A model comparison of flow and lateral sediment trapping in estuaries

Two different models for the distribution of flow and sediment over the cross-section of a tidally dominated channel are compared. The first is a state-of-the-art numerical model that solves the three-dimensional shallow water equations with prognostic density field. The second is an idealized model which includes residual and semi-diurnal tidalmotions and uses a diagnostic residual density gradient as baroclinic forcing. For bothmodels, an off-line sediment module is used to compute the lateral mean sediment distribution. For fairly high values of vertical diffusivity (~ 0.01 m2 s-1), a good qualitative agreement is found for residual flow patterns. The agreement of the amplitude of the semi-diurnal velocity components is satisfactory as well, although the phase distributions show deviations. The lateral mean sediment distributions are rather similar, and stem from a balance that is predominantly governed by mean concentration and residual currents. The flow patterns only differ qualitatively for either very low or very high tidal velocities. The sediment distributions only deviate for low tidal flow regimes

Effect of bottom stress formulation and tidal forcing on modeled flow and sediment trapping in cross-sections of tide-dominated estuaries

Field data collected in cross-sections of tide-dominated estuaries reveal that flow and suspended sedimentconcentration show pronounced spatial and temporal behavior, which depend on factors like tidal discharge,density gradients and the geometry of the cross-section. Models are capable of reproducing and explainingmany aspects of the observations, but also marked discrepancies occur between model results and data. Theobjective of the present study is to systematically investigate the sensitivity of model output to formulationsof physical processes. This is done by comparing two types of models. The first is a numerical model (NM)that solves the full shallow water equations with prognostic salt dynamics. The second is an IM that solves areduced set of equations for tidal water motion and uses a diagnostic salinity field. The IM can be used as atool to interpret the complex output of the NM. The NM, on the other hand, can be used to probe the limits ofapplicability of the IM and may give hints on further improvements of the IM

Preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography

Much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean. Here the process of internal-tide generation and dissipation by tidal flow over an isolated Gaussian topography is examined, using two-dimensional numerical simulations employing the MITgcm. Four different topographies are considered, for five different amplitudes of barotropic forcing, thereby allowing a variety of combinations of key nondimensional parameters. While much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes, here attention is focused on parameters dependent on the flow amplitude. For narrow topography, large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency. Tall narrow topographies are found to be the most conducive to mixing. Dissipation rates in these calculations are most efficient for the narrowest topography

Modelling the transverse distribution of velocity and suspended sediment in tidal estuaries

An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage. Examples are the Western Scheldt River Estuary and the Chesapeake Bay. Within these environments complex patterns of velocity and suspended sediments are observed in the transversal plane (across-estuary and vertical), and sediments are trapped laterally (across-estuary). The transverse structure of velocity is relevant to the transport of salt, sediment, contaminants, oxygen and other material. High sediment concentrations affect water quality, ecology and wildlife, and may cause siltation of navigation channels and harbors. This work aims at a fundamental understanding of the transverse distributions of estuarine velocity and suspended sediment. The thesis provides two-dimensional (cross-sectional) analytical models to identify the effect of individual forcing mechanisms on the transverse distribution of velocity and suspended sediment in tidally-dominated estuaries. The models are based on the shallow water equations and sediment mass balance. Considered are the residual and the semi-diurnal tidal components of the along-estuary, across-estuary and vertical velocity and of the suspended sediment concentration. The models apply to partially to well-mixed tidal estuaries, relatively uniform along-channel conditions and weakly to moderately nonlinear flow. Horizontal density gradients are prescribed based on numerical or observational data. The analytical flows are decomposed into components induced by individual mechanisms. Considered are tides, horizontal residual density gradients, river discharge, stokes return flow, wind, the earth’s rotation, tidal variations in the across-channel density gradient and channel curvature. In addition, two tidally rectified along-channel residual flow mechanisms are considered, which result from net advection of along-channel tidal momentum by the Coriolis-induced transverse tidal flow and by the density-induced transverse tidal flow, respectively. The models were validated against observations in the James River and Chesapeake Bay, and against a three-dimensional numerical model for various estuarine conditions. An important finding is that the residual across-channel density gradient is crucial for the lateral distribution and trapping of sediment in many estuarine cross-sections. The gradient tends to trap sediments in fresher areas of the cross-section. Tidal variations in the across-channel density gradient were found to cause a double circulation pattern in the transverse tidal flow during slack tides. The gradient also affects along-channel residual velocity via density-induced tidal rectification. This rectification component features landward currents in the channel and seaward currents over the slopes, and is particularly effective in deeper water. Coriolis-induced tidal rectification was found to induce residual flows that are up-estuary to the right and down-estuary to the left of an estuarine channel (looking up-estuary in the northern hemisphere). The process fundamentally changes the transverse structure of along-channel residual flow for stronger tides or steeper channels, as the flow becomes internally asymmetric. For weaker tides, along-channel residual flows are typically dominated by a gravitational circulation, i.e., landward flow in the channel and seaward flow over the shoals, or river flow. Stokes return flow, which resembles river flow, is particularly important for strong tides in shallow wate

Preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography

Much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean. Here the process of internal-tide generation and dissipation by tidal flow over an isolated Gaussian topography is examined, using two-dimensional numerical simulations employing the MITgcm. Four different topographies are considered, for five different amplitudes of barotropic forcing, thereby allowing a variety of combinations of key nondimensional parameters. While much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes, here attention is focused on parameters dependent on the flow amplitude. For narrow topography, large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency. Tall narrow topographies are found to be the most conducive to mixing. Dissipation rates in these calculations are most efficient for the narrowest topography

Transverse structure of tidal and residual flow and sediment concentration in estuaries: sensitivity to tidal forcing and water depth

An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped estuarine crosssections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model. The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter space considered (average tidal velocity amplitude, 0.1–1 m s-1 and maximum water depth, 10–60 m). Its skills are lower for along-channel residual flows if nonlinearities are moderate to high (strong tides in deep estuaries) and for transverse flows and residual sediment concentrations if the Ekman number is small (weak tides in deep estuaries). An important new aspect of the analytical model is the incorporation of tidal variations in the across-channel density gradient, causing a double circulation pattern in the transverse flow during slack tides. The gradient also leads to a new tidally rectified residual flow component via net advection of along-channel tidal momentum by the density-induced transverse tidal flow. The component features landward currents in the channel and seaward currents over the slopes and is particularly effective in deeper water. It acts jointly with components induced by horizontal density differences, Coriolisinduced tidal rectification and Stokes discharge, resulting in different along-channel residual flow regimes. The residual across-channel density gradient is crucial for the residual transverse circulation and for the residual sediment concentration. The clockwise densityinduced circulation traps sediment in the fresher water over the left slope (looking up-estuary in the northern hemisphere). Model results are largely consistent with available field data of well-mixed estuaries

Transverse structure of tidal flow, residual flow and sediment concentration in estuaries: sensitivity to tidal forcing and water depth

An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped estuarine cross-sections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model. The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter space considered (average tidal velocity amplitude, 0.1-1 m s - 1 and maximum water depth, 10-60 m). Its skills are lower for along-channel residual flows if nonlinearities are moderate to high (strong tides in deep estuaries) and for transverse flows and residual sediment concentrations if the Ekman number is small (weak tides in deep estuaries). An important new aspect of the analytical model is the incorporation of tidal variations in the across-channel density gradient, causing a double circulation pattern in the transverse flow during slack tides. The gradient also leads to a new tidally rectified residual flow component via net advection of along-channel tidal momentum by the density-induced transverse tidal flow. The component features landward currents in the channel and seaward currents over the slopes and is particularly effective in deeper water. It acts jointly with components induced by horizontal density differences, Coriolis-induced tidal rectification and Stokes discharge, resulting in different along-channel residual flow regimes. The residual across-channel density gradient is crucial for the residual transverse circulation and for the residual sediment concentration. The clockwise density-induced circulation traps sediment in the fresher water over the left slope (looking up-estuary in the northern hemisphere). Model results are largely consistent with available field data of well-mixed estuaries

Lateral entrapment of sediment in tidal estuaries: an idealized model study

Two physical mechanisms leading to lateral accumulation of sediment in tidally dominated estuaries are investigated, involving Coriolis forcing and lateral density gradients. An idealized model is used that consists of the three-dimensional shallow water equations and sediment mass balance. Conditions are assumed to be uniform in the along-estuary direction. A semidiurnal tidal discharge and tidally averaged density gradients are prescribed. The erosional sediment flux at the bed depends both on the bed shear stress and on the amount of sediment available in mud reaches for resuspension. The distribution of mud reaches over the bed is selected such that sediment transport is in morphodynamic equilibrium, that is, tidally averaged erosion and deposition of sediment at the bed balance. Analytical solutions are obtained by using perturbation analysis. Results suggest that in most estuaries lateral density gradients induce more sediment transport than Coriolis forcing. When frictional forces are small (Ekman number E 0.02), the lateral density gradient mechanism dominates and entraps sediment in areas with fresher water. Results also show that the lateral sediment transport induced by the semidiurnal tidal flow is significant when frictional forces are small (E 0.02). Model predictions are in good agreement with observations from the James River estuary