the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Mar 2024
Variation of sediment supply by periglacial debris flows at Zelunglung in the eastern syntaxis of Himalayas since the 1950 Assam Earthquake
Abstract. Periglacial debris flows boosted by strong earthquakes or climatic warming in alpine mountains play a crucial role in sediment delivery from hillslopes and downslope channels into rivers. Rapid and massive sediment supply to rivers by the debris flows has profoundly influenced the evolution of the alpine landscape. Nonetheless, there is a dearth of knowledge concerning the roles tectonic and climatic factors played in the intensified sediment erosion and transportation. In order to increase our awareness of the mass wasting processes and glacier changes, five debris flows that occurred at the Zelunglung catchment of the eastern syntaxis of the Himalayas since 1950 Assam earthquake are investigated in detail by field surveys and long-term remote sensing interpretation. Long-term seismic and meteorological data indicate that the four events of 1950–1984 were the legacies of the earthquake, and recent warming events drove the 2020 event. The transported sediment volume indexed with a non-vegetated area on the alluvial fan reduced by 91 % to a stable low level nearly 40 years after 1950. It is reasonable to hypothesize that tectonic and climatic factors alternately drive the sediment supplies caused by the debris flows. High concentrations of coarse grains, intense erosion, and extreme impact force of the 2020 debris flow raised concerns about the impacts of such excess sediment inputs on the downstream river evolution and infrastructure safety. In regard to the hydrometeorological conditions of the main river, the time to evacuate the transported coarse sediments is approximately two orders of magnitude of the recurrence period of periglacial debris flows.
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RC1: 'Comment on egusphere-2024-312', Anonymous Referee #1, 14 Mar 2024
Dear authors,
you describe an interesting 'longer term' link between earthquake occurrence and massive periglacial surface processes.
I would mainly recommend a general (and professional) English review, and adding some lines about the physical relationship
between the seismic and (peri) glacial processes. It is also important to highlight the relatively large distance (if I see well) between
the 1950 Assam earthquake epicenter (or distance to activated fault) and the glacier valley.
sincerely yours
reviewer H
Citation: https://doi.org/10.5194/egusphere-2024-312-RC1 -
AC1: 'Reply on RC1', Kaiheng Hu, 20 Mar 2024
Thank you very much! We are looking forward to your valuable comments.
Citation: https://doi.org/10.5194/egusphere-2024-312-AC1
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AC1: 'Reply on RC1', Kaiheng Hu, 20 Mar 2024
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RC2: 'Comment on egusphere-2024-312', Aaron Bufe, 21 Mar 2024
Hu and co-authors present an analysis of periglacial debris flows in a small catchment draining the Namche Barwa massif in the Himalaya using historical and modern remote sensing applications. I have some suggestions for improvements. In particular, I am not convinced by the discussion about the causes for the debris flows.
Motivation of the paper: First I suggest to sharpen the motivation of the paper. According to the introduction (L61 ff), “little is known about the roles the extreme hazards play in incrementing sediment erosion, transportation, and the control of the hazards between tectonic and climatic factors.” I actually do not quite understand what is meant here.
Second, it is not made clear in the following text how that knowledge gap is addressed by the study. Questions that would be good to answer in the introduction are: Why was that particular study area chosen? How do the approaches advance the gap that you are proposing? In the discussion or conclusion, you can also come back to that point.
Methodology: I do not quite understand how you can take the non-vegetated area as a proxy for debris flow volume (e.g. L135). To get a volume from an area, I believe you need some estimate of thickness. How do you get that?
Causes of debris flows: First the discussion of causes for the debris flows is a bit confusing to me. In the abstract you write that the four events 1950 – 1984 were legacies of the 1950 Assam earthquake. The suggested link between the 1950 Earthquake and the 1950 debris flow seems solid (as per Fig. 15). The suggestion that the other events were also preconditioned by that earthquake seems less obvious to me. Is the only evidence that none of the other earthquakes could have caused the debris flows (L381ff)? What about other possible triggers, such as an extreme precipitation events? I am by no means an expert, but the argument for the influence of the 1950 earthquake on later debris flows doesn’t seem very convincing to me. By the way, in the discussion (L381ff), it did not become clear that you actually suggest that the 1950 – 1984 events were all triggered by the earthquake. I only got that from the abstract.
Also, in the abstract you say that “recent warming events drove the 2020 event”. And you start your discussion stating that either earthquakes or climate change increase the occurrence of debris flows. I do not think you have evidence to say that unless you have a much longer time series. Lets assume the null-hypothesis that debris flow events occur randomly within some average recurrence interval. With the few events you study here, I would challenge you to show that the distribution of earthquakes is statistically distinguishable from a random occurrence. Similarly, I would challenge that a single debris flow after a single warming episode is evidence enough to conclude this flow is caused by the warming. I suggest to formulate these links much more carefully and “suggest” a link.
Line comments
L105: Just visually. Fig. 2 doesn’t particularly look like precipitation is increasing at all. How did you calculate that increase?
L132: I would start the methods section with a summary sentence of the measurements that you are trying to make.
L159ff: I suggest to move the entire section on glacier changes before the methods. This is not your work as far as I understand, so it’s a bit odd to have that as part of the other results. You could move it together with the study area section
L336: Can you explain where the interpretation of “some kind of dilute or hyper-concentrated flow” comes from?
L351ff: A lot of that section reads like a discussion. I suggest to move it there.
Another note on this paragraph: You write of three surges (L168) but then only explicitly note two of them (L169ff and L172ff). I guess the third surge is the one you describe in L179? Can you make that explicit?
For all of the figures with geographic reference, it has to be clear where they are from with respect to the region. Figure 1 is missing a regional overview map. It was unclear to me where Figures 4 – 6 are taken within the study area or what extent Figs. 8-11 have. You can either mark their positions in Figure 1, or have little insets with every figure that show where in the study area that figure/picture is located. Also, not all figures have information on orientation and, if relevant, scale (e.g. Fig. 4-6. 7c&d, Photo of Fig. 9 missing north arrow etc.)
Figure 1:
- The yellow text in panels b&c and the white text in panel a was hard to read on my printout
Figure 4
- The year numbers are a bit hard to see against the grey background
Figure 6
- There is no explanation in the figure or caption what T1-T2 are. Even if it is quite obvious, it would be good to define explicitly
- The North Arrow and scale-bar is really hard to see
Fig 8c
- The person as scalebar is really hard to see
Figure 12
- Can you give some more information in the figure caption about where the ‘on-site’ sample is and when the sample was taken?
I hope these comments are useful and remain with best wishes
Aaron Bufe
Citation: https://doi.org/10.5194/egusphere-2024-312-RC2 -
AC2: 'Reply on RC2', Kaiheng Hu, 29 Mar 2024
Thank you very much! These comments are very useful. We try to find more substantial links between the debris flows and earthquakes or warming events. We will improve our figures.
Thank you again!
Kaiheng Hu, on behalf of all the authors
Citation: https://doi.org/10.5194/egusphere-2024-312-AC2
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RC3: 'Comment on egusphere-2024-312', Natalie Barbosa, 15 Apr 2024
The manuscript “Variation of sediment supply by periglacial debris flows at Zelunglung in the easter syntaxis of Himalayas since 1950 Assam Earthquake” by Hu et al., describes the occurrence of five debris flows that impacted the Zelunglung alluvial fan using a combination of field surveys, historical aerial imagery, and UAV flights.
The manuscript contributes with observations on long-term changes in vegetation at the alluvial fan interpreted as a proxy for debris flow activity at the catchment. The manuscript aims to estimate relative qualitative debris flow activity compared to the 1950 event. Also, a detailed description of the 2020 debris flow event is presented using UAV photogrammetry. The authors discuss the results considering seismic activity, the Zelunglung glacier surge dynamics, and precipitation and air temperature. The manuscript exemplifies the complex interactions between tectonic and climate in debris flows triggering factors in glaciated areas. Despite the presented remote sensing interpretation, field observation, and literature review, further considerations are needed to strengthen the conclusions, particularly the conclusion referring to the influence of the 1950 earthquake in further debris flows until 1990.
The presented historical aerial imagery is a valuable source of information that could be explored to strengthen the manuscript. For example, if available, the source and affected area of the historical periglacial debris flows (1950,1968,1972,1984) could be identified.
I missed in the manuscript a clear explanation about why you consider that the 1950 earthquake has a stronger influence than the glacier surges in the triggering of the debris flows after 1950 and, therefore, impacting the vegetation changes between 1950 and 1990 (line 484). In lines 187-188, it is stated that the debris flows were triggered by the glacier instability. Also, the 1950 debris flow coincides with a glacier surge.
The photo interpretation from Planet Lab images shows an ice-rock residual under the detachment area of the 2020 debris flow. The authors conclude that the entrained volume is at least 16 times the initiated volumes (line 227), consistent with the 1.14 Mm3 previously presented by Peng et al., (2022). This is highly relevant for the calibration of Debris Flow models.
The proposed methodology involved a considerable amount of remote sensing data manual interpretation and general information on the uncertainty of the manual mapping of non-vegetation areas was given. Regardless, I missed a short sentence on the reconstruction of the DSM from the UAV surveys (e.g., software use, ground control points, alignment), and the uncertainty on the elevation change that propagates to the presented volumes (line 346).
The manuscript is well structured and the figures are illustrative.
Line comments:
Line 130-131: Did you present results on the surveys in 2021 and 2022 in this manuscript?
Figure 4: Can you add a north arrow and a scale? Maybe you can try to use a different font color. The years of the images are hard to see.
Figure 9: c) Which distance is presented on the x-axis? distance from the outlet?
Figure 11: Could you please extend the cross-section before the bridge to include the deposition areas? You could also include some information on the deposited particle sizes to exemplify.
Line 341. The maximum erosive depth of 20.47m is in the main channel or correspond to lateral erosion? Please include the mean erosive depth at the channel.
Line 346. Could you please discuss how much underestimated is the volume you are presenting compared to Peng et al., (2022) and what is the expected error from the photogrammetric workflow?
Figure 12: The figure is not referenced in the text. Maybe you can merge Figure 12 and Figure 9 and include the location of the on-site sample.
Figure 13: Can you add a north arrow?
Figure 14: Could you also include in Figure d) the other 4 debris flows for comparison?
I hope the comments help improve your manuscript.
Kind regards,
Natalie Barbosa.
Citation: https://doi.org/10.5194/egusphere-2024-312-RC3
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