3D Numerical Modeling of Hydrodynamics and Sediment Transport in Estuaries

As a USEPA recommended hydrodynamic and transport model, EFDC model has been widely used in modeling estuarine and coastal hydrodynamics and transport. EFDC employs sigma coordinate transformation to deal with irregular water depth. However, it is well known that this coordinate transformation introduces additional terms and produces computation errors in calculation of horizontal pressure gradient terms when a steep bottom slope exists in water domains. Errors in pressure gradient calculation can cause errors in velocity field and ultimately results in errors in spurious transports. In this study, a new algorithm is presented to reduce the numerical errors induced by the horizontal pressure gradient term near steep topography. The basic concept of this algorithm is to re-organize the pressure terms in sigma coordinate system to avoid the subtractions of two large horizontal pressure terms. To accomplish this objective, the 4th order Lagrangian interpolation method was firstly used in sigma space to obtain concentration in the corresponding z-level of the water column. Secondly, the horizontal concentration difference was determined. Finally, the horizontal pressure gradient in the water column was directly calculated from the horizontal concentration gradient. A stepwise bottom boundary condition was adopted for steep slopping bottom boundary. The algorithm has been used to enhance the EFDC model. The model code has been tested in three test cases: 1) flat bottom basin, 2) steep sloping channel, and a coastal shelf. Results indicate that conventional approach in current EFDC dealing with horizontal pressure gradient terms causes spurious surface elevation and velocity field. In comparison, the employment of the algorithm presented in this study this study significantly reduced numerical errors in predicting surface elevation and currents in navigation channels and coastal shelves. Equations for estimating horizontal diffusion coefficient in 3D numerical modeling of estuarine transport have been evaluated in this study in a shallow tidal river. In the application of a 3D hydrodynamic model to Little Manatee River located in Florida of USA, the popular Smagoringsky diffusion scheme was shown to result in the underestimate salinity in comparison with field observations. Another horizontal diffusion equation by Overton et al was also unable to provide satisfactory results of salinity variations in the shallow and narrow river. In an analytic test case of a non-tidal uniform flow channel, Smargorinsky equation results in unreasonable zero horizontal diffusion and no salinity intrusion in the nontidal one dimensional tidal river. An enhanced horizontal diffusion equation was presented in this study. Decoupled from the horizontal eddy viscosity, the enhanced horizontal diffusion equation is composed of the Smargorinsky equation with addition of a non-tidal background horizontal diffusion to account for the effects of shallow and narrow effects of streams. The enhanced equation has been calibrated with field observations of hourly surface and bottom salinity at two field stations during 2/15/2005-2/28/2005. It was also satisfactorily verified with field observations for the period of 3/1/2005-6/30/2005. Model predictions of salinity and currents fields from model predictions were presented to support water research. The enhanced horizontal diffusion equation will be helpful for more accurate modeling of other water quality constituents in tidal rivers. A 3D sediment transport model is applied to Apalachicola Bay to predict temporal and spatial distributions of sediment concentrations in water columns. The model is coupled with the 3D hydrodynamic model in the EFDC model code that provides information on estuarine circulations and salinity transport. The hydrodynamic model has been calibrated with field observations of water levels and salinity. The sediment transport model solves the transport equation with source and sinks terms to represent sediment deposition and re-suspension. The model is capable of predicting dye transport and fecol coliform. Basing on the collection of field observation and data analysis, the main driving force for sediment resuspension in the bay is found to be surface wind drive current. The calibrated hydrodynamic model then was used to simulate the total suspended sediments (TSS) transportation and get a satisfying result. The calibrated model can serve as an effective tool for environmental scientists and resources managers to examine effects of management scenarios on estuarine sediments transport and the aquatic ecosystem.