Modeling hydrodynamics and salt transport in the Alafia River estuary, Florida during May 1999–December 2001

Hydrodynamics and salt transport processes in the Alafia River, a tributary to Tampa Bay in southwest Florida, were studied using an efficient laterally averaged model that is unconditionally stable with respect to gravity waves, bottom and wall frictions, and the vertical eddy viscosity term. The simulation domain is the downstream 24 km of the river that contains the entire tidal reach and a portion of the upstream sloping channel where the riverbed is higher than the tidal water elevations. Boundary conditions at the upstream boundary are specified with measured freshwater inflow, while those at the downstream boundary are specified with measured real-time data of surface elevation and salinity profile. The model was calibrated and verified with water surface elevations and salinities measured at three stations within the simulation domain for the period between May 1999 and December 2001. The measurement station at the upstream boundary gauges about 79% of the Alafia River watershed. The remaining 21% of the watershed is not gauged due to the tidal influence. This study used the Hydrological Simulation Program-Fortran (HSPF) [Bicknell et al., 1997. Hydrological Simulation Program-Fortran, User’s Manual for Version 11. EPA/600/SR-97/080. U.S. Environmental Protection Agency, National Exposure Research Laboratory, Research Triangle Park, NC 27711.] model to estimate freshwater runoff from the downstream 21% of the watershed. Model simulations show that runoff from the un-gauged portion of the Alafia River watershed is an important factor controlling the salinity distribution in the Alafia River even though the majority of the Alafia River watershed is gauged. Based on model results, an empirical relationship that relates the location of the freshwater–saltwater interface at the surface layer with the total daily freshwater inflow is obtained. The empirical relationship suggests that the location of the freshwater–saltwater interface at the surface layer is inversely proportional to the logarithm of the total daily freshwater inflow. If the total daily freshwater inflow is reduced by m percent, the location of the interface at the surface layer migrates roughly 1.782 ln[100/(100 m)] kilometers in the upstream direction in the Alafia River.