Low-frequency current oscillations triggered by wave-current interactions in a mixed-energy tidal inlet

Waves and tidal currents are key factors for sediment transport and morphodynamic evolution of mixed-energy tidal inlets. The intratidal interaction between waves and currents in tidal inlets is non-linear and highly variable in space and time. Despite the potential relevance of these interactions, the mechanisms of wave-current interaction and the effects on sediment transport in tidal inlets remain elusive. In this study, we combine in-situ measurements and numerical modeling of a micro-tidal mixed-energy inlet that show the formation of low-frequency current oscillations (LFO), which could be related to macro-vortices. Wave parameters and current velocity measurements were collected during the month of March 2018, using three Acoustic Doppler Current Profilers (ADCP) placed in the inner estuary and along the terminal lobe of Matanzas Inlet, FL. Examination of field observations revealed low-frequency (15-30 minutes) current oscillations of amplitudes up to 0.6 m/s, at the offshore limit of the main channel during episodes of strong ebb tidal currents (~0.7 m/s) and moderate wave conditions (significant wave heights ~1.5 m). These LFO were correlated with the radiation stresses, the intensity of ebb tidal currents, and ADCP backscatter, raising the question of a possible linkage to sediment transport. A numerical model of an idealized tidal inlet was implemented to analyze the two-dimensional horizontal distribution of these LFO. Simulation results suggest that these LFO are related to the formation of macro-vortices (up to 100 m in diameter) associated with current velocity gradients. Additionally, the LFO intensity is proportional to the magnitude of the cross-shore radiation stress gradients, and tidal range.