Axial convergence fronts in a barotropic tidal inlet—sand shoal inlet, VA

An axial convergence front system in Sand Shoal Inlet, VA, is analyzed. The fronts, which are parallel to the main channel and mostly within the channel, occur during different tidal stages within a 13-h observation period. A 25-ft boat is used to tow an acoustic Doppler current profiler to measure velocity profiles along an hour-glass shaped ship track. Salinity and temperature are measured using conductivity–temperature–depth sensors. A harmonic-statistic analysis is used to analyze the tide, tidal velocity, and mean velocity. The transverse convergence and divergence of velocity are calculated. The rms errors of the harmonic-statistic analysis of the elevation and velocity are about 0.28 m and 0.13 m/s (with a maximum velocity of over 2 m/s), respectively. On average, about 83%, 95%, and 70% of the variabilities of the elevation, longitudinal and transverse velocities, respectively, can be explained by the M2 tidal and subtidal constituents. Strong transverse velocity convergences are identified by the analysis and are generally consistent with the observed front positions. The analysis shows that the front system is apparently generated by a combination of several mechanisms including (1) differential rotation of the tidal ellipses and spatial variations of the major axes of the tidal ellipses, owing to the strong bottom friction, and (2) a strong geometric convergence at the inlet. Density effect is found to be negligible and the planetary vorticity tilt effect is also unimportant because of a much higher relative vorticity. The observed front system is distinguished from the conventional estuarine axial convergence fronts which require a strong along channel density gradient. Therefore, in estuaries and coastal embayment where both tides and density gradients are important and where cross-channel bathymetry change is present, the frontal genesis can be a combination of several processes including the tidal convergence discussed here.