Widening and Deepening the Main Navigational Channel of the Lower St. Johns River (Northeasten Florida): Simulation of the Pre- and Post-Condition Hydrodynamics

A large-scale hydrodynamic model is used to provide boundary conditions in support of a localized transport study. The hydrodynamic model domain includes the lower 170 km of the St. Johns River and its adjacent salt marshes along with the Atlantic Intracoastal Waterway (AICWW), which interconnects the St. Mary's Inlet, Nassau Sound, Fort George Inlet, Mayport, and St. Augustine Inlet, amongst all other tidal inlets along the southeastern United States seaboard. We apply an unstructured, finite element mesh which describes the AICWW and its associated estuaries, including the adjacent salt marshes, with element sizes of about 25 – 75 m. The main navigational channel of the Lower St. Johns River is described with element sizes of about 50–150 m. Element sizes increase offshore and away from the local region of interest, ultimately to on the order of tens of kilometers, where the hydrodynamics are known to be simpler. We utilize the unstructured modeling approach to simulate tidal hydrodynamics within the entire domain for the: existing conditions of the Lower St. Johns River; and the various post-condition scenarios being proposed by the United States Army Corps of Engineers. It is learned that tidally driven flows in the main navigational channel are influenced by the salt marshes surrounding the lower river reaches. We conclude that these salt marshes must be incorporated into the model domain in order to properly simulate tidal discharge in the Lower St. Johns River.