## 1 Introduction  
Compared with physical models of sediment transport, numerical models have several advantages, such as low running costs, fast simulation speed, and convenient parameter adjustment. Thus, numerical models have wide applications in the planning, design, and assessment of estuarine and coastal engineering [1; 2; 3; 4; 5]. Previous efforts to model sediment transport have conducted lots of work based on non-uniform sediment that exists in natural rivers and coastal waters [2]. Research showed that models considering multiple sediment classes appeared to be better suited to simulate suspended sediment transport in the Tagus estuary than models using a single grain size [6]. In the numerical experiments, the computed results improved significantly when they used non-uniform sediment transport formulas or a modified approach [7]. Research also demonstrated that a model considering non-uniform sediment transport yielded the results that were consistent with measurements, while a model that only considered uniform sediment under-predicted or over-predicted the sediment transport rates [8; 9]. It has been found that dividing sediments into a number of classes and separately simulating each sediment size class improved predictions [10]. In contrast, they found that the traditional approach of adjusting the sediment in a multi-disperse mixture to consider all particle sizes does not accurately predict the distribution of volumetric sediment concentrations. As a result, models using non-uniform sediment parameters show better simulation reliability than uniform sediment methods. Zhejiang, where the estuary of one of the world's biggest river lies, is one of the most densely distributed areas of estuaries in China. The flow structure and sediment transport are complicated under the combined influence of natural and anthropic factors. Quantification of sediment transport in tidal estuaries is essential to support both theoretical, as well as numerical modelling, of tidal estuarine morphodynamic evolution. Obtaining high-resolution data of sediment transport is hard. Both physical modelling and numerical simulation methods have been employed to investigate the sediment transport problem in estuaries. [PERSON] et al. [11] elaborated on the theory and design of different river engineering models, including estuarine and coastal conditions. [PERSON] et al. [12] proposed a small-scale riverine model, and [PERSON] et al. [13] reviewed its applicability. Flow in estuary is subject to the combined effect of tide and residual runoff, and sediment flux comes in both directions, thus creating a complex dynamic environment. Compared with inland rivers, the sediment transport in tidal estuaries is more complex and therefore more difficult to simulate [14; 15; 16]. The estuarine zone is where the land and sea interact. Various physical processes, such as current, waves, and sediments, are coupled and add to computation complexity. In recent years, a few non-uniform sediment numerical models have been proposed in the literature. [PERSON] et al. [17] established a two-dimensional non-uniform sediment transport model for the Oujiang estuary and Wenzhou Bay by using the sediment transport capacity formula for uniform sediment. [PERSON] et al. [18; 19] developed two models for non-uniform sediment transport in channels by considering hiding-exposure effects among different size classes. [PERSON] et al. [20; 21] proposed a one-dimensional and a three-dimensional model for non-uniform sediment transport in rivers by using a formula for sediment transport capacity that did not consider the interaction between fractions. [PERSON] et al. [22] described a two-dimensional model for non-uniform sediment transport in alluvial channels by using empirical values for the critical bed-shear stresses for erosion and deposition. [PERSON] et al. [23] presented a two-dimensional model for mixed-grain size sediment transport by considering grain-sorting effects in non-uniform sediment. [PERSON] et al.