of the glacier tongue and the mean glacier elevation for most regions, with the notable exception of the inner Tibetan Plateau. Glacier-wide mass balances do not differ significantly between debris-free and debris-covered glaciers in 7 of the 12 regions analyzed. Lake-terminating glaciers have more negative mass balances than the regional averages, the influence of lakes being stronger on small glaciers than on large glaciers.
... The supraglacial hydrology of debris-covered glaciers has received increasing attention in recent years. For example, it is welldocumented that the formation of proglacial moraine-dammed lakes, and the consequent presence of a local base-level, can be facilitated by the development, growth, and coalescence of supraglacial ponds into a chain of linked ponds Mertes et al., 2016;Sakai, 2012;Thompson et al., 2012). Melt rates are disproportionately high at pond margins due to the continued horizontal and vertical incision (Miles et al., 2016;Sakai et al., 2000), and recent studies have found that supraglacial ponds are expanding to cover an increasing proportion of the surface of debris-covered glaciers (Gardelle et al., 2012;Watson et al., 2016). ...
Article
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While the supraglacial hydrology of debris-covered glaciers is relatively well studied, almost nothing is known about how water is transported beneath the glacier surface. Here, we report the results of sixteen fluorescent dye tracing experiments conducted in April–May 2018 over the lowermost 7 km of the high-elevation, debris-covered Khumbu Glacier, Nepal, to characterise the glacier's surface and subsurface drainage system. Dye breakthroughs indicated a likely highly sinuous and channelised subsurface hydrological system draining water from the upper part of the ablation area. This flowpath was distinct from the linked chain of supraglacial ponds present along much of the glacier's lower ablation area, through which water flow was extremely slow (∼0.003 m s ⁻¹ ), likely reflecting the study's timing during the pre-monsoon period. Subsurface drainage pathways emerged at the glacier surface close to the terminus, and flowed into small near-surface englacial reservoirs that typically delayed meltwater transit by several hours. We observed rapid pathway changes resulting from surface collapse, indicating a further distinctive aspect of the drainage of debris-covered glaciers. We conclude that the surface and subsurface drainage of Khumbu Glacier is both distinctive and dynamic, and argue that further investigation is needed to refine the characterisation and test its regional applicability to better understand future Himalayan debris-covered glacier meltwater delivery to downstream areas.
... Carrivick and Tweed, 2013;Cook and Quincey, 2015), and glacier thinning results in the development of supraglacial lakes, particularly on debris-covered glaciers (e.g. Benn et al., 2001;Thompson et al., 2012;Mertes et al., 2016). Consequently, there has been a general trend of increasing glacial lake number and size in many regions in recent times (e.g. ...
Article
Glacial Lake Outburst Floods (GLOFs) represent a significant threat in deglaciating environments, necessitating the development of GLOF hazard and risk assessment procedures. Here, we outline a Multi- Criteria Decision Analysis (MCDA) approach that can be used to rapidly identify potentially dangerous lakes in regions without existing tailored GLOF risk assessments, where a range of glacial lake types exist, and where field data are sparse or non-existent. Our MCDA model (1) is deskbased and uses freely and widely available data inputs and software, and (2) allows the relative risk posed by a range of glacial lake types to be assessed simultaneously within any region. A review of the factors that influence GLOF risk, combined with the strict rules of criteria selection inherent to MCDA, has allowed us to identify 13 exhaustive, nonredundant, and consistent risk criteria. We use our MCDA model to assess the risk of 16 extant glacial lakes and 6 lakes that have already generated GLOFs, and found that our results agree well with previous studies. For the first time in GLOF risk assessment, we employed sensitivity analyses to test the strength of our model results and assumptions, and to identify lakes that are sensitive to the criteria and risk thresholds used. A key benefit of the MCDA method is that sensitivity analyses are readily undertaken. Overall, these sensitivity analyses lend support to our model, although we suggest that further work is required to determine the relative importance of assessment criteria, and the thresholds that determine the level of risk for each criterion. As a case study, the tested method was then applied to 25 potentially dangerous lakes in the Bolivian Andes, where GLOF risk is poorly understood; 3 lakes are found to pose 'medium' or 'high' risk, and require further detailed investigation.
... Still, supraglacial water storage is increasing for many Himalayan glaciers (e.g. Thompson et al., 2012;Watson et al., 2016). This is expected as climate warms and debris-covered glaciers stagnate, which are precursors to proglacial lake formation . ...
Article
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A set of supraglacial ponds filled rapidly between April and July 2017 on Changri Shar Glacier in the Everest region of Nepal, coalescing into a ∼180 000 m2 lake before sudden and complete drainage through Changri Shar and Khumbu glaciers (15–17 July). We use PlanetScope and Pléiades satellite orthoimagery to document the system's evolution over its very short filling period and to assess the glacial and proglacial effects of the outburst flood. We also use high-resolution stereo digital elevation models (DEMs) to complete a detailed analysis of the event's glacial and geomorphic effects. Finally, we use discharge records at a stream gauge 4 km downstream to refine our interpretation of the chronology and magnitude of the outburst. We infer largely subsurface drainage through both of the glaciers located on its flow path, and efficient drainage through the lower portion of Khumbu Glacier. The drainage and subsequent outburst of 1.36±0.19×106 m3 of impounded water had a clear geomorphic impact on glacial and proglacial topography, including deep incision and landsliding along the Changri Nup proglacial stream, the collapse of shallow englacial conduits near the Khumbu terminus and extensive, enhanced bank erosion at least as far as 11 km downstream below Khumbu Glacier. These sudden changes destroyed major trails in three locations, demonstrating the potential hazard that short-lived, relatively small glacial lakes pose.
... The southernmost 6.5 km of the glacier is nearly stagnant (Quincey et al., 2009) and has a low surface slope of ∼ 4 • . The terrain of this glacier, its wasting processes and the evolution of surface lakes have been well studied through a series of previous publications (Benn et al., 2000(Benn et al., , 2001Thompson et al., 2012Thompson et al., , 2016, as have the debris properties including limited measurements of debris thickness (Nicholson and Benn, 2012). ...
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Shallow ground-penetrating radar (GPR) surveys are used to characterize the small-scale spatial variability of supraglacial debris thickness on a Himalayan glacier. Debris thickness varies widely over short spatial scales. Comparison across sites and glaciers suggests that the skewness and kurtosis of the debris thickness frequency distribution decrease with increasing mean debris thickness, and we hypothesize that this is related to the degree of gravitational reworking the debris cover has undergone and is therefore a proxy for the maturity of surface debris covers. In the cases tested here, using a single mean debris thickness value instead of accounting for the observed small-scale debris thickness variability underestimates modelled midsummer sub-debris ablation rates by 11%–30%. While no simple relationship is found between measured debris thickness and morphometric terrain parameters, analysis of the GPR data in conjunction with high-resolution terrain models provides some insight into the processes of debris gravitational reworking. Periodic sliding failure of the debris, rather than progressive mass diffusion, appears to be the main process redistributing supraglacial debris. The incidence of sliding is controlled by slope, aspect, upstream catchment area and debris thickness via their impacts on predisposition to slope failure and meltwater availability at the debris–ice interface. Slope stability modelling suggests that the percentage of the debris-covered glacier surface area subject to debris instability can be considerable at glacier scale, indicating that up to 32% of the debris-covered area is susceptible to developing ablation hotspots associated with patches of thinner debris.
... Additionally, compared with land-terminating glaciers, lake-terminating glaciers shrink faster in Sikkim (Basnett et al., 2013), have more negative rates of elevation changes in Bhutan, Everest region and West Nepal and have more negative mass balances in the Everest region . Terminal lakes develop when supraglacial ponds coalesce into a single lake dammed by a terminal moraine (e.g., Benn et al., 2012;Thompson et al., 2012). Eventually, the terminal lakes enhance the frontal ablation by dynamic thinning, calving and thermo-erosion (e.g., Benn et al., 2012). ...
Thesis
High Mountain Asia (HMA) hosts the largest glacierized area outside the polar regions. Approximately 15 % of the ~100 000 km² of HMA glaciers is covered by a debris layer of various thickness. The influence of this debris on the HMA glacier response to climate change remains debated. In principle, the presence of a thick layer of debris reduces the melt of the ice beneath it, due to the insulating effect. However, other processes such as ablation of bare ice cliff faces, subaqueous melt of supraglacial ponds and internal ablation due to englacial hydrology could substantially contribute to enhance the debris-covered glacier mass losses. The aim of this PhD work is to assess the impact of the debris on glacier mass balance in HMA. Up to now, the influence of the debris cover has been assessed through glacier front position changes or on a restricted sample of glaciers, and no large scale study of the influence of the debris cover on the glacier-wide mass balance is available.As a starting point, we derived glacier mass changes for the period 2000-2016 for the entire HMA, with an unprecedented resolution, using time series of digital elevation models (DEMs) derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) optical satellite imagery. We calculated a total mass loss of -16.3 ± 3.5 Gt yr-1 (-0.18 ± 0.04 m w.e. yr-1) with contrasted rates of regionally-averaged mass changes ranging from -0.62 ± 0.23 m w.e. yr-1 for the eastern Nyainqentanglha to +0.14 ± 0.08 m w.e. yr-1 for the western Kunlun Shan.At the scale of HMA, the pattern of glacier mass changes is not related to the presence of debris, but is linked with the climatology. Consequently, we studied the influence of the debris-cover on mass balance within climatically homogeneous regions. Based on the mass balances of individual glaciers larger than 2 km² (more than 6 500 glaciers, which represent 54% of the total glacierized area), we found that debris-covered glaciers have significantly more negative mass balances for four regions out of twelve, a significantly less negative mass balance for one region and non-significantly different mass balances for the remaining seven regions. The debris-cover is generally a less significant predictor of the mass balance than the slope of the glacier tongue or the glacier mean elevation. The influence of the debris is not completely clear and complicated to untangle from the effect of the other morphological parameters, because heavily debris-covered tongues are situated at lower elevations than debris-free tongues, where ablation is higher.However, such a statistical analysis of the influence of the debris-cover on the glacier-wide mass balance variability is not very informative in terms of glaciological processes. In order to better constrain the contribution of the different ablation processes on debris-covered tongues, work at a finer scale is required. For the debris-covered tongue of Changri Nup Glacier, Everest region, Nepal, we quantified the contribution of ice cliffs to the ablation budget. Using a combination of very high resolution DEMs derived from Pléiades images and an unmanned aerial vehicle, we found that ice cliffs contributed to ~23 ± 5 % of the total net ablation of the tongue, over two contrasted years, although they occupy only 7 to 8 % of its area. Ice cliffs are large contributors to the ablation of a debris-covered tongue, but they cannot alone explain the so-called debris cover anomaly, i.e. the fact that debris free and debris covered tongues have similar thinning rates. This anomaly is probably due to smaller emergence velocity over debris-covered tongues than over debris-free tongues, resulting in similar thinning rates, despite less negative surface mass balance rates. We advocate for more measurements of ice thickness of debris-covered tongues in order to better understand their dynamics.
... Still, supraglacial water storage is increasing for many Himalayan glaciers (e.g. Thompson et al., 2012;Watson et al., 2016). This is expected as climate warms and debris-covered glaciers stagnate, precursors to proglacial lake formation . ...
Article
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A set of supraglacial ponds rapidly filled between April and July 2017 on Changri Shar Glacier in the Everest region of Nepal, coalescing into a ~180,000m² lake before sudden and complete drainage through Changri Shar and Khumbu Glaciers 15–17 July. We use a suite of PlanetScope and Pléiades satellite orthoimagery to document the system's evolution over its very short filling period and to assess the glacial and proglacial effects of the outburst flood. We additionally use high resolution stereo digital elevation models (DEMs) to complete a detailed analysis of the event's ablative and geomorphic effects. Finally, measurement of the flood’s passage at a stream gauge 4km downstream enables a refined interpretation of the chronology and overall magnitude of the outburst. We infer largely subsurface drainage through both glaciers located on its flowpath, and efficent drainage through Khumbu Glacier. The drainage and subsequent outburst of 1.36&pm;0.19x10⁶m³ impounded water had a clear geomorphic impact on glacial and proglacial topography at least as far as 11km downstream, including deep incision and landsliding along the Changri Nup proglacial stream, the collapse of shallow englacial conduits near the Khumbu terminus and extensive, enhanced bank erosion below Khumbu Glacier. These sudden changes led to the rerouting of major trails in three locations, demonstrating the potential hazard that short-lived, relatively small glacial lakes pose.
... Normally, two types of glacial lakes are recognized such as (1), proglacial lakes and (2), supraglacial lakes. The proglacial lakes often grow downstream of steep glaciers, where water is collected behind former moraines (Richardson and Reynolds 2000;Komori 2008) due to the recession of the glacier terminus Westoby et al. 2014) whereas, the supraglacial lakes develop on the surface of glacier itself and grow by coalescence of small ponds (Richardson and Reynolds 2000;Benn et al. 2012; Thompson et al. 2012). These glacial lakes have the capacity to impound large volumes of meltwater (e.g. in excess of 106-107 m 3 ) following the continuous lake expansion Janský et al. 2009;Somos-Valenzuela et al. 2014). ...
Article
Using high resolution Google EarthTM images in conjunction with Landsat images, the glaciers and lakes in the Baspa basin are classified to explore the recent changes. A total number of 109 glaciers (187 ± 3.7 km2) are mapped and subsequently classified as compound valley glaciers, simple valley glaciers, cirques, niches, glacieretes and ice aprons. The compound and simple valley glaciers contribute 67.1 ± 1.3% and 19.8 ± 0.3% to the total glacier cover of the basin. Similarly, a total number of 129 glacial lakes (0.360 ± 0.007 km2) are identified. From 1976 to 2011, the compound valley glaciers have lost a small area of 10.3 ± 0.03% at a rate of 0.41 ± 0.002 km2 a-1 whereas the niche glaciers have lost higher area of 40.1 ± 0.001% at a rate of 0.04 ± 0.0001 km2 a-1. Change detection of two benchmark glacial lakes revealed a progressive expansion during recent decades. The Baspa Bamak proglacial lake has expanded from 0.020 ± 0.0004 km2 (2000) to 0.069 ± 0.001 km2 (2011). Due to the complete loss of source ice, another glacial lake has expanded from 0.09 ± 0.001 km2 (1994) to 0.10 ± 0.002 km2 (2011). During the study period, the mean annual temperature that is Tavg, Tmin and Tmax have increased significantly at the 95% confidence level by 1.5 oC (0.070 °C a-1), 1.8 oC (0.076 °C a-1) and 1.6 oC (0.071 °C a-1) from 1985 to 2008. However, the precipitation has decreased significantly from 1976 and 1985 to 2008.
... a series of previous publications ( Benn et al., 2000Benn et al., & 2001Thompson et al., 2012Thompson et al., & 2016, as 115 ...
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Shallow ground penetrating radar (GPR) surveys are used to characterize the small-scale spatial variability of supraglacial debris thickness on a Himalayan glacier. Debris thickness varies widely over short spatial scales. Comparison across sites and glaciers suggests that the skewness and kurtosis of the debris thickness frequency distribution decrease with increasing mean debris thickness, and we hypothesise that this is related to the degree of gravitational reworking the debris cover has undergone, and is therefore a proxy for the maturity of surface debris covers. In the cases tested here, using a single mean debris thickness value instead of accounting for the observed small-scale debris thickness variability underestimates modelled midsummer sub-debris ablation rates by 11–30 %. While no simple relationship is found between measured debris thickness and morphometric terrain parameters, analysis of the GPR data in conjunction with high-resolution terrain models provides some insight to the processes of debris gravitational reworking. Periodic sliding failure of the debris, rather than progressive mass diffusion, appears to be the main process redistributing supraglacial debris. The incidence of sliding is controlled by slope, aspect, upstream catchment area and debris thickness via their impacts on predisposition to slope failure and meltwater availability at the debris-ice interface. Slope stability modelling suggests that the percentage of the debris-covered glacier surface area subject to debris instability can be considerable at glacier scale, indicating that up to 22 % of the debris covered area is susceptible to developing ablation hotspots associated with patches of thinner debris.
... Ngozumpa Glacier also has a complex englacial drainage system, which effects the evacuation of meltwater and spatial patterns of surface mass loss (Benn et al., 2017). The moraine-dammed lake at its terminus, commonly referred to as Spillway Lake, experienced rapid expansion after 2001 (Thompson et al., 2012), but has recently experienced a reduction in lake area that is thought to be temporary, as a result of local sediment redistribution on the surrounding slopes ( Thompson et al., 2016). Thakuri et al. (2014) reported a surface area loss of À1. 4% from 19624% from to 20114% from , and King et al. (2017 reported substantial surface lowering in the high-altitude clean-ice areas of the glacier from 2000 to .812°E, Figure 1) originates in the Western Cwm below Mount Everest (8,848 m a.s.l.) and terminates at 4,876 m a.s.l. ...
Article
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Debris-covered glaciers are ubiquitous in the Himalaya, and supraglacial debris significantly alters how glaciers respond to climate forcing. Estimating debris thickness at the glacier scale, however, remains a challenge. This study inverts a subdebris melt model to estimate debris thickness for three glaciers in the Everest region from digital elevation model difference-derived elevation change. Flux divergences are estimated from ice thickness and surface velocity data. Monte Carlo simulations are used to incorporate the uncertainties associated with debris properties, flux divergence, and elevation change. On Ngozumpa Glacier, surface lowering data from 2010 to 2012 and 2012 to 2014 are used to calibrate and validate the method, respectively. The debris thickness estimates are consistent with existing in situ measurements. The method performs well over both actively flowing and stagnant parts of the glacier and is able to accurately estimate thicker debris (>0.5 m). Uncertainties associated with the thermal conductivity and elevation change contribute the most to uncertainties of the debris thickness estimates. The surface lowering associated with ice cliffs and supraglacial ponds was found to significantly reduce debris thickness, especially for thicker debris. The method is also applied to Khumbu and Imja-Lhotse Shar Glaciers to highlight its potential for regional application.
... Surface lowering causes these debris-covered glaciers to develop very low or even reversed long-profile topographical gradients through their ablation areas, which promotes the formation of supraglacial water bodies. These ponds and lakes influence the seasonal transport of water through the glacial system, and can expand and coalesce to form substantial supraglacial or proglacial, moraine-dammed lakes that may eventually pose a potential flood hazard ( Thompson et al. 2012;Watson et al. 2016). Such features are commonly bordered by steep, debris-free ice cliffs, which progressively backwaste and, if connected to a supraglacial pond or lake, may undergo thermoerosional notch development at the ice-water interface, promoting the onset of calving processes ( Fig. 2) ( Hambrey et al. 2008;Thompson et al. 2016). ...
Conference Paper
High Mountain Asia contains the largest volume of glacier ice outside the Polar regions, and contain the headwaters of some of the largest rivers in central Asia. These glaciers are losing mass at a mean rate of between –0.18 m and –0.5 m water equivalent per year. While glaciers in the Himalaya are generally shrinking, those in the Karakoram have experienced a slight mass gain. Both changes have occurred in response to rising air temperatures due to Northern Hemisphere climatic change. In the Westerly influenced Indus catchment, glacier meltwater makes up a large proportion of the hydrological budget, and loss of glacier mass will ultimately lead to a decrease in water supplies. In the monsoon-influenced Ganges and Brahmaputra catchments, the contribution of glacial meltwater is relatively small compared to the Indus, and the decrease in annual water supplies will be less dramatic. Therefore, enhanced glacier melt will increase river flows until the middle of the 21st 28 Century, but in the longer-term into the latter part of this century, river flows will decline as glaciers shrink. Declining meltwater supplies may be compensated by increases in precipitation, but this could exacerbate the risk of flooding.
... With the increase in air temperature and changing precipitation, mountain glaciers have been widely retreating, thinning, and even disappearing in recent decades; this facilitates the formation and rapid development of glacial lakes [1][2][3][4]. The continued increase in size of glacial lakes may result in the sudden breaching of lake dams and can produce catastrophic glacial lake outburst floods (GLOFs), causing severe damage to downstream settlements as well as other structures and the natural environment worldwide [5,6]. In particular, some GLOFs in transboundary basins exert significant impacts on bordering countries. ...
Article
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Due to recent global climate change, glacial lake outburst floods (GLOFs) have become a serious problem in many high mountain areas. Accurately and rapidly mapping glacial lakes is the basis of other glacial lake studies that are associated with water resources management, flood hazard assessment, and climate change. Most glacial lake detection studies have mainly used medium to coarse resolution images, whose application is limited to large lakes. Because small glacial lakes are abundant and because changes in these lakes are small and occur around the lake shores, fine-resolution satellite imagery is required for adequate assessments. In addition, the existing detection methods are mainly based on simply applying a threshold on various normalized difference water indices (NDWIs); this cannot give appropriate results for glacial lakes that have a wide range of turbidity, mineral, and chlorophyll content. In the present study, we propose a region-dependent framework to overcome the spectral heterogeneity of glacial lake areas using a nonlocal active contour model that is integrated with the NDWI. As the first trial, the glacial lakes were detected using high-resolution GaoFen-2 multispectral imagery in the test site of Altai Mountains (northern Xinjiang Province). The validation of the results was carried out using the manually digitized lake boundaries. The average probabilities of false positives PFP and false negatives PFN were found to be 0.0106 and 0.0039, respectively. After taking into consideration the spectral features of the water and making slight NDWI threshold adjustments, this method can also be used for lake detection in any glaciated environment elsewhere in the world.
... Figure 7.4 shows an image of glacial lake on the moraine of the Ngozumpa glacier in the Everest region of Nepal taken a day apart on May 28, 2013 andMay 29, 2013, showing the level of drainage that occurred within a day. Recent studies have shown that sustained melting and ice loss in the decades to come owing to increasing temperature across high-altitude regions such as the Everest ( Shea et al. 2014) will only accelerate the rapid formation of such potentially hazardous lakes ( Thompson et al. 2012). ...
... Glacier retreat over recent decades has led to the formation of new ice-dammed lakes in many regions (IPCC 2012 and e.g. Schomacker 2010;Wang et al. 2011;Thompson et al. 2012). Many of these have drained suddenly producing debrisflows and floods extending 10s to 100s of kilometres downstream of source (e.g.Clague & Evans 2000;Richardson & Reynolds 2000;Kershaw et al. 2005;Russell et al. 2006). ...
Article
A prominent thrust-moraine system formed in the inner van Mijenfjorden, Svalbard, during a surge event in a tributary fjord, creating a large temporary lake. Based on geomorphological, sedimentological, stratigraphical and chronological data, the lake began to form shortly after 648–551 cal. a BP. At its maximum, the lake covered an estimated area of 77 km2 with a water volume of 1.2 km3. Lake sediment up to 80 cm thick was rapidly deposited on top of terrestrial and marine sediments. At its maximum extent, the short-lived lake was the largest of any known Holocene lake in Svalbard. Modern river discharge would fill the lake to its highest shoreline at 23 m a.s.l. in only one season. Drainage was stepwise, as evidenced by four shorelines and abandoned drainage channels. This study has taken advantage of a unique suite of data available for such an ice-dammed lake. The results demonstrate the power of a multidisciplinary approach for recognizing lake events in the geological record, which is essential given the low preservation potential of such sediments. http://onlinelibrary.wiley.com/doi/10.1111/bor.12302/full
... Processes of glacier melt and the formation of glacier lakes is controlled by a multitude of factors including local energy balance, topography, geology, aspect, valley dimensions, moraine stability, debris thickness and the presence of surface features such as melt ponds and ice cliffs Buri et al., 2016;Harrison et al., 2006;Huggel et al., 2002;Immerzeel et al., 2014;Lejeune et al., 2013;Portocarrero, 2014;Reid et al., 2012;Reid and Brock, 2010;Thompson et al., 2012). The impact of these variables has been studied extensively in other areas, particularly the Himalaya, where large debris-covered glacier tongues are common Buri et al., 2016;Immerzeel et al., 2014;Scherler et al., 2011;Shroder et al., 2000). ...
Article
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The glaciers of the Cordillera Blanca, Peru, are rapidly retreating and thinning as a result of climate change, altering the timing, quantity and quality of water available to downstream users. Furthermore, increases in the number and size of proglacial lakes associated with these melting glaciers is increasing potential exposure to glacier lake outburst floods (GLOFs). Understanding how these glaciers are changing and their connection to proglacial lake systems is thus of critical importance. Most satellite data are too coarse for studying small mountain glaciers and are often affected by cloud cover, while traditional airborne photogrammetry and lidar are costly. Recent developments have made unmanned aerial vehicles (UAVs) a viable and potentially transformative method for studying glacier change at high spatial resolution, on demand and at relatively low cost. Using a custom designed hexacopter built for high-altitude (4000–6000 m a. s. l. ) operation, we completed repeat aerial surveys (2014 and 2015) of the debris-covered Llaca Glacier tongue and proglacial lake system. High-resolution orthomosaics (5 cm) and digital elevation models (DEMs) (10 cm) were produced and their accuracy assessed. Analysis of these datasets reveals highly heterogeneous patterns of glacier change. The most rapid areas of ice loss were associated with exposed ice cliffs and meltwater ponds on the glacier surface. Considerable subsidence and low surface velocities were also measured on the sediments within the pro-glacial lake, indicating the presence of extensive regions of buried ice and continued connection to the glacier tongue. Only limited horizontal retreat of the glacier tongue was observed, indicating that measurements of changes in aerial extent alone are inadequate for monitoring changes in glacier ice quantity.
... Comparisons of the NZ LGM lakes can also be made with the massive, debris-covered glaciers in the Everest region, central Himalaya, where very large lakes are forming as supraglacial ponds coalesce. Glacial lake development across the central Himalaya is similar in process to that identified in New Zealand, where ice surface lowering and increasing glacier stagnation has been highlighted to promote supraglacial pond formation and their potential to coalesce into larger lakes where a low glacier surface gradient exists (Watanabe et al., 1994;Richardson and Reynolds, 2000;Quincey et al., 2007;R€ ohl, 2008;Thompson et al., 2012;King et al., 2017King et al., , 2018. The evolutionary trajectory of these large proglacial lakes can be attributed to the development of supraglacial ponds (Kirkbride, 1993;Watanabe et al., 1994;R€ ohl, 2008;Sakai, 2012;Carrivick and Tweed, 2013;Watson et al., 2018). ...
Article
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Proglacial lakes can affect the stability of mountain glaciers and can partly disengage glacier behaviour from climatic perturbations. However, their role in controlling the onset and progression of deglaciation from the Last Glacial Maximum (LGM) remains poorly understood. This lack of understanding is partly because the evidence required to consistently and robustly identify the location and evolution of ice-contact lakes is not standardised. In this paper we therefore firstly present a new set of criteria for identifying the landform and sedimentary evidence that defines and characterises ice-marginal lakes. Secondly, we then apply these key criteria with the aid of high-resolution topographic mapping to produce the first holistic definition and assessment of major proglacial lake landforms and sediments pertaining to the end of the LGM across South Island, New Zealand. The major findings of this assessment can be grouped to include that: (i) The localised constraints to proglacial lake extent were topography, glacier size and meltwater/sediment fluxes, (ii) Lake damming was initiated by outwash fan-heads that interrupted water and sediment flows down-valley, and (iii) New Zealand LGM lakes were unequivocally in contact with a calving ice margin. These findings will be useful for reconstructing ice dynamics and landscape evolution in this region.
... Transmission of thermal energy from fetch of glacial lakes connected to glaciers causes submerged ice melt resulting calving of glacier terminus, which provides space for upward pro-glacial lake expansion Sakai et al., 2009). Sub-aerial melting, water line melting, and ice calving are the drivers for the expansion of pro-glacial or supraglacial lakes in the Himalayas (Mertes et al., 2017;Thompson et al., 2012). The expansion of glacial lakes not directly connected with glaciers depends on water from glacier melt, ice melt, or precipitation (Salerno et al., 2016). ...
Article
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The retreat and shrinkage of glaciers due to climate change are the causes for the formation and expansion of glacial lakes in the Himalayas. This study presents the rapidly expanding glacial lakes in Nepal Himalayas between 1988 and 2018 based on the published glacial lake inventories produced from Landsat imageries (30 m). Glacier-fed end moraine-dammed glacial lakes whose surface area was ≥0.1 km 2 in 2018 with an expansion rate of more than 30% in 1988-2018 were regarded as rapidly expanding glacial lakes. The results show that 19 rapidly expanding glacial lakes are heterogeneously distributed in different sub-basins of Nepal. Among the sub-basins, Dudh Koshi sub-basin has a maximum (5) number of rapidly expanding glacial lakes. The total surface area of these 19 glacial lakes expanded by ~133%, from 4.12±0.61 km 2 in 1988 to 9.62±1.04 km 2 in 2018. Regular monitoring of rapidly expanding glacial lakes is required because the rapid expansion heightens the risk of Glacial Lake Outburst Flood (GLOF) by developing more potential flood volume and the expanding lakes can reach sites of possible avalanches.
... 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). Increasing trends of pond development of 17-52% a −1 have been reported for the last few decades, for example in the Khumbu region of Nepal Himalaya (Watson et al., 2016;Chand and Watanabe, 2019). ...
Article
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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.
... It is highly likely that the lake drain might have resulted from a piping failure in the ice-cored moraine, given the scars left by the outburst floodwaters in the dammed moraine. Whereas the field-based assessment indicated a piping failure as the likely cause of the 2014 GLOF impacting Gya Village, the possibility of future GLOFs resulting from impact waves triggered by the calving of the glacier snout Sakai et al., 2009;Thompson et al., 2012) and mass wasting (Allen et al., 2016a;Ashraf et al., 2012) or a combination of both can not be ruled out. Downstream, the flood completely damaged two houses and partially 6 other buildings. ...
Article
This study assessed spatiotemporal changes at Gya Glacier, the associated development of a proglacial lake, and reconstructed the 2014 outburst flood that struck Gya Village in the Trans-Himalayan region of Ladakh, India. This study analyzed and for the first time successfully reconstructed a Glacial Lake Outburst Flood (GLOF) event in the Trans-Himalayan region of Ladakh. Glacier and glacial lakes changes were quantified using remote sensing data supplemented with field observations. Glacier ice-thickness and glacier-bed overdeepenings were modeled using a shear-stress based model, GlabTop (Glacier-bed Topography). The reconstruction of the 2014 GLOF and the potential hazard assessment of Gya Lake were carried out using the hydrodynamic model HEC-RAS; results were validated against ground-collected data. Temporal evaluation of satellite data revealed a 45.6% loss in the total glacier area between 1969 and 2019. The earliest snow-free image available for the region shows that a proglacial lake existed as early as 1969 with an area of 3.06 ha. The lake has expanded to ~11 ha in 2019. Results from the GlabTop model suggest that the lake could grow further up to 12 ha in the future. Field-based geomorphic indicators suggest that the 2014 GLOF event resulted from a piping failure of the frontal moraine destroying numerous agricultural fields, some buildings, downstream infrastructure, and eroded natural channel embankments. The reconstruction of the event revealed that 25% of the lake waters drained out with a peak discharge of 470 m3s-1, inundating an area of ~4 km2 around Gya Village. However, a complete breaching of the terminal moraine could result in an event that would be 5.5 times larger than the 2014 GLOF. Therefore, this study could be useful not only in planning disaster-resilient infrastructure around proglacial lake environments in the cold-arid Ladakh but also in framing mitigation plans to reduce risk for vulnerable downstream communities.
... Since the 1990s, the retreat rate of glaciers in different mountain ranges has accelerated significantly [79], and during the same period, researchers have found that the number and area of glacial lakes have increased rapidly [5,9,43,80]. For example, related studies have pointed out that the area of the Ngozumpa glacial lakes in the Himalayan in Nepal had been expanding at a rate of 10% per year since 2001 and reached a total area of 0.3 km 2 in 2009; these researchers stated that the main reason for this rapid expansion of glacial lakes was the rapid melting and retreat of glaciers and the break-up of glacial ice tongues [81]. In 2010, glacial lakes directly supplied by modern glacial meltwater in the high-mountain Asia region accounted for 78% of all glacial lakes and 83% of the total area of all glacial lakes [43,82]. ...
Article
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The Himalayan, Karakoram, and Hindu Kush (HKH-TMHA) are the three main mountain ranges in the high-mountain Asia region, covering the China–Pakistan Economic Corridor (CPEC). In this study, we identified glacial lakes in the HKH-TMHA region based on multitemporal Landsat images taken from 1990 to 2020. We analyzed the spatial distribution and evolution of glacial lakes in the HKH-TMHA region from the perspective of their elevation, size, and terrain aspect; then, we described their temporal changes. The results showed that approximately 84.56% of the glacial lakes (84.1% of the total lake area) were located at elevations between 4000 m and 5500 m, and glacial lakes with areas ranging from 0.01–0.5 km2 accounted for approximately 95.21% of the number and 63.01% of the total area of glacial lakes. The number (38.64%) and area (58.83%) of south-facing glacial lakes were largest in HKH-TMHA and expanded significantly over time. There were 5835 (664.84 ± 89.72 km2) glacial lakes in 1990; from 1990 to 2020, the number of glacial lakes in the HKH-TMHA region increased by 5974 (408.93 km2) in total; and the annual average increase in the area of glacial lakes reached 13.63 km2 (11.15%). In 2020, the total number of glacial lake reached to 9673 (899.66 ± 120.63 km2). In addition, most glacial lakes were located in the Eastern Himalayan, China, and the Indus Basin. Based on the precipitation and temperature analyses performed in our study area, we found inconsistent climate characteristics and changes in the three mountain ranges. In general, the daily precipitation (temperature) increased by 1.0766 mm (1.0311 °C), 0.8544 mm (0.8346 °C), and 0.8245 mm (−0.1042 °C) on the yearly, summer, and winter scales, respectively. Glacial melting and climate change are common contributors to glacial lake expansion. The investigation of glacial lakes in this region can provide basic supporting data for research on glacial lake-related disasters, land cover, and climate change in the high-mountain Asia region.
... It is quite evident from the comparative study of different satellite images from 1962,1975,1988,2000,2010, and 2018 that a higher magnitude of deglaciation has resulted in faster expansion of GLC over the past decades. Several other Himalayan proglacial morainedammed lakes have been expanding similarly due to associated glacier retreats [65][66][67]. ...
Article
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The Sikkim Himalayan glaciers and glacial lakes are affected by climate change like other parts of the Himalayas. As a result of this climate variability in the Sikkim Himalaya, a detailed study of the Gurudongmar lake complex (GLC) evolution and outburst susceptibility assessment is required. Glacial lake volume estimation and lake outburst susceptibility assessment were carried out to reveal different characteristics for all four lakes (GL-1, GL-2, GL-3, and GL-4) from the lake complex. Each of these lakes has a moderate to very high potential to outburst. As the dam of GL-1 provides no retention capacity, there is a very high potential of a combined effect with the sudden failure of the moraine-dams of GL-2 or GL-3 located upstream. Temporal analysis of GLC using optical remote sensing data and in-field investigations revealed a rapidly increasing total lake area by ~74 ± 3%, with an expansion rate of +0.03 ± 0.002 km2 a−1 between 1962 and 2018 due to climate change and ongoing glacier retreat. The overall lake area expansion rates are dependent on climate-driven factors, and constantly increasing average air temperature is responsible for the enlargement of the lake areas. Simultaneously, changes in GLC expansion velocity are driven by changes in the total amount of precipitation. The deficit in precipitation probably triggered the initial higher rate from 1962 to 1988 during the winter and spring seasons. The post-1990s positive anomaly in precipitation might have reduced the rate of the glacial lake area expansion considerably.
... Spatially heterogeneous melt patterns caused by the development and evolution of supra-glacial lakes and other surface features result in substantial mass losses over time [36]. The determination of these features is the basis for the monitoring of glacier changes [37]. ...
Article
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Glaciers and numerous glacial lakes that are produced by glacier melting are key indicators of climate change. Often overlooked, supra-glacial lakes develop in the melting area in the low-lying part of a glacier and appear to be highly variable in their size, shape, and location. The lifespan of these lakes is thought to be quite transient, since the lakes may be completely filled by water and burst out within several weeks. Changes in supra-glacial lake outlines and other surface features such as supra-glacial rivers and crevasses on the glaciers are useful indicators for the direct monitoring of glacier changes. Synthetic aperture radar (SAR) is not affected by weather and climate, and is an effective tool for study of glaciated areas. The development of the Chinese GaoFen-3 (GF-3) SAR, which has high spatial and temporal resolution and high-precision observation performance, has made it possible to obtain dynamic information about glaciers in more detail. In this paper, the classical Canny operator, the variational B-spline level-set method, and U-Net-based deep-learning model were applied and compared to extract glacial lake outlines and other surface features using different modes and Chinese GF-3 SAR imagery in the Mount Everest Region of the Himalayas. Particularly, the U-Net-based deep-learning method, which was independent of auxiliary data and had a high degree of automation, was used for the first time in this context. The experimental results showed that the U-Net-based deep-learning model worked best in the segmentation of supra-glacial lakes in terms of accuracy (Precision = 98.45% and Recall = 95.82%) and segmentation efficiency, and was good at detecting small, elongated, and ice-covered supra-glacial lakes. We also found that it was useful for accurately identifying the location of supra-glacial streams and ice crevasses on glaciers, and quantifying their width. Finally, based on the time series of the mapping results, the spatial characteristics and temporal evolution of these features over the glaciers were comprehensively analyzed. Overall, this study presents a novel approach to improve the detection accuracy of glacier elements that could be leveraged for dynamic monitoring in future research.
... These parameters are available mostly for the Himalaya, Karakorum, (e.g. Han et al., 2010;Thompson et al., 2012;Brun et al., 2018;Nicholson et al., 2018;Rounce et al., 2018;Buri et al., 2021) and to a lesser degree the European Alps (Reid and Brock, 2014;Mölg et al., 2019), but are lacking for other mountain regions where debris-covered glaciers are common (Röhl, 2006). In the Andes of Argentina and Chile, specifics about the evolution of debris layers, and their influence in glacier surface morphology, overall dynamics and mass changes, is particularly limited despite the abundance of debris-covered ice in some sectors such as the Central Andes (see e.g. ...
Article
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A number of glaciological observations on debris-covered glaciers around the globe have shown a delayed length and mass adjustment in relation to climate variability, a behavior normally attributed to the ice insulation effect of thick debris layers. Dynamic interactions between debris cover, geometry and surface topography of debris-covered glaciers can nevertheless govern glacier velocities and mass changes over time, with many glaciers exhibiting high thinning rates in spite of thick debris cover. Such interactions are progressively being incorporated into glacier evolution research. In this paper we reconstruct changes in debris-covered area, surface velocities and surface features of three glaciers in the Patagonian Andes over the 1958–2020 period, based on satellite and aerial imagery and Digital Elevation Models. Our results show that debris cover has increased from 40 ± 0.6 to 50 ± 6.7% of the total glacier area since 1958, whilst glacier slope has slightly decreased. The gently sloping tongues have allowed surface flow velocities to remain relatively low (<60 m a ⁻¹ ) for the last two decades, preventing evacuation of surface debris, and contributing to the formation and rise of the ice cliff zone upper boundary. In addition, mapping of end of summer snowline altitudes for the last two decades suggests an increase in the Equilibrium Line Altitudes, which promotes earlier melt out of englacial debris and further increases debris-covered ice area. The strongly negative mass budget of the three investigated glaciers throughout the study period, together with the increases in debris cover extent and ice cliff zones up-glacier, and the low velocities, shows a strong linkage between debris cover, mass balance evolution, surface velocities and topography. Interestingly, the presence of thicker debris layers on the lowermost portions of the glaciers has not lowered thinning rates in these ice areas, indicating that the mass budget is mainly driven by climate variability and calving processes, to which the influence of enhanced thinning at ice cliff location can be added.
... Although all the lakes were manually checked and edited, due to the limitation of available images and other factors, the conditions for glacial-lake mapping were not perfectly consistent for each year. For example, the image dates were not consistent across the whole HMA region because of atmospheric disturbances, and there were also influences from varying lake characteristics, image quality (Bhardwaj et al., 2015;Thompson et al., 2012), ice, and shadows that obscured the lakes, which all contributed to detection errors in the lake extent and their annual variation. Generally, these errors were objective and acceptable as a result of the nature of the limited remote-sensing data. ...
Article
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Atmospheric warming is intensifying glacier melting and glacial-lake development in High Mountain Asia (HMA), and this could increase glacial-lake outburst flood (GLOF) hazards and impact water resources and hydroelectric-power management. There is therefore a pressing need to obtain comprehensive knowledge of the distribution and area of glacial lakes and also to quantify the variability in their sizes and types at high resolution in HMA. In this work, we developed an HMA glacial-lake inventory (Hi-MAG) database to characterize the annual coverage of glacial lakes from 2008 to 2017 at 30 m resolution using Landsat satellite imagery. Our data show that glacial lakes exhibited a total area increase of 90.14 km2 in the period 2008–2017, a +6.90 % change relative to 2008 (1305.59±213.99 km2). The annual increases in the number and area of lakes were 306 and 12 km2, respectively, and the greatest increase in the number of lakes occurred at 5400 m elevation, which increased by 249. Proglacial-lake-dominated areas, such as the Nyainqêntanglha and central Himalaya, where more than half of the glacial-lake area (summed over a 1∘ × 1∘ grid) consisted of proglacial lakes, showed obvious lake-area expansion. Conversely, some regions of eastern Tibetan mountains and Hengduan Shan, where unconnected glacial lakes occupied over half of the total lake area in each grid, exhibited stability or a slight reduction in lake area. Our results demonstrate that proglacial lakes are a main contributor to recent lake evolution in HMA, accounting for 62.87 % (56.67 km2) of the total area increase. Proglacial lakes in the Himalaya ranges alone accounted for 36.27 % (32.70 km2) of the total area increase. Regional geographic variability in debris cover, together with trends in warming and precipitation over the past few decades, largely explains the current distribution of supraglacial- and proglacial-lake area across HMA. The Hi-MAG database is available at https://doi.org/10.5281/zenodo.4275164 (Chen et al., 2020), and it can be used for studies of the complex interactions between glaciers, climate and glacial lakes, studies of GLOFs, and water resources.
... When the lake outburst event happened, most subglacial water was drained, and subglacial drainage channels perhaps closed towards the end of the summer. This rapid evolution of the drainage system was caused by the boundary condition change, namely the sudden decrease in water pressure as the lake at the glacier terminus drained [41]. Water 2020, 12, x FOR PEER REVIEW 11 of 16 ...
Article
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Early recognition of glacial lake outburst floods (GLOFs) is required for timely and cost-effective remedial efforts to be implemented. Although the formation of ice-dammed lakes is known to begin as a pond or river that was blocked by ice from the glacier terminus, the relationship between glacier dynamics and lake development is not well understood. Using a time-series of Sentinel-1A synthetic aperture radar (SAR) data acquired just before and after the lake outburst event in 2018, information is presented on the dynamic characteristics of Kyagar Glacier and its ice-dammed lake. Glacier velocity data derived from interferometry show that the glacier tongue experienced an accelerated advance (maximum velocity of 20 cm/day) just one month before the lake outburst, and a decreased velocity (maximum of 13 cm/day) afterward. Interferometric and backscattering properties of this region provide valuable insight into the diverse glaciated environment. Furthermore, daily temperature and total precipitation data derived from the ECMWF re-analysis (ERA)Interim highlight the importance of the sustained high-temperature driving force, supporting empirical observations from previous studies. The spatial and temporal resolution offered by the Sentinel-1A data allows variations in the glacier surface motion and lake evolution to be detected, meaning that the interaction mechanism between the glacial lake and the associated glacier can be explored. Although the glacier surge provided the boundary conditions favorable for lake formation, the short-term high temperatures and precipitation caused the melting of ice dams and also a rapid increase in the amount of water stored, which accelerated the potential for a lake outburst.
... As we are currently in an interglacial phase, it is unlikely that projected warming will cause extensive postglacial lake formation on the order of that observed during the Deglacial and early Holocene; although, melt of the remaining Greenland and Antarctic ice caps could lead to lake formation in these areas over multimillennial timescales (Pattyn et al., 2018). With warming in the near future, recession of smaller ice caps and glaciers currently in progress will likely accelerate, hastening observed formation and expansion of moraine-dammed lakes in mountainous regions (Sakai 2012;Thompson et al., 2012). The frequency of outburst floods resulting from breach of these moraine dams is also expected to increase with future warming, posing a significant human threat (Harrison et al., 2018). ...
Article
The northern mid- to high-latitudes have the highest total number and area of lakes on Earth. Lake origins in these regions are diverse, but to a large extent coupled to glacial, permafrost, and peatland histories. The synthesis of 1207 northern lake initiation records presented here provides an analog for rapid landscape-level change in response to climate warming, and its subsequent attenuation by physical and biological feedback mechanisms. Our compilation reveals two peaks in northern lake formation, 13,200 and 10,400 years ago, both following rapid increases in North Atlantic air temperature. Placing our findings within the context of existing paleoenvironmental records, we suggest that solar insolation-driven changes in climate (temperature and water balance) that led to deglaciation and permafrost thaw likely contributed to high rates of northern lake formation during the last Deglacial period. However, further landscape development and stabilization dramatically reduced rates of lake formation beginning ∼10,000 years ago. This suggests that temperature alone may not control future lake development; rather, multiple factors must align to enable a landscape to respond with an increase in lake area. We propose that land surfaces strongly geared toward increased lake formation were highly conditioned by glaciation. Thus, it is unlikely that warming this century will cause lake formation as rapid or as widespread as that during the last Deglacial period.
... Evidence from sediment coring of proglacial Lake Gokyo demonstrates that mass movements into lakes have occurred, even at high elevations (4750 m) [32]. As glacial melt accelerates, the formation of supraglacial lakes on the glacier surface becomes a hazard [33] as these lakes are isolated from meltwater release points. If supraglacial lakes suddenly break their banks, they can release significant water flow and downstream flooding [19]. ...
Article
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In 2019, the National Geographic and Rolex Perpetual Planet Everest expedition successfully retrieved the greatest diversity of scientific data ever from the mountain. The confluence of geologic, hydrologic, chemical and microbial hazards emergent as climate change increases glacier melt is significant. We review the findings of increased opportunity for landslides, water pollution, human waste contamination and earthquake events. Further monitoring and policy are needed to ensure the safety of residents, future climbers, and trekkers in the Mt. Everest watershed.
... Past studies in the Everest region were used the coarse resolution satellite images, i.e., Landsat (30 m) (Shrestha et al. 2017;Khadka et al. 2018), Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2, 10 m) (Salerno et al. 2012) to map the glacial lakes. Additionally, studies were lake specific (Benn et al., 2000(Benn et al., , 2001Bolch et al., 2008b;Thompson et al., 2012). Recently, a study was carried out using 30-m Landsat images to understand the development of supraglacial lakes on the surface of debris-covered glaciers in the Everest region from 1989-2017 by Chand and Watanabe (2019), which showed the most rapid increase in area and persistent lake on the Ngozumpa Glacier. ...
Article
Several glacial lakes are developing and increasing in their size with the warming trend of temperature. Some of the glacial lakes are potential for glacial lake outburst floods (GLOFs) in the Himalaya region. This study presents the inventory of the glacial lakes of the Ngozompa Glacier basin, a source of Dudh Koshi River using 2-m resolution satellite imageries of 2016. An index-based approach with manual correction was applied to the WorldView and GeoEye images for mapping all the glacial lakes with size >20 m2. The inventory revealed the 1,022 glacial lakes with a total area of 2.67 ± 0.14 km2. The supraglacial lakes were found most frequently among all types of glacial lakes, and they accounted for 96% of the total number of the lakes, however, unconnected glacial lakes accounted for 71% of the total area of the glacial lakes. The distribution of the size of the lakes was not normal and dominated by the small size lakes. The number of lakes with a size of one pixel (900 m2) and four pixels (3,600 m2) of Landsat image accounted for 86%, 94%, respectively showed the usefulness of a high-resolution dataset to observe small features. Similarly, inventory exhibited the presence of relatively large glacial lakes at the terminus of the glacier, which indicates the potentiality for the development of large glacial lakes. This inventory presented the highly accurate map of the glacial lakes of the Ngozompa Glacier basin, which can be used as a reference for future study.
... Lake 632 expansion therefore enhances glacial mass loss and meltwater production while the lake is 633 underlain or dammed by ice(Carrivick and Tweed, 2013;Röhl, 2008). Once calving is triggered, it 634becomes the dominant method of subsequent lake growth(Röhl, 2008;Thompson et al., 2012). 635Calving into a proglacial lake progresses from notch development and roof collapse to large-scale, 636full-height slab calving enabled by the lake deepening to the glacier bed (Kirkbride and Warren, 637 1997; Thompson et al., 2012). ...
Article
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.
... Based on the final generated lake inventory data, we used the slope of linear regression of lake area (over the grid cell of 135 1°×1°) versus image date to qualify the yearly lake area changes during the study period. The approach to change analysis was predicted on using a Theil-Sen estimator, which chooses median slope among all the derived fitted lines to smooth the annual time series of data (Kumar, 1968;Song et al., 2018) Although the lake mapping was mainly conducted in the same period to ensure year-to-year consistency to the degree possible, the smoothing approach was still necessary because of the annual variation in the lake extent attributable to a variety of sources including adverse weather conditions, varying lake 140 characteristics and image quality (Bhardwaj et al., 2015;Thompson et al., 2012). As such, conventional linear trends for the annual layers to estimate year-to-year changes is not reliable. ...
Preprint
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Abstract. Climate change is intensifying glacier melting and lake development in High Mountain Asia (HMA), which could increase glacial lake outburst flood hazards and impact water resource and hydroelectric power management. However, quantification of variability in size and type of glacial lakes at high resolution has been incomplete in HMA. Here, we developed a HMA Glacial Lake Inventory (Hi-MAG) database to characterize the annual coverage of glacial lakes from 2008 to 2017 at 30 m resolution using Landsat satellite imagery. It is noted that a rapid increase in lake number and moderate area expansion was influenced by a large population of small glacial lake (≤ 0.04 km<sup>2</sup>), and faster growth in lake number occurred above 5300 m elevation. Proglacial lake dominated areas showed significant lake area expansion, while unconnected lake dominated areas exhibited stability or slight reduction. Small glacial lakes accounted for approximately 15% of the lake area in Eastern Hindu Kush, Western Himalaya, Northern/Western Tien Shan, and Gangdise Mountains, but contributed > 50 % of lake area expansion in these regions over a decade. Our results demonstrate proglacial lakes are a main contributor while small glacial lakes are an overlooked element to recent lake evolution in HMA. Regional geographic variability of debris cover, together with trends in warming and precipitation over the past few decades, largely explain the current distribution of supra- and proglacial lake area across HMA. The Hi-MAG database are available at: https://doi.org/10.5281/zenodo.3700282 , it can be used for studies on glacier-climate-lake interactions, glacio-hydrologic models, glacial lake outburst floods and potential downstream risks and water resources.
... Latest studies show that the frequency of GLOFs on the Tibetan Plateau has a tendency to increase (Richardson and Reynolds, 2000;Liu et al., 2014). Therefore, researchers begin to focus on the formation and development of glacial lakes (Chen et al., 2007;Sakai and Fujita, 2010;Salerno et al., 2012;Thompson et al., 2012;Mergili et al., 2013;Nie et al., 2013;Wang and Zhang, 2014), and the assessment of hazards posed by GLOFs (Wang et al., 2008;X. Wang et al., 2012;Haemmig et al., 2014;Lamsal et al., 2014;Shrestha and Nakagawa, 2014;Somos-Valenzuela et al., 2015). ...
... Glacial lakes are important indicators of the regional glacier dynamics in response to climate warming and changing precipitation [1][2][3]. With the continuous retreating and thinning of mountain glaciers in the Tibetan Plateau, a large number of glacial lakes have developed and experienced rapid expansion in recent decades, leading to the increased hazard risk of glacial lake outburst floods (GLOFs) [4]. ...
Article
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Glacial lakes are an important component of the cryosphere in the Tibetan Plateau. In response to climate warming, they threaten the downstream lives, ecological environment, and public infrastructures through outburst floods within a short time. Although most of the efforts have been made toward extracting glacial lake outlines and detect their changes with remotely sensed images, the temporal frequency and spatial resolution of glacial lake datasets are generally not fine enough to reflect the detailed processes of glacial lake dynamics, especially for potentially dangerous glacial lakes with high-frequency variability. By using full time-series Sentinel-1A/1B imagery over a year, this study presents a new systematic method to extract the glacial lake outlines that have a fast variability in the southeastern Tibetan Plateau with a time interval of six days. Our approach was based on a level-set segmentation, combined with a median pixel composition of synthetic aperture radar (SAR) backscattering coefficients stacked as a regularization term, to robustly estimate the lake extent across the observed time range. The mapping results were validated against manually digitized lake outlines derived from Gaofen-2 panchromatic multi-spectral (GF-2 PMS) imagery, with an overall accuracy and kappa coefficient of 96.54% and 0.95, respectively. In comparison with results from classical supervised support vector machine (SVM) and unsupervised Iterative Self-Organizing Data Analysis Technique Algorithm (ISODATA) methods, the proposed method proved to be much more robust and effective at detecting glacial lakes with irregular boundaries that have similar backscattering as the surroundings. This study also demonstrated the feasibility of time-series Sentinel-1A/1B SAR data in the continuous monitoring of glacial lake outline dynamics.
... Glacial lakes, by contrast, are an appropriate archive of negative mass balances, and hence atmospheric warming, because they form and grow when glaciers waste back from their former frontal position and expose accumulation space for meltwater (Grabs and Hanisch, 1993;Mool, 1995;Yamada and Sharma, 1993). Thousands of such glacial lakes have thus appeared behind moraines or in bedrock depressions and cirques, and many more are recently forming as supraglacial ponds Miles et al., 2017;Thompson et al., 2012;Watson et al., 2016). Mapping glacial lakes from satellite imagery, assessing their geometric and topographic properties, and tracking their changes over time has become a core field of research in Himalayan glaciology (Table 1.1). ...
Thesis
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The Himalayas are a region that is most dependent, but also frequently prone to hazards from changing meltwater resources. This mountain belt hosts the highest mountain peaks on earth, has the largest reserve of ice outside the polar regions, and is home to a rapidly growing population in recent decades. One source of hazard has attracted scientific research in particular in the past two decades: glacial lake outburst floods (GLOFs) occurred rarely, but mostly with fatal and catastrophic consequences for downstream communities and infrastructure. Such GLOFs can suddenly release several million cubic meters of water from naturally impounded meltwater lakes. Glacial lakes have grown in number and size by ongoing glacial mass losses in the Himalayas. Theory holds that enhanced meltwater production may increase GLOF frequency, but has never been tested so far. The key challenge to test this notion are the high altitudes of >4000 m, at which lakes occur, making field work impractical. Moreover, flood waves can attenuate rapidly in mountain channels downstream, so that many GLOFs have likely gone unnoticed in past decades. Our knowledge on GLOFs is hence likely biased towards larger, destructive cases, which challenges a detailed quantification of their frequency and their response to atmospheric warming. Robustly quantifying the magnitude and frequency of GLOFs is essential for risk assessment and management along mountain rivers, not least to implement their return periods in building design codes. Motivated by this limited knowledge of GLOF frequency and hazard, I developed an algorithm that efficiently detects GLOFs from satellite images. In essence, this algorithm classifies land cover in 30 years (~1988–2017) of continuously recorded Landsat images over the Himalayas, and calculates likelihoods for rapidly shrinking water bodies in the stack of land cover images. I visually assessed such detected tell-tale sites for sediment fans in the river channel downstream, a second key diagnostic of GLOFs. Rigorous tests and validation with known cases from roughly 10% of the Himalayas suggested that this algorithm is robust against frequent image noise, and hence capable to identify previously unknown GLOFs. Extending the search radius to the entire Himalayan mountain range revealed some 22 newly detected GLOFs. I thus more than doubled the existing GLOF count from 16 previously known cases since 1988, and found a dominant cluster of GLOFs in the Central and Eastern Himalayas (Bhutan and Eastern Nepal), compared to the rarer affected ranges in the North. Yet, the total of 38 GLOFs showed no change in the annual frequency, so that the activity of GLOFs per unit glacial lake area has decreased in the past 30 years. I discussed possible drivers for this finding, but left a further attribution to distinct GLOF-triggering mechanisms open to future research. This updated GLOF frequency was the key input for assessing GLOF hazard for the entire Himalayan mountain belt and several subregions. I used standard definitions in flood hydrology, describing hazard as the annual exceedance probability of a given flood peak discharge [m3 s-1] or larger at the breach location. I coupled the empirical frequency of GLOFs per region to simulations of physically plausible peak discharges from all existing ~5,000 lakes in the Himalayas. Using an extreme-value model, I could hence calculate flood return periods. I found that the contemporary 100-year GLOF discharge (the flood level that is reached or exceeded on average once in 100 years) is 20,600+2,200/–2,300 m3 s-1 for the entire Himalayas. Given the spatial and temporal distribution of historic GLOFs, contemporary GLOF hazard is highest in the Eastern Himalayas, and lower for regions with rarer GLOF abundance. I also calculated GLOF hazard for some 9,500 overdeepenings, which could expose and fill with water, if all Himalayan glaciers have melted eventually. Assuming that the current GLOF rate remains unchanged, the 100-year GLOF discharge could double (41,700+5,500/–4,700 m3 s-1), while the regional GLOF hazard may increase largest in the Karakoram. To conclude, these three stages–from GLOF detection, to analysing their frequency and estimating regional GLOF hazard–provide a framework for modern GLOF hazard assessment. Given the rapidly growing population, infrastructure, and hydropower projects in the Himalayas, this thesis assists in quantifying the purely climate-driven contribution to hazard and risk from GLOFs.
Article
Reduction of ice masses concerning global warming is significantly changing geomorphology in high mountains. Formation of supraglacial lakes is one of such essential indications. Therefore, in the present study, we attempted to understand regional morphodynamics of supraglacial lakes, distributed in 17 glaciers within the Everest Himalaya. An average of 0.08 km²/yr lake size change rate was noticed during the studied year. Decadal (2010–2019) changes after studying fine spatial resolution satellite images revealed that only 161 out of total 9117 lakes were static, and are mostly concentrated at the lower part of the ablation area with an alarming rate of surface area increase. On the other hand, a new cluster of lakes appeared at higher elevations gradually. We collected here empirical evidence of regional morphodynamics and key controlling factors to stabilize them. The parameters, viz., spatio-temporal distribution of lakes, their domain wise variation, multi-temporal (Seasonal to long-term) changes, lake density estimation, and lake stability index were estimated and mapped. Finally, new lake formations at higher elevation were triggered by gradual increase in temperature, decrease in glacier surface velocity, slope and ice thickness. Moreover, from the feature selection techniques, it is established that lake stability index are mostly controlled by ice thickness followed by the surface velocity and slope.
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In April and May 2019, as a part of the National Geographic and Roxel Perpetual Planet Everest Expedition, the most interdisciplinary scientific ever was launched. This research identified changing dynamics, including emergent risks resulting from natural and anthropogenic change to the natural system. We have identified compounded risks to ecosystem and human health, geologic hazards, and changing climate conditions that impact the local community, climbers, and trekkeers in the future. This review brings together perspectives from across the biological, geological, and health sciences to better understand emergent risks on Mt. Everest and in the Khumbu region. Understanding and mitigating these risks is critical for the ~10,000 people living in the Khumbu region, as well as the thousands of visiting trekkers and the hundreds of climbers who attempt to summit each year.
Article
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Supraglacial ponds and ice cliffs can dramatically enhance ablation rates on debris-covered glaciers. Supraglacial ponds can also coalesce, forming moraine-dammed lakes at risk of glacial lake outburst flood (GLOF). Given Bhutanese glaciers have some of the highest ice loss rates in the Himalaya and GLOF vulnerability is high, we seek to advance our understanding of the spatial distribution and evolution of supraglacial ponds and ice cliffs. Here, we use high-resolution (3 m) Planet Labs satellite imagery to provide the first short-term, high-resolution dataset of supraglacial pond and ice cliff evolution for three glaciers along the Bhutan–Tibet border from 2016 to 2018. A total of 5754 ponds and 2088 ice cliffs were identified. Large intra-annual changes were observed, with ponded area changes and drainage events coinciding with the seasonality of the Indian Summer Monsoon. On average, ~19% of the total number of ponds had a coincident ice cliff. Pond spatial distribution was driven by ice-surface velocities, with higher numbers of ponds found in areas of low velocity (<8 m a ⁻¹ ). Our study provides the first detailed, quantitative investigation of supraglacial ponds and ice cliffs in Bhutan, providing a framework for further monitoring in this understudied, yet important, region of the Himalaya.
Article
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To understand the mechanism of simultaneous drainage event related to supraglacial lakes on a debris-covered glacier, we investigated water-level variations of supraglacial lakes on the southern Inylchek Glacier in Kyrgyzstan. To examine these variations, we used daily aerial images for 2017–2019 from an uncrewed aerial vehicle that were converted to 15 cm-digital surface models and ortho-images. Our main results are as follows: (1) When one lake drained, the water levels of other lakes simultaneously increased, indicating that drainage water is shared with several lakes through a main englacial conduit. In one drainage event, a branched off englacial conduit clearly connected to a main englacial conduit. (2) Sometimes several lakes discharged simultaneously, indicating that several lakes had connected to a main englacial conduit that had opened. Such cases can cause larger-scale drainage than that from the opening of a branched off englacial conduit. (3) Simultaneous drainage occurred twice in the same year, each time through a different conduit, indicating that the main englacial conduit can be abandoned and reused. (4) In some lakes, the water level on the hydraulic gradient line increased gradually with nearly the same increase rate just before drainage. Such an increase may be an indicator of a possible simultaneous drainage event.
Article
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The thickness of supraglacial debris cover controls how it impacts the ablation rate of underlying glacier ice, yet this quantity remains challenging to measure, particularly at glacier scales. We present a relatively straightforward, and cost-effective method to estimate debris thickness exposed above ice cliffs using simplified geometrical measurements from a high-resolution digital surface model (DSM), derived from a terrestrial photographic survey and a Structure from Motion with Multi-View Stereo workflow (SfM-MVS). As the ice surface relief beneath the debris cover is unknown, we assume it to be horizontal and provide error bounds based on characteristic ice-surface slope at the visible debris/ice interface. Debris thickness around the three sampled ice cliffs was highly variable (interquartile range of 0.80–2.85 m) and negatively skewed with a mean thickness of 2.08 ± 0.68 m. Manual, and high-frequency radar, determinations of debris thickness in the same area show similar thickness distributions, but statistically different mean debris thickness, due to local heterogeneity. Debris thickness values derived in this study all exceed estimates from satellite surface temperature inversions. Wider application of the method presented here would provide useful data for improving debris thickness approximations from satellite imagery.
Article
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Debris-covered glaciers (DCGs) are characterised by distinct hydrological systems that differ fundamentally from those observed on clean-ice valley glaciers. To date, most studies of DCG hydrology have focused on supraglacial hydrology, given that surface streams are broadly accessible and repeat observations can lead to conceptual models of channel evolution. Few have characterised englacial conduits and their layout, and none have directly investigated potential subglacial drainage networks in any setting. In this review, we summarise the current state of knowledge relating to DCG hydrology with a global focus, and present our own field observations to illustrate the distinct nature of DCG landforms on a receding high-elevation glacier in the Himalaya. We draw on recent work that has gone some way towards providing a process-based understanding of the formation and evolution of englacial and subglacial hydrological pathways and consider the role that DCG hydrology plays in regulating water supplies to downstream communities, contrasting this information with clean-ice examples. We conclude by identifying important knowledge gaps that might be considered priorities for future research into DCG hydrology.
Article
The Himalaya mountain range is characterized by highly glacierized, complex, dynamic topography. The ablation area of Himalayan glaciers often features a highly heterogeneous debris mantle comprising ponds, steep and shallow slopes of various aspects, variable debris thickness, and exposed ice cliffs associated with differing ice ablation rates. Understanding the composition of the supraglacial debris cover is essential for a proper understanding of glacier hydrology and glacier-related hazards. Until recently, efforts to map debris-covered glaciers from remote sensing focused primarily on glacier extent rather than surface characteristics and relied on traditional whole-pixel image classification techniques. Spectral unmixing routines, rarely used for debris-covered glaciers, allow decomposition of a pixel into constituting materials, providing a more realistic representation of glacier surfaces. Here we use linear spectral unmixing of Landsat 8 Operational Land Imager (OLI) images (30 m) to obtain fractional abundance maps of the various supraglacial surfaces (debris material, clean ice, supraglacial ponds and vegetation) across the Himalaya around the year 2015. We focus on the debris-covered glacier extents as defined in the database of global distribution of supraglacial debris cover. The spectrally unmixed surfaces are subsequently classified to obtain maps of composition of debris-covered glaciers across sample regions. We test the unmixing approach in the Khumbu region of the central Himalaya, and we evaluate its performance for supraglacial ponds by comparison with independently mapped ponds from high-resolution Pléiades (2 m) and PlanetScope imagery (3 m) for sample glaciers in two other regions with differing topo-climatic conditions. Spectral unmixing applied over the entire Himalaya mountain range (a supraglacial debris cover area of 2254 km2) indicates that at the end of the ablation season, debris-covered glacier zones comprised 60.9 % light debris, 23.8 % dark debris, 5.6 % clean ice, 4.5 % supraglacial vegetation, 2.1 % supraglacial ponds, and small amounts of cloud cover (2 %), with 1.2 % unclassified areas. The spectral unmixing performed satisfactorily for the supraglacial pond and vegetation classes (an F score of ∼0.9 for both classes) and reasonably for the debris classes (F score of 0.7). Supraglacial ponds were more prevalent in the monsoon-influenced central-eastern Himalaya (up to 4 % of the debris-covered area) compared to the monsoon-dry transition zone (only 0.3 %) and in regions with lower glacier elevations. Climatic controls (higher average temperatures and more abundant precipitation), coupled with higher glacier thinning rates and lower average glacier velocities, further favour pond incidence and the development of supraglacial vegetation. With continued advances in satellite data and further method refinements, the approach presented here provides avenues towards achieving large-scale, repeated mapping of supraglacial features.
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The Bolivian Andes have experienced sustained and widespread glacier area reduction and volume loss in recent decades. This study finds that from 1986 to 2018 glacier areas have shrunk from 529 km2 to 281 km2 (49 %) in the Bolivian Cordillera Oriental. Glacier melting and recession has been accompanied by the development of proglacial lakes, which can pose a glacial lake outburst flood (GLOF) risk to downstream communities. Therefore, glacier bed topographies were extracted and illustrate the potential development of 68 future lakes. Eight of these lakes possess populations downstream. A simple geometric model (MC-LCP) was used to model GLOFs from these potential future lakes, illustrating that ~1100 to ~2900 people could be affected by flooding if these lakes were to appear and to burst. The rest of this work is dedicated on the estimation of the risk from current, already existing lakes. Multi-Criteria Decision Analysis (MCDA) was used to rapidly identify potentially dangerous proglacial lakes in regions around the world without existing tailored GLOF risk assessments, where a range of proglacial lake types exist, and where field data are sparse or non-existent. After testing the robustness of the MCDA model against a number of past GLOFs, it was applied to the Bolivian Cordillera Oriental. From the 25 lakes possessing populations downstream, 3 lakes were found to pose ‘medium’ or ‘high’ risk, and required further detailed investigation. Since no attempt has yet been made to model GLOF inundation downstream from these proglacial lakes, 2m resolution DEMs were generated from stereo and tri-stereo SPOT 6/7 satellite images to drive a hydrodynamic model (HEC-RAS 5.0.3) of GLOF flow. The model was tested against field observations of a 2009 GLOF from Keara, in the Cordillera Apolobamba, and was shown to reproduce realistic flood depths and inundation. The model was then used to model GLOFs from Pelechuco lake (Cordillera Apolobamba) and Laguna Arkhata and Laguna Glaciar (Cordillera Real). In total, ~1100 to ~2200 people could be directly affected by outburst flooding.
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Rapid changes of glacial lakes are among the most visible indicators of global warming in glacierized areas around the world. The general trend is that the area and number of glacial lakes increase significantly in high mountain areas and polar latitudes. However, there is a lack of knowledge about the current state of glacial lakes in the High Arctic. This study aims to address this issue by providing the first glacial lake inventory from Svalbard, with focus on the genesis and evolution of glacial lakes since the end of the Little Ice Age. We use aerial photographs and topographic data from 1936 to 2012 and satellite imagery from 2013 to 2020. The inventory includes the development of 566 glacial lakes (total area of 145.91 km2) that were in direct contact with glaciers in 2008–2012. From the 1990s to the end of the 2000s, the total glacial lake area increased by nearly a factor of six. A decrease in the number of lakes between 2012 and 2020 is related to two main processes: the drainage of 197 lakes and the merger of smaller reservoirs into larger ones. The changes of glacial lakes show how climate change in the High Arctic affect proglacial geomorphology by enhanced formation of glacial lakes, leading to higher risks associated with glacier lake outburst floods in Svalbard.
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In recent decades, the downwasting of several debris-covered glaciers in the Himalaya has led to the formation of large and potentially hazardous moraine- dammed lakes. The frequency of Glacial Lake Outburst Flood (GLOF) events in the Himalaya has steadily increased since the 1970s and as global temperatures continue to rise this trend is set to continue in the future. Downwasting of the debris-covered Ngozumpa Glacier in the Khumbu Himal, Nepal, has resulted in the abandonment of the lateral and terminal moraine crests, leaving them standing several tens of metres above the glacier surface. The moraines have exerted a control on the drainage of meltwater from the glacier surface and have encouraged ponding of meltwater on the glacier surface. The present study examines the evolution of perched supraglacial ponds on the Ngozumpa Glacier and assesses how the growth of these ponds affects the rate of downwasting of the glacier surface. The expansion rates of perched ponds can be rapid, up to 21,609 m ² a⁻¹, but the growth of these ponds tends to be terminated when contact is made with the englacial drainage network. The thesis documents for the first time a complete cycle of perched supra glacial pond growth and drainage and also provides direct evidence for internal ablation during pond drainage, a process that has only been inferred in previous research. The western lateral moraine has dammed back drainage from the western tributary valleys, resulting in the formation of laterally-dammed lakes. The research presented here examines the processes and rates of paraglacial reworking of the Ngozumpa moraines in order to assess the stability and longevity of the moraine dam. Approximately 1 km from the Ngozumpa terminus a large Spillway Lake has formed. Meltwater from upglacier is channelled into the lake and exits the glacier surface through an over-spill channel cut down through the western lateral moraine. The level of the Spillway Lake is thereby controlled by the height of the spillway channel through the western lateral moraine. The rate of expansion of the Spillway Lake is lower than that of the perched ponds upglacier, but as the Spillway Lake continues to enlarge and surface downwasting of the glacier surface proceeds, the lake could enter a period of rapid and unstable growth. By analogy with other glaciers in the Khumbu Rimal, it is possible that a large and potentially hazardous lake will form on the Ngozumpa within the next two decades.
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The present study investigates thermal resistances on debris-covered glaciers around Mt. Everest and in the Lunana region of Bhutan, using satellite images taken by ASTER and NCEP/NCAR reanalysis data. The thermal resistance is defined as the thickness divided by the thermal conductivity of a debris layer, and is an important index to the evolution of glacial lakes through the melting process. This index is obtained from surface temperature and heat balance on the debris layers. Since the net radiation is a dominant energy source on the Himalayan glaciers, thermal resistances are calculated by neglecting turbulent heat flux in heat balance. We evaluate errors of thermal resistances using field meteorological data and multitemporal ASTER data. The result shows that above errors are unlikely to a#ect the spatial pattern of thermal resistances. About half of ,/ target glaciers without moraine-dammed lakes have larger thermal resistances than 1 glaciers with the lakes. Spatial distribution of thermal resistances shows the large increases toward glacier termini on the glaciers without lakes, whereas relatively small and uniform values on those with lakes. These results imply that the di#erence in magnitudes and distribution of thermal resistances on debris-covered glaciers are related to di#erent evolutionary stages of the glacial lakes in the Himalayas. The present study demonstrates the possibility that ASTER data provide thermal resistance distribution over many glaciers for glacial lake studies without simultaneous field observations.
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Analyses of maximum temperature data from 49 stations in Nepal for the period 1971-94 reveal warming trends after 1977 ranging from 0.06°to 0.12°C yr-1 in most of the Middle Mountain and Himalayan regions, while the Siwalik and Terai (southern plains) regions show warming trends less than 0.03°C yr-1. The subset of records (14 stations) extending back to the early 1960s suggests that the recent warming trends were preceded by similar widespread cooling trends. Distributions of seasonal and annual temperature trends show high rates of warming in the high-elevation regions of the country (Middle MOuntains and Himalaya), while low warming or even cooling trends were found in the southern regions. This is attributed to the sensitivity of mountainous regions to climate changes. The seasonal temperature trends and spatial distribution of temperature trends also highlight the influence of monsoon circulation. The Kathmandu record, the longest in Nepal (1921-94), shows features similar to temperature trends in the Northern Hemisphere, suggesting links between regional trends and global scale phenomena. However, the magnitudes of trends are much enhanced in the Kathmandu as well as in the all-Nepal records. The authors' analyses suggest that contributions of urbanization and local land use/cover changes to the all-Nepal record are minimal and that the all-Nepal record provides an accurate record of temperature variations across the entire region.
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The ablation amount for entire ice cliffs reaches about 2017( of that at the whole debris-covered area, the ablation rate at the ice cliff mainly depends on shortwave radiation, which differs widely with the orientation of all ice cliff. Therefore, the distribution of ice cliffs ill relation to their orientation was observed oil a debris-covered glacier. The south-facing, cliffs were small in area because they have low slope angles and tended to be covered with debris. The north-facing cliffs. oil the other hand, were large in the studied area and maintain a slope angle larger than the repose angle of debris. They, therefore, are stable. Longwave radiation from the debris surface opposite the ice cliffs was larger on the lower portion of ice cliffs than on the upper portion in every azimuth. This difference in longwave radiation maintained a steep slope angle on ice cliff. Shortwave radiation was stronger at the upper portion of ice cliffs than at tire lower portion due to tire local shading effect, causing gentle sloping of ice clift's. This was especially pronounced at cliffs facing to south. Therefore, the dependency of the ice cliff angle in orientation call be explained by tire difference in local radiation between the upper and lower portion of the ice cliff.
Article
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There are many supraglacial ponds on debris-covered glaciers in the Nepal Himalayas. The heat absorbed at the surface of a pond was estimated from heat budget observations on the Lirung Glacier in Langtang Valley, Nepal. The results indicated an average heat absorption of 170 W m-2 during the summer monsoon season. This rate is about 7 times the average for the whole debris-covered zone. Analysis of the heat budget for a pond suggests that at least half of the heat absorbed at a pond surface is released with the water outflow from the pond, indicating that the water warmed in the pond enlarges the englacial conduit that drains water from the pond and produces internal ablation. Furthermore, the roof of the conduit could collapse, leading to the formation of ice cliffs and new ponds, which would accelerate the ablation of the debris-covered glacier.
Article
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Analyses of maximum temperature data from 49 stations in Nepal for the period 1971–94 reveal warming trends after 1977 ranging from 0.06 to 0.12C yr 1 in most of the Middle Mountain and Himalayan regions, while the Siwalik and Terai (southern plains) regions show warming trends less than 0.03C yr 1. The subset of records (14 stations) extending back to the early 1960s suggests that the recent warming trends were preceded by similar widespread cooling trends. Distributions of seasonal and annual temperature trends show high rates of warming in the high-elevation regions of the country (Middle Mountains and Himalaya), while low warming or even cooling trends were found in the southern regions. This is attributed to the sensitivity of mountainous regions to climate changes. The seasonal temperature trends and spatial distribution of temperature trends also highlight the influence of monsoon circulation. The Kathmandu record, the longest in Nepal (1921–94), shows features similar to temperature trends in the Northern Hemisphere, suggesting links between regional trends and global scale phenomena. However, the magnitudes of trends are much enhanced in the Kathmandu as well as in the all-Nepal records. The authors' analyses suggest that contributions of urbanization and local land use/cover changes to the all-Nepal record are minimal and that the all-Nepal record provides an accurate record of temperature variations across the entire region.
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The thermal structure and hydrodynamics of supraglacial Tsho Rolpa Lake (27 degrees 51'N, 86 degrees 29'E) in the Nepal Himalaya were examined in the premonsoon of 1996. We continuously measured flow velocity, water temperature, and turbidity with moored self-recording current meters, temperature data loggers, and turbidimeters. Vertical measurements (every 0.2 m in depth) of water temperature and turbidity were also made by lowering a self-recording sonde, Tsho Rolpa Lake (surface area, 1.39 km(2) at present) has increased in size since late 1950s (surface area, 0.23 km(2) in 1958) by both glacial ice melt below the lake bottom and the retreat of the glacier terminus. Lake stratification is defined by suspended sediment concentration (SSC) rather than by temperature. The suspended sediment is mostly silt and clay (d < 15.6 mu m), which is supplied by the meltwater discharge from a subaqueous tunnel portal into the lake at the glacier terminus. The sediment is transported to the deepest zone of the lake by underflows and diffused by advection by the other currents into the upper zone. The observations revealed that a diurnal valley wind produces a vertical water circulation in the quasi-isopycnal surface layer, which is about 27 m deep. This circulation transports the surface water, heated mostly by solar radiation, toward the glacier terminus and consequently forces relatively warm water (>similar to 5 degrees C) into continual contact with the glacier terminus. This warm water contact could induce calving of the upper glacier-ice by increasingly melting the subaqueous lower part.
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Across the surface of debris-covered glaciers in the Nepal Himalayas there are many lakes on the order of 1 km in scale. Some of the lakes, dammed by the end/lateral moraine, have been expanded from ponds on the order of 100 m in length. Tsho Rolpa Glacier Lake in the headwater region of the Rolwaling Valley, Eastern Nepal, was probably a small pond in the early 1950s. The present expansion rate of the glacial lake is unknown. From our investigation of the hydrological and meteorological conditions in 1993-1994, the calculated heat balance in the lake showed a deepening rate of about 1.2 m yr 21 all through the year. This is comparable to the mean deepening rate of the bottom basin since 1952. The vertical expansion thus is still continuing. Annual volume expansion including the calving ice (horizontal expansion) volume was 3% of the present total volume of the lake.
Article
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Glaciological investigations were carried out in 1994 on the glaciers in Hidden Valley, Mukut Himal, Nepal Himalayas, in order to make a comparison with observations made in 1974. Most of the glaciers were found to have retreated by 30-60 m in terminus elevation over the 20 years between the two studies. Rikha Samba Glacier, the longest glacier in the valley, has retreated by about 200 m. The areal average of the amount of surface lowering and the volume loss of the glacier wa estimated to be 12.6 m ice equivalent and 13% of the obtained as an average for 20 years, which is one of the largest negative values amongst small glaciers of the world.
Article
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Glacier lakes are a common phenomenon in high mountain areas. Outbursts from glacier lakes have repeatedly caused the loss of human lives as well as severe damage to local infrastructure. In several high mountain ranges around the world, a grave uncertainty about the hazard potential of glacier lakes still exists, especially with respect to the effects of accelerating rates of glacier retreat as a consequence of atmospheric warming. Area-wide detection and modeling of glacier lake hazard potentials is, therefore, a major challenge. In this study, an approach integrating three scale levels allows for the progressive focus on critical glacier lakes. Remote sensing methods for application in glacier lake hazard assessment are presented, and include channel indexing, data fusion, and change detection. Each method matches the requirements of a certain scale level. For estimating potential disaster amplitudes, assessments must be made of maximum discharge and runout distance of outbursts floods and debris flows. Existing empirical relations are evaluated and complementary ones as derived from available data are proposed. Tests with observations from a recent outburst event from a moraine-dammed lake in the Swiss Alps show the basic applicability of the proposed techniques and the usefulness of empirical relations for first hazard assessments. In particular, the observed runout distance of the debris flow resulting from the outburst does not exceed the empirically estimated maximum runout distance. A list of decision criteria and related remote sensing techniques are discussed in conclusion. Such a list is an essential tool for evaluating the hazard potential of a lake. A systematic application of remote sensing based methods for glacier lake hazard assessment is recommended.
Article
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Glacial hazards such as ice avalanches, glacial lake outburst floods, and debris flows have caused severe damage,in populated,mountain,regions such as the Swiss Alps. Assessment,of such hazards must consider basic glaciological, geomorphological, and hydraulic principles together with experience gained from previous events. An ap- proach is presented here to assess the maximum,event magnitude,and probability of occurrence,of glacial hazards. Analysis of magnitude,is based on empirical relationships derived from published case histories from the Swiss Alps and other mountain,regions. Probability of occurrence,is difficult to estimate because of rapid changes in the nature of glacial systems, the low frequency of events, and the high complexity of the involved processes. Here, the probability is specified in qualitative and systematic terms based on indicators such as dam type, geometry, and freeboard height (for glacial lakes) and tendency of avalanche repetition, precursor events, and increased water supply to the glacier bed (for ice avalanche,events). The assessment,procedures are applied to a recent lake outburst with subsequent,debris flow and to an ice avalanche,in the Swiss Alps. The results yield reasonable event maxima,that were not exceeded,by actual events. The methods,provide first-order assessments,and may,be applied in dynamic,mountain,environments,where population and infrastructure growth,require continuous,evaluation of hazards. Key words: glacial hazards, lake outburst, debris flow, ice avalanche, hazard assessment procedure, probability of occur-
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Repeat photographs and field survey reveal the mechanism of short-term ice-cliff evolution at Maud Glacier, a temperate lake-calving glacier in New Zealand. Calving is cyclic, each cycle involving four stages: (1) waterline melting and collapse of the roof of a sub-horizontal notch at the cliff foot; (2) calving of ice flakes from the cliff face leading to a growing overhang from the waterline upwards, and crack propagation from the glacier surface; (3) large but infrequent calving of slabs in response to the developing overhang, returning the cliff to an “initial” vertical profile; (4) rare subaqueous calving ofa submerged ice foot. Results indicate that the rate-controlling process is the speed of waterline melting, and that calving rate is independent of water depth (at least at time-scales of weeks to months). Slowly calving lake-terminating glaciers have mass balances more negative than land-terminating glaciers, but nevertheless advance and retreat in response to mass-balance driven changes in ice velocity.
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New dates for late Quaternary glaciations in the Himalayas show that, during the last glacial cycle, glaciations were not synchronous throughout the region. Rather, in some areas glaciers reached their maxima at the global glacial maximum of c.18-20 ka bp, whereas in others glaciers were most extensive at c.60-30 ka bp. Comparison of these data with palaeoclimatic records from adjacent regions suggest that, on millennial timescales, Himalayan glacier fluctuations are controlled by variations in both the South Asian monsoon and the mid-latitude westerlies. The Himalayas and Tibetan Plateau form the largest mountain mass on earth, stretching over 2000 km east-west from Burma to Afghanistan and over 1000 km north-south from the deserts of Central China to the Indo-Gangetic Plain, and with an average elevation of 6000 m (Fig. 1). This vast area of high ground exerts an important influence on regional and global climate, in several ways (Murakami 1987). First, it has mechanical eVects on atmospheric circulation, particularly in winter when it splits the surface westerly winds into northern and southern branches and prevents the southward flow of cold continental air towards the Indian Subcontinent. The mechanical eVects also extend farther afield due to the en- hancement of the northern westerly jet stream in the lee of the plateau. Second, heating of the plateau in summer raises air
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Over the past few years there has been an increase in understanding of glacier-impounded or 'ice-dammed' lake behaviour. The spectacular jokulhlaup (catastrophic flood) from Grimsvotn, Iceland in November 1996 has both raised the profile of such events and emphasized the need for awareness of the processes involved. This review summarizes the extent of current knowledge of ice-dammed lakes, highlighting key developments and outlining areas of study still subject to difficulties. Controls on ice-dammed lake formation and persistence are identified, and cycles of jokulhlaup activity are related to glacier fluctuations. Ice-dammed lake drainage trigger mechanisms are reviewed and recent progress in the understanding of such mechanisms is emphasized. Controls on jokulhlaup routing and the development and character of jokulhlaup conduits are discussed and recent advances in jokulhlaup prediction, hydrograph modelling and peak discharge estimation are assessed. A process-based schematic model, drawing on published research, links ice-dammed lake occurrence and drainage to jokulhlaup characteristics. It is demonstrated that ice-dammed lake and ice-dam characteristics ultimately control seven key jokulhlaup attributes which determine the potential impact of jokulhlaups on both landscape and human activity in glaciated regions.
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Glacial lake outburst floods (GLOFs) are common natural hazards in the Himalaya. These floods, usually of large magnitude, can severely affect fragile mountain ecosystems and their limited economic activities. In this study, GLOF hazard in the Sagarmatha region (national park and buffer zone) was assessed using dam break and hydrodynamic modeling. The available data from the Dig Tsho GLOF of 1985 were used to validate many of the model outputs. The technique was further applied to GLOF hazard assessment of Imja Lake, the largest and potentially most dangerous glacial lake in the region. The peak outflow discharge of an Imja GLOF is estimated at 5463 m³/s. The peak discharge attenuates to about 2000 m³/s at the boundary of the buffer zone at about 45 km from the outburst site. Finally, a GLOF vulnerability rating map was prepared and an assessment of vulnerable settlements was carried out. The study was found to be a cost-effective means of obtaining preliminary information on the extent and impact of possible GLOF events-information that is useful for developing plans for early warning systems and implementing management plans.
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Supraglacial Tsho Rolpa Lake in the Nepal Himalaya has been increasing rapidly in size since the 1950s, corresponding to the mountain-glacier shrinkage after the Little Ice Age. The lake basin expansion results from the subsidence by dead-ice melt below the bottom of the lake, and the retreat of the glacier terminus. Field observations of Tsho Rolpa in 1996 revealed that the retreat of glacier terminus is connected to a wind-induced vertical circulation of surface water heated by solar radiation. In order to clarify the mechanism of the lake expansion associated with sedimentary processes, we measured bottom sedimentation rate with some sediment traps, and vertical suspended sediment concentration (SSC) and water temperature, and analyzed the grain size of suspended and trapped sediments. The sediments, mostly composed of clay-sized grains, are dominantly supplied by glacier-melt water inflow at the glacier terminus. Sedimentary processes of such fine sediment comprise: (1) suspended-sediment fallout from intrusion of horizontal currents; (2) sediment sorting by sediment-laden underflows; and (3) the debris supply from the ice collapse at the glacier terminus. The (1) and (2) processes produce the density stratification of the lake, accompanied by a pycnocline at a depth of about 27 m. The existence of the pycnocline builds up the vertical water circulation in the surface layer to enhance the glacier-melt at the terminus. With respect to the subsidence of the lake bottom, nearly molecular thermal diffusion is probably dominant near the bottom of the deepest point, which results from the kinetic-energy dissipation of sediment-laden underflows. The stable existence of the bottom turbid water throughout the year could cause continuous dead-ice melt below the lake bottom.
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Generalized numerical models of sub-debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change, as empirical methods are site-specific and strongly dependent upon the conditions prevailing during the measurement period. We present a modified surface energy-balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables. Despite numerous simplifications, the model performs well and modelled melt rates give a good match to observed melt rates, suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick. This is a useful improvement on previous models which are inappropriate for thick debris cover.
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Comparison of the terminus locations of Rongbuk Glacier, Mount Everest, measured in 1966 and 1997 shows that in the past 30 years the glacier has retreated 170-270 m, equivalent to a retreat speed of 5.5-8.7 m a−1. During summer 1997, a 15 m firn core was recovered from Dasuopu glacier (28°23′N, 85°44′E; 7000 m a.s.l.) on the northwest margin of Xixabangma Feng, Xizang (Tibet). The seasonal variations of δ18O values in the core indicate that monsoon signals are clearly recorded in the glacier. δ18O values are controlled by the amount effect in the monsoon season; more negative δ18O is representative of the monsoon season in snow layers. Analysis of the relationship between ice-core δ18O, sampled from 6500 m a.s.l. on the north side of Mount Everest, and instrumental series representing regional-scale precipitation, atmospheric circulation and temperature suggests a change in the relative influence of these parameters on δ18O since the 1940s. The results of the comparison add to and lengthen the sparse array of instrument data available for the Tibetan (Qinghai-Xizang) Plateau and demonstrate a recent decline in moisture flux for at least the southern part of the plateau. Glacier retreat, associated with a recent increase in temperature in the region, is coincident with this period of decreased moisture flux.
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Englacial cave systems were mapped using speleological techniques in three debris-covered glaciers in the Khumbu Himal, Nepal. Detailed three-dimensional mapping of the cave systems and observations of relationships with structures in the surrounding ice show conduits formed by a mechanism directly analogous to speleogenesis in limestone karst. The highest, oldest parts of all passages developed along debris-filled crevasse traces with hydraulic conductivity in the range 10−4 to 10−5 m s−1. Conduits form when these hydraulically efficient pathways bridge between areas with different hydraulic potential. They then evolve by grading (through head-ward migration of nick points and vertical incision) to local base level, often the surface of supraglacial lakes. Most supraglacial lakes on Himalayan glaciers are perched above the elevation of the terminal stream, and exist for a few years before draining through englacial conduits. As a result, near-surface drainage evolution is frequently interrupted by base-level fall, and conduits may record multiple phases of incision. Conduits commonly migrate laterally during incision, undermining higher levels of the ice and encouraging collapse. Voids can be created by fluvial processes and collapse of crevassed ice. The oft-noted resemblance of the surface morphology of debris-covered glaciers to karst landscapes thus extends to the subsurface, and karst hydrology provides a framework for understanding englacial drainage.
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Annual-layer thickness data, spanning AD 1534-2001, from an ice core from East Rongbuk Col on Qomolangma (Mount Everest, Himalaya) yield an age-depth profile that deviates systematically from a constant accumulation-rate analytical model. The profile clearly shows that the mean accumulation rate has changed every 50-100 years. A numerical model was developed to determine the magnitude of these multi-decadal-scale rates. The model was used to obtain a time series of annual accumulation. The mean annual accumulation rate decreased from ∼0.8 m ice equivalent in the 1500s to ∼0.3 m in the mid-1800s. From ∼1880 to ∼1970 the rate increased. However, it has decreased since ∼1970. Comparison with six other records from the Himalaya and the Tibetan Plateau shows that the changes in accumulation in East Rongbuk Col are broadly consistent with a regional pattern over much of the Plateau. This suggests that there may be an overarching mechanism controlling precipitation and mass balance over this area. However, a record from Dasuopu, only 125 km northwest of Qomolangma and 700 m higher than East Rongbuk Col, shows a maximum in accumulation during the 1800s, a time during which the East Rongbuk Col and Tibetan Plateau ice-core and tree-ring records show a minimum. This asynchroneity may be due to altitudinal or seasonal differences in monsoon versus westerly moisture sources or complex mountain meteorology.
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Changes in the area and bathymetry of Imja Glacial Lake and in the elevation of its damming moraine, Khumbu region, Nepal Himalaya are investigated. Previously reported changes in the lake area have been updated by multi-temporal ASTER images, which revealed a decreased expansion rate after 2000. A provisional expansion of the lake observed in 2004, from which some studies concluded an accelerated lake expansion due to global warming, has, from 2005, subsided to the glacier surface. Bathymetric changes for the period 1992–2002 that were first obtained for Himalayan glacial lakes suggest that the melting of debris-covered ice beneath the lake is insignificant in terms of the increase in lake volume, and that the retreat of a glacier in contact with the lake by calving is essential for the lake's expansion. Changes in the height of a damming moraine for the period 2001–2007 suggest a continuous surface lowering near the lake, though the lowering rates are smaller than those for the period 1989–1994.
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Field surveys of supraglacial ponds on debris-covered glaciers in the Nepal Himalaya clarify that ice-cliff calving occurs when the fetch exceeds $80 m. Thermal undercutting is important for calving processes in glacial lakes, and subaqueous ice melt rates during the melt and freeze seasons are therefore estimated under simple geomorphologic conditions. In particular, we focus on the differences between valley wind-driven water currents in various fetches during the melt season. Our results demonstrate that the subaqueous ice melt rate exceeds the ice-cliff melt rate when the fetch is >20 m and water temperature is 2–48 8C. Calculations suggest the onset of calving due to thermal undercutting is controlled by water currents driven by winds at the surface of the lake, which develop with expanding water surface.
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