Fortnightly variations of the lateral structure of flow and hydrography at the Chesapeake Bay entrance

Abstract

Hydrographic and velocity data collected during 11 surveys at the Chesapeake Bay entrance were used to assess fortnightly variations in the lateral structure of the subtidal water exchange and density stratification. The bay entrance extends from Cape Henry to Fishermans Island and features two channels, Chesapeake Channel and North Channel. The channels are separated by Middle Ground (<10m deep) and Six-Meters shoal. Water density data were available in 8 of the 11 surveys, but current velocity data were available for all surveys. The structure of density and velocity were analyzed by comparing river discharge and wind forcing. Seasonal river input modified the eddy viscosity coefficient, through stratification changes, and became the main factor determining the velocity structure. Bathymetry, wind forcing and fortnightly tidal variations were the other modifiers. Scenarios with similar river input and wind forcing conditions showed consistent structures and they were clearly influenced by neap and spring tidal conditions. Stronger vertical stratification occurred during springs rather than during neaps in the Chesapeake Channel, in contrast to what is observed in the North Channel and to what typically would be expected elsewhere from fortnightly variability. Consistent with the vertical stratification pattern, increased transverse density gradients and two-layer circulation developed over Middle Ground and Six-Meters Shoal during spring tides. Confined to the Chesapeake Channel longitudinal velocity seems to result in larger and closer to the channel transverse density gradient during springs than during neaps and consequently larger transverse velocities.