ADCP-flow velocity profile to interpret hydromorphological features of China’s Yangtze Three-Gorges valley

In 2002, a field investigation was conducted along the Three-Gorges valley of the upper Yangtze catchment by ADCP (Acoustic Doppler Current Profile SONTEK-500). Data obtained when surveying were accompanied with discharge of <15000 m3/s in the valley and characterize the unique river-flow velocity profile and riverbed morphology. Taking into consideration the relationship between average flow velocity and fluvial variables, four distinct river sections can be highlighted, i.e. Chongqing-Wanxian, Wanxian-Fengjie, Fengjie-Zigui and Gezhou reservoir area (up- downstream). The average flow velocity coincides with river width from Chongqing to Wanxian. High-flow velocity ranging from 3.0 to 4.0 m/s is recorded at many sites, where the wider river channel (> 1000 m) and shallower water depth (<20 m) occur and large-size gravel shoals prevail. Alternated low-flow velocity (<1.5 m/s) appears at those river sections with deep water (> 50 m) and U-shaped river-channel morphology. Mapping the river cross-section area at those sites can determine that smaller cross-section area accelerates the flow velocity. From Wanxian to Fengjie, the average flow velocity ranging from 3.0 to 4.5 m/s fluctuates in a good agreement with water depth. The high-flow velocity is associated with narrower river-channel, where V-shaped gorges valley occurs with small cross-section area. Further downstream from Fengjie to Zigui, the low flow velocity is linked to deep river channel characterized by W-shaped valley morphology of large cross-section area, in general. The average flow velocity is 2.5-3.5 m/s, and maximum can reach 6.0 m/s near Wu-Gorges. Our survey had also detected a slow-flow velocity (mostly <1.0m/s) in the river channel of about 100 km long in the Gezhou reservoir downstream. Heavy siltation to 20 m thick above the former riverbed and about 20 km extending upstream from the Dam site occurs above Gezhou Dam. The backwater can reach 150 km due to elevated water level to 27 m by the damming at the end of 1970s, and riverbed erosion below the dam reaches 15-20 m. In addition, our survey records the deeper water river valley from Fengjie to Yichangranging from 70 to 80 m (> 100 m; maximum) in the gorges valley (30-40 m below the present mean sea level). This contrasts the relative shallow water river-channel above Fengjie, i.e. 20-30 m in general and 50-60 m, maximum at gorges site. This ADCP investigation displays the hydromorphological feature in the Three-Gorges valley, and most importantly, it accumulates invaluable dataset for the post-dam study in the near future.