Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

Huber, Marius L.; Lupker, Maarten; Gallen, Sean F.; Christl, Marcus; Gajurel, Ananta P.

doi:https://doi.org/10.5194/esurf-8-769-2020

Articles | Volume 8, issue 3

https://doi.org/10.5194/esurf-8-769-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esurf-8-769-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 8, issue 3

Research article

| Highlight paper

|

22 Sep 2020

Research article | Highlight paper |

| 22 Sep 2020

Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

Marius L. Huber, Maarten Lupker, Sean F. Gallen, Marcus Christl, and Ananta P. Gajurel

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Final revised paper (published on 22 Sep 2020)
Supplement to the final revised paper
Preprint (discussion started on 20 Mar 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'Supplement title', Marius Lothar Huber, 20 Mar 2020
SC2: 'Comments from Paul A. Carling', Paul Carling, 25 Mar 2020
- AC1: 'Reply to Paul Carling's comments', Maarten Lupker, 02 Apr 2020
RC1: 'Review of Huber et al., 2020', Georgina Bennett, 15 Apr 2020
- AC2: 'Intermediate reply', Maarten Lupker, 22 Apr 2020
RC2: 'Review of Huber et al (ESurf) - 2020', Anonymous Referee #2, 17 Apr 2020
- AC3: 'Intermediate reply', Maarten Lupker, 22 Apr 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Maarten Lupker on behalf of the Authors (22 Jun 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (02 Jul 2020) by Wolfgang Schwanghart

RR by Anonymous Referee #3 (11 Jul 2020)

Suggestions for revision or reasons for rejection

This paper has already been reviewed at the Interactive comment stage. The authors are considering a series of large boulders prominent in the lower terraces of the Trishuli and Sunkoshi valleys in Nepal Himalayas. Based on the lithology and on the cosmogenic radionucleide exposure dating of these boulders, the authors found in both valleys a significant cluster at ~ 5 ka, corresponding to the Early Climatic Holocene Optimum. Combining travel distances of the boulders (> 10 km up to ~40 km) and estimated peak discharges ca. 103 to 105 m3/s based on hydraulic parameters, the authors discuss the main potential triggers (earthquakes, climate change) to generate such large floods. They eventually interpret these large boulders as the result of past GLOFs events, an interpretation which seems relevant. More generally, this paper confirms that infrequent events have long-lasting consequences on valley bottoms and channel morphology, and the possible coupling between large-infrequent events and bedrock incision patterns in Himalayan rivers, as demonstrated in former studies.

This is an excellent paper, very convincing, well written and illustrated, which has been significantly improved and completed after the Discussion stage. Most improvements were brought about paleo-discharge estimations, origin of the boulders timing and boulder emplacement mechanisms, together with additional references.

Therefore, my comments for late final technical corrections are:
1) A few points are missing:
- Line 403, the authors refer to climate variability, but they refer only to “modulation of glacier extent and proglacial lake volumes”. Referring on recent LLOFs following monsoon rainfall events could also be relevant (i.e. Tatopani (1998) in the Kali Gandaki as a good example), even if they then consider these LLOF cannot provide discharge high enough to move such large boulders.
- Line 663: Why not adding roads and related infrastructure (i.e. bridges) to hydropower? This is a great concern for a country like Nepal where road development is progressing very rapidly. See the paper (this is just one example): McAdoo B.G., Quak M., Gnyawali K.R., Adhikari B.R., Devkota S., Rajbhandari P.L., Sudmeier-Rieux K., 2018. Roads and landslides in Nepal: How development affects environmental risk. Natural Hazards and Earth Systems Sciences, 18, 3203-3210. DOI: 10.5194/nhess-18-3203-2018
2) Very minor technical corrections
- line 232, correct spelling: « below »
- line 399: exact reference is Fort, 1987 (and not Fort et al., 1987).
- line 462: add the bracket after the reference Stolle et al. 2017.

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ED: Publish subject to minor revisions (review by editor) (13 Jul 2020) by Wolfgang Schwanghart

Dear authors,
I can now inform you that an additional reviewer has evaluated your manuscript "Timing of exotic, far-travelled boulder emplacement and paleo-outburst flooding in the central Himalaya". As you will see, the reviewer has only few minor comments which underscores the careful revision that you have done in response to the reports of two reviewers and Paul Carling. I am thus happy to accept your paper subject to minor revisions which enable you to account for the comments by the reviewer (see reviewer comments) and a few minor comments by myself. These are detailed in the following paragraphs:

L391f: The analysis of recorded meteorological floods could be potentially complemented by referencing the paper by Rakhecha (2002, Rakhecha, P. R.: Highest floods in India. The Extremes of the Extrems: Extraordinary Floods. Proceedings of a symposium held at Reykjavik. Iceland, July 2000, pp. 167–172., 2002.), who present an envelope curve for extreme floods conditional on drainage area. I think that this paper likely strengthens your argument that present day meteorological floods are likely unable to have discharges necessary to transport the boulders analyzed in this study.

L423: A more recent publication that underscores the possibility of Mw 9 earthquakes is by Stevens and Avouac (2016, Stevens, V. L. and Avouac, J.-P.: Millenary Mw > 9.0 earthquakes required by geodetic strain in the Himalaya, Geophys. Res. Lett., 43(3), 2015GL067336, doi:10.1002/2015GL067336, 2016.)

Though listed in the discussion version, the reference below has not been in the final version:
Fort, M., Cossart, E., Arnaud-Fassetta, G., 2010. Hillslope-channel coupling in the Nepal Himalayas and 520 threat to man-made structures: The middle Kali Gandaki valley. Geomorphology 124, 178–199.
Considering the orientation of the final version, I think the reference could be re-incorporated in (1) chapter 5.4 because it is a good example of “minor” LLOF, yet with displacement of large boulders now acting as bed armouring, and/or (2) in chapter 5.5 because it also shows (i) the regulation of fluvial incision, and (ii) the incidence on the future on the road network.

I am looking forward to the final revised version of the manuscript!

Best regards,
Wolfgang Schwanghart

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AR by Maarten Lupker on behalf of the Authors (21 Jul 2020) Author's response Manuscript

ED: Publish as is (23 Jul 2020) by Wolfgang Schwanghart

ED: Publish as is (11 Aug 2020) by Tom Coulthard (Editor)

AR by Maarten Lupker on behalf of the Authors (13 Aug 2020) Manuscript

Short summary

Large boulders found in two Himalayan valleys show signs of long fluvial transport (>10 km). Paleo-discharges required to mobilize these boulders exceed typical monsoon discharges. Exposure dating shows that a cluster of these boulders was emplaced ca. 5 kyr ago. This period is coeval with a weakening of the Indian monsoon and glacier retreat in the area. We, therefore, suggest that glacier lake outburst floods are likely mechanisms that can explain these exceptional transport processes.