Numerical simulation of sediment transport characteristics under tidal action in the Qiantang estuary

The Qiantang River Estuary (QRE) is a typical alluvial estuary featured by intense tidal bores, high suspended sediment concentration, and drastic bed erosion-deposition processes. Understanding the variation characteristics of sediment concentration under interactive runoff–tidal current dynamics is of great significance for estuarine disaster prevention, resource utilization and protection, as well as estuarine regulation engineering. Conventional numerical models for estuarine sediment transport neglect the density variation induced by hyper-concentrated sediment mixtures and the feedback effect of riverbed evolution on flow dynamics, which impairs the mass conservation properties of model systems. Relevant studies specifically targeting strong-tidal estuaries remain relatively insufficient. In this study, an unstructured triangular-mesh two-dimensional coupled hydro-sediment numerical model is developed to simulate flow regimes and sediment transport processes under tidal bores. The hydrodynamic module adopts water–sediment mixture flow equations, which fully consider the impacts of sediment transport and riverbed deformation on flow. The sediment module employs a non-equilibrium suspended load transport framework. The governing equations are discretized and solved via an explicit finite-volume method. Interface advective fluxes are calculated using the Roe scheme based on approximate Riemann solutions, while central difference is adopted for diffusive flux computation. The proposed model is comprehensively calibrated and validated against in-situ field measurements of the QRE. Simulated results of tidal level, flow velocity, suspended sediment concentration and riverbed deformation are in good agreement with observed data, demonstrating that the numerical scheme reliably represents the actual hydrodynamic and sedimentary conditions of the QRE. Finally, the validated model is applied to investigate sediment transport characteristics within the QRE driven by tidal bore. Results reveal that sediment concentration exhibits dramatic intra-tidal temporal fluctuations, and presents a significantly positive correlation with tidal range over the semi-lunar tidal cycle. Longitudinally, the estuary forms a distinct turbidity maximum zone, with elevated sediment concentration spanning from Cangqian to the Cao’e River mouth and relatively low values at the upstream and downstream extremities. Transverse sediment distribution and its tidal variability are closely coupled with cross-sectional geomorphic configurations. The findings in this study provide a theoretical reference for enriching sediment transport mechanisms in strong-tidal estuaries and support practical estuarine regulation and management.