Glacial Lake Outburst Floods: Geomorphological Agents and Hazardous Phenomena

The existence of glacial lakes in the Southeastern Tibetan Plateau (SETP) is a potential hazard to the downstream regions, as the failure of some lakes has potential to result in disastrous glacial lake outburst flood (GLOF) events of high-magnitude. In the present study, we conducted a comprehensive investigation for Bienong Co, an end moraine-dammed glacial lake in SETP, including its area evolution analysis, basin morphology simulation, water volume estimation, possible outburst triggers analysis, and one- and two-dimensional hydrodynamic simulation. The results show that the area of Bienong Co was 1.15 ± 0.05 km2 in August 2020, which has remained generally stable over the past four decades. However, it exhibits the high risk of GLOFs due to its larger area, the steep and high moraine dam, the close distance to its mother glacier, and the surrounding steep slopes. The lake basin is relatively flat at bottom and steep on both flanks, and the slope near the glacier (16.5°) is steeper than that near the moraine dam (11.3°), with a maximum lake depth of ~181.04 m and a water volume of ~1.02 × 108 m3 in August 2020. Four scenarios of GLOFs based on different breach depths, breach widths and failure times were simulated using the hydrodynamic model of MIKE 11 and MIKE 21 to predict the potential impacts on downstream areas. An extreme-magnitude GLOF would have a catastrophic impact on the downstream region, with most of the settlements, all bridges and majority of Jiazhong Highway along the flow channel being completely submerged. However, in a low-magnitude GLOF, most settlements would be safe or partially inundated. It means that most of the residents in the flow channel of Bienong Co can avoid the damage caused by a low-magnitude GLOF (smaller breach depth of dam). Although three settlements in the downstream area are at risk of being completely submerged in a low-magnitude GLOF event, the flooding arrives late and people have enough time to escape. Finally, the maximum depths of glacial lakes with similar areas were compared for 16 glacial lakes with measured bathymetry data in the Himalayas and Bienong Co in SETP, according to the regional division of maritime and continental zones. The results show that glacial lakes located in maritime regions have larger depths than those in continental regions, and Bienong Co is the deepest glacial lake comparing others in the Himalayas. Therefore, a huge amount of water could be discharged by a potential GLOF event of Bienong Co, creating a serious hazard, which should be taken seriously in the future. Overall, this study of Bienong Co could provide a new understanding of the moraine-dammed glacial lakes in the SETP and a reference of GLOFs to the local government.

The Himalayan region is a fragile ecosystem with a steep landscape where natural disasters such as the Glacial Lake Outburst Floods (GLOFs) event have been frequently observed that cause significant damage in valleys downstream areas. GLOF is one of the major unexpected serious hazards that can release a large volume of water with debris cover in a very short interval of time that caused loss of lives, properties, and severe damage to downstream infrastructures. In the present study, lake-terminating South Lhonak lake (Lat 27° 54′ 41.88″ N and Long 88° 11′ 44.65″ E) is located at an altitude of 5205 m above mean sea level (a.s.l.) in the Teesta basin, Sikkim has been identified as highest potentially dangerous glacial lake based on the criteria of area, elevation, slope, distance from the glacier and volume of water held, with maximum surface area 1.39 km2 acquired on October 2020. Spatio-temporal analysis of the glacial lake using Landsat 5 and Landsat-8 satellite imagery shows that the glacial lake has been grown from 0.48 to 1.39 km2 over the past 30 years. The depth of South Lhonak glacial lake is 39.54 m which has been calculated by using Huggle’s formula. This study evaluates the one-dimensional hydrodynamic modeling for GLOF simulation at different downstream sites from the glacial lake. The HEC-GeoRAS has been used for the creation of river geodatabase. Consequently, simulation was carried out using HECRAS model at multiple sections along the river. The GLOF scenario was revealed due to overtopping failure of the frontal moraine, producing peak discharge, and releasing water volume of 54.96 × 106 m3. The result of this study reveals that the predicted peak discharges of the lake have been estimated for breach widths of 30, 40, and 60 m, and breach formation time has also been considered as 40, 30, and 20 min, respectively. The estimated flood peak was 14,658.60 m3/s at the GLOF site, and the same got mitigated to 11,978.77 m3/s at the downstream end, respectively. The study’s findings will aid in the formulation of risk management plans and risk reduction strategies in the event of a GLOF hazard.KeywordsSouth Lhonak lakeGlacial Lake outburst floodsHEC-RAS modelHEC-GeoRASRiver cross-sectionFlood hydrograph

<p>In contrast to glaciers in other areas of the world, the Karakoram glaciers appear to be stable or increasing mass in response to global climate change, a phenomenon known as the 'Karakoram anomaly.' Many glaciers are experiencing irregular, frequent, and rapid frontal advances (surges), which cause natural hazards by obstructing river channels forming ice-dammed lakes, consequent outburst floods and posing threats downstream over the region. Predicting the phenomenon to protect downstream communities remains challenging around the globe. The determination of the surge characteristics, timing and evolution of lakes and GLOFs is fundamental to flood control and disaster management. This study documents 179 glacial lake outburst floods (GLOFs) that occurred from 1533 to 2020 in five major valleys. Sixty-four of the events took place after 1970, and 37 of these had remote sensing imagery that covered the GLOF formation to breaching sequence. Thirty-six glaciers were associated with GLOFS due to ice-front advance building ice barriers in rivers. The Kayger and Khurdopin glaciers are the most hazardous examples, responsible for 31.8% of major GLOFs in the Karakoram. Using a cross-correlation feature-tracking technique on remote sensing imagery, we analyzed ten surge glaciers and documented six surge events from 1990 to 2019. Results show periodic surge cycles for the Khurdopin, Kyager, Shishper, and Chilinji glaciers of c. 15–20 years, with a surge velocity in the mid-2010s higher than that of the late 1990s for all studied glaciers. The higher velocity of a glacier increases the risk of flooding downstream of the terminus because the transfer of a huge ice mass towards the terminus during the surge is a key factor for conduit development, formation and reformation of a series of ice-dammed lakes, thus determining the magnitude and frequency of outburst flood events. The response of Karakorum glaciers to global warming and climate forcing, comprising a continuum of glacier mass gain, ice thinning, and ice advance has resulted in lake formation and ice dam failures. We predict the frequency of GLOFs will increase in the future. These findings support the increasing anomalous behavior of glaciers in the Karakoram region. A conceptual model of ice-dammed lake formation and GLOF initiation in response to glacier surging is presented to synthesize the detailed observations.</p>

Abstract. Hazard and risk from glacial lake outburst floods (GLOFs) in Bhutan have traditionally been assessed with limited consideration of the downstream exposure and vulnerability associated with individual lakes. However, exposure and vulnerability are key components of risk, and when explicitly attributed to each lake, can provide a more robust basis for prioritising hazard investigations and mitigation efforts. We modelled hypothetical GLOF scenarios for all glacial lakes with an area greater than 0.05 km2 and located within 1 km of a glacier terminus. We then determined GLOF risk by explicitly accounting for downstream impacts using depth–velocity outputs at each exposed element affected by the simulated GLOF from each lake, as well as the vulnerability of the affected community. Our study shows that approximately > 11 000 people, > 2500 buildings, > 250 km of road, > 400 bridges and ∼ 20 km2 of farmland are exposed to potential GLOF in Bhutan. We classified lake130 (Thorthormi Tsho) as a very high hazard glacial lake in Bhutan, five lakes as high hazard and 22 other lakes as moderate hazard. Among these high hazard glacial lakes, three of them: lake93 (Phudung Tsho), lake251, and lake278 (Wonney Tsho) were not recognised as being high hazard in previous studies. Five downstream local government administrative units (LGUs) were associated with very high GLOF risk, while eight others are associated with high GLOF risk. Five of these very high and high risk LGUs had not been previously documented as being at risk from GLOF. Our study underscores the significance of integrating potential inundation mapping and downstream exposure data to define high hazard glacial lakes. We recommend strengthening and expanding the existing GLOF preparedness and risk mitigation efforts in Bhutan, particularly in the LGUs, as having high GLOF risk identified in this study, to reduce potential future damage and loss.

Glacial lake outburst floods (GLOFs) are among the most severe cryospheric hazards in the Third Pole, encompassing the Tibetan Plateau and surrounding Himalayas, Hindu Kush, and Tianshan Mountains. Recent studies on glacial lake changes and GLOF characteristics and risks in this region have shown scattered and insufficiently detailed features. Here, we conduct an appraisal of the GLOF risks by combining high-resolution satellite images, case-by-case high-precision GLOF modeling, and detailed downstream exposure data. The glacial lake changes from 2018 to 2022 in the region were primarily driven by the accelerated expansion of proglacial lakes. The GLOF frequency has exhibited a significant increasing trend since 1980, with intensified activity in Southeastern Tibet and the China-Nepal border area over the past decade. Approximately 6,353 km2 of land could be at risk from potential GLOFs, posing threats to 55,808 buildings, 105 hydropower projects, 194 km2 of farmland, 5,005 km of roads, and 4,038 bridges. This study directly responds to the need for local disaster prevention and mitigation strategies, highlighting the urgent requirement of reducing GLOF threats in the Third Pole and the importance of regional cooperation.

Glacial lake outburst floods (GLOFs) are high-magnitude events that occur when the dam of a glacial lake fails, releasing huge volumes of water and entrained debris. The increasing frequency of GLOFs, driven by the ongoing effects of climate change, raises concerns about the long-term stability of high-mountain regions in Nepal and across the Himalayas. While the immediate impacts of catastrophic GLOFs are often devastating, the secondary hazards they trigger are frequently overlooked. These secondary hazards, including landslides, geomorphic instability, and stream channel destabilization, pose significant challenges to local communities. Although GLOF events typically last only minutes to hours, the geohazard cascades they initiate may affect communities for years or even decades. A recent GLOF event that caused extensive damage to infrastructure and farmland in Thame, a small mountain village in Nepal's Khumbu Himal region, demonstrated this chain of cascading hazards. Following the catastrophic outburst of two glacial lakes on August 16, 2024, the village now faces increased landslide risk due to significant stream-channel incision below the settlement. The geologic layers beneath the town are susceptible to slow-moving, deep-seated rotational landslides, particularly when lateral support is reduced by stream incision. As a result, the fluvial terrace on which the village is built is becoming increasingly likely to fail from landsliding. Field investigations in fall 2024 collected drone imagery and ground-based photographs of the flood deposits and affected downstream areas. These data were used to develop high-resolution photogrammetric topographic models, enabling reconstruction of the flood dynamics and the evolution of similar past events. Analysis of sediment deposits further reveals how GLOFs interact with ongoing geomorphic processes, contributing to landscape transformation over time. By integrating field observations with photogrammetric modeling, this study highlights the cascading nature of hazards following GLOFs and their role in shaping mountain landscapes.

Shrinking glaciers in the Hindu Kush–Karakoram–Himalaya–Nyainqentanglha (HKKHN) region have formed several thousand moraine-dammed glacial lakes1–3, some of these having grown rapidly in past decades3,4. This growth may promote more frequent and potentially destructive glacial lake outburst floods (GLOFs)5–7. Testing this hypothesis, however, is confounded by incomplete databases of the few reliable, though selective, case studies. Here we present a consistent Himalayan GLOF inventory derived automatically from all available Landsat imagery since the late 1980s. We more than double the known GLOF count and identify the southern Himalayas as a hotspot region, compared to the more rarely affected Hindu Kush–Karakoram ranges. Nevertheless, the average annual frequency of 1.3 GLOFs has no credible posterior trend despite reported increases in glacial lake areas in most of the HKKHN3,8, so that GLOF activity per unit lake area has decreased since the late 1980s. We conclude that learning more about the frequency and magnitude of outburst triggers, rather than focusing solely on rapidly growing glacial lakes, might improve the appraisal of GLOF hazards. Melting glaciers are increasing Himalayan glacial lakes and potentially the risk of outburst floods. An advanced automated algorithm identifies glacial lake outburst floods from Landsat images since the late 1980s to improve understanding of these events and trends in their frequency.

The glaciers of the Hindu Kush-Himalayas (HKH) are nature's renewable storehouse of fresh water. They serve as the perennial sources of rivers that are used as renewable sources for irrigation, drinking water, energy, and industry. However, these glaciers are retreating; the resultant long-term loss of natural fresh water storage will have as yet uncalculated effects. More immediately, as they retreat, glacial lakes can form behind the exposed and unstable terminal moraines. Sudden breaching of these can result in the discharge of huge amounts of water and debris -- a glacial lake outburst flood or GLOF -- often with catastrophic effects downstream.There is still no detailed inventory of glaciers and glacial lakes, of GLOF events or of potential GLOF sites, in the HKH region. These two publications are designed to start filling this gap. Supported by the United Nations Environment Programme Regional Resource Centre for Asia and the Pacific (UNEP/RRC-AP), ICIMOD has used its expertise in the area of geographic information systems (GIS) to create a comprehensive inventory and GIS database of glaciers and glacial lakes in Nepal and Bhutan using available maps, satellite images, aerial photographs, reports, and field data on different scales. In Nepal, 3,252 glaciers were identified and 2,323 glacial lakes, of which 20 were considered potentially dangerous. In Bhutan, 677 glaciers were identified and 2,674 glacial lakes, of which 24 were considered potentially dangerous.These results will provide the basis for the planning and prioritisation of disaster mitigation efforts as well as a guide for infrastructure planning. They will help scientists, planners, and decision-makers in many areas, particularly water resources and land-use planning. The publications include a description of the methods used to identify glaciers, glacial lakes, and glacial lakes that may pose a threat; as well as an inventory (and maps) of the glaciers and glacial lakes in each country. The database and analysis are the first to cover the whole of the countries on a large scale.

Abstract. The Himalayas of South Asia are home to many glaciers that are retreating due to climate change and causing the formation of large glacial lakes in their absence. These lakes are held in place by naturally deposited moraine dams that are potentially unstable. Specifically, an impulse wave generated by an avalanche or landslide entering the lake can destabilize the moraine dam, thereby causing a catastrophic failure of the moraine and a glacial lake outburst flood (GLOF). Imja-Lhotse Shar Glacier is amongst the glaciers experiencing the highest rate of mass loss in the Mount Everest region, in part due to the expansion of Imja Tsho. A GLOF from this lake may have the potential to cause catastrophic damage to downstream villages, threatening both property and human life, which prompted the Nepali government to construct outlet works to lower the lake level. Therefore, it is essential to understand the processes that could trigger a flood and quantify the potential downstream impacts. The avalanche-induced GLOF process chain was modeled using the output of one component of the chain as input to the next. First, the volume and momentum of various avalanches entering the lake were calculated using Rapid Mass Movement Simulation (RAMMS). Next, the avalanche-induced waves were simulated using the Basic Simulation Environment for Computation of Environmental Flow and Natural Hazard Simulation (BASEMENT) model and validated with empirical equations to ensure the proper transfer of momentum from the avalanche to the lake. With BASEMENT, the ensuing moraine erosion and downstream flooding was modeled, which was used to generate hazard maps downstream. Moraine erosion was calculated for two geomorphologic models: one site-specific using field data and another worst-case based on past literature that is applicable to lakes in the greater region. Neither case resulted in flooding outside the river channel at downstream villages. The worst-case model resulted in some moraine erosion and increased channelization of the lake outlet, which yielded greater discharge downstream but no catastrophic collapse. The site-specific model generated similar results, but with very little erosion and a smaller downstream discharge. These results indicated that Imja Tsho is unlikely to produce a catastrophic GLOF due to an avalanche in the near future, although some hazard exists within the downstream river channel, necessitating continued monitoring of the lake. Furthermore, these models were designed for ease and flexibility such that local or national agency staff with reasonable training can apply them to model the GLOF process chain for other lakes in the region.

Inventory and recently increasing GLOF susceptibility of glacial lakes in Sikkim, Eastern Himalaya

In Hindu Kush Himalayas (HKH) region, observations on glacial fluctuations indicate substantial retreat of glaciers, especially in Pakistan, Nepal, India and China. Recent global temperature rise has allegedly been responsible for the depletion of these glaciers and consequently creation of lakes on their terminus. Several of these lakes have burst and caused flooding or Glacial Lake Outburst Floods (GLOFs) in the recent past. GLOFs have a potential of releasing millions of cubic meters of water in a few hours causing catastrophic flooding downstream and damaging whatever comes into their way. Hazard mapping of elements at risk may enhance the capacity of the vulnerable communities to face these disasters in order to reduce their devastating impacts. Study of GLOFs hazard in Hunza River basin of Pakistan using geospatial techniques consisting of satellite remote sensing, Geographical Information System (GIS), hydraulic and hydrology tools is proposed in this paper. Geospatial techniques have a capacity to acquire information regarding the status of glaciers and glacier lakes at spatial and temporal resolutions beyond the capability of infrequent and point scale in-situ monitoring. Remotely sensed satellite data along with ground based data are used in this study for risk mapping of Passu Lake in Hunza River basin, Pakistan. The outcomes of the proposed study will be helpful for GLOFs risk management and an overall strategy to address possible risks from future GLOFs events in the country.

Glacial lake outburst floods threaten China-Nepal connectivity: Synergistic study of remote sensing, GIS and hydrodynamic modeling with regional implications

Glacial lakes are not only the important refresh water resources in alpine region, but also act as a trigger of many glacial hazards such as glacial lake outburst flood (GLOF) and debris flow. Therefore, glacial lakes play an important role on the cryosphere, climate change and alpine hazards. In this paper, the issues of glacial lake were systematically discussed, then from the view of glacial lake inventory and glacial lake hazards study, the glacial lake was defined as natural water mainly supplied by modern glacial meltwater or formed in glacier moraine’s depression. Furthermore, a complete classification system of glacial lake was proposed based on its formation mechanism, topographic feature and geographical position. Glacial lakes were classified as 6 classes and 8 subclasses, i.e., glacial erosion lake (including cirque lake, glacial valley lake and other glacial erosion lake), moraine-dammed lake (including end moraine-dammed lake, lateral moraine-dammed lake and moraine thaw lake), ice-blocked lake (including advancing glacier-blocked lake and other glacier-blocked lake), supraglacial lake, subglacial lake and other glacial lake. Meanwhile, some corresponding features exhibiting on remote sensing image and quantitative indices for identifying different glacial lake types were proposed in order to build a universal and operational classification system of glacial lake.

<p>No continuously updated glacier and glacial lake inventories exist for Norway. Previous inventories have been developed for the time periods of 1947-1985, 1988-1997 and 1999-2006 for glaciers and 1988-1997, 1999-2006, 2014 and 2018 for glacial lakes, by manual digitization, and semi-automated mapping. However, these methods are both time consuming and do not allow for an analysis of glacial lake behaviour on shorter timescales or on a seasonal basis. Therefore, one aim of this study is to present consistent inventories for glaciers and glacial lakes in Norway using semi-automated mapping and machine learning techniques applied on satellite imagery of different spatial and temporal resolution (Landsat 30m, 16 days, and Sentinel 10m, 5 days). An automated method that allows frequent monitoring of glacier variables can provide essential knowledge for the understanding of glacial lake dynamics in a changing climate.</p><p>In addition to glacial lake inventories, smaller ice caps with active glacial lakes are investigated more closely, aiming at following the development of glacial lakes throughout seasons. Here we are also analyzing the suitability of PlanetScope imagery compared to the Sentinel and Landsat imagery to detect the known glacial lake outburst flood events and identify currently unrecognized hazard-prone glacial lakes. Since the field-based investigations of glacial lake changes (especially of the ice-dammed lakes) are sparse in Norway, developing methods for remote-sensed, automated monitoring of glacial lake changes and glacial lake outburst floods is essential in order to develop early warning systems, detect potentially hazardous lakes and prevent human losses and damages to infrastructure and local businesses.</p>

Hydrodynamic moraine-breach modeling and outburst flood routing - A hazard assessment of the South Lhonak lake, Sikkim