Hydroacoustic insights into bedload and suspended sediment dynamics in Nepal's Himalayan rivers

Coarse sediment transport in Himalayan rivers exerts a first-order control on river morphology, flood hazard, and the longevity of hydropower infrastructure. Variations in gravel or sand export from the mountain front govern downstream channel incision and aggradation rates, directly modifying channel conveyance capacity and flood risk on the Gangetic Plains. These highly mobile gravel-bed rivers draining the Himalayas are prone to abrupt channel switching driven by the mobility of coarse bedload, exacerbating flood hazard. Despite their importance, current flood models and hydropower development plans remain limited by uncertainties in bedload flux estimates. 

To understand the controls on sediment yield from the mountain front, we monitor both suspended sediment load through direct sampling, and bedload through acoustic and seismic monitoring at the mountain front of the Karnali and West Rapti Rivers in Nepal. Two Aquarian hydrophones connected to AudioMoth data loggers are installed at each site to monitor bedload transport for the duration of an entire monsoon season. This acoustic dataset is combined with seismic data from a DiGOS geophone at each site, water level data from the Department of Hydrology and Meteorology in Nepal, and manual suspended sediment concentration sampling. These complementary approaches independently approximate the sand and gravel fractions respectively, allowing us to evaluate the accuracy of current approximation methods for bedload in Himalayan rivers.  

The results indicate that suspended sediment loads are primarily supply-limited, showing a pronounced seasonal hysteresis in suspended sediment concentration with peaks that record local events such as landslides and storms in the catchment. In contrast, the bedload flux appears to be transport-limited, closely tracking the river discharge. Acoustic data show initial bedload movement at lower discharges, with an abrupt increase in bedload transport signal at a threshold discharge. At higher discharges there is a saturation of the signal, which may reflect boundary conditions of bedload transport or data clipping. These contrasting signals indicate independent transport mechanisms for suspended load and bedload, with a high degree of variability in suspended load relative to a much more predictable bedload flux. Seasonal fluctuations in this bedload-to-suspended-load ratio demonstrate that bedload estimates based on suspended sediment measurements can substantially misrepresent total sediment fluxes, with major implications for flood early warning systems and urban planning.