Intratidal and Residual Flows Around Inlets of a Reef Lagoon

Abstract

Wave-driven circulation and inlet dynamics were investigated in a Caribbean fringing reef lagoon via velocity transect data collected with a towed ADCP along two lagoon-inlet connections over diurnal tidal cycles. The inlets sampled were Boca Grande inlet, 700 m wide, and to the north of Boca Chica, 250 m wide. Flow reconstructions with semidiurnal (D2) and diurnal (D1) tidal bands plus a residual showed that the strongest flows were seaward and located at the inlets. Maximum outflows were 0.6 m/s in the center of Boca Chica and 0.7 m/s at the southern end of Boca Grande. Differences in inlet width and varying bathymetry between the two inlets caused atypical flow circulation, including inflow and residual recirculation. Boca Chica featured converging outflow, with across-lagoon flows (East-West) dominating over along-lagoon flows (North-South). Boca Grande displayed a highly variable, complex vertical structure with 2-layer diverging flows. Other characteristics of Boca Grande included counter-clockwise vorticity appearing at the inlet and inflow at the north end of the inlet. Dynamics related to residual flows were Bernoulli-like, with advective accelerations predominantly balancing pressure gradients.