Friction dominated exchange in a Florida estuary

Abstract

The typically observed gravitational circulation in estuaries with lateral variations in bathymetry consists of a combined distribution of vertically and horizontally sheared flows. The distribution features inflow at depth and outflow at the surface and along the sides. However, theoretical results of density-driven exchange flows dominated by frictional effects display a laterally sheared distribution with inflow occupying the deepest portion of the cross-section and outflow over the shoals. The main purpose of this investigation was to obtain observational evidence in support of theoretical results. A tidal cycle survey was conducted on February 24, 2009, to collect current velocity and hydrographic profile data along a cross-estuary transect. Observations from Hillsborough Bay were compared qualitatively to numerical model and analytical solution results. The observed residual exchange flow pattern compared favorably with the results from a numerical model and an analytical solution that used a condition controlled by friction. The relative importance of friction was explored at tidal and subtidal timescales. Intratidally, frictional influences were observed in the spatial distribution of tidal current amplitude and phase, as well as in potential energy anomaly (stratification) variations. Subtidally, frictional influences were observed through the spatial distributions of tidally averaged stratification, turbulent kinetic energy dissipation and eddy viscosity values. The main finding of this study was that relatively weak tidal currents (<0.3 m/s) can still produce dominant frictional influences in the dynamics. Results indicated that bathymetry shaped by rather wide shoals and relatively narrow channel mold the frictionally dominated, laterally sheared net flow.