Continuously stratified flow dynamics over a hollow

Acoustic Doppler current profiles (ADCP) and density profiles were measured over three lower Chesapeake Bay bathymetric depressions (hollows) in order to determine the effects of a hollow on a continuously stratified flow. Measurements showed an acceleration of the near-bottom flow as it moved toward the deepest part of the hollows, in contrast to the deceleration expected from two-dimensional Bernoulli-type dynamics. The acceleration was attributed to lateral water intrusions that were most apparent during floods. The presence of lateral water intrusions was corroborated by observations in a transverse section that crossed the deepest part of the hollows. The observed deceleration of the along-hollow flow as it moved toward decreasing bathymetry was attributed to lateral flows toward the shoals. The current acceleration toward the deepest part of the hollows during flood, favored by vertical and lateral water intrusions, was deflected toward the right (looking landward) owing to Coriolis accelerations. In general during ebb and flood, the dynamics over the hollows could be explained with three-dimensional Bernoulli-type dynamics modified by Coriolis acceleration. At only one of the hollows, Cape Charles hollow, the sampling strategy allowed resolution of internal lee waves with periodicities of 1.42 h during flood and ebb tide periods. These internal waves were produced by the interaction of the sloping pycnocline and tidal currents. The formation of internal waves over the leeward side favored the development of vertical shear in the horizontal velocity. Large vertical shears enhanced turbulence and internal waves should have increased form drag resulting in locations in the water column of weakest along-hollow velocities. The phase of the semidiurnal tidal currents increased monotonically from the depth equivalent to the maximum depth of the shoals surrounding the hollow toward the bottom and surface, in contrast to the expected increment from bottom to surface over an estuary influenced by bottom friction.