Impact assessment of Three Gorges Dam’s impoundment on river dynamics in the north branch of Yangtze River estuary, China

As one of the world’s largest hydropower projects, the Three Gorges Dam (TGD)’s 156–175 m height storing test in flood recession seasons during 2006–2011 has aroused international attention. The novel approaches of cross-wavelet transform and wavelet coherence were firstly introduced to reveal the significant coherence and time lag property in time–frequency space between river dynamics in hydrological stations along Yangtze River. Then, using the optimum lag time nodes, corresponding regression models between river dynamics in these stations were established and utilized to assess the river dynamics in stations downstream under the impact of TGD’s impoundment. Results showed that the average decrease ratio of 24.3 % in the Three-Gorge reservoir outflow was caused in TGD’s impounding periods during 2006–2011. The corresponding average decrease ratios of 25.1, 16.5 and 1.9 % in Yichang flow, Datong flow (with 6 days lag), and estuarine Santiao river stage (with 32 days lag), respectively, were caused by TGD’s impoundment during 2006–2011. In addition, the impact weights of multiple driving factors on estuarine river dynamics were discussed, and the main factor of the combined impacts of estuarine tidal fluctuation and sea level change while the secondary factor of the upper stream inflow were demonstrated. A further suggestion about the optimization design of TGD’s safety impounding scheme was proposed to prevent estuarine saline water intrusion by guaranteeing the average daily increase of Three Gorges reservoir water level lower than 1.11 m/day during TGD’s impounding period.