the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Oct 2023
A glacial lake outburst flood hazard assessment for the Phochhu River Basin, Bhutan
Tandin Wangchuk
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.
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Tandin Wangchuk and Ryota Tsubaki
Status: open (until 05 Dec 2023)
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RC1: 'Comment on nhess-2023-125', Stefan Ram, 19 Nov 2023
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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
reply
(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 7th 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
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AC1: 'Reply on RC1', Ryota Tsubaki, 28 Nov 2023
reply
Tandin Wangchuk and Ryota Tsubaki
Tandin Wangchuk and Ryota Tsubaki
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