Suspended sediment dynamics in an urban, mountain catchment in Nepal

Abstract. Urban mountain catchments are highly vulnerable to erosion and sedimentation due to steep terrain, intense rainfall, and rapid land-use change at the urban fringe. However, event-scale sediment transport remains poorly understood in these regions, particularly in data-scarce areas such as the Himalayas. This study presents the first high-frequency, event-based analysis of suspended sediment transport in the Nakkhu River, a rapidly urbanizing catchment in Kathmandu Valley, Nepal. Using optical backscatter sensors and targeted field sampling during the 2023 monsoon, we analysed how rainfall, antecedent moisture, and human disturbance shaped sediment responses. Sediment transport was highly episodic, with two extreme storms accounting for nearly half of the seasonal suspended sediment load. Analysis of SSC–discharge hysteresis patterns revealed event-specific variability shaped by rainfall intensity, antecedent conditions, and hydrologic connectivity. Peak SSC often lagged peak discharge during low-flow events, suggesting upstream sediment sources. In contrast, high-intensity storms produced rapid sediment delivery, likely from hillslopes, mining zones, and in-channel deposits. Low-frequency (daily) monitoring underestimated sediment loads by approximately 30 % compared to 30-minutes interval data. This study provides the first 30-minute interval event-scale analysis of suspended sediment transport in Kathmandu Valley, revealing how sediment responses vary across the monsoon season in relation to rainfall intensity, discharge dynamics, and antecedent conditions. These insights, including lagged sediment peaks during low-flow events and rapid sediment flushing during intense storms, highlight the value of sub-hourly monitoring for capturing within-event variability and identifying short-lived sediment sources in urban mountain watersheds.