Preprints
https://doi.org/10.5194/esurf-2021-19
https://doi.org/10.5194/esurf-2021-19

  08 Mar 2021

08 Mar 2021

Review status: this preprint is currently under review for the journal ESurf.

Controls on the grain size distribution of landslides in Taiwan: the influence of drop height, scar depth and bedrock strength

Odin Marc1,2, Jens M. Turowski2, and Patrick Meunier3 Odin Marc et al.
  • 1Géosciences Environnement Toulouse (GET), UMR 5563, CNRS/IRD/CNES/UPS, Observatoire Midi-Pyrenees, 14 Avenue Edouard Belin, 31400 Toulouse, France
  • 2German Research Center for Geoscience, GFZ-Potsdam, Section 4.6, Geomorphology, Telegrafenberg, Potsdam, Germany
  • 3Laboratoire de Géologie, Ecole Normale Supérieure, 15 Rue Lhomond, Paris, France

Abstract. The size of grains delivered to river by hillslopes processes is thought to be a key factor to better understand sediment transport, long-term erosion as well as sedimentary archives. Recently, models have been developed for the grain size distribution produced in soil, but they may not apply to active orogens where high erosion rates on hillslopes are driven by landsliding. Until now relatively few studies have focused on landslide grain size distributions. Here we present grain size distribution 5 (GSD) obtained by the grid-by-number sampling on 17 recent landslide deposits in Taiwan, and we compare it to the geometrical and physical properties of the landslides, such as their width, area, rock-type, drop height and estimated depth. All slides occurred in slightly metamorphosed sedimentary units, except two, which occurred in younger unmetamorphosed shales, with rock strength expected to be 3 to 10 times weaker from their metamorphosed counterparts. We found that 4 deposits displayed a strong grain-size segregation on their deposit with downslope toe deposits 3 to 10 times coarser than apex 10 deposits. In 3 cases, we could also measure the GSD inside the landslides that presented percentiles 3 to 10 times finer than the surface of the deposit. Both observations could be due to either kinetic sieving or deposit reworking after the landslide failure but we cannot explain why only some deposits had a strong segregation. Averaging this spatial variability we found the median grainsize of the deposits to be strongly negatively correlated to drop height, scar width and depth. However, previous work suggest that regolith particles and bedrock blocks should coarsen with increasing depth, opposite to our observation. 15 Accounting for a model of regolith coarsening with depth, we found that the ratio of the original bedrock block size and the D50 was proportional the potential energy of the landslide normalized to its bedrock strength. Thus the studied landslides agree well with a published, simple fragmentation model, even if that model was calibrated on much larger and much stronger rock avalanches than those featured in our dataset. This scaling may thus serve for future model of grain size transfer from hillslopes to river, trying to better understand landslide sediment evacuation and coupling to river erosional dynamics.

Odin Marc et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esurf-2021-19', Mikaël Attal, 23 Mar 2021
    • AC1: 'Preliminary Reply on RC1 comments', Odin Marc, 25 Mar 2021
  • RC2: 'Comment on esurf-2021-19', Anonymous Referee #2, 01 Apr 2021

Odin Marc et al.

Data sets

Grain Size Distribution of 17 Taiwanese landslide deposits Odin Marc, Jens M. Turowski, and Patrick Meunier https://www.hydroshare.org/resource/ade683be61e54fa5b60da97418a5f3df/

Odin Marc et al.

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Short summary
The size of rock fragments delivered to river is essential to understand erosion and sediment transport and their related hazards. In mountains landslides deliver these rock fragments, but few studies have analyzed which properties of the landslides controlled the resulting sizes. Here we present measurement on 17 landslides from Taiwan and show that their grain sizes depend on rock-strength, landslide depth and its drop height, thus validating and updating a previous theory on fragmentation.