Evaluation of the Impact on St. Johns River Circulation and Salinity for the Jacksonville Harbor Deepening Project

Jacksonville Harbor, already a major U.S. port, is presently undergoing a general reevaluation study by the U.S. Army Corps of Engineers-Jacksonville District (USACE-SAJ) that is evaluating various plans to deepen the navigation channel depth from its current depth of 40 feet (12.2 meters) to a maximum depth of 50 feet (15.24 meters) from the river mouth at the Atlantic Ocean to a point 14 miles (22.5 km). Deepening the existing channel would allow for larger vessels to access the Jacksonville Port, thereby reducing transportation costs and providing increased navigational safety while minimizing or avoiding negative impacts to environmental resources. To evaluate the potential impacts of a channel-deepening project on the St. Johns River estuary system, a three-dimensional hydrodynamic and transport model of Jacksonville Harbor was developed by using the Environmental Fluid Dynamic Code (EFDC) model.The EFDC hydrodynamic and salinity model, validated for the Jacksonville Harbor deepening project area, provided the means to assess the direct impacts of channel modifications to salinity and water circulation in the main stem of the lower St. Johns River from various channel-deepening scenarios during a 6-year evaluation period (1996-2001). The 6-year evaluation period includes the lowest river flow during any 3-year period in the river's 78-year flow record to ensure that assessed project impacts are greater than those during an average year. Therefore, this study's evaluation presents conservative estimates of the impacts of the channel deepening. The hydrodynamic model results examine the effects of proposed channel-deepening projects on water level, salinity and water age throughout the model domain and provides tools for analyses of biological, chemical, and ecological impacts. This paper presents the cumulative impacts in the future (50 years after the project completion) from proposed channel-deepening alternatives and other projects, including the Mayport Deepening Project for the U.S. Navy, freshwater withdrawals in the St. Johns River, and future sea level rise.