the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Feb 2025
Assessing economic impacts of future GLOFs in Nepal’s Everest region under different SSP scenarios using three-dimensional simulations
Abstract. This study investigates simulated glacial lake outburst floods (GLOFs) at five glacial lakes in the Everest region of Nepal using the three-dimensional model OpenFOAM. It presents the evolution of GLOF characteristics in the 21st century considering different moraine breach scenarios and two Shared Socioeconomic Pathways scenarios. The results demonstrate that in low-magnitude scenarios, the five lakes generate GLOFs that inundate between 0.35 and 2.23 km2 of agricultural land with an average water depth of 0.9 to 3.58 meters. These GLOFs reach distances of 59 to 84 km, affect 30 to 88 km of roads, and inundate 183 to 1,699 buildings with 1.2 to 4.9 m of water. In higher scenarios, GLOFs can extend over 100 km and also affect larger settlements in the foothills. Between 80 and 100 km of roads, between 735 and 1,989 houses and between 0.85 and 3.52 km2 of agricultural land could be inundated, with average water depths of up to 10 meters. The high precision of the 3D flood modeling, with detailed simulations of turbulence and viscosity, provides valuable insights into 21st-century GLOF evolution, supporting more accurate risk assessments and effective adaptation strategies.
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RC1: 'Comment on egusphere-2025-50', Adam Emmer, 21 Mar 2025
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This study models potential future GLOFs from three existing and two future glacial lakes in Nepal. This study brings novelty while integrating OpenFOAM modelling with SSP scenarios. The study details the impacts of potential GLOF scenarios on people and infrastructure while it fails to justify some of the basic assumptions.
Strikingly, the breach scenarios (Table 1) are defined regardless moraine dam geometry, physical limits of breach development, internal structure and possibly overdeepened bedrock terrain. Why 30, 60 and 90 m? Why not 20, 40 or 60 m? Or 10, 20 and 30 m? It is important to highlight that anyhow sophisticated modelling outcomes are totally dependent on rather arbitrary definition of these breach scenarios. It is also important to highlight that these scenarios have different probabilities for individual studied lakes and that some are not even realistic.
Now what is called BR1 (lower boundary; 30 m breach depth) is already pretty harsh scenario and the term “lower boundary” is misleading in this context. How many examples of 30 m deep breaches do we have from lakes of similar size and topographic setting? I don't think about many. The BR2 (upper boundary; 60 m breach depth) is not only unlikely but also unrealistic for lakes with flat and wide dam geometry (such as Imja which dam height is 55 m, according to 10.5194/hess-29-733-2025 or 35 m according to 10.5194/hess-19-1401-2015).
For a comparison, the breach which developed during the 2023 South Lhonak GLOF – the largest GLOF from a moraine-dammed lake in High Mountain Asia in past decades – is 55 m deep (see 10.1126/science.ads2659). The use of as extreme scenario as BR3 (90 m breach depth) needs special justification on a case-by-case basis.
In conclusion, in the study of 5 lakes, breach scenarios should rather be tailored to specific dam properties of individual studied lakes and I encourage the authors to address this issue (I recommend major revisions). Thank you.
Citation: https://doi.org/10.5194/egusphere-2025-50-RC1 -
AC1: 'Reply on RC1', Wilhelm Furian, 27 Mar 2025
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Dear Mr Emmer,
We sincerely thank you for recognizing the potential and novelty of our study. You are absolutely right that the morphological breach parameters have not been sufficiently explained, and that this lack of detail reduces the clarity of our approach. We will address your comments by incorporating the following clarifications into our manuscript, which we hope will improve the transparency of our methodology and clarify our assumptions and choices.
Regarding your second paragraph:
You are absolutely right in pointing out that our rationale for choosing these exact breach parameters was not sufficiently explained. We address this in the following paragraphs:
Due to the requirements of working with OpenFOAM, several concessions have to be made when creating breach scenarios. As we are working with this 3D numerical model, several options that can be included in 2D approaches are not available to us. We cannot define a desired hydrograph for each lake or couple a designated breach model to the simulation. Rather, the hydrograph in OpenFOAM results from the breach scenario and the lake volume and is not specified directly.
Therefore, in our approach, the DEM for each simulation run, including the moraine breach, has to be manually created and transformed into an STL surface. As we ran almost 100 simulations, it would not be feasible to define individual moraine breaches for every lake in every scenario, taking into account, e.g., the changing moraine structure, the growing lake volume and depth etc.
We fully agree that the magnitude of the simulated floods mostly depends on the chosen breach scenarios and mention this in Table 4. However, we recognize the need to further explain the reasoning behind our choices and argue for their validity:
Since we chose to use a 3D model to improve the simulation accuracy, we had to limit the number of breach scenarios in order not to increase the computational time beyond the 10-12 weeks that the simulations already had to run (excluding the time needed to set-up the simulations).
The chosen breach scenarios follow 10.1016/j.jhydrol.2021.126208 (Sattar et al. 2021), who provide numerous GLOF scenarios for Lower Barun Lake. We excluded the lower and higher estimates of their study, as the higher ones seem unrealistic and the impact of the lower scenarios would be too small to justify the longer computation time. We therefore chose two main depths: 30 and 60 m, and a third extreme depth of 90 m.The 30m breach was chosen because it represents a significant, but not extreme, moraine breach. The 60m breach was chosen as the upper limit of GLOF events (as seen at South Lhonak Lake, 10.1126/science.ads2659). The 90m scenario was chosen as a potential GLOF of extreme magnitude, theoretically possible at all lakes except Imja Tsho. However, due to its unlikely nature, we did not evaluate this scenario further. Instead, we provide the results in the supplementary material.
As a degree of generalization is required for large-scale OpenFOAM simulations, we use these breach parameters for all five lakes. Apart from the fact that it would not be feasible to individually create different moraine breaches for our 3D approach, many of the parameters needed to delineate individual breaches are not easily quantifiable for the future scenarios: Internal moraine ice can melt, and moraines can be damaged by earthquakes or lowered by internal piping, etc.
However, this is not made clear in our manuscript and we will add a subchapter detailing the reasoning behind the choice of these breach parameters and justifying their use for each lake.We do not aim to investigate the probabilities of specific events, as it would be beyond the scope of our study to approximate the necessary parameters for the whole of the 21st century. However, this is not made sufficiently clear in our manuscript and we thank you for pointing this out. We will include a clearer description of the objectives of our study and a more substantial justification for our choice of breach parameters.
Regarding your third paragraph:
We agree that the phrase "lower boundary" is misleading for a 30m breach, as it may well be the lower boundary in this study, but not in the wider scientific context. We will rephrase the relevant paragraphs.
You are absolutely right, a 60m breach would be very unlikely at Imja Tsho, as it would represent more than a complete moraine incision. We used an approach similar to the recently published 10.5194/hess-29-733-2025, where “the maximum breach depth is considered to reach the marine dam’s maximum height and extend from the dam crest down to the point where the hummocky terrain ends” (p. 737). However, we have failed to explain this in detail and will therefore adapt our manuscript to exclude the 60m scenario for this lake.
According to 10.5194/hess-29-733-2025, the results of the 30m breach are within realistic boundaries at Imja Tsho. Our study found ~11,800 m3 s−1 peak discharge for a 30m breach, while Chen et al. (2025) estimated a peak discharge of between 8,000 and 30,000 m3 s−1 for a full breach, with a mean of ~15,000 m3 s−1.
For Tsho Rolpa, both 30m and 60m breaches are possible and our results (regarding discharge and inundation depth) are in good agreement with previous studies:
10.13101/ijece.5.123 found 90,000 m3 s−1 peak discharge in the highest scenario vs. 81,000 m3 s−1 peak discharge in our highest scenario
10.5194/hess-29-733-2025 give 13,000 to 15,000 m3 s−1 as the peak discharge vs. the ~12,300 m3 s−1 peak discharge in our study
10.5194/piahs-387-59-2024 used 20m and 40m breach depth, and for both inundation depth and discharge our results agree well with theirs.We will adapt the relevant parts of the manuscript and include a subchapter comparing the performance of our model, thus justifying the use of the 30m and 60m breach for four lakes and the use of 30m at Imja Tsho.
Regarding your fourth paragraph:
We have already mentioned the unrealistic nature of the 90m breach in the manuscript, but we will rephrase the sentences to emphasise that we are excluding it from our analysis for this reason.
We thank you again for your valuable comments, which help to improve the quality of our manuscript. We hope that our clarifications and adjustments address your concerns and look forward to your response.
Best regards -
Wilhelm FurianCitation: https://doi.org/10.5194/egusphere-2025-50-AC1
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AC1: 'Reply on RC1', Wilhelm Furian, 27 Mar 2025
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CC1: 'Comment on egusphere-2025-50', Nitesh Khadka, 28 Mar 2025
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The paper provides new insights into GLOF modeling and the resulting socio-economic consequences in the Nepal Himalaya. I have a few suggestions:
- How does the used model depict accurate GLOF rheology (how sediments are incorporated in the flow)?
- Can we expect inundation of 20 m only for large lakes such Ngojumba Tsho?
- Trekking trails are shown as roads, such as in Phakding (Figure 8)? Will it not have impact when carrying out the socio-economic (monetary) assessment?
Citation: https://doi.org/10.5194/egusphere-2025-50-CC1
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