Effects of high river discharge on decadal morphological evolution of the inner Yangtze Estuary

Morphological evolution of estuaries and deltas at the decadal timescale is becoming a global issue in recent decades due to their economic and environmental significances. Present study explores the decadal morphological evolution under high river discharge and decreasing river sediment. Quantitative analysis of bathymetric data indicates that frequent river floods in the 1990s enhanced erosion of the inner estuary superimposed with river sediment decline. A process-based modeling approach (Delft3D) is applied to investigate the physical mechanisms of river flooding on morphological change. Hydrodynamic simulations indicate that the water level gradient and residual transport in the inner estuary increase with river discharge. High water level gradient occurs simultaneously with peak ebb flow, and this status can last for about 5-6 hours. This hydrodynamic condition with sufficient long period facilitates channel erosion and sandbar incision. Morphological simulations indicate that erosion along the main channels is enhanced under higher river discharge, especially in the upstream part. The enhanced erosion can be offset by the increase in sediment load. River flooding superimposed with further decreased sediment supply in the future may induce more significant modifications of channel-shoal systems within the inner Yangtze Estuary than the present.