THE EFFECT OF WAVE-CURRENT INTERACTION ON TIDALLY FORCED ESTUARINE CIRCULATION

Reductions in the velocity and surface elevation of tidal flows that have been observed in the Delaware Bay, a coastal plain estuary on the eastern coast of the United States, correlate well with local wind events. While there are several mechanisms that could account for this phenomenon, we consider hydrodynamic wave-current interaction to be a reasonable explanation. In order to theoretically model the interaction between tides and surface gravity waves, and eliminate the need for a reference velocity required by the Grant and Madsen (1979) wave-current interaction model, we propose a process-oriented model that couples the mechanism of wave-current interaction directly to the current dynamics of the tidal flow. The tidal model uses an eddy viscosity concept to parameterize the turbulent stresses and further characterizes the eddy viscosity in terms of a flow dependent shear velocity. Within the bottom boundary layer of the surface gravity wave, the turbulent stresses are also parameterized by an eddy viscosity that is coupled to the shear velocity of the tidal model. Results of the coupled model indicate, that as expected, wave-current interaction increases the bottom friction felt by the tidal flow and usually reduces the volume flux. However, if the physical bottom roughness is sufficiently small (less than 30 cm) then it is possible to enhance the volume flux in a long, deep estuary, contrary to expectations. A simple one dimensional, linearized friction model shows this potentially dramatic effect can be attributed to an increase in the bottom friction that then tunes the estuary closer to its natural resonance. Obviously an estuary at resonance maximizes the volume flux that must pass through the estuary mouth. The effect of wave-current interaction is to provide a possible means of increasing the bottom friction and enhancing the volume flux.