Study on the Evolution of the Sandy-gravel Riverbed and the Characteristics of Channel hydrodynamic force after the Operation of the Three Gorges Project

Juntao Liu^1,2Yang Yunping^1*Ming Li³Xinbo Liu⁴Chao Ji¹Yongsheng Liu^5*

• ¹Tianjin Research Institute of Water Transport Engineering, Tianjin, China
• ²Tianjin Water Carriage Engineering Reconnaissance Designing Academy, Tianjin, China
• ³Changjiang Waterway Institute of Planning and Design (Wuhan), Wuhan, China
• ⁴Watershed Hub Operation Management Center, Wuhan, China
• ⁵Hunan Provincial Territorial Space Survey and Monitoring Institute, Changsha, Anhui Province, China

Post-commissioning of the cascade reservoir system, substantial riverbed scouring manifested downstream of the primary dam structure. Notable geomorphological adjustments in both the riverbed and adjacent sandbars were observed, particularly under equivalent discharge conditions with depressed water levels.. Changes in the hydrodynamic environment related to "steep slopes and rapid flow" remained unclear. We used a section of the Yangtze River (120 km) downstream of the Three Gorges Dam (TGD) and flow sediment, and riverbed topography data (from 2022–2023) to analyze the intensity and distribution of riverbed scouring and deposition and changes in water surface gradient under the same flow rate and low-water levels. The low-water and bankfull channels of the Yichang–Dabujie reaches experienced a strong cumulative scouring trend (99.5% concentrated in the low-water channel), and scouring intensity significantly decreased. The sandbar area decreased with a correlated increase in deep-water channels. After implementation, the sandbar and deep-water channel fluctuation range decreased. A cumulative downward trend in flow rate to a low-water level occurred. Sandy sections declined more than sandy-gravel riverbed reach, with an increased water surface gradient, known as the “steep slope” phenomenon. As the sandy to gravel riverbed transition increased, the amplified superimposed “rapid flow” phenomenon increased. Downstream sandy riverbed sections continue to experience strong scouring. Gravel riverbed sections will maintain low-level scouring and remain stable without large-scale human interference. The "steep slope and rapid flow" phenomenon as a dominantin sandy-gravel transition reaches. This stusry systematically elucidated riverbed evolution processes, hydrodynamic regime alterations, and morphological trends downstream of mega-reservoirs, offering critical implications for hydraulic engineering design and navigational safety management.