HR: 0830h
AN: OS41C-0812    [PDF]
TI: A Parallel Double Front System along the Main Channel of a Barotropic Tidal Inlet
AU: * Li, C
EM: chunyan@skio.peachnet.edu
AF: Skidaway Institute of Oceanography, 10 Ocean Science, Savannah, GA 31411
AB: In an estuary with a significant longitudinal density gradient, fronts can occur during flood stage if a cross channel shear of flow exists. In a wide estuary, models have suggested convergence on the right hand side when facing the downstream direction, because of Coriolis effect, favoring a single front line changing its position with tidal phase. If a front system occurs during different tidal stages including ebb and appears in pairs on both sides of a channel, then neither of the above mechanisms can explain it. Here I report such a front system observed in a barotropic tidal inlet - Sand Shoal Inlet, VA. The front system is observed during different tidal stages within a 13-hour observation period. A 25-ft boat is used to tow an acoustic Doppler current profiler (ADCP) to measure velocity profiles along an hour-glass shaped ship track. 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 $M_2$ 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 fronts discussed here. Presented here also is a barotropic 3D analytic tidal model allowing arbitrary cross channel depth functions to demonstrate the interaction between frictional tide and bathymetry to generate such a front system. The model result suggests that without a density gradient and Coriolis force, an axial front system can develop in a tidal channel with significant cross channel depth variations. Depending on the bathymetry function, a parallel double front can occur on both sides of the channel. The front positions can also vary with tidal phase.
DE: 4203 Analytical modeling
DE: 4235 Estuarine processes
DE: 4528 Fronts and jets
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting