Preprints
https://doi.org/10.5194/egusphere-2022-1347
https://doi.org/10.5194/egusphere-2022-1347
12 Jan 2023
 | 12 Jan 2023

Geomorphological and hydrological controls on sediment export in earthquake-affected catchments in the Nepal Himalaya

Emma L. S. Graf, Hugh D. Sinclair, Mikaël Attal, Boris Gailleton, Basanta Raj Adhikari, and Bishnu Raj Baral

Abstract. Large earthquakes can contribute to mountain growth by building topography, but also contribute to mass removal from mountain ranges through widespread mass wasting. On annual to decadal timescales, large earthquakes also have the potential to significantly alter fluvial sediment dynamics if a significant volume of the sediment generated reaches the fluvial network. In this contribution, we focus on the Melamchi-Indrawati and Bhote Koshi rivers in central Nepal, which have both experienced widespread landsliding associated with the 2015 Gorkha (Nepal) earthquake. Using a time series of high-resolution satellite imagery, we have mapped exposed gravel along the river from 2012–2021 to identify zones of active channel deposition and document changes over time. Counter to expectations, we show negligible increases in coarse sediment accumulation in both catchments since the Gorkha earthquake. However, an extremely high concentration flow event on 15 June 2021 caused an approximately four-fold increase in exposed gravel along a 30 km reach of the channel with up to 12 m of channel aggradation in the Melamchi-Indrawati rivers; this event was localised and did not impact the neighbouring Bhote Koshi catchment. Based on published reports, new helicopter based photography and satellite data, we demonstrate that this event was sourced from a localised rainfall event between 4500 and 4800 m, and that the majority of the sediment was supplied from sources that were unrelated to the landslides generated by the Gorkha earthquake.

Emma L. S. Graf et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1347', Anonymous Referee #1, 30 Jan 2023
  • RC2: 'Comment on egusphere-2022-1347', Alexander Densmore, 08 Feb 2023
  • RC3: 'Comment on egusphere-2022-1347', Oliver Francis, 14 Feb 2023
  • AC1: 'Comment on egusphere-2022-1347', Emma Graf, 07 Apr 2023

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1347', Anonymous Referee #1, 30 Jan 2023
  • RC2: 'Comment on egusphere-2022-1347', Alexander Densmore, 08 Feb 2023
  • RC3: 'Comment on egusphere-2022-1347', Oliver Francis, 14 Feb 2023
  • AC1: 'Comment on egusphere-2022-1347', Emma Graf, 07 Apr 2023

Emma L. S. Graf et al.

Emma L. S. Graf et al.

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Cited

Latest update: 09 Dec 2023
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Do earthquakes result in higher levels of sediment in rivers in the years following the event? There are several reasons to believe this could be the case. Previous studies have even provided evidence of increased sediment loads in mountainous areas after major earthquakes. However, a study by Graf et al. challenges this perspective. They demonstrate that despite the thousands of landslides triggered by the 2015 Gorkha Earthquake in Nepal, there were only minimal increases in coarse sediment in river channels during the subsequent years. Instead, the study reveals a different story. In June 2021, a single, exceptionally heavy rainfall event mobilized large volumes of sediment, leading to riverbed elevation by as much as 12 meters. This effect greatly overshadowed any sedimentary impact left by the earthquake along the river corridor. The study underscores that earthquake evidence may not always be found in the sedimentary records of alluvial areas. In high-mountainous regions, records of riverbed elevation may be strongly influenced by unique and localized events.
Short summary
Using satellite images, we show that, unlike other examples of earthquake affected rivers, the rivers of Central Nepal experienced little increase in sedimentation following the 2015 Gorkha earthquake. Instead, a catastrophic flood occurred in 2021 that buried towns and agricultural land under up to 10 meters of sediment. We show that, in this example, intense storms remobilised glacial sediment from high elevations causing much greater impact than flushing of earthquake-induced landslides.