Modeling the formation of a sand bar within a large funnel-shaped, tide-dominated estuary: Qiantangjiang Estuary, China

Abstract

The Qiangtangjiang Estuary (the outer part being known as Hangzhou Bay) located on the east coast of China is a large funnel-shaped, tide-dominated and well-mixed estuary. The equilibrium estuarine morphology has been attained and characterized by a large sand bar having a total length of 125km and an elevation of 10m above the average adjacent seabed. In order to investigate the physical processes governing the formation of this morphological feature, two-dimensional depth-averaged process-based morphodynamic modeling (Delft3D) was carried out on a schematized funnel-shaped domain with exponentially decreasing widths based on the dimensions of the Qiangtangjiang Estuary. The model simulated a 6000-year period, the output showing the development of a sand bar that reached equilibrium within about 3000years. The general shape, size and position of the modeled sand bar are consistent with the observations. Short-term simulations of hydrodynamic and sediment transport processes at the initial stage indicate that, in response to the interactions between river discharge and tidal currents, which are strongly influenced by the funnel-shape, the sand bar developed in the transition zone between the river-dominated upper estuary and the flood-dominated lower estuary where sediment transport pathways converge. A series of sensitivity analyses suggest that the estuarine convergence rate, sediment grain size, and river discharge are the main controlling factors of sand bar formation. Similar to other large funnel-shaped, tide-dominated estuaries of the world, a sufficient supply of fine cohesionless sediment (derived from the adjacent Changjiang Estuary), a large river discharge, and a strong shoreline convergence rate have shaped the large sand bar in the Qiangtangjiang Estuary.