Geomorphological and hydrological controls on sediment export in earthquake-affected catchments in the Nepal Himalaya
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: final response (author comments only)
- 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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This paper asks a very straightforward question regarding what the legacy of earthquake triggered landslides are on the mobilization of sediment in river channels adjacent to landslides in the years after an earthquake. Recent events in Taiwan, China, and New Zealand all highlight the potential for earthquake-triggered landslide debris to cascade into river systems and dramatically change river morphology. This study exploits the 2015 Gorkha (Nepal) earthquake and examines changes in valley bottom gravel storage for two rivers, the Bhote Kosi and the Melachi-Indrwati, as a proxy for sediment storage.
Notably, the study shows that earthquake-triggered landslides don’t influence valley bottom sediment storage in a clear way, despite the fact that many of the landslides are adjacent to river channels. Moreover, a huge flood event in 2021, which did influence the valley bottom sediment storage in a clear and dramatic way on one of the two rivers, transported sediment that was pretty clearly not sourced from earthquake triggered landslides. Rather, very likely, the legacy of previous landslide dams (and importantly the fine sediment they impound) is likely the dominate source of sediment to river channels.
All of this suggests that how earthquakes influence river channels through the landslides they trigger is a bit more nuanced than has been articulated previously. While the legacy of landslides is important, in this example, the importance derives not from the sediment landslides generate directly (which is generally quite coarse) but rather through the sediment that is impounded behind landslide dams (which is by definition mobile in the river and hence finer).
I love this study because it takes a critical and objective look at an issue that I think many have opinions about but few bother to actually explore with data. I just have a one substantial comment below, and then a minor comment.
Figure 4-5 (and text pertaining to): why not simply calculate the width of the exposed gravel orthogonal to the river channel as a function of the distance downstream? Plotting area versus distance is confusing because it relies on a discretization, and hence will be more difficult to reproduce in the future. I think a more straightforward thing would be to plot width versus distance, which when numerically integrated in the downstream direction would yield area. There is much less ambiguity about this measurement, and it is much more straightforward to interpret. Also, once you have area, you can easily get to volume with an assumption about aggradation depth.
Figure 6. again, to me thinking about changes in width is much easier to interpret than changes in the area of segments that have arbitrary lengths.
338-344: A similar case was documented in California in the Sierra Nevadas in 2018 and summarized in this study:
Collins, Brian D., et al. "Linking mesoscale meteorology with extreme landscape response: Effects of narrow cold frontal rainbands (NCFR)." Journal of Geophysical Research: Earth Surface 125.10 (2020): e2020JF005675.
In this work there is weather radar available that demonstrates exactly what you are speculating about here - a local zone of extremely intense precipitation. In the case of the Sierras, the culprit is a Narrow Cold Frontal Rainband (NCFR). I don’t know if these also occur commonly in the Himalaya, but regardless, this paper provides support for the basic idea you are proposing so I think it’s worth citing.