Long-term assessment of hydropeaking and cumulative impacts on sediment transport, grain size dynamics, channel stability and water resource management.

Hydropeaking refers to rapid and frequent changes in flow discharge and water levels caused by the operation of hydropower dams. This phenomenon is driven by the need to meet fluctuating electricity demands, with water being released or stored to generate power as needed. High energy demand leads to hydropeaking, significantly impacting water resource management in downstream areas. This study investigated the influence of increasing hydropower generation (18 reservoirs, total storage capacity of 2910 million m) on hydropeaking in Central Vietnam's Vu Gia Thu Bon (VGTB) River basin. An analytical method (statistics, frequency analysis combined with field surveys and experiments) was used to quantify the cascading operation on an hourly water level scale (2018-2021) at six hydrological stations within the VGTB River system. Key findings indicate that (1) hydropeaking levels are high in the VGTB River system and have increased significantly over the last decade, particularly in recent years; (2) hydropeaking predominantly occurs spatially along the Vu Gia River (12 reservoirs, total storage capacity of 1335 million m), mainly between the Hoi Khach and Ai Nghia stations, with 380 days of daily water level differences ranging from 1 m to 2 m during the dry season; and (3) the water levels at the hydrological stations of the Thu Bon River (0.3-1.34 m) are more stable and exhibit less variation than those in the Vu Gia River (1.1-1.8 m). Rapid and frequent changes in water levels accelerate riverbank erosion, contributing to channel degradation and altering the physical shape of both rivers over time. Furthermore, this study addresses the lack of understanding regarding upstream impacts on sediment transport by emphasizing the changes in grain size distribution (d50) caused by hydropeaking. Specifically, the grain size (d50) in the Bung tributary, with its numerous reservoirs, was significantly greater than that in the Vu Gia and Thu Bon Rivers, with d50 values of 2.5801 mm, 0.5062 mm, and 0.3952 mm, respectively. To minimize the effects of hydropeaking, dam operators and flood management authorities must adjust hydropower generation to better align with natural flow patterns, gradually release and control water, and enhance the adaptability of downstream communities.