Fortnightly variability in the transverse dynamics of a coastal plain estuary

Current velocity and water density profiles were obtained along two cross-estuary transects with the purpose of determining the fortnightly variability of the transverse dynamics in a partially stratified coastal plain estuary. The profiles were measured with a towed acoustic Doppler current profiler and a conductivity-temperature-depth recorder in the James River estuary, Virginia. The cross-estuary transects were sampled during the spring tides of October 26-27, 1996, and the ensuing neap tides of November 2-3, 1996. The transects were ∼4 km long, featured a bathymetry that consisted of a channel flanked by shoals, and were sampled repeatedly during two semidiurnal tidal cycles (25 hours) in order to separate semidiurnal, diurnal, and subtidal signals from the observations. This work concentrates on the subtidal transverse dynamics. The transverse baroclinic pressure gradients were larger during neap tides than during spring tides. During spring tides the advective accelerations were predominantly greater than the Coriolis accelerations, most markedly over the edges of the channel. Both effects combined with frictional influences to balance the pressure gradient in the transverse direction. During neap tides, advective accelerations were not as dominant over Coriolis accelerations as during spring tides. Also, during neap tides, Coriolis played a more relevant role, compared to spring tides, in combining with friction to balance the pressure gradient. This behavior was indicative of the momentum balance approaching gravitational circulation modified by the Earth's rotation, weak friction, and nonlinear advection during neap tides. The balance became more influenced by nonlinear advection and friction and less influenced by the Earth's rotation during spring tides. These results showed that transverse dynamics of a partially stratified estuary are far from being in geostrophic balance.