A modeling study of the Satilla River estuary,Georgia. I: Flooding-drying process and water exchange over the salt marsh-estuary-shelf complex

The flooding-drying process over the intertidal zone of the Satilla River estuary of Georgia was examined using a three-dimensional (3-D) primitive equations numerical model with Mellor and Yamada's (1982) level 2.5 turbulent closure scheme. The model was forced by the semi-diurnal M₂, S₂, and N₂ tides and freshwater discharge at the upstream end of the estuary. The intertidal salt marsh was treated using a 3-D wet-dry point treatment technique that was developed for the σ-coordinate transformation estuary model. Good agreement was found between model-data comparison at anchor monitoring sites and also along the estuary that suggested that the model provided a reasonable simulation of the temporal and spatial distribution of the 3-D tidal current and salinity in the Satilla River estuary. Numerical experiments have shown that the flooding-drying process plays a key role in the simulation of tidal currents in the main river channel and in water transport over the estuarine-salt marsh complex. Ignoring this process could lead to a 50% under-estimation of the amplitude of tidal currents. The model results also revealed a complex spatial structure of the residual flow in the main channel of the river, with characteristics of multiple eddy-like cell circulations. These complicated residual currents are formed due to tidal rectification over variable topography with superimposition of inertial effects, asymmetry of tidal currents, and baroclinic pressure gradients. Water exchanges over the estuary-intertidal salt marsh complex are asymmetric across the estuary, and tend to vary periodically on the northern side while quickly washing out of the marsh zone on the southern side. Strong Stokes’ drifting velocity was predicted in the estuary, so that the Lagrangian trajectories of particles were characterized by strong nonlinear processes that differ significantly from those estimated by the Eulerian residual currents.