Tidal and nontidal exchange at a subtropical inlet

Abstract

A tidal-cycle study at Destin Inlet, Northwest Florida, investigated intratidal and residual flow structures for the first time across the inlet. Underway current velocity profiles were combined with hydrographic station profiles at neap tides to document the appearance of tidal fronts, the distribution of tidal currents across two cross-sections, and the residual, or non-tidal, flow structure at the same cross-sections. Intratidal variations of water density and velocity showed the presence of fronts both in 1) late ebb-early flood and 2) late flood-early ebb tidal stages. Late ebb-early flood tidal intrusion fronts brought about depth-independent changes in water density >10kg/m3 in <2h. Their counterparts, late flood-early ebb plume-like fronts, produced similar magnitude of variations in density but were depth-dependent. Diurnal tidal current distributions across the inlet followed a general behavior of a damped wave with strongest currents appearing near the surface and near the deepest part of the cross-sections (thalweg). However, curvature effects seemed to modify this behavior locally by shifting the maximum tidal current away from the thalweg. Frictional and curvature effects on the diurnal flows were confirmed by an analytical solution for tidal currents. The observed density gradients drove residual flows that were vertically sheared, with outflow at the surface and inflow near the bottom. Such residual flow distributions were reproduced by an analytical solution that diagnosed the flow structure as inherent of a dynamically narrow inlet with relatively weak frictional effects.