The alteration of lateral circulation under the influence of human activities in a multiple channel system, Changjiang Estuary

Extensive coastal engineering activities including channel dredging and constructions of large infrastructure have greatly altered hydrodynamics in the Changjiang Estuary. While the changes in longitudinal circulation have been well-documented, limited studies have focused on the alteration in lateral circulation and its impact on estuarine circulation in the Changjiang Estuary. Lateral circulation in an estuary plays an important role in modulating estuarine circulation, sediment transport, and morphological change. Different from many of previous numerical studies, we utilize an unstructured-grid numerical model configured in a way to allow inundation over dikes and tidal flats. Numerical model experiments show significant changes in lateral circulation and stratification along the North Passage in response to the altered geometry and bathymetry due to the channel deepening and the dike construction. Both the channel deepening and the dike construction led to an enhanced stratification. Further analysis suggests that the along-channel, the lateral, and the vertical (including up- and downwelling and vertical mixing) effects are all important in modulating the intra-tidal stratification-destratification process. The channel deepening enhances both the lateral and the vertical effects around the high-water slack. The construction of dikes that completely blocks the lateral water exchange between the North Passage and surrounding tidal flats, changes the system from a multiple-channel system into a single-channel system. With dike construction, the estuarine circulation tends to have a horizontally sheared pattern with weaker friction and a stronger stratification.