Annual Sediment Transport Dynamics in the Narayani Basin, Central Nepal: Assessing the Impacts of Erosion Processes in the Annual Sediment Budget

Abstract

Identifying the roles of erosional processes in the denudation of mountain ranges requires a better understanding of erosional sensitivity to climatic, topographic, or lithologic controls. We analyzed erosion in the Narayani River basin (draining central Nepal and presenting contrasted lithologic and geochemical signatures in its outcropping rocks and a wide variety of erosional processes and climatic conditions) to assess the relative contributions of erosion processes to the annual sediment export. By combining acoustic Doppler current profiler measurements with depth profiles and daily surface samplings of the suspended load, we propose a simplified model to precisely calculate sediment fluxes at the basin outlet. We estimate an equivalent erosion rate of 1.8−0.2+0.35 mm/year for the year 2010, similar to the average value of 1.6−0.2+0.35 mm/year estimated from 15 years of records and long‐term (~ky) denudation rates of 1.7 mm/year derived from cosmogenic nuclides. The stability of erosion is attributed to efficient buffering behavior and spatial integration in the drainage system. Strong relations between rainfall events and the sediment export suggest that the system is mainly supply limited. Combining physical calculation of sediment fluxes with grain size analyses and geochemical tracers (hydroxyl isotopic compositions, carbonate contents, and total organic carbon content), we estimate that glacial and soil erosion do not contribute more than 10% and a few percentage, respectively, to the total budget and are only detectable during premonsoon and early monsoon periods. During the monsoon, erosion by landslides and mass wasting events overwhelms the sediment budget, confirming the dominant role of these erosional processes in active mountain chains.

Scale breaks of suspended sediment rating in large rivers in Germany induced by organic matter

“…Limiting the water sampling to the top 30 cm slightly underestimates the average SSC in a channel cross section (and thus the suspended sediment load). However, we argue that the rating behavior does not significantly change compared to depthintegrated measurements (Morin et al, 2018).…”

Long-term erosion of the Nepal Himalayas by bedrock landsliding: the role of monsoons, earthquakes and giant landslides

“…The Gorkha earthquake may also not have been representative, as it was relatively deep (15 km) and did not rupture the surface (Avouac et al, 2015). In contrast, paleo-seismological investigations have shown that large surface-rupturing earthquakes (> 100 km long) have occurred along the Himalayan range (Mugnier et al, 2013;Bollinger et al, 2014). Earthquakes shallower than the Gorkha event would likely produce stronger ground motions and thus trigger more landslides and also potentially more large landslides.…”

“…Studies of paleo-ruptures in central Nepal, constrained by historical damage or dated fault scarps, have revealed complex earthquake intervals (Mugnier et al, 2013;Bollinger et al, 2014Bollinger et al, , 2016. Specifically, data from historical reconstructions, accounting for blind ruptures, suggest that at least six large earthquakes affected central Nepal in the last ∼ 1000 years, possibly eight if we consider ruptures from eastern and western Nepal that may have propagated to central Nepal (Mugnier et al, 2013;Bollinger et al, 2016). However, these ruptures have poorly constrained magnitudes, varying from M w ∼ 7.5 to 8.5, and have uncertain return times (Mugnier et al, 2013).…”

“…Specifically, data from historical reconstructions, accounting for blind ruptures, suggest that at least six large earthquakes affected central Nepal in the last ∼ 1000 years, possibly eight if we consider ruptures from eastern and western Nepal that may have propagated to central Nepal (Mugnier et al, 2013;Bollinger et al, 2016). However, these ruptures have poorly constrained magnitudes, varying from M w ∼ 7.5 to 8.5, and have uncertain return times (Mugnier et al, 2013). Dated deformation of river ter-races in the last 4500 years indicates relatively regular surface rupturing of the Main Frontal Thrust (MFT) by great earthquakes every 650-850 years (Bollinger et al, 2014).…”

Integrating suspended sediment flux in large alluvial river channels: Application of a synoptic Rouse-based model to the Irrawaddy and Salween rivers

“…It enables a two-dimensional synoptic map of sediment concentration, flux, and grain-size distribution across morphologically complex river channels, where depth and flow velocity often show significant lateral variations ( Fig. 6) and where averaging across the channel Morin et al, 2018;Santini et al, 2019) would likely result in significant errors of the calculated sediment flux and mean composition.…”