Junyu Tao

A GPU-accelerated shallow water model with a local time-step (LTS) is employed in this work to examine how the Coriolis forces affect the tidal level difference and, consequently, the water–sediment exchange between Hangzhou Bay (HZB) and the Yangtze River Estuary. The model is applied to both idealized and realistic estuary configurations to analyze tidal level gradients between the two neighboring estuaries under different flow conditions and with and without the Coriolis force condition. The model’s accuracy in predicting tidal levels and currents was validated against field data. It is shown that the tidal level gradient is negative during flood tide, indicating a mass transfer trend from south to north, whereas the tidal level gradient is positive during ebb tide, indicating a north-to-south mass transfer. Considering sediment originates mainly from the riverine side, the sediment mass transfer may occur mainly during ebb tide, and the direction is from the Yangtze River to the HZB. This finding provides numerical evidence for previous recognition that sediment in HZB mainly comes from the Yangtze River Estuary. A comparison of the idealized and realistic estuary configurations further indicates that the contrasting bed topography enhances tidal level gradients. The findings show that by causing tidal phase changes and asymmetric tidal range modifications, the Coriolis force increases lateral water level gradients (up to 0.7 m) between the Yangtze Estuary and Hangzhou Bay. Idealized modeling further demonstrates that higher Coriolis coefficients promote sediment exchange and exacerbate water level fluctuations across estuaries. Without the Coriolis effect, the tide level distribution in adjacent estuaries is symmetrical. In the Northern and Southern Hemispheres, the tide level distribution in adjacent estuaries is the opposite. In addition, this study has shown that changes in river flow have a limited effect on water levels at stations farther from the estuary’s flow intake and therefore have a negligible effect on the water level gradient in adjacent estuaries farther away. However, topography differences have a significant effect on water level gradients in neighboring estuaries. These studies emphasize the significance of the Coriolis force in regulating sediment transport pathways in estuaries.

In this paper, a computationally efficient shallow water model is developed for sediment transport in the Yangtze Estuary by considering mixed cohesive and non-cohesive sediment transport. It is firstly shown that the model is capable of reproducing tidal-hydrodynamics in the estuarine region. Secondly, it is demonstrated that the observed temporal variation of suspended sediment concentration (SSC) for mixed cohesive and non-cohesive sediments can be well-captured by the model with calibrated parameters (i.e., critical shear stresses for erosion/deposition, erosion coefficient). Numerical comparative studies indicate that: (1) consideration of multiple sediment fraction (both cohesive and non-cohesive sediments) is important for accurate modeling of SSC in the Yangtze Estuary; (2) the critical shear stress and the erosion coefficient is shown to be site-dependent, for which intensive calibration may be required; and (3) the Deepwater Navigation Channel (DNC) project may lead to enhanced current velocity and thus reduced sediment deposition in the North Passage of the Yangtze Estuary. Finally, the implementation of the hybrid local time step/global maximum time step (LTS/GMaTS) (using LTS to update the hydro-sediment module but using GMaTS to update the morphodynamic module) can lead to a reduction of as high as 90% in the computational cost for the Yangtze Estuary. This advantage, along with its well-demonstrated quantitative accuracy, indicates that the present model should find wide applications in estuarine regions.

It is generally believed that sediment erosion and deposition can’t occur simultaneously, which is also reflected in the classical Partheniades-Krone formulas used to calculate erosion and deposition flux. In this study, the erosion and deposition fluxes of cohesive sediment are integrated in the tidal period respectively, and when they are equal, the corresponding suspended cohesive sediment concentration is called ‘tidal average saturated concentration of cohesive sediment’. Theoretical analysis of the factors affecting the saturated concentration indicates that a large erosion coefficient results in a high saturated concentration level. The corresponding critical erosion and deposition shear stresses (i.e., τe and τd) at saturated concentration have many possibilities. Therefore, it is understandable that good agreement of suspended sediment concentration between simulation and observation have been obtained by adjusting τe and τd in the previous numerical simulation calibration. According to the relative magnitude of τe and τd at saturated concentration, the erosion and deposition fluxes of cohesive sediment can be divided into four situations: weak erosion (i.e., τe  > τd), intense erosion (i.e., τe  < τd), intense deposition (i.e., τe  < τd), and weak deposition (i.e., τe > τd ). A two-dimensional numerical model is applied to calculate the temporal and spatial variation of the saturated concentration of cohesive sediment in the Yangtze Estuary. Simulation results shows the following findings. 1) The impact of the fraction of the kth size class in the surface (top) layer of bed material on erosion flux of non-uniform cohesive sediment is necessary to be considered. Otherwise, the calculated saturated concentration of cohesive sediment is greater than the measured. 2) The differences between saturated concentration and the field calculated/measured suspended sediment concentration can be applied to infer bed erosion/deposition characteristics to some extent, and compared it with the measured erosion/deposition result, which in turn verifies the values of  τe and τd in the model. This finding provides insights for the following research on transport and diffusion of cohesive sediment in estuary and coastal areas.

Direct numerical simulations of lock-exchange turbidity currents with small Reynolds number were often assumed to well represent turbidity currents with large Reynolds number. Here, this assumption is examined using a layer-averaged numerical model. It is shown that in the initial stage the current front position converges if the Reynolds number approaches certain threshold values. However, at later stages, turbidity currents with larger Reynolds number propagate faster and farther. This is because, by definition, a larger Reynolds number corresponds to more sediment mass carried by the current, and thus higher driving force. Furthermore, turbidity currents with the same Reynolds number can correspond to very different front positions and deposition profiles, as the Reynolds number depends on both current thickness and concentration.