the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Apr 2024
Timing, Causes, and Ecological Impacts of the 1991 Glacial Lake Outburst Flood at Rijieco in the Eastern Himalayas
Abstract. We investigate and reconstruct a publicly lesser-known glacial lake outburst flood that happened at a moraine-dammed proglacial lake – Rijieco (27.963° N, 88.9° E) in the eastern Himalayas, Tibet, China, more than thirty years ago. Satellite images interpret that from 1977 to 1991, the surface area of Rijieco increased from 0.34 km2 to 0.43 km2, along with a reduction of 0.27 km2 in the glacial area, and subsequently dropped to 0.24 km2 after the GLOF. Further inspection of the 1991 images narrows down the date of occurrence of the Rijieco GLOF to between 21 September – 7 October 1991. The most probable triggering mechanism of the GLOF is an avalanche from the south-west part of the glacial lake because the local hydrometeorological data show precipitation of 100 mm higher than the multi-year mean in the preceding two months and an elevated temperature anomaly in the month of the disaster, combined with a sufficient topographic potential of the slope. Based on the field-measured dimension of the dam gap (31 m high and 75 m wide) and local topography, it is estimated that nearly 6 million m3 of impounded water was released during the GLOF. The reconstruction of the outburst flood with the HEC-RAS 2D hydrodynamic model shows that the flood peak discharge at the dam was 2900 m3/s and then attenuated first to around 200 m3/s at the alluvial fan and 45 m3/s at the entrance to the Duoqing lake, an inland lake about 50 km away from the dam. The sudden release of the impounded water and large volumes of entrained sediment shortly expanded the Duoqing water area by 12.2 % and strongly disturbed its margin vegetation. The transported sediments silted an area of 1.0×106 m2 of the channel and flood plain where new vegetation has not yet recovered. The harm of GLOFs to the lake ecosystem in the high-altitude Himalayan region may not be repaired in a short period. This study reveals short-term geomorphic impacts of GLOFs and noticeable but less mentioned long-term ecological impacts on a Tibetan inland lake system.
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RC1: 'Comment on egusphere-2024-884', Anonymous Referee #1, 07 May 2024
General Comments
Originality: Fair
This study seeks to investigate a historical Glacial Lake Outburst Flood (GLOF) that occurred in 1991 at lake Rijieco. They aim to combine remotely sensed data with climatic data and hydrological modelling to determine the magnitude of the GLOF along with its potential triggers. There is merit in the historical investigation of GLOF events to further our understanding of landform susceptibility and trigger mechanisms. The modelling of the 1991 GLOF magnitude and impact, although not novel does provide a useful insight into reconstructing this event, however this is overshadowed by the papers poor scientific quality in other aspects.
Scientific Quality: Poor
Scientific understanding:
I note the authors have sought to address knowledge gaps in the current understanding of GLOF’s however they have omitted multiple key references explaining these and have drastically oversimplified key characteristics associated with GLOF’s. Critically, there is no key distinction found herein between “susceptibility” and “triggers”, hindering the scientific argument underpinning this paper. Susceptibility is linked to the glacial lake dam and can relate to: lake volume, dam composition, dam morphology and lake freeboard. This paper fails to address these elements of the dam at Rijieco and this is critical when assessing any glacial lake related hazard. Trigger mechanisms refer to an event or series of events that instigate a hazard, which in the case of GLOFs comprise: avalanches, calving events, melting of ice cores, seepage, inappropriate engineering works and earthquakes (debateable). Although the authors try address these they are far too definitive in their interpretations without sufficient evidence of the triggers. Given that the event occurred over 30 years ago, a rigorous method should be applied to consider which if any of these triggers is more likely to occur than the other. Of note is that the explanation of the hazard chain is not in fact feasible (Figure 7), displaying a rockfall of 1 vertical kilometre and 8 horizontal kilometres into the lake. This figure and associated text in particular oversimplifies the hazard chain with little evidence to support these inferences from current datasets and omits extensive work done by multiple academics to explain these hazards in detail.
Furthermore, process identification forms a key component of any hazard assessment as without a proper understanding of the susceptibility of a land system as well as potential triggers for an associated hazard at a site-specific scale, the assessment is unlikely to be representative (Emmer et al., 2013; Racoviteanu et al., 2022). A glacial hazard assessment should aim to combine a suite of datasets, both remotely sensed and (if warranted) geophysical to accurately ascertain the factors inherent to the risk (Richardson and Reynolds, 2000; Reynolds, 2006; Racoviteanu et al., 2022). This should aim to link the susceptibility of landform to the potential trigger mechanisms of that site with consideration of how those factors may develop with climatic changes over a number of timescales (Wang et al., 2018; Emmer et al., 2020; Racoviteanu et al., 2022; Reynolds, 2023). Given the justification for this study is that of hazard assessment, I would expect to see a comprehensive hazard assessment including the following steps (Reynolds, 2023):
- Desk study
- Gap analysis
- Event mapping
- Process identification
- Neo-tectonic provinces and seismicity
- Regional geological analysis
- Landslide susceptibility mapping
- Glacial hazard assessment
- Hydrological analyses
- Sediment management
- Other analyses identified from gap analysis
Although elements of this are addressed I would argue that they are not done in a sufficient way to truly represent a hazard assessment.
Ultimately this study does not undertake a rigorous hazard assessment for this site and based upon inconsistencies in their explanation of GLOF mechanics. There are elements of GLOF reconstruction but these are undermined by the authors explanations of likely GLOF mechanisms. I would argue that this work does not truly assess either the historical GLOF or any other hazards in the region.
Data collection and analysis:
Although interesting, the remotely sensed data fails to acknowledge uncertainties, particularly in the resolution of the various sensors used. Furthermore, there are instances where higher resolution sensors were freely available from USGS and were not used (Sentinel, etc.,), this hinders interpretation of processes occurring. I feel there is a lack of acknowledgement in regard to the resolution of sensors and the scale of the changes to the geomorphology and development of these landforms, this is critical for a rigorous method. This has led to a large number of assumptions as to the nature of the GLOF throughout the paper based upon historical imagery that is not of a sufficient resolution, hindering the argument.
The field campaign (Figure 8) does not in fact yield useful data in terms of hazard assessment and to accurately assess any hazard a comprehensive assessment should be undertaken following established guidelines (see Reynolds, 2023).
Significance: Poor
This manuscript does not contribute to changing our scientific understanding of a subject substantially or to introducing new practical applications of broad relevance. I agree that this site warrants investigation, to a degree, but the application of this has note been done sufficiently to draw any meaningful conclusions.
Presentation Quality: Poor
The figures presented within this manuscript have numerous inconsistencies and are not of publication standard. This is explained in detail in the specific comments.
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AC1: 'Reply on RC1', Kaiheng Hu, 16 Jul 2024
Thank you very much for your valuable comments. To assess the historical GLOF properly, we will enhance our hazard analysis more systematically and robustly and improve the figures.
Citation: https://doi.org/10.5194/egusphere-2024-884-AC1
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RC2: 'Comment on egusphere-2024-884', Anonymous Referee #2, 11 Jul 2024
The authors investigated the glacial lake outburst of Rijieco that occurred in eastern Himalayas thirty years ago and examined its timing, causes, and ecological impacts by using satellite images and field investigation. They found that the outburst flood had both short-term geomorphic and long-term ecological impacts on a Tibetan inland lake system. Since it occurred thirty years ago, there is limited satellite data and it is difficult to identify the triggering mechanism. Information about the damage on downstream community and infrastructures is also difficult to find. I personally think that more in-depth analysis is still needed before it can be considered for publication.
Major comments:
1, The authors analyzed the triggering mechanism and give three possible scenarios. I think more evidence can be given to validate these assumptions. Although there are limited satellite images in the study area thirty years ago, more satellite images in adjacent years after the GLOF (e.g. 1992, 1993) can still be used and more evidence about ice avalanche or glacier calving may be found.
2, Does the analysis on the long-term ecological impacts make sense in such a remote area?
3, Several Figures (Figure 4, 5, 9) are not clear enough and should be improved. For example, Figure 4 is too dark. The language should be more concise.
Minor comments
Line 42, More than 600 GLOF events have been reported in the HMA. In line 45, As many as seventy-nine events have been recorded there? I am confused about which number is true?
Line 47, the Central and Eastern parts of what?
Line 196, 106? Or 106
Line 217, brackets
Line 476, rephrase this sentence
Fig.1 the extent of b in a (triangle) looks much larger than 1b.
Fig.4 is unclear, and the glacial lake is too small to see its changes
Fig.5 is unclear. The three quadrilaterals look like different. Do they have different scales?
Fig. 12, The flow direction of two figures is opposite and both of them are not representative.
Line 42, More than 600 GLOF events have been reported in the HMA. In line 45, As many as seventy-nine events have been recorded there? I am confused about which number is ture?
Line 47, the Central and Eastern parts of what?
Line 196, 106? Or 106
Line 217, brackets
Line 476, rephrase this sentence
Fig.1 the extent of b in a (triangle) looks much larger than 1b.
Fig.4 is unclear, and the glacial lake is too small to see its changes
Fig.5 is unclear. The three quadrilaterals look like different. Do they have different scales?
Fig. 12, The flow direction of two figures is opposite and both of them are not representative.
Citation: https://doi.org/10.5194/egusphere-2024-884-RC2 -
AC2: 'Reply on RC2', Kaiheng Hu, 16 Jul 2024
Thank you very much for your comments. We will carry out a more in-depth analysis to validate the assumptions regarding the triggering mechanisms and address other comments.
Citation: https://doi.org/10.5194/egusphere-2024-884-AC2
-
AC2: 'Reply on RC2', Kaiheng Hu, 16 Jul 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-884', Anonymous Referee #1, 07 May 2024
General Comments
Originality: Fair
This study seeks to investigate a historical Glacial Lake Outburst Flood (GLOF) that occurred in 1991 at lake Rijieco. They aim to combine remotely sensed data with climatic data and hydrological modelling to determine the magnitude of the GLOF along with its potential triggers. There is merit in the historical investigation of GLOF events to further our understanding of landform susceptibility and trigger mechanisms. The modelling of the 1991 GLOF magnitude and impact, although not novel does provide a useful insight into reconstructing this event, however this is overshadowed by the papers poor scientific quality in other aspects.
Scientific Quality: Poor
Scientific understanding:
I note the authors have sought to address knowledge gaps in the current understanding of GLOF’s however they have omitted multiple key references explaining these and have drastically oversimplified key characteristics associated with GLOF’s. Critically, there is no key distinction found herein between “susceptibility” and “triggers”, hindering the scientific argument underpinning this paper. Susceptibility is linked to the glacial lake dam and can relate to: lake volume, dam composition, dam morphology and lake freeboard. This paper fails to address these elements of the dam at Rijieco and this is critical when assessing any glacial lake related hazard. Trigger mechanisms refer to an event or series of events that instigate a hazard, which in the case of GLOFs comprise: avalanches, calving events, melting of ice cores, seepage, inappropriate engineering works and earthquakes (debateable). Although the authors try address these they are far too definitive in their interpretations without sufficient evidence of the triggers. Given that the event occurred over 30 years ago, a rigorous method should be applied to consider which if any of these triggers is more likely to occur than the other. Of note is that the explanation of the hazard chain is not in fact feasible (Figure 7), displaying a rockfall of 1 vertical kilometre and 8 horizontal kilometres into the lake. This figure and associated text in particular oversimplifies the hazard chain with little evidence to support these inferences from current datasets and omits extensive work done by multiple academics to explain these hazards in detail.
Furthermore, process identification forms a key component of any hazard assessment as without a proper understanding of the susceptibility of a land system as well as potential triggers for an associated hazard at a site-specific scale, the assessment is unlikely to be representative (Emmer et al., 2013; Racoviteanu et al., 2022). A glacial hazard assessment should aim to combine a suite of datasets, both remotely sensed and (if warranted) geophysical to accurately ascertain the factors inherent to the risk (Richardson and Reynolds, 2000; Reynolds, 2006; Racoviteanu et al., 2022). This should aim to link the susceptibility of landform to the potential trigger mechanisms of that site with consideration of how those factors may develop with climatic changes over a number of timescales (Wang et al., 2018; Emmer et al., 2020; Racoviteanu et al., 2022; Reynolds, 2023). Given the justification for this study is that of hazard assessment, I would expect to see a comprehensive hazard assessment including the following steps (Reynolds, 2023):
- Desk study
- Gap analysis
- Event mapping
- Process identification
- Neo-tectonic provinces and seismicity
- Regional geological analysis
- Landslide susceptibility mapping
- Glacial hazard assessment
- Hydrological analyses
- Sediment management
- Other analyses identified from gap analysis
Although elements of this are addressed I would argue that they are not done in a sufficient way to truly represent a hazard assessment.
Ultimately this study does not undertake a rigorous hazard assessment for this site and based upon inconsistencies in their explanation of GLOF mechanics. There are elements of GLOF reconstruction but these are undermined by the authors explanations of likely GLOF mechanisms. I would argue that this work does not truly assess either the historical GLOF or any other hazards in the region.
Data collection and analysis:
Although interesting, the remotely sensed data fails to acknowledge uncertainties, particularly in the resolution of the various sensors used. Furthermore, there are instances where higher resolution sensors were freely available from USGS and were not used (Sentinel, etc.,), this hinders interpretation of processes occurring. I feel there is a lack of acknowledgement in regard to the resolution of sensors and the scale of the changes to the geomorphology and development of these landforms, this is critical for a rigorous method. This has led to a large number of assumptions as to the nature of the GLOF throughout the paper based upon historical imagery that is not of a sufficient resolution, hindering the argument.
The field campaign (Figure 8) does not in fact yield useful data in terms of hazard assessment and to accurately assess any hazard a comprehensive assessment should be undertaken following established guidelines (see Reynolds, 2023).
Significance: Poor
This manuscript does not contribute to changing our scientific understanding of a subject substantially or to introducing new practical applications of broad relevance. I agree that this site warrants investigation, to a degree, but the application of this has note been done sufficiently to draw any meaningful conclusions.
Presentation Quality: Poor
The figures presented within this manuscript have numerous inconsistencies and are not of publication standard. This is explained in detail in the specific comments.
-
AC1: 'Reply on RC1', Kaiheng Hu, 16 Jul 2024
Thank you very much for your valuable comments. To assess the historical GLOF properly, we will enhance our hazard analysis more systematically and robustly and improve the figures.
Citation: https://doi.org/10.5194/egusphere-2024-884-AC1
-
RC2: 'Comment on egusphere-2024-884', Anonymous Referee #2, 11 Jul 2024
The authors investigated the glacial lake outburst of Rijieco that occurred in eastern Himalayas thirty years ago and examined its timing, causes, and ecological impacts by using satellite images and field investigation. They found that the outburst flood had both short-term geomorphic and long-term ecological impacts on a Tibetan inland lake system. Since it occurred thirty years ago, there is limited satellite data and it is difficult to identify the triggering mechanism. Information about the damage on downstream community and infrastructures is also difficult to find. I personally think that more in-depth analysis is still needed before it can be considered for publication.
Major comments:
1, The authors analyzed the triggering mechanism and give three possible scenarios. I think more evidence can be given to validate these assumptions. Although there are limited satellite images in the study area thirty years ago, more satellite images in adjacent years after the GLOF (e.g. 1992, 1993) can still be used and more evidence about ice avalanche or glacier calving may be found.
2, Does the analysis on the long-term ecological impacts make sense in such a remote area?
3, Several Figures (Figure 4, 5, 9) are not clear enough and should be improved. For example, Figure 4 is too dark. The language should be more concise.
Minor comments
Line 42, More than 600 GLOF events have been reported in the HMA. In line 45, As many as seventy-nine events have been recorded there? I am confused about which number is true?
Line 47, the Central and Eastern parts of what?
Line 196, 106? Or 106
Line 217, brackets
Line 476, rephrase this sentence
Fig.1 the extent of b in a (triangle) looks much larger than 1b.
Fig.4 is unclear, and the glacial lake is too small to see its changes
Fig.5 is unclear. The three quadrilaterals look like different. Do they have different scales?
Fig. 12, The flow direction of two figures is opposite and both of them are not representative.
Line 42, More than 600 GLOF events have been reported in the HMA. In line 45, As many as seventy-nine events have been recorded there? I am confused about which number is ture?
Line 47, the Central and Eastern parts of what?
Line 196, 106? Or 106
Line 217, brackets
Line 476, rephrase this sentence
Fig.1 the extent of b in a (triangle) looks much larger than 1b.
Fig.4 is unclear, and the glacial lake is too small to see its changes
Fig.5 is unclear. The three quadrilaterals look like different. Do they have different scales?
Fig. 12, The flow direction of two figures is opposite and both of them are not representative.
Citation: https://doi.org/10.5194/egusphere-2024-884-RC2 -
AC2: 'Reply on RC2', Kaiheng Hu, 16 Jul 2024
Thank you very much for your comments. We will carry out a more in-depth analysis to validate the assumptions regarding the triggering mechanisms and address other comments.
Citation: https://doi.org/10.5194/egusphere-2024-884-AC2
-
AC2: 'Reply on RC2', Kaiheng Hu, 16 Jul 2024
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