Effects of the South Passage Channel Regulation Project on Saltwater Intrusion in the Yangtze Estuary

Saltwater intrusion threatens freshwater availability in the Yangtze Estuary. The effects of the Deep Waterway Project in the North Passage have been extensively studied, whereas the influence of the Channel Regulation Project in the South Passage (SPCRP) recently implemented has received little attention. A calibrated depth-averaged Delft3D model is used to compare no-project (C0), Phase-1 (C1), and Phases 1 + 2 (C12) scenarios without dredging and quantify changes in flow division, salinity, and cross-sectional salt transport. The results show that Phase-1 has a significant effect on hydrodynamic response. Under spring-tide conditions, the South Passage share of the combined North and South Passage discharge decreases from 0.54 (C0) to 0.24 (C1) and 0.21 (C12), while the net water flux at the upstream South Passage section decreases from 2002 m3/s (C0) to 767 m3/s (C1) and 711 m3/s (C12). Salinity increases of up to 3.63 ppt in the South Passage and 3.03 ppt in the North Passage occur under C1 relative to C0, whereas Phase-2 produces only localized reductions of up to 0.87 ppt and 0.81 ppt relative to C1. Changes are concentrated in the South and North Passages, while the North Channel remains relatively unaffected. At the upstream South Passage section, spring-tide salinity transport is landward in all scenarios, increasing from −4345 ppt·m3/s (C0) to −9788 ppt·m3/s (C1) and −9844 ppt·m3/s (C12), despite seaward net water flux. Neap-tide transport is seaward. The spring-tide landward transport is controlled by a strongly negative resolved tidal covariance, consistent with higher flood-tide than ebb-tide salinity. These results provide a basis for linking estuarine projects with saltwater intrusion risk management.