Sediment Transport and Morphodynamics at the Confluence of Yangtze River and Poyang Lake, China

The effect of dam operations on variation of upstream flow and sediment load at large river confluences was scarcely investigated in the literature. Three Gorges Dam (TGD), one of the largest dams in the world, has caused more and more effects on the environmental and ecological processes of the downstream Yangtze River as well as its tributaries, e.g., its largest tributary, the Poyang Lake.

In recent years, a significantly decreasing of the magnitude and frequency of the backwater of the Yangtze River and the similar situation of sediment to the Poyang Lake was observed, which it is expected to be related with the advanced and lengthened dry seasons in the Lake. To investigate the effect of TGD on the variation of flow and sediment load at the confluence of the Yangtze River and the Poyang Lake, a field study was conducted in August 2018 in high flow conditions at this confluence. During this field campaign, an acoustic Doppler velocity profiling (ADCP) was used to measure the three-dimensional water velocities, water depth and backscatter intensity, which could be used to analyse the transport of suspended sediment after a proper calibration by sonar equation. Water and bed sediment sampling were collected for the analysis of sediment texture. A Multi-Parameter Water Quality detecting equipment was used for the measurement of turbidity and total dissolved solids at the confluence.

Preliminary results showed that: (1) large differences in terms of turbidity and total dissolved solids were found between Yangtze River and Poyang Lake; (2) on the side of Yangtze River, the concentration of total suspended solids (TSS) was 74 mg/L and it was almost constant over the depth, while on the side of Poyang Lake, the TSS concentration was 13 mg/L and it was increasing with depth; (3) the distribution of backscatter intensity showed a high concentration of sediment in the flow separation region because of secondary currents; (4) about confluence morphodynamics, the classical scour hole was not observed, but there was a long and deep channel downstream of the junction.