Preprints
https://doi.org/10.5194/egusphere-2023-2157
https://doi.org/10.5194/egusphere-2023-2157
26 Oct 2023
 | 26 Oct 2023

Downstream rounding rate of pebbles in the Himalaya

Prakash Pokhrel, Mikael Attal, Hugh D. Sinclair, Simon M. Mudd, and Mark Naylor

Abstract. Sediment grains are progressively rounded during their transport down a river. For more than a century, Earth scientists have used the roundness of pebbles within modern sediment, and of clasts within conglomerates, as a key metric to constrain the sediment's transport history and source area(s). However, the current practices of assessment of pebble roundness are mainly qualitative and based on time consuming manual measurement methods. This qualitative judgement provides the transport history only in a broad sense, such as classifying distance as 'near' or 'far'. In this study, we propose a new model that quantifies the relationship between roundness and the transport distance. We demonstrate that this model can be applied to the clasts of multiple lithologies including modern sediment as well as conglomerates deposited by ancient river systems. We present field data from two Himalayan catchments in Nepal. We use the Normalized Isoperimetric Ratio (IRn) which relates a pebble's area (A) to its perimeter (P), to quantify roundness. The maximum analytical value for IRn is 1, and IRn is expected to increase with transport distance. We propose a non-linear roundness model based on our field data, whereby the difference between a grain's IRn and the maximum value of 1 decays exponentially with transport distance, mirroring Sternberg's model of mass loss or size reduction by abrasion. This roundness model predicts an asymptotic behaviour for IRn, and the distance over which IRn approaches the asymptote is controlled by a rounding coefficient. Our field data suggest that the roundness coefficient for granite pebbles is eight times that of quartzite pebbles. Using this model, we reconstruct the transport history of a Pliocene paleo-river deposit preserved at the base of the Kathmandu intermontane Basin. These results, along with other sedimentary evidence, imply that the paleo-river was much longer than the length of the Kathmandu Basin, and that it must have lost its headwaters through drainage capture. We further explore the extreme rounding of clasts from Miocene conglomerate of the Siwaliks Zone and find evidence of sediment recycling.

Journal article(s) based on this preprint

17 Apr 2024
Downstream rounding rate of pebbles in the Himalaya
Prakash Pokhrel, Mikael Attal, Hugh D. Sinclair, Simon M. Mudd, and Mark Naylor
Earth Surf. Dynam., 12, 515–536, https://doi.org/10.5194/esurf-12-515-2024,https://doi.org/10.5194/esurf-12-515-2024, 2024
Short summary
Prakash Pokhrel, Mikael Attal, Hugh D. Sinclair, Simon M. Mudd, and Mark Naylor

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2023-2157', Joakim Edlund, 30 Oct 2023
    • AC1: 'Reply on CC1', Prakash Pokhrel, 06 Feb 2024
  • RC1: 'Comment on egusphere-2023-2157', Laure Guerit, 21 Nov 2023
    • AC2: 'Reply on RC1', Prakash Pokhrel, 06 Feb 2024
  • RC2: 'Comment on egusphere-2023-2157', Anonymous Referee #2, 30 Nov 2023
    • AC3: 'Reply on RC2', Prakash Pokhrel, 06 Feb 2024

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2023-2157', Joakim Edlund, 30 Oct 2023
    • AC1: 'Reply on CC1', Prakash Pokhrel, 06 Feb 2024
  • RC1: 'Comment on egusphere-2023-2157', Laure Guerit, 21 Nov 2023
    • AC2: 'Reply on RC1', Prakash Pokhrel, 06 Feb 2024
  • RC2: 'Comment on egusphere-2023-2157', Anonymous Referee #2, 30 Nov 2023
    • AC3: 'Reply on RC2', Prakash Pokhrel, 06 Feb 2024

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
AR by Prakash Pokhrel on behalf of the Authors (06 Feb 2024)  Author's response   Author's tracked changes   Manuscript 
ED: Publish subject to minor revisions (review by editor) (21 Feb 2024) by Rebecca Hodge
AR by Prakash Pokhrel on behalf of the Authors (26 Feb 2024)  Author's response   Author's tracked changes   Manuscript 
ED: Publish as is (26 Feb 2024) by Rebecca Hodge
ED: Publish as is (05 Mar 2024) by Tom Coulthard (Editor)
AR by Prakash Pokhrel on behalf of the Authors (06 Mar 2024)

Journal article(s) based on this preprint

17 Apr 2024
Downstream rounding rate of pebbles in the Himalaya
Prakash Pokhrel, Mikael Attal, Hugh D. Sinclair, Simon M. Mudd, and Mark Naylor
Earth Surf. Dynam., 12, 515–536, https://doi.org/10.5194/esurf-12-515-2024,https://doi.org/10.5194/esurf-12-515-2024, 2024
Short summary
Prakash Pokhrel, Mikael Attal, Hugh D. Sinclair, Simon M. Mudd, and Mark Naylor
Prakash Pokhrel, Mikael Attal, Hugh D. Sinclair, Simon M. Mudd, and Mark Naylor

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Short summary
Pebbles become increasingly rounded during downstream transport in rivers due to abrasion. This study quantifies pebble roundness along the length of two Himalayan rivers. We demonstrate that roundness increases with downstream distance and that the rates are dependent on rock type. We apply this to reconstructing travel distances, and hence size of ancient Himalayan. Results show that the ancient river network was larger than the modern indicating that there has been river capture.