| The lateral entrapment of suspended sediment exerts tremendous influence on the ecological environment and morphodynamics in tidal estuaries, and received much attention of many researchers. Early researches mainly focused on field observation and lab analysis. Recent years, the numerical models or analytical models on hydrodynamics and sediment transport have been paid much attention.The dominant tidal hydrodynamics in both James River estuary and the South Channel of the Yangtze estuary is semi-diurnal tidal flow. In James River estuary, the dominant residual driven factor is horizontal density gradient, the river discharge is very weak, while in the South Channel of the Yangtze estuary, the dominant residual driven factor is river discharge, the horizontal density gradient is very weak. Correspondingly, most sediment is trapped over the south shallow shoal of James River estuary, while trapped over the north deep channel of the South Channel of the Yangtze estuary. To explain the sediment entrapment mechanism of tidal estuary, an analytical model mainly considering M2tidal flow and horizontal density gradient was established by Huijts et al.(2006) to model the hydrodynamics and lateral sediment transport in tidal estuaries. With this model, they explained the mechanism of the sediment entrapment on the left shallow shoal of James River estuary, Chesapeake Bay. As the river discharge is weak in James river, thus it has been neglected in this model. Besides, they also neglected several factors such as wind shear stress, tidal nonlinear advection, M4tidal flow as well as sediment spatial settling lag, which are potentially important in the sediment entrapment of many tidal estuaries. Therefore, the aim of our research is to further study the model of Huijts et al.(2006), by considering M2and M4tidal flow, horizontal density gradient, wind shear stress, tidal rectification, sediment spatal settling lag and so on, and expecting to analysis the lateral sediment entrapment mechanism of more hydrodynamics types of tidal estuaries.In our research, the model estuary is infinitely long with an arbitrary lateral depth profile. The hydrodynamics is governed by the three-dimensional shallow water navier-stokes equations on the f-plane. At the surface, the rigid lid approximation is applied, and a residual wind shear stress is prescribed. At the bottom, we assume no slip and impermeability condition. The water motion is driven by M2tidal flow and M4tidal flow at the seaward and driven by river discharge at the landward. The sediment hydrodynamics is computed by sediment mass conservation equations. The sediment is non-cohesive, and consisted of a single class of fine particles, thus the settling velocity is constant. At the surface, the diffusive and settling sediment flux balance each other. At the bottom, the exchange capacity of sediment between river bed and the water balance over a tidal period. The vertical distribution of SSC (suspended sediment concentration) is depended on the bed shear stress of hydrodynamics. The lateral distribution of SSC is depended on the morphodynamic equilibrium condition, which assumes that the net lateral sediment transport over a tidal cycle is zero.To solve the model, all the equations and correspondingly boundary conditions are scaled and non-dimensionalized, firstly. And then, perturbation analysis was done on all the equations to linearize the partial differential equations. To verify every single driven factor induced flow field, we used the non-structure ocean model (FVCOM) to obtain the numerical solution of each driven factor, including M2tidal flow, M4tidal flow and residual flow induced by wind shear stress, river discharge, nonlinear tidal advection as well as horizontal density gradient. The verification results showed that, all the numerical solutions and analytical solutions are qualitatively in good agreement. Thus, our analytical model is reasonable and could be used to discuss more estuary mechanisms.In James River estuary, the dominant tidal dynamics is M2tidal flow, the dominant residual flow is induced by horizontal density gradient. The river discharge is weak. Under the driver of lateral density gradient, a lateral clock-wise circulation was formed, which entraps much of sediment over the left shallow shoal. Moreover, both the observation and model results showed that, part of the sediment are also trapped over the right shoal, which is mainly caused by the M4tidal flow and spatial settling lag effect. The former effect transport sediment to the right shoal, while the latter one transport sediment from deep channel to shallow shoal. The sediment transport of M2tidal flow and river discharge is relatively weak.In the South Channel of Yangtze estuary, the dominant tidal dynamics is M2tidal flow, the dominant residual flow is induced by river discharge. The horizontal density gradient is very weak. Deflected by the Coriolis force, the lateral residual flow is almost an unclock-wise circulation, which transports much of sediment to the right of the channel and causing the entrapment of sediment over the right shallow channel. Moreover, the M2sediment transport, which points to the right channel, is also very strong. The sediment transport of M4tidal flow and wind induced residual flow is relatively weak.From the study results above, the dominant sediment transport factor in James River estuary is lateral density gradient. The dominant sediment transport factors in South Channel of Yangtze estuary are river discharge and M2tidal flow. Although the trapping area between these two estuaries is different, both of them are trapped in the shallow shoal or channel. Therefore, we further developed the above analytical model to compute the lateral morphologic evolution, calling morphological model. In the morphological model, the morphodynamic equilibrium assumption is dropped. The exchange between suspended sediment and bed sediment is free. The remaining boundary conditions are same as the lateral entrapment analytical model.To discuss the sediment entrapment and morphologic evolution trend of different hydrodynamic types of estuaries, the analytical model was applied to an idealized estuary with several scenarios. Each scenario corresponds to each individual type of estuaries according to the research of Prichard (1952).1) Regarding to plain estuaries like Upper Chesapeake Bay, the hydrodynamics is controlled by M2tidal flow, which is also the dominant lateral sediment transport factor. The M2tidal flow transport sediment from central channel to the left and right shoals and entraps sediment over the two shallow shoals, which is generally consistent with the observation of Upper Chesapeake Bay. In this type of estuary, the central channel and shallow shoals would be deposited slightly, the left and right of central channel will be eroded slightly.2) Regarding to plain estuaries like James River estuary, the hydrodynamics is controlled by M2tidal flow and horizontal density gradient, and both of them are the dominant lateral sediment transport factors. The former one transports sediment from right channel to left one, and the latter one transports sediment from central deep channel to shallow shoals. Thus, most sediment is trapped over the left channel, while part of sediment trapped over the right channel, which is generally consistent with the observation of James River estuary. In this type of estuary, the left channel would be deposited, while the right channel will be eroded.3) Regarding to runoff-dominanted estuaries, the hydrodynamics is controlled by M2tidal flow and river discharge, and both of them transport sediment from left channel to right one. Thus, most sediment is trapped over the right channel, which is generally consistent with the observation of the South Channel of Yangtze estuary. In this type of estuary, the right channel would be deposited, while the left channel will be eroded.4) Regarding to the transition estuaries between runoff-dominanted and plain estuary, the hydrodynamics is controlled by M2tidal flow, horizontal density gradient and river discharge. We set the horizontal density gradient constantly as (dp/dx, dp/dy)=5x10-4kg m-4. If the river discharge is weak, such as500m3s-1, the dominant lateral sediment transport factors are horizontal density gradient and M2tidal flow, most of sediment are trapped over the left channel. If the river discharge is abundant, such as5000m3s-1, the dominant lateral sediment transport factors are river discharge and M2tidal flow, most of sediment are trapped over the right channel. We also found that if the river discharge is3500m3s-1, the sediment transport due to river discharge and lateral density gradient banlance each other, thus the dominant lateral sediment transport factors is M2tidal flow, sediment could be trapped over two shallow shoals.Meanwhile, we also modelled the lateral entrapment and morphologic evolution of South Channel of Yangtze estuary according to three extreme weather or hydrodynamics events, i.e. storm surge, salt water intrusion and flood. Although our analytical model is applicable to many types of tidal estuaries, several potentially important mechanisms need improvement, i.e., the along-estuary variation of hydrodynamics and sediment movement, more frequencies tidal flow, middle-and long-term morphological evolution modelling. |
