the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jun 2026
Two-Dimensional Hydrodynamic Modelling of Glacial Lake Outburst Flood Scenarios from Chubda Tsho: Breach Sensitivity and Downstream Flood Dynamics in the Chamkhar Chhu Basin, Bhutan
Abstract. Glacial lake outburst floods (GLOFs) are high-magnitude, short-duration hazards in glacier-fed Himalayan basins, where steep valley confinement can amplify downstream flood intensity. Despite the identification of Chubda Tsho as a potentially dangerous glacial lake in Bhutan, hydrodynamic assessments of potential outburst floods from this lake have not yet been reported. This study evaluates potential GLOF scenarios from Chubda Tsho using inflows generated by a calibrated and independently validated Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS), coupled with two-dimensional (2D) simulations in the Hydrologic Engineering Center's River Analysis System (HEC-RAS). The model was calibrated against the July 2007 flood (NSE = 0.90) and validated using the August 2015 event (NSE = 0.56). Hydrodynamic performance was assessed using an extreme-event hydrograph from Cyclone Aila (2009) and compared with observed inundation extent. Results show good agreement, with a Critical Success Index (CSI) of 0.67, a total area difference of 2.56%, and close correspondence between simulated (1.60 km²) and observed (1.56 km²) inundation extents within the 16.14 km² Chamkhar Valley model domain. The Chamkhar Valley contains approximately 8.41 km² of habitable area. Three breach scenarios representing 50%, 75%, and 100% lake-volume release were simulated using empirically derived breach relationships. Flood waves reach the Chamkhar Valley between approximately 1 h 50 min and 2 h 15 min after breach initiation. Peak discharge increases from 2,114 m³ s⁻¹ to 5,258 m³ s⁻¹, accompanied by nonlinear increases in peak depth (8.4–12.8 m) and velocity (5.0–7.6 m s⁻¹). While inundation extent remains constrained by valley geometry, flood depth and velocity increase disproportionately with breach magnitude. Results indicate that downstream hydraulic intensity is strongly influenced by breach magnitude, while warning time remains limited in confined Himalayan valleys. This highlights the value of validated 2D hydrodynamic modelling for GLOF hazard assessment in Himalayan catchments with limited observational data.
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Status: open (until 16 Jul 2026)
- RC1: 'Comment on egusphere-2026-2584', Anonymous Referee #1, 07 Jun 2026 reply
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RC2: 'Comment on egusphere-2026-2584', Anonymous Referee #2, 24 Jun 2026
reply
Summary
The study by Namgay et al. (2026) models three different glacial lake outburst flood (GLOF) scenarios originating from Chubda Tsho, located in the headwaters of the Chamkar Chu basin, using HEC-HMS and HEC-RAS. While the topic is highly relevant to local communities and disaster risk management in Bhutan, the manuscript currently offers limited scientific novelty in terms of methodology, analysis, and contribution to the existing body of knowledge on GLOF hazard and risk assessments. I encourage the authors to better articulate the novelty and significance of their work and to address the following comments before the manuscript can be considered for publication.
Major Comments
1. Chubda Tsho glacial lake characterization
The manuscript provides insufficient information on Chubda Tsho itself. The authors should include details on:
- The origin and formation history of the lake.
- Lake type (e.g., moraine-dammed, ice-contact, etc.).
- Historical lake expansion and development rates.
- The basis on which the lake was identified as a potentially dangerous glacial lake.
- Current lake area and volume, including methods used to derive these estimates.
- The area–volume relationship applied in the study.
- The reference year(s) used for lake area and volume estimates.
- Potential future lake expansion and corresponding volume increases.
- The likely maximum extent of future lake growth.
- In addition, more information on the parent glacier (Chubda Glacier) should be provided, including available estimates of glacier mass balance, velocity, thickness, and recent changes derived from remote sensing studies.
2. Flood and rainfall events used for calibration and validation
The manuscript provides insufficient information regarding the flood events used in the modelling framework (e.g., the 2007 flood, the 2015 event, and Cyclone Aila). The authors should clearly describe:
- The characteristics of each event.
- The hydrometeorological parameters used.
- Data sources and the agencies or organizations responsible for data collection.
- The rationale for selecting certain events for calibration and others for validation.
A more transparent explanation of the calibration and validation strategy is necessary.
3. Hydrograph assumptions and model validation
A fundamental difference exists between rainfall-induced flood hydrographs and GLOF hydrographs. The manuscript states that: “Hydrodynamic validation relied on the extreme rainfall-driven flood associated with Cyclone Aila (2009), in the absence of documented historical GLOF observations for the study basin.” This approach requires stronger justification because rainfall floods and moraine-dam breach floods are governed by different hydrological and hydraulic processes and often exhibit substantially different hydrograph shapes. I recommend validating the modelling framework against the well-documented 1994 Lugge Tsho GLOF in Bhutan, for which inundation extent and other observational datasets are available. Such validation would provide greater confidence in the model's ability to simulate GLOF dynamics.
Furthermore, the manuscript assumes that: “Synthetic outburst hydrographs were constructed assuming a single-peak breach process, with peak discharge occurring near the midpoint of breach development.” The authors should provide stronger justification for this assumption. Moraine-dam breach hydrographs are often characterized by a very steep rising limb and rapid peak discharge development. The current assumptions may not adequately capture the hydrograph characteristics of a moraine-dam failure. A comparison with documented GLOF hydrographs from similar moraine-dammed lakes would strengthen the analysis.
4. Downstream risk assessment
The study focuses primarily on flood propagation but provides limited assessment of downstream impacts. I recommend conducting a more comprehensive downstream risk assessment, similar to the approach adopted by Rinzin et al. (2023), including:
- Exposure of settlements and infrastructure.
- Potential impacts on hydropower facilities, roads, bridges, and agricultural land.
- Population at risk.
- Hazard and risk zonation.
Such an analysis would significantly enhance the practical relevance of the study.
5. Present-day and future GLOF scenarios
The manuscript would be substantially strengthened by incorporating future GLOF scenarios in addition to the present-day assessment. Given the ongoing expansion of Chubda Tsho, the authors should evaluate how future increases in lake area and volume may influence potential outburst magnitude, peak discharge, inundation extent, and downstream impacts. This could be achieved by projecting plausible future lake extents and volumes based on observed lake growth rates, glacier retreat, and topographic constraints.
Importantly, such an analysis could provide a clear element of scientific novelty for the study. While many existing GLOF assessments focus solely on present-day conditions, a comparative evaluation of present and future GLOF hazards would offer valuable insights into how climate-driven lake evolution may alter downstream flood risk over time. This would significantly enhance the study's relevance for long-term disaster risk reduction, infrastructure planning, and climate adaptation in the Chamkar Chu basin.
By explicitly comparing present-day and future hazard and risk scenarios, the authors would move beyond a conventional GLOF modelling exercise and provide a more forward-looking assessment of evolving cryospheric hazards.
6. Figure quality and visualization
All figures are currently below publication quality and require substantial improvement. In particular, Figures 3–7 and Figures 10–12 should be revised to improve:
- Resolution and readability.
- Map layout and cartographic quality.
- Color schemes and contrast.
- Label clarity and font size.
- Figure captions and explanatory content.
High-quality visualizations are essential for effectively communicating the modelling results and their implications.
Overall Recommendation
The study addresses an important hazard issue for the Chamkar Chu basin and downstream communities. However, the manuscript would be substantially strengthened by providing a more comprehensive characterization of Chubda Tsho and its parent glacier, improving the justification and validation of the modelling framework, incorporating future lake development and GLOF scenarios, expanding the downstream risk assessment, and enhancing figure quality. Most importantly, the authors should clearly demonstrate the scientific novelty and added value of the study relative to existing GLOF modelling research.
Rinzin, S., Zhang, G., Sattar, A., Wangchuk, S., Allen, S. K., Dunning, S., and Peng, M.: GLOF hazard, exposure, vulnerability, and risk assessment of potentially dangerous glacial lakes in the Bhutan Himalaya, J. Hydrol., 619, 129311, https://doi.org/10.1016/j.jhydrol.2023.129311, 2023.
Citation: https://doi.org/10.5194/egusphere-2026-2584-RC2
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Review egusphere-2026-2584
The authors have chosen one of the lakes identified as potentially hazardous in Bhutan for a hydrodynamic simulation to assess flood extents under a variety of breach scenarios. They use the HEC modelling suite for both the estimated discharge peaks from a hydrological model (if I understand correctly) as well as the hydrodynamical modelling and come to the general conclusion that flood waves in the Chamkar Valley are rapid, leaving little time for early warning. In general, such studies are interesting and required to get a better understanding of potential risks and general preparedness. The manuscript is also very well written, and the figures are generally clear, hence leaving hardly anything for me to comment on in detail. However, there are four crucial shortcomings, that lead me to suggest a thorough rethinking of the scope before resubmitting.