Comment on tc-2022-62

Glacial lake outburst floods (GLOFs) are a serious potential threat to the safety of life and property in downstream areas. In this study, moraine-dammed glacial lakes in the Yi’ong Zangbo River basin were recognized based on Landsat ETM+/TM/OLI images in 2000 and 2019. Also, GLOFs for the Jionglaco, the largest glacial lake in this basin, were simulated using the one-dimensional hydrodynamic model. The results show that the total number and area of moraine-dammed glacial lakes in this basin increased by 10 (10.52%) and 5.49 km2 (48.24%) from 2000 to 2019, respectively, in which the area of the Jionglaco increased by 3.22 km2. The peak discharge at the breach outlet for five scenarios with different combinations of breach width (80 and 120 m), depth (2.5 and 5 m), and flood time (1.5 and 3 h) are 489 , 1,327.43, 444.32, 617.47, and 1,570.61 m3/s. With the addition of baseflow in the river, the peak discharge at bridge site 15,138.93 km from Jionglaco, is 1,040.89, 1,724.00, 1,024.85, 1,162.25, and 1,990.52 m3/s. The combination of baseflow in river and the GLOF discharge results in an increasing peak discharge in the further downstream region. However, the arrival of peak discharge in downstream areas is delayed, which increases the chances of people escaping.

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.

Cryosphere-related hazards are a growing but largely neglected threat for rural settlements, agrarian land use and local livelihoods in the cold-arid Trans-Himalayan region of Ladakh. Despite the growing number of studies on cryosphere-related hazards across High Mountain Asia and other glacierized mountain regions, the occurrence, frequency and magnitude of glacial lake outburst floods (GLOFs) are almost entirely overlooked for the region of Ladakh. Due to the small size and high elevational location of glaciers above 5200 m a.s.l. also the glacial lakes are of small size and some of them are almost permanently ice-covered. In the recent past several GLOF events occurred which destroyed infrastructure and agricultural area. It becomes obvious that even these small glacial lakes might be a permanent threat for local livelihoods and socioeconomic development. This is even more problematic as the number and size of lakes has significantly increased over the past decades. Many of these lakes are dammed by ice-cored moraines which tend to become instable due to climate warming. A comprehensive inventory of glacial lakes for the entire Trans-Himalayan region of Ladakh was carried out. This includes several almost permanently ice-covered high altitude lakes, which have to be detected by visual image interpretation. Changes in the extent and number of glacial lakes have been quantified for the years 1969, 1993, 2000/02 and 2018 in order to assess the potential threat of future GLOFs in the region. A total of 192 glacial lakes cover an area of 5.93 ± 0.70 km2 with an estimated water volume of about 61.11 ± 8.5 million m3, including 127 proglacial (PG) and 56 lakes located on recent moraines (RM) were mapped in 2018. The change detection analyses also indicated the disappearance of 22 glacial lakes (decrease by more than 90%) between 1969 and 2018. The lake development of selected former reported GLOF events were analysed in detail to reconstruct lake level changes which possibly indicate earlier GLOF events. Based on high temporal resolution remote sensing data, a sophisticated monitoring concept needs to be realized to indicate the development of short-lived lakes on glaciers or on debris landforms with buried ice or fast glacial lake growth.

<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>

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

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.

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.

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.

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.

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

Glacial lake outburst floods (GLOFs) pose an increasing hazard to communities living downstream of glaciated areas of the Northern Patagonia Icefield (NPI). The proliferation of GLOF events poses a growing hazard to human development and infrastructure which is rapidly expanding into formerly isolated areas. The outlet glaciers on the eastern flank of the NPI, in particular, have experienced an increase in the frequency of GLOFs, threatening downstream communities with the potential for future events. Using a satellite-derived DEM and UAV imagery, we evaluated flood parameters of a 16 March 1989 GLOF event in the Valle Soler. Calculated flood volume was in excess of 140 × 106 m3 while the peak discharge was approximately 20,000 m3/s. This newly calculated flood discharge value distinguishes this event as the largest recorded glacial moraine outburst flood in the literature and is significantly larger than previous estimates (1,800–2,000 m3/s) for this incident.

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

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.