| Under the effects of dynamic field in estuaries, hydrodynamic and sediment processes are complex in respects influencing factors and boundary conditions. And numerical simulation become an important method. Most estuaries are muddy estuaries with high content of cohesive sediment in the suspended sediment in China, and the cohesive sediment transport can dominate the evolution of estuaries. Therefore, the investigations on the disciplines of the cohesive sediment transport and applications in numerical simulations can provide important theoretical significance.This paper summarizes the progress in disciplines of erosion and deposition for cohesive sediment. And a concept of relative dry bulk density for bed materials has been employed into the classic formulae of the erosion source term (EST) for cohesive sediment, which is applied to describe the relationship between the EST and consolidation degree. Then two improved formulae are derivated for loose bed and natural bed materials, respectively. After the two formulae calibrated by data from existing laboratory erosion experiment, the formula for natural bed materials was then validated against the erosion experiment data upon nature cohesive sediment and fine quartz sand documented in the literature, which indicates that the formula can be applied to calculate the EST for cohesive sediment in numerical simulation.A two-dimensional (2D) numerical model to simulate the processes of hydrodynamic and sediment transport in estuaries is presented, which employs a finite volume method (FVM) based on an unstructured triangular mesh. In this model, the governing equations comprise the2D shallow turbid water equations, the nonuniform sediment transport equations for graded sediments and the bed evolution equation. A couple solution method is applied to predict the hydrodynamic process, the suspended sediment concentrations and morphological evolution, simultaneously. An optimized Runge-Kutta time stepping method is applied to maintain the stability of solution and provide the second-order accuracy in time, while the Roe's approximate Riemann solver and the MUSCL scheme are employed for the spatial reconstruction of the state variables in FVM to provide the second-order accuracy in space. In this model, a wetting and drying approach in the literature refined for the unstructured triangular mesh is applied, and the minimum water depth is used to determine the wetting and drying. In these simulations, certain criterias were applied to differentiate cohesive sediment and non-cohesive sediment. And the source term of the sediment transport for cohesive and non-cohesive sediment are treated with different methods, respectively. In addition, the validated improved formula of the EST for the natural bed materials has also been applied in the model.The model are calibrated and validated with three different cases, respectively. In the case on a classic generalized flume experiment, the calculated longitudinal bed profile in the flume agreed well with the observed profile, which indicates that the model can predict the sediment transport well. In the estuary simulation case, the predicted processes of hydrodynamics and sediment transport in the Haihe Estuary generally agreed well with the observed data during the spring and neap tidal circles in October1995. Different treatment methods in the mode of sediment transport have also been investigated, which indicates that it is necessary to consider the transport of both cohesive and non-cohesive sediments in estuaries. Finally, the model was validated against data from a flushing experiment in the Taojialu Sluice during in May2002, which indicates that the simulation capability of predicting bed evolution driven by runoff and tidal currents.The calibrated and validated model was applied to simulate the processes of hydrodynamics and transport of both cohesive and non-cohesive sediments during June1982and May1983in the Yellow River Estuary (YRE), and the calculated results agreed well with the observed data. The characteristics of hydrodynamic and sediment transport were analyzed with the comparisons. Then, the uniform and nonuniform sediment transport mode of sediment transport have been investigated, which indicates that it is necessary to treat sediment with nonuniform mode in estuaries like the YRE. In addition, the transport mode of both cohesive and non-cohesive sediments was verified to provide higher simulation accuracy in estuaries. This study can provide a reference for the cohesive sediment transport and morphological evolution in estuaries. |
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