Himalayan Hazards Hub

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.

The Lunana region in Bhutan, which hosts four large glacial lakes with significant hazard potential, has undergone rapid changes over the past decade. Using PlanetScope satellite scenes, we mapped ice velocities at monthly intervals from 2017 to 2023. We reveal that the disintegration of Thorthormi Glacier’s terminus in 2022 coincided with year-on-year acceleration with mean surface velocities as high as 448 ±10.0 m a-1 by 2021, and seasonal variability in surface velocity magnitude >144.6 ±10.0 m a-1. This acceleration is attributed to a reduction in basal drag as the terminus reached flotation, evidenced by the calving of tabular icebergs. While Bechung, Raphstreng, and Lugge exhibited a similar interannual velocity trend, the upper regions of Bechung and Raphstreng showed a higher seasonal range (31% and 19.9% from their mean) compared to Lugge (4.2%). In the upper regions we also find a decelerating velocity trend (3.5 – 20.6% over the 6 years), which is attributed to surface thinning and reducing driving stresses. We show that accelerating trends in velocity can be a precursor to higher rates of retreat and rapid lake expansion, demonstrating the importance of continuous monitoring of lake-terminating glacier ice velocities in the Himalaya.

Modelling complex mass flow processes, such as glacial lake outburst floods (GLOFs), for hazard and risk assessments requires extensive data and computational resources. Researchers often rely on low-resolution, open-access datasets and parameters derived from plausibility due to the difficulty involved in conducting direct measurements. This results in considerable uncertainties in forward modelling, potentially limiting the accuracy and reliability of predictions. To determine the sensitivity of the model outputs stemming from input parameter uncertainties in the forward modelling, we selected 9 parameters relevant to GLOF modelling and performed a total of 84 simulations, each representing a unique GLOF scenario in the physically based r.avaflow model. Our results indicate that mass-movement-triggered moraine-dammed GLOF modelling outputs are notably sensitive to five parameters, which are, in order of importance: (1) volume of mass movement entering the lake, (2) DEM datasets, (3) origin of mass movement, (4) entrainment coefficient, and (5) basal friction angle. The GLOF output parameter resulting from the volume of mass movement entering the lake has the greatest coefficient of variation (CV) (47 %), while the internal friction angle had the lowest CV (0.4 %). For future GLOF modelling, we recommend carefully considering the output uncertainty stemming from the sensitive input parameters identified here, some of which cannot be constrained before a GLOF and which must be addressed using statistical approaches.

Climate change is causing Himalayan glaciers to shrink rapidly and natural hazards to increase, whilst downstream exposure is growing. Glacier shrinkage promotes the formation of glacial lakes, which can suddenly drain and produce glacier lake outburst floods (GLOFs). Bhutan is one of the most vulnerable countries globally to these hazards. Here we use remotely sensed imagery to quantify changes in supraglacial water storage on Tshojo Glacier, Bhutan, where previous supraglacial pond drainage events have necessitated downstream evacuation. Results showed a doubling of both total ponded area (104,529 m2 to 213,943 m2) and its standard deviation (64,808 m2 to 158,550 m2) between the periods 1987-2003 and 2007-2020, which was predominantly driven by increases in the areas of the biggest ponds. These ponds drained regularly and have occupied the same location since at least 1967. Tshojo Glacier has remained in the first stage of proglacial lake development for 53 years, which we attribute to its moderate slopes and ice velocities. Numerical modelling shows that pond outbursts can reach between ~6 and 47 km downstream, impacting the remote settlement of Lunana. Our results highlight the need to better quantify variability in supraglacial water storage and its potential to generate GLOFs, as climate warms.