Abstract
This study seeks to advance understanding of residual, or subtidal, exchange flow structures at different inlets of a multiple-inlet estuary. For this purpose, the study used towed Acoustic Doppler Current Profiler (ADCP) measurements and hydrographic profiles during one diurnal tidal cycle. The spatial structure of ocean-estuary exchange was observed at the two main inlets of the Charlotte Harbor estuary, Florida, which is influenced by well-defined subtropical seasons: wet and dry. The <6 m deep San Carlos Bay inlet was sampled in late August during the wet season and the <16 m deep Boca Grande Pass was sampled in March during the dry season. Results revealed distinct subtidal flow distributions at the two inlets: vertically sheared exchange flows in San Carlos Bay, and laterally sheared exchange flows in Boca Grande Pass. Non-dimensional densimetric tidal Froude and Ekman numbers, as well as analytical model results, suggested the main drivers of the contrasting flow patterns. Vertically sheared flows at San Carlos Bay displayed relatively small densimetric tidal Froude and Ekman numbers, associated with density gradients as the main driving agent. Boca Grande Pass showed relatively large tidal Froude number because of the influence of tidal stresses, and relatively large Ekman number caused by friction. These differences were attributed to the contrasting characteristics of the two inlets, including their bathymetry, tidal forcing and river discharge influence. Such differences are explained by the non-dimensional numbers, which should be applicable to other inlets forced by tides and density gradients.
•Contrasting exchange flow structures observed at two inlets of a subtropical estuary.•Vertically and laterally sheared flows driven by bathymetry and seasonal river discharge.•Non-dimensional Froude and Ekman numbers in a dynamical framework explain estuarine exchange flow dynamics.