Comment on egusphere-2026-4202

Abstract

Abstract. In April, 2015 the Mw 7.8 Gorkha Earthquake hit central Nepal, causing widespread co- and post-seismic landsliding. Such surface processes have the potential to increase erosion and perturb fluvial systems for years to decades. Although mass wasting is known to supply coarse material to channels, its impact on the grain size characteristics of suspended sediments remains poorly understood. Here, we present time-series grain size distribution data collected during 2015–2018 from four hydrologic stations using a nested catchment approach in the Koshi basin, where abundant landslides occurred after the Gorkha event. We analyse post-earthquake grain size distribution of riverine suspended sediment and landslide deposit using an end-member modelling analysis. Our results exhibit statistically distinctive signals. We find that one of the coarsest grain size end members (~400 mm) is particularly dominant in the two smallest catchments in the monsoon immediately following the earthquake in 2015. This grain size end member is also abundant in our co-seismic landslide deposit samples, indicating that the higher presence of this end member at our upstream stations likely relates to sediment supply from co- or post-seismic landslides. The earthquake signal in the suspended sediment is only present during the 2015 monsoon season, suggesting that the fine sediment transport in central Nepal was only perturbated by the earthquake for about one season. This provides novel and valuable process-based insight that elevated landslide activity leads to coarsening of suspended load in small catchments. However, it seems unlikely that this signal is recorded in far downstream depositional basins as it is lost during transport.

Authors 7

1. David Puhl corresponding

   University of British Columbia · GFZ Helmholtz Centre for Geosciences

   Affiliation as printed

   Department of Geography, University of British Columbia, Vancouver, BC, Canada

   Section 4.6 Geomorphology, German Research Centre for Geosciences GFZ, Helmholtz Zentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

2. Jens M. Turowski

   GFZ Helmholtz Centre for Geosciences

   Affiliation as printed

   Section 4.6 Geomorphology, German Research Centre for Geosciences GFZ, Helmholtz Zentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

3. Christoff Andermann

   Centre National de la Recherche Scientifique · Géosciences Rennes · GFZ Helmholtz Centre for Geosciences · Université de Rennes

   Affiliation as printed

   Section 4.6 Geomorphology, German Research Centre for Geosciences GFZ, Helmholtz Zentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

   Univ Rennes, CNRS, Géosciences Rennes, UMR 6118, 35000 Rennes, France

4. Michael Dietze Aachen Department of Geography

   GFZ Helmholtz Centre for Geosciences · RWTH Aachen University

   Affiliation as printed

   Department of Geography, RWTH Aachen University, Wüllnerstraße 5b, D-52062 Aachen, Germany

   Section 4.6 Geomorphology, German Research Centre for Geosciences GFZ, Helmholtz Zentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

5. Kristen L. Cook

   Institut des Sciences de la Terre · Institut de Recherche pour le Développement · Université Grenoble Alpes

   Affiliation as printed

   ISTerre, Université Grenoble Alpes, IRD, Grenoble, France

6. Gen K. Li

   University of California, Santa Barbara

   Affiliation as printed

   Department of Earth Science, University of California, Santa Barbara, CA 93106, USA

7. Basanta R. Adhikari

   Tribhuvan University · Institute of Engineering · Pulchowk Campus

   Affiliation as printed

   Department of Civil Engineering, Pulchowk Campus, Institute of Engineering, Tribhuvan University, Nepal