Tidal Flat Morphodynamic Processes of the Yangtze Estuary and Their Engineering Implications

The intertidal flats are classified as attached bar, spit and isolated bar in relation to the land, and flat, sharp bank and eroded cliff according to the shape of the cross-shore profile. Tidal currents on the flat are basically back and forth along the river channel banks but gyratory on the seaward side of the Chongming Island. The flow velocity on the intertidal flat is gradually reduced with increasing elevation. The river discharge strengthens ebb flows and modifies current asymmetry especially on the lower flat in neap tide, although hydrodynamics over the tidal flat is tide-dominated. The wave height on the tidal flat is normally limited to a few decimeters although it changes with water depth, slope and wind. Suspended sediment concentration over the tidal flat is typically hundreds to thousands of mg/1. Although the delta has grown rapidly in history, the rate of growth is different in different periods. A maximum advancing rate of 330 m/ a was recently found in the central part of the river mouth towards the sea. In view of the natural conditions, reclamation of higher intertidal flat (above the mean tidal level) in advancing coasts is suggested, which would leave broad wetlands for wild lives. In addition, some possible influences of coastal engineering projects and the future natural backgrounds of engineering under reduction in riverine sediment supply and sea level rise are addressed.