A glacial lake outburst flood hazard assessment for the Phochhu River Basin, Bhutan

Wangchuk, Tandin; Tsubaki, Ryota

doi:https://doi.org/10.5194/nhess-2023-125

Preprints

https://doi.org/10.5194/nhess-2023-125

Preprints

10 Oct 2023

| 10 Oct 2023

Status: a revised version of this preprint was accepted for the journal NHESS and is expected to appear here in due course.

A glacial lake outburst flood hazard assessment for the Phochhu River Basin, Bhutan

Tandin Wangchuk and Ryota Tsubaki

Abstract. The melting of glaciers has led to an unprecedented increase in the number and size of glacial lakes, particularly in the Himalayan region. A Glacial Lake Outburst Flood (GLOF) is a natural hazard in which water from a glacial or glacier-fed lake is swiftly discharged. GLOFs can significantly harm life, infrastructure, and settlements located downstream, and can cause considerable ecological, economic, and social impacts. Based on a dam breach model, BREACH, and a hydrodynamic model, HEC-RAS, we examined the potential consequences of a GLOF originating from the Thorthomi glacial lake, located within the Phochhu River Basin, one of Bhutan’s largest and rapidly expanding glacial lakes. Our analysis revealed that, following a breach, the Thorthomi glacial lake will likely generate a peak flow of 16,360 m³ s⁻¹ within four hours. Such discharge could potentially cause considerable damage, with an estimated 245 hectares of agricultural land and over 1,277 buildings at risk for inundation. Our results emphasize an urgent need for understanding and preparing for the potential consequences of a GLOF from Thorthomi Lake in order to mitigate ecological, economic, and social impacts on downstream areas. Our findings provide valuable insights for policymakers and stakeholders involved in disaster management and preparedness.

Received: 24 Jul 2023 – Discussion started: 10 Oct 2023

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Tandin Wangchuk and Ryota Tsubaki

Status: closed

RC1:
'Comment on nhess-2023-125', Stefan Ram, 19 Nov 2023

The authors describe a GLOF simulation by coupling a dam breach model with a hydrodynamic model. They give a good overview about their methodologies which are up to date and widely used by scientists. The input data is suitable and well described. Due to a lack of data, many assumptions had been made on the glacial lake, the main driver of the model. Those assumptions are based on widely agreed formulations but they still don’t guarantee a perfect result.
The structure of the manuscript is clear at the beginning. However, from section 6 I feel there is some inconsistency. I expect section 6 (prediction of the Thortomi GLOF) to be similar structured as section 5 (reconstruction of the 1994 Lygge GLOF). This ensures an easier comparison between both simulations. Additionally, I wouldn’t put 6.5.1 and especially Table 4. In the result section as they are describing input data for the breach analysis. The conclusions are reasonable and clearly explained.
This work fills a gap of many GLOF studies in Bhutan that were already published. Its methodologies are very good and give robust results. There are some minor issued that need to be addressed before publishing this manuscript.
Minor issues:
1 – DSM
It’s not clear to me what elevation data was exactly used. I guess its AW3D Standard 2.5m DSM right? A 2.2m does not exist. The 2.2m is due to reprojection of the 2.5m WGS84 original DSM to UTM. Please indicate that.
Usually DTMs (not DSM) are needed for flood modelling. FABDEM and MERIT are also some kind of DTM as buildings and vegetation are removed. I see you did some editing to the riverbed so to flow propagation should be correct. But inundation mapping where you did not touch the DSM data might lead to wrong flood extents due to hedges and especially bridges that are still represented in the elevation data.
If you say "other" freely (line 209), then people might think that AW3D 2.5m is freely available but I think it’s not. Can you please clarify.
2 – Figures
Please revise figure 14 and figure 15. Make sure that the class breaks if the colour ramp are equal in all tiles of the figures. With an individual colorization in each image it is not possible to compare visually.
And the colour camp is inverted in section (d) of figure 15, please correct.
3 - Input data
I can’t find the amount of inflow at Mo Chu river. In figures 13, 14 and 15 it is clearly visible that there is flow that can’t be triggered by the GLOF. Especially for the interpretation of hydrographs downstream of Punaka Dzhong this is very important.
4 - Language
Generally, the language is very good and easy to understand.
I have only seen the acronym PFV was introduced in line 330 but it was never used again even though it is possible.
5 - Computation
I know HEC can simulate sub grid scale but 20m computation grid on a 2.5m raster seems to high as you lose the full power of the DSM resolution. And small structures like hedges and buildings might get lost.
6 - Other
Line 249: “Here, some small contributions from…”
Can you roughly indicate the contributions?
It’s strange to measure a peak flow of 2,539m³/s so far downstream while the estimated peak discharge was 1,800 to 2,500 m³/s.

Citation: https://doi.org/10.5194/nhess-2023-125-RC1
- AC1: 'Reply on RC1', Ryota Tsubaki, 28 Nov 2023
  
  (Hereafter, Italic typeface represents the comments from the reviewer) The authors describe a GLOF simulation by coupling a dam breach model with a hydrodynamic model. They give a good overview about their methodologies which are up to date and widely used by scientists. The input data is suitable and well described. Due to a lack of data, many assumptions had been made on the glacial lake, the main driver of the model. Those assumptions are based on widely agreed formulations, but they still don’t guarantee a perfect result.
  The structure of the manuscript is clear at the beginning. However, from section 6 I feel there is some inconsistency. I expect section 6 (prediction of the Thortomi GLOF) to be similar structured as section 5 (reconstruction of the 1994 Lygge GLOF). This ensures an easier comparison between both simulations. Additionally, I wouldn’t put 6.5.1 and especially Table 4. In the result section as they are describing input data for the breach analysis.
  (Hereafter, the bold type sentences are our reply to each comment.) Thank you for the suggestions to improve the structural consistency between sections 5 and 6. Section 6.5.1 and Table 4 describe data used in the following dam breach process modelling. However, the data described in section 6.5.1 and shown in Table 4 is also our important output obtained with the method described in section 6.2, so we locate section 6.5.1 (section 6.4.1 in the revised manuscript) in the result section.
  The conclusions are reasonable and clearly explained.
  This work fills a gap of many GLOF studies in Bhutan that were already published. Its methodologies are very good and give robust results. There are some minor issued that need to be addressed before publishing this manuscript.
  Minor issues:
  1 – DSM
  It’s not clear to me what elevation data was exactly used. I guess its AW3D Standard 2.5m DSM right? A 2.2m does not exist. The 2.2m is due to reprojection of the 2.5m WGS84 original DSM to UTM. Please indicate that.
  Usually DTMs (not DSM) are needed for flood modelling. FABDEM and MERIT are also some kind of DTM as buildings and vegetation are removed. I see you did some editing to the riverbed so to flow propagation should be correct. But inundation mapping where you did not touch the DSM data might lead to wrong flood extents due to hedges and especially bridges that are still represented in the elevation data.
  If you say "other" freely (line 209), then people might think that AW3D 2.5m is freely available but I think it’s not. Can you please clarify.
  The elevation data used is AW3D Standard 2.5m resolution DSM. The resolution changes depending on the location and 2.2 m is based on the actual resolution we used in this study. Additional explanations on the data resolution will be added to the revised document.
  While DTMs are required for the flood modelling, most of the freely available data represented the topography of the study area very poorly. Channel correction (riverbed) for the middle reach was made to enable proper flow propagation, and in other areas of the study reach, the DSM represented the channel adequately. The bridges and structures along the river channel were removed from the elevation data by ourselves. A description will be added in the revised manuscript.
  2 – Figures
  Please revise figure 14 and figure 15. Make sure that the class breaks if the colour ramp are equal in all tiles of the figures. With an individual colorization in each image, it is not possible to compare visually.
  And the colour camp is inverted in section (d) of figure 15, please correct.
  Figures 14 and 15 are revised to correct the error and to improve clarity.
  3 - Input data
  I can’t find the amount of inflow at Mo Chu river. In figures 13, 14 and 15 it is clearly visible that there is flow that can’t be triggered by the GLOF. Especially for the interpretation of hydrographs downstream of Punaka Dzhong this is very important.
  The inflow from Mochhu is not taken into consideration since the normal flow rate of the Mochhu River was is small (not for the future Thortomi GLOF but e.g. 96 m³/s during the 1994 Lugge GLOF, added in the revised manuscript) compared with the flow rate of GLOF. The inundation extent and depth in the Mochhu River is due to the backwater flow from the Phochhu River. This point will be added in the text.
  4 - Language
  Generally, the language is very good and easy to understand.
  I have only seen the acronym PFV was introduced in line 330 but it was never used again even though it is possible.
  Thank you very much for carefully checking the consistency. The acronym PFV in line 330 will be deleted in the revised manuscript.
  5 - Computation
  I know HEC can simulate sub grid scale but 20m computation grid on a 2.5m raster seems to high as you lose the full power of the DSM resolution. And small structures like hedges and buildings might get lost.
  We agree that using a larger computational grid size undermines the details of the topography represented in the high-resolution DSM. We tried to run a simulation with 10 m computational grid but it took over a week to complete the simulation. To reduce the computational time, 20 m grid was used in this study.
  6 - Other
  Line 249: “Here, some small contributions from…”
  Can you roughly indicate the contributions?
  It’s strange to measure a peak flow of 2,539m³/s so far downstream while the estimated peak discharge was 1,800 to 2,500 m³/s.
  The contribution from the Mochhu River (96 m³/s on 7^th October 1994) will be indicated in the revised manuscript. Regarding the contributions from the Dangchhu River, since there are no gauging stations in the river, it is difficult to guess the contribution from this river. The estimation of 1,800 to 2,500 m³/s was in the previous study. Our estimation is 2,455 m³/s and shows better agreement with the field record. (Not discussed here but we must also consider the uncertainty in the field record too).
  Finally, we would like to express our thanks to Mr. Stefan Ram for his careful and constructive comments on our manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-125-AC1
RC2:
'Comment on nhess-2023-125', Rayees Ahmed, 08 Dec 2023

The research article entitled “A glacial lake outburst flood hazard assessment for the Phochhu River Basin, Bhutan” addresses the concerning trend of glacial lake expansion in the Himalayan region due to glacier melting. It specifically discusses the potential risks associated with Glacial Lake Outburst Floods (GLOFs) and analyzes the potential consequences of a GLOF from the Thorthomi glacial lake in Bhutan's Phochhu River Basin using the BREACH dam breach model and HEC-RAS hydrodynamic model. The idea of the paper is appealing and well structured, however I would like authors to incorporate the following major and minor comments before it is considered for publication in NHESS.
Major Comments
In the Introduction section you have mentioned that approximately 17 million m³ of lake water was artificially released from the Thorthomi glacial lake which has lowered the water level of the lake. Did you consider this reduction of volume in your study because the volume of the water is the essential component for the GLOF simulation? How much is the current volume of water that you have used as an input to route the potential GLOF? The reduction in lake water volume due to engineering efforts may decrease the intensity of GLOF inundation parameters/flood wave (like flood extent, velocity, depth, etc).
Section 5.2.1. If the moraine dam is unconsolidated with loose material, then there may be a chance of piping failure as well. I suggest authors consider this mode of failure as well. Furthermore, you have not mentioned how you have calculated the parameters in Table 1. I would like to see the methods of calculation of these parameters in the revised paper. This will help other researchers to calculate these parameters while working on GLOF simulations.
Section 5.2.2. How did you prepare the breach hydrograph (inflow hydrograph)? Which parameters have you considered to prepare a breach hydrograph and how did you calculate those parameters? You need to provide a detailed methodology in this section.
You did not mention the percentage of water volume released from the lake.
Why have you separated the methodology and results for both lakes? You have mixed the methodology and results which makes it very difficult to understand what you are actually trying to convey. You need to be systematic and clear while revising the manuscript.
You should move all the literature to the Introduction section. In the methodology section, you need to be clear and crisp like why and how you have selected the method and how you have calculated them. I have noticed that you have started each section by referring to huge text derived from the literature.
The equation-derived depth and volume always provide overestimated values when you talk about GLOF simulation. Since you have mentioned bathymetry was not possible so have used empirical equations (section 6.2.2) but why have used equations proposed by Sakai (2012) to estimate the maximum depth and volume of the Thorthomi glacial lake when we have some recent integrated empirical equations developed by Qi et al., 2022 and
Line 375 why not piping failure?
Regarding bathymetry data, I suggest authors go through the GLOBathy data Khazaei et al, 2022 or other global bathymetry datasets for the comparative analysis.
Where is uncertainty analysis? The data and mythology utilized have several associated uncertainties. How did you carry out the uncertainty analysis?
You should provide a section on the limitations of the study and future research gaps.
Overall, the manuscript has several major and minor issues which need thorough revision.
Minor comments
Since you have envaulted the downstream impacts of the potential GLOF or you can say elements at risk therefore I suggest “Risk assessment” instead of hazard in the title.
Merge Figures 1 and 3
Figure 2 should be more illustrative
More key findings should be added in the abstract section.
You need to give language check throughout the text.
Line 90 you do not need to write the abbreviation of GLOF everywhere. Mention it in the beginning and then write the short form only I,e GLOF.
Line 135 include some recent studies
Line 160: Revisit the sentence it is not clear.
Repetition of the sentences like 330.
Table 4 M³ should be m³
Again, in Table 2 you have not mentioned how you have calculated these parameters.
Fig 9 nothing is clear.
Fig 16. The color scheme for Lulc is confusing.
The authors should check the grammar and language throughout the text.

Citation: https://doi.org/10.5194/nhess-2023-125-RC2
- AC2: 'Reply on RC2', Ryota Tsubaki, 06 Jan 2024
  
  The research article entitled “A glacial lake outburst flood hazard assessment for the Phochhu River Basin, Bhutan” addresses the concerning trend of glacial lake expansion in the Himalayan region due to glacier melting. It specifically discusses the potential risks associated with Glacial Lake Outburst Floods (GLOFs) and analyzes the potential consequences of a GLOF from the Thorthomi glacial lake in Bhutan's Phochhu River Basin using the BREACH dam breach model and HEC-RAS hydrodynamic model. The idea of the paper is appealing and well structured, however I would like authors to incorporate the following major and minor comments before it is considered for publication in NHESS.
  (Hereafter, bold-italic typefaced sentences indicate our replies to each comment. Line-numbers in our reply correspond to the line-number in the revised manuscript we will submit)
  
  Thank you for carefully reviewing our manuscript. We reply to each comment below.
  Major Comments
  In the Introduction section you have mentioned that approximately 17 million m3 of lake water was artificially released from the Thorthomi glacial lake which has lowered the water level of the lake. Did you consider this reduction of volume in your study because the volume of the water is the essential component for the GLOF simulation? How much is the current volume of water that you have used as an input to route the potential GLOF? The reduction in lake water volume due to engineering efforts may decrease the intensity of GLOF inundation parameters/flood wave (like flood extent, velocity, depth, etc).
  As indicated in Table 2, water volume for the Thorthomi glacial lake was estimated as 400 million m3.
  Regarding water discharge from GLOF, we agree that such efforts is important to reduce the damage caused by a GLOF. However, our understanding of the damage potential for worst-case scenarios is still limited and was the focus of our study. Due to challenging working conditions and health issues, discharging water safely is even challenging today (lines 65-66).
  Section 5.2.1. If the moraine dam is unconsolidated with loose material, then there may be a chance of piping failure as well. I suggest authors consider this mode of failure as well.
  We agree that there is a potential for piping failures. However, wave overtopping is not a common trigger for dam failure (Neupane et al. 2019). Awal et al. (2010) summarized the causes of 20 GLOF events and reported that 80% of GLOF overtopping was caused by ice and rock avalanches. Begam et al. 2018 also deemed overtopping as the major failure type for GLOF events. Following these previous research studies, for our study, we selected overtopping as the cause of a GLOF. In the revised manuscript, this point is discussed in lines 94-96.
  Furthermore, you have not mentioned how you have calculated the parameters in Table 1. I would like to see the methods of calculation of these parameters in the revised paper. This will help other researchers to calculate these parameters while working on GLOF simulations.
  
  Section 5.2.2. How did you prepare the breach hydrograph (inflow hydrograph)? Which parameters have you considered to prepare a breach hydrograph and how did you calculate those parameters? You need to provide a detailed methodology in this section.
  To estimate the breach hydrograph used in Section 5.2.2, we used the method described in Section 5.2.1. A review of GLOF modelling is provided in Section 3.
  You did not mention the percentage of water volume released from the lake.
  For the worst-case scenario considered in this study, approximately 70% of total lake water was released. To describe this scenario, changes have been made in the manuscript (line 460).
  Why have you separated the methodology and results for both lakes? You have mixed the methodology and results which makes it very difficult to understand what you are actually trying to convey. You need to be systematic and clear while revising the manuscript.
  We understand that separating the Methods and Results sections is a typical paper format, especially for studies with one evaluated hypothesis. Our study consists of three study components: (1) comparing available methods (glacial lake bathymetry estimations, the dam break model, floods and others) and data, (2) validating composition (largely in Section 5), and (3) applying the method to a Thorthormi GLOF event (Section 6).　We prioritize study components in the manuscript structure rather than the separation of Methods and Results. We have outlined the structure of the manuscript in the Introduction (lines 107-109).
  You should move all the literature to the Introduction section. In the methodology section, you need to be clear and crisp like why and how you have selected the method and how you have calculated them. I have noticed that you have started each section by referring to huge text derived from the literature.
  As indicated in the previous comment, this study consists of three study components. We could move all of the scientific literature reviews for each study component to the Introduction, however, if we do so, we are afraid that the connection between the review and each study component will become difficult for the reader to follow. We have outlined the structure of the manuscript in the Introduction (lines 107-109).
  The equation-derived depth and volume always provide overestimated values when you talk about GLOF simulation. Since you have mentioned bathymetry was not possible so have used empirical equations (section 6.2.2) but why have used equations proposed by Sakai (2012) to estimate the maximum depth and volume of the Thorthomi glacial lake when we have some recent integrated empirical equations developed by Qi et al., 2022 and
  Thank you for introducing a great paper. When we conducted our study, this work was not available. We now mention it in lines 443-447.
  Line 375 why not piping failure?
  Based on a review of previous studies (Neupane et al. 2019, Awal et al. 2010, Begam et al. 2018) and to discuss the worst-case scenario, we discussed overtopping failure. This point has been added to lines 94-96.
  Regarding bathymetry data, I suggest authors go through the GLOBathy data Khazaei et al, 2022 or other global bathymetry datasets for the comparative analysis.
  Thank you for introducing a great paper. When we conducted our study, this work was unavailable. The dataset developed by Khazaei et al. 2022 does not cover the lake under consideration in this study, and the dataset is not specific to glacial lakes.
  Where is uncertainty analysis? The data and mythology utilised have several associated uncertainties. How did you carry out the uncertainty analysis?
  A brief analysis of the uncertainty determined through prediction and the confidence band has been described in lines 436-440.
  You should provide a section on the limitations of the study and future research gaps.
  The last part in the Conclusion provides limitations and the need for further research (lines 584-602), as well as the required engineering correspondence (lines 589-590).
  Overall, the manuscript has several major and minor issues which need thorough revision.
  Minor comments
  Since you have envaulted the downstream impacts of the potential GLOF or you can say elements at risk therefore I suggest “Risk assessment” instead of hazard in the title.
  Thank you for your comment. Our manuscript discusses hazards as well as potential damage caused by the Thorthomi GLOF. For the title, we considered replacing “hazard” with “risk” but decided to use “hazard” because 1) an assessment of Thorthomi GLOF hazards was the main point of our manuscript (a risk assessment was also important, but supplemental compared to the hazards assessment), 2) we did not discuss the probability of damage (sometimes “risk” is used for such contexts), and 3) the word “hazard” can also include“risk” (which, of course, depends on definitions).
  Merge Figures 1 and 3
  We prepared Figure 1 for the Introduction and Figure 3 of the target study site. We think keeping the present composition helps separate the introductory portion of the manuscript and the specific target of our study.
  Figure 2 should be more illustrative
  We understand the effectiveness of illustrative explanations. Figure 2 is a simple representation of the process flowchart highlighting required models and data. We think this simplified figure makes the overall structure and content of the paper simple to understand for all types of readers.
  More key findings should be added in the abstract section.
  Important results from our study include: (1) selecting and composing available good methods (glacial lake bathymetry estimations, dam break models, floods and others) and data, (2) validating the composition, and (3) applying the method to a Thorthormi GLOF event. Since these are the key components of our study, we used the beginning half of the abstract to explain this information.
  
  Main findings were reflected in the abstract section.
  You need to give language check throughout the text.
  We attempted to describe our results in clear language and sent the manuscript for proof editing prior to submission. The journal provides English editing during the proofing phase, so we believe the language will be as clear as possible prior to publication.
  Line 90 you do not need to write the abbreviation of GLOF everywhere. Mention it in the beginning and then write the short form only I,e GLOF.
  Thank you for pointing out this information. We have made the necessary changes.
  Line 135 include some recent studies
  Taylor et al, 2023 was cited in line 25. Three other recent studies were referenced in lines 94 to 96. Qi et al. 2022 was referenced in line 444.
  Line 160: Revisit the sentence it is not clear.
  The sentence has been edited (lines 173 to 175).
  Repetition of the sentences like 330.
  The connection of sentences has been improved (line 358-359).
  Table 4 M3 should be m3
  Units in the table have been corrected.
  Again, in Table 2 you have not mentioned how you have calculated these parameters.
  We have indicated estimated parameter sources below the table. Most of the parameters were obtained from published papers and reports. Parameters estimated by our study were obtained from the terrain model and the regression analysis discussed in the paper.
  Fig 9 nothing is clear.
  The journal suggests that figures be clear for readers with color vision deficiencies. Colors and textures were chosen following this suggestion.
  Fig 16. The color scheme for Lulc is confusing.
  The journal suggests that figures be clear for readers with color vision deficiencies. Colors and textures were chosen following this suggestion. The figure is revised to improve the visibility.
  The authors should check the grammar and language throughout the text.
  We attempted to describe our results in clear language and sent the manuscript for proof editing prior to submission. The journal provides English editing during the proofing phase, so we believe the language will be as clear as possible prior to publication.
  Lastly, we thank you for fair and constructive comments, which helped improve the manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-125-AC2
AC3: 'Comment on nhess-2023-125', Ryota Tsubaki, 23 Jan 2024

Dear all
Authors had replied to all comments, and waiting some further feedbacks.
Best regards,
Ryota Tsubaki

Citation: https://doi.org/10.5194/nhess-2023-125-AC3

Status: closed

RC1:
'Comment on nhess-2023-125', Stefan Ram, 19 Nov 2023

The authors describe a GLOF simulation by coupling a dam breach model with a hydrodynamic model. They give a good overview about their methodologies which are up to date and widely used by scientists. The input data is suitable and well described. Due to a lack of data, many assumptions had been made on the glacial lake, the main driver of the model. Those assumptions are based on widely agreed formulations but they still don’t guarantee a perfect result.
The structure of the manuscript is clear at the beginning. However, from section 6 I feel there is some inconsistency. I expect section 6 (prediction of the Thortomi GLOF) to be similar structured as section 5 (reconstruction of the 1994 Lygge GLOF). This ensures an easier comparison between both simulations. Additionally, I wouldn’t put 6.5.1 and especially Table 4. In the result section as they are describing input data for the breach analysis. The conclusions are reasonable and clearly explained.
This work fills a gap of many GLOF studies in Bhutan that were already published. Its methodologies are very good and give robust results. There are some minor issued that need to be addressed before publishing this manuscript.
Minor issues:
1 – DSM
It’s not clear to me what elevation data was exactly used. I guess its AW3D Standard 2.5m DSM right? A 2.2m does not exist. The 2.2m is due to reprojection of the 2.5m WGS84 original DSM to UTM. Please indicate that.
Usually DTMs (not DSM) are needed for flood modelling. FABDEM and MERIT are also some kind of DTM as buildings and vegetation are removed. I see you did some editing to the riverbed so to flow propagation should be correct. But inundation mapping where you did not touch the DSM data might lead to wrong flood extents due to hedges and especially bridges that are still represented in the elevation data.
If you say "other" freely (line 209), then people might think that AW3D 2.5m is freely available but I think it’s not. Can you please clarify.
2 – Figures
Please revise figure 14 and figure 15. Make sure that the class breaks if the colour ramp are equal in all tiles of the figures. With an individual colorization in each image it is not possible to compare visually.
And the colour camp is inverted in section (d) of figure 15, please correct.
3 - Input data
I can’t find the amount of inflow at Mo Chu river. In figures 13, 14 and 15 it is clearly visible that there is flow that can’t be triggered by the GLOF. Especially for the interpretation of hydrographs downstream of Punaka Dzhong this is very important.
4 - Language
Generally, the language is very good and easy to understand.
I have only seen the acronym PFV was introduced in line 330 but it was never used again even though it is possible.
5 - Computation
I know HEC can simulate sub grid scale but 20m computation grid on a 2.5m raster seems to high as you lose the full power of the DSM resolution. And small structures like hedges and buildings might get lost.
6 - Other
Line 249: “Here, some small contributions from…”
Can you roughly indicate the contributions?
It’s strange to measure a peak flow of 2,539m³/s so far downstream while the estimated peak discharge was 1,800 to 2,500 m³/s.

Citation: https://doi.org/10.5194/nhess-2023-125-RC1
- AC1: 'Reply on RC1', Ryota Tsubaki, 28 Nov 2023
  
  (Hereafter, Italic typeface represents the comments from the reviewer) The authors describe a GLOF simulation by coupling a dam breach model with a hydrodynamic model. They give a good overview about their methodologies which are up to date and widely used by scientists. The input data is suitable and well described. Due to a lack of data, many assumptions had been made on the glacial lake, the main driver of the model. Those assumptions are based on widely agreed formulations, but they still don’t guarantee a perfect result.
  The structure of the manuscript is clear at the beginning. However, from section 6 I feel there is some inconsistency. I expect section 6 (prediction of the Thortomi GLOF) to be similar structured as section 5 (reconstruction of the 1994 Lygge GLOF). This ensures an easier comparison between both simulations. Additionally, I wouldn’t put 6.5.1 and especially Table 4. In the result section as they are describing input data for the breach analysis.
  (Hereafter, the bold type sentences are our reply to each comment.) Thank you for the suggestions to improve the structural consistency between sections 5 and 6. Section 6.5.1 and Table 4 describe data used in the following dam breach process modelling. However, the data described in section 6.5.1 and shown in Table 4 is also our important output obtained with the method described in section 6.2, so we locate section 6.5.1 (section 6.4.1 in the revised manuscript) in the result section.
  The conclusions are reasonable and clearly explained.
  This work fills a gap of many GLOF studies in Bhutan that were already published. Its methodologies are very good and give robust results. There are some minor issued that need to be addressed before publishing this manuscript.
  Minor issues:
  1 – DSM
  It’s not clear to me what elevation data was exactly used. I guess its AW3D Standard 2.5m DSM right? A 2.2m does not exist. The 2.2m is due to reprojection of the 2.5m WGS84 original DSM to UTM. Please indicate that.
  Usually DTMs (not DSM) are needed for flood modelling. FABDEM and MERIT are also some kind of DTM as buildings and vegetation are removed. I see you did some editing to the riverbed so to flow propagation should be correct. But inundation mapping where you did not touch the DSM data might lead to wrong flood extents due to hedges and especially bridges that are still represented in the elevation data.
  If you say "other" freely (line 209), then people might think that AW3D 2.5m is freely available but I think it’s not. Can you please clarify.
  The elevation data used is AW3D Standard 2.5m resolution DSM. The resolution changes depending on the location and 2.2 m is based on the actual resolution we used in this study. Additional explanations on the data resolution will be added to the revised document.
  While DTMs are required for the flood modelling, most of the freely available data represented the topography of the study area very poorly. Channel correction (riverbed) for the middle reach was made to enable proper flow propagation, and in other areas of the study reach, the DSM represented the channel adequately. The bridges and structures along the river channel were removed from the elevation data by ourselves. A description will be added in the revised manuscript.
  2 – Figures
  Please revise figure 14 and figure 15. Make sure that the class breaks if the colour ramp are equal in all tiles of the figures. With an individual colorization in each image, it is not possible to compare visually.
  And the colour camp is inverted in section (d) of figure 15, please correct.
  Figures 14 and 15 are revised to correct the error and to improve clarity.
  3 - Input data
  I can’t find the amount of inflow at Mo Chu river. In figures 13, 14 and 15 it is clearly visible that there is flow that can’t be triggered by the GLOF. Especially for the interpretation of hydrographs downstream of Punaka Dzhong this is very important.
  The inflow from Mochhu is not taken into consideration since the normal flow rate of the Mochhu River was is small (not for the future Thortomi GLOF but e.g. 96 m³/s during the 1994 Lugge GLOF, added in the revised manuscript) compared with the flow rate of GLOF. The inundation extent and depth in the Mochhu River is due to the backwater flow from the Phochhu River. This point will be added in the text.
  4 - Language
  Generally, the language is very good and easy to understand.
  I have only seen the acronym PFV was introduced in line 330 but it was never used again even though it is possible.
  Thank you very much for carefully checking the consistency. The acronym PFV in line 330 will be deleted in the revised manuscript.
  5 - Computation
  I know HEC can simulate sub grid scale but 20m computation grid on a 2.5m raster seems to high as you lose the full power of the DSM resolution. And small structures like hedges and buildings might get lost.
  We agree that using a larger computational grid size undermines the details of the topography represented in the high-resolution DSM. We tried to run a simulation with 10 m computational grid but it took over a week to complete the simulation. To reduce the computational time, 20 m grid was used in this study.
  6 - Other
  Line 249: “Here, some small contributions from…”
  Can you roughly indicate the contributions?
  It’s strange to measure a peak flow of 2,539m³/s so far downstream while the estimated peak discharge was 1,800 to 2,500 m³/s.
  The contribution from the Mochhu River (96 m³/s on 7^th October 1994) will be indicated in the revised manuscript. Regarding the contributions from the Dangchhu River, since there are no gauging stations in the river, it is difficult to guess the contribution from this river. The estimation of 1,800 to 2,500 m³/s was in the previous study. Our estimation is 2,455 m³/s and shows better agreement with the field record. (Not discussed here but we must also consider the uncertainty in the field record too).
  Finally, we would like to express our thanks to Mr. Stefan Ram for his careful and constructive comments on our manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-125-AC1
RC2:
'Comment on nhess-2023-125', Rayees Ahmed, 08 Dec 2023

The research article entitled “A glacial lake outburst flood hazard assessment for the Phochhu River Basin, Bhutan” addresses the concerning trend of glacial lake expansion in the Himalayan region due to glacier melting. It specifically discusses the potential risks associated with Glacial Lake Outburst Floods (GLOFs) and analyzes the potential consequences of a GLOF from the Thorthomi glacial lake in Bhutan's Phochhu River Basin using the BREACH dam breach model and HEC-RAS hydrodynamic model. The idea of the paper is appealing and well structured, however I would like authors to incorporate the following major and minor comments before it is considered for publication in NHESS.
Major Comments
In the Introduction section you have mentioned that approximately 17 million m³ of lake water was artificially released from the Thorthomi glacial lake which has lowered the water level of the lake. Did you consider this reduction of volume in your study because the volume of the water is the essential component for the GLOF simulation? How much is the current volume of water that you have used as an input to route the potential GLOF? The reduction in lake water volume due to engineering efforts may decrease the intensity of GLOF inundation parameters/flood wave (like flood extent, velocity, depth, etc).
Section 5.2.1. If the moraine dam is unconsolidated with loose material, then there may be a chance of piping failure as well. I suggest authors consider this mode of failure as well. Furthermore, you have not mentioned how you have calculated the parameters in Table 1. I would like to see the methods of calculation of these parameters in the revised paper. This will help other researchers to calculate these parameters while working on GLOF simulations.
Section 5.2.2. How did you prepare the breach hydrograph (inflow hydrograph)? Which parameters have you considered to prepare a breach hydrograph and how did you calculate those parameters? You need to provide a detailed methodology in this section.
You did not mention the percentage of water volume released from the lake.
Why have you separated the methodology and results for both lakes? You have mixed the methodology and results which makes it very difficult to understand what you are actually trying to convey. You need to be systematic and clear while revising the manuscript.
You should move all the literature to the Introduction section. In the methodology section, you need to be clear and crisp like why and how you have selected the method and how you have calculated them. I have noticed that you have started each section by referring to huge text derived from the literature.
The equation-derived depth and volume always provide overestimated values when you talk about GLOF simulation. Since you have mentioned bathymetry was not possible so have used empirical equations (section 6.2.2) but why have used equations proposed by Sakai (2012) to estimate the maximum depth and volume of the Thorthomi glacial lake when we have some recent integrated empirical equations developed by Qi et al., 2022 and
Line 375 why not piping failure?
Regarding bathymetry data, I suggest authors go through the GLOBathy data Khazaei et al, 2022 or other global bathymetry datasets for the comparative analysis.
Where is uncertainty analysis? The data and mythology utilized have several associated uncertainties. How did you carry out the uncertainty analysis?
You should provide a section on the limitations of the study and future research gaps.
Overall, the manuscript has several major and minor issues which need thorough revision.
Minor comments
Since you have envaulted the downstream impacts of the potential GLOF or you can say elements at risk therefore I suggest “Risk assessment” instead of hazard in the title.
Merge Figures 1 and 3
Figure 2 should be more illustrative
More key findings should be added in the abstract section.
You need to give language check throughout the text.
Line 90 you do not need to write the abbreviation of GLOF everywhere. Mention it in the beginning and then write the short form only I,e GLOF.
Line 135 include some recent studies
Line 160: Revisit the sentence it is not clear.
Repetition of the sentences like 330.
Table 4 M³ should be m³
Again, in Table 2 you have not mentioned how you have calculated these parameters.
Fig 9 nothing is clear.
Fig 16. The color scheme for Lulc is confusing.
The authors should check the grammar and language throughout the text.

Citation: https://doi.org/10.5194/nhess-2023-125-RC2
- AC2: 'Reply on RC2', Ryota Tsubaki, 06 Jan 2024
  
  The research article entitled “A glacial lake outburst flood hazard assessment for the Phochhu River Basin, Bhutan” addresses the concerning trend of glacial lake expansion in the Himalayan region due to glacier melting. It specifically discusses the potential risks associated with Glacial Lake Outburst Floods (GLOFs) and analyzes the potential consequences of a GLOF from the Thorthomi glacial lake in Bhutan's Phochhu River Basin using the BREACH dam breach model and HEC-RAS hydrodynamic model. The idea of the paper is appealing and well structured, however I would like authors to incorporate the following major and minor comments before it is considered for publication in NHESS.
  (Hereafter, bold-italic typefaced sentences indicate our replies to each comment. Line-numbers in our reply correspond to the line-number in the revised manuscript we will submit)
  
  Thank you for carefully reviewing our manuscript. We reply to each comment below.
  Major Comments
  In the Introduction section you have mentioned that approximately 17 million m3 of lake water was artificially released from the Thorthomi glacial lake which has lowered the water level of the lake. Did you consider this reduction of volume in your study because the volume of the water is the essential component for the GLOF simulation? How much is the current volume of water that you have used as an input to route the potential GLOF? The reduction in lake water volume due to engineering efforts may decrease the intensity of GLOF inundation parameters/flood wave (like flood extent, velocity, depth, etc).
  As indicated in Table 2, water volume for the Thorthomi glacial lake was estimated as 400 million m3.
  Regarding water discharge from GLOF, we agree that such efforts is important to reduce the damage caused by a GLOF. However, our understanding of the damage potential for worst-case scenarios is still limited and was the focus of our study. Due to challenging working conditions and health issues, discharging water safely is even challenging today (lines 65-66).
  Section 5.2.1. If the moraine dam is unconsolidated with loose material, then there may be a chance of piping failure as well. I suggest authors consider this mode of failure as well.
  We agree that there is a potential for piping failures. However, wave overtopping is not a common trigger for dam failure (Neupane et al. 2019). Awal et al. (2010) summarized the causes of 20 GLOF events and reported that 80% of GLOF overtopping was caused by ice and rock avalanches. Begam et al. 2018 also deemed overtopping as the major failure type for GLOF events. Following these previous research studies, for our study, we selected overtopping as the cause of a GLOF. In the revised manuscript, this point is discussed in lines 94-96.
  Furthermore, you have not mentioned how you have calculated the parameters in Table 1. I would like to see the methods of calculation of these parameters in the revised paper. This will help other researchers to calculate these parameters while working on GLOF simulations.
  
  Section 5.2.2. How did you prepare the breach hydrograph (inflow hydrograph)? Which parameters have you considered to prepare a breach hydrograph and how did you calculate those parameters? You need to provide a detailed methodology in this section.
  To estimate the breach hydrograph used in Section 5.2.2, we used the method described in Section 5.2.1. A review of GLOF modelling is provided in Section 3.
  You did not mention the percentage of water volume released from the lake.
  For the worst-case scenario considered in this study, approximately 70% of total lake water was released. To describe this scenario, changes have been made in the manuscript (line 460).
  Why have you separated the methodology and results for both lakes? You have mixed the methodology and results which makes it very difficult to understand what you are actually trying to convey. You need to be systematic and clear while revising the manuscript.
  We understand that separating the Methods and Results sections is a typical paper format, especially for studies with one evaluated hypothesis. Our study consists of three study components: (1) comparing available methods (glacial lake bathymetry estimations, the dam break model, floods and others) and data, (2) validating composition (largely in Section 5), and (3) applying the method to a Thorthormi GLOF event (Section 6).　We prioritize study components in the manuscript structure rather than the separation of Methods and Results. We have outlined the structure of the manuscript in the Introduction (lines 107-109).
  You should move all the literature to the Introduction section. In the methodology section, you need to be clear and crisp like why and how you have selected the method and how you have calculated them. I have noticed that you have started each section by referring to huge text derived from the literature.
  As indicated in the previous comment, this study consists of three study components. We could move all of the scientific literature reviews for each study component to the Introduction, however, if we do so, we are afraid that the connection between the review and each study component will become difficult for the reader to follow. We have outlined the structure of the manuscript in the Introduction (lines 107-109).
  The equation-derived depth and volume always provide overestimated values when you talk about GLOF simulation. Since you have mentioned bathymetry was not possible so have used empirical equations (section 6.2.2) but why have used equations proposed by Sakai (2012) to estimate the maximum depth and volume of the Thorthomi glacial lake when we have some recent integrated empirical equations developed by Qi et al., 2022 and
  Thank you for introducing a great paper. When we conducted our study, this work was not available. We now mention it in lines 443-447.
  Line 375 why not piping failure?
  Based on a review of previous studies (Neupane et al. 2019, Awal et al. 2010, Begam et al. 2018) and to discuss the worst-case scenario, we discussed overtopping failure. This point has been added to lines 94-96.
  Regarding bathymetry data, I suggest authors go through the GLOBathy data Khazaei et al, 2022 or other global bathymetry datasets for the comparative analysis.
  Thank you for introducing a great paper. When we conducted our study, this work was unavailable. The dataset developed by Khazaei et al. 2022 does not cover the lake under consideration in this study, and the dataset is not specific to glacial lakes.
  Where is uncertainty analysis? The data and mythology utilised have several associated uncertainties. How did you carry out the uncertainty analysis?
  A brief analysis of the uncertainty determined through prediction and the confidence band has been described in lines 436-440.
  You should provide a section on the limitations of the study and future research gaps.
  The last part in the Conclusion provides limitations and the need for further research (lines 584-602), as well as the required engineering correspondence (lines 589-590).
  Overall, the manuscript has several major and minor issues which need thorough revision.
  Minor comments
  Since you have envaulted the downstream impacts of the potential GLOF or you can say elements at risk therefore I suggest “Risk assessment” instead of hazard in the title.
  Thank you for your comment. Our manuscript discusses hazards as well as potential damage caused by the Thorthomi GLOF. For the title, we considered replacing “hazard” with “risk” but decided to use “hazard” because 1) an assessment of Thorthomi GLOF hazards was the main point of our manuscript (a risk assessment was also important, but supplemental compared to the hazards assessment), 2) we did not discuss the probability of damage (sometimes “risk” is used for such contexts), and 3) the word “hazard” can also include“risk” (which, of course, depends on definitions).
  Merge Figures 1 and 3
  We prepared Figure 1 for the Introduction and Figure 3 of the target study site. We think keeping the present composition helps separate the introductory portion of the manuscript and the specific target of our study.
  Figure 2 should be more illustrative
  We understand the effectiveness of illustrative explanations. Figure 2 is a simple representation of the process flowchart highlighting required models and data. We think this simplified figure makes the overall structure and content of the paper simple to understand for all types of readers.
  More key findings should be added in the abstract section.
  Important results from our study include: (1) selecting and composing available good methods (glacial lake bathymetry estimations, dam break models, floods and others) and data, (2) validating the composition, and (3) applying the method to a Thorthormi GLOF event. Since these are the key components of our study, we used the beginning half of the abstract to explain this information.
  
  Main findings were reflected in the abstract section.
  You need to give language check throughout the text.
  We attempted to describe our results in clear language and sent the manuscript for proof editing prior to submission. The journal provides English editing during the proofing phase, so we believe the language will be as clear as possible prior to publication.
  Line 90 you do not need to write the abbreviation of GLOF everywhere. Mention it in the beginning and then write the short form only I,e GLOF.
  Thank you for pointing out this information. We have made the necessary changes.
  Line 135 include some recent studies
  Taylor et al, 2023 was cited in line 25. Three other recent studies were referenced in lines 94 to 96. Qi et al. 2022 was referenced in line 444.
  Line 160: Revisit the sentence it is not clear.
  The sentence has been edited (lines 173 to 175).
  Repetition of the sentences like 330.
  The connection of sentences has been improved (line 358-359).
  Table 4 M3 should be m3
  Units in the table have been corrected.
  Again, in Table 2 you have not mentioned how you have calculated these parameters.
  We have indicated estimated parameter sources below the table. Most of the parameters were obtained from published papers and reports. Parameters estimated by our study were obtained from the terrain model and the regression analysis discussed in the paper.
  Fig 9 nothing is clear.
  The journal suggests that figures be clear for readers with color vision deficiencies. Colors and textures were chosen following this suggestion.
  Fig 16. The color scheme for Lulc is confusing.
  The journal suggests that figures be clear for readers with color vision deficiencies. Colors and textures were chosen following this suggestion. The figure is revised to improve the visibility.
  The authors should check the grammar and language throughout the text.
  We attempted to describe our results in clear language and sent the manuscript for proof editing prior to submission. The journal provides English editing during the proofing phase, so we believe the language will be as clear as possible prior to publication.
  Lastly, we thank you for fair and constructive comments, which helped improve the manuscript.
  
  Citation: https://doi.org/10.5194/nhess-2023-125-AC2
AC3: 'Comment on nhess-2023-125', Ryota Tsubaki, 23 Jan 2024

Dear all
Authors had replied to all comments, and waiting some further feedbacks.
Best regards,
Ryota Tsubaki

Citation: https://doi.org/10.5194/nhess-2023-125-AC3

Tandin Wangchuk and Ryota Tsubaki

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Short summary

A Glacial Lake Outburst Flood (GLOF) is a natural hazard in which water from a glacier-fed lake is swiftly discharged, and causes serious harm to life, infrastructure, and communities. We used numerical models to predict the potential consequences of a GLOF from the Thorthomi glacial lake in Bhutan. We found that If the GLOF occurs, the lake could release massive flood water within four hours, posing a considerable risk. Study findings help to mitigate the impacts of future GLOF.


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