The effects of channels and shoals on exchange between the Chesapeake Bay and the adjacent ocean

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The role of bathymetric changes in determining the transport of water and salt in the lower Chesapeake Bay (LCB) was investigated using high‐resolution acoustic Doppler current profiler (ADCP) and conductivity‐temperature‐depth profiles. A cross‐channel transect was repeated eight times during neap tides on October 6–7, 1993, which illustrated the lateral structure of the longitudinal and transverse flow fields and the intratidal variations in the flow structure across the LCB. Amplitude and phase of the M2 tidal component, as well as the mean flow velocity, were calculated using least squares fitting at every point of a uniform grid obtained from the ADCP data. The results differ from the classical two‐layer pattern of estuarine circulation modified by Coriolis effects but are consistent with recent hydrographic observations. Semidiurnal flow was highest over the navigational channels, and lateral gradients were strongest in regions of sharp bathymetric changes. The phase lag of the semidiurnal flow also showed lateral and vertical gradients that represented advances at the bottom with respect to the surface and over the shoals in relation to the channels. The section of the water column measured indicated a mean outflow of 0.7×104 m3/s and a mean inflow of 1.3×104 m3/s. The apparent gain of water by the estuary during the period of observation can be explained by meteorologically forced net barotropic inflow. The depth‐averaged mean longitudinal flow consisted of inflow in the navigational channels and outflow over the shoals. The mean transverse flow showed near‐surface convergence over the channels. We propose a possible explanation for the observed flow and density structure as follows: the barotropic and baroclinic forcing interact with the bathymetry to extend the inflow from the bottom to the surface, thereby inducing a transverse circulation that yields near‐surface convergence over the channels.