Tidal Current-Induced Vorticity and Subtidal Circulation Fields in the Delaware Bay and Adjacent Inner Shelf Waters

Using the two-dimensional depth integrated version of the Advanced Circulation (ADCIRC) Model, this research studied the tidal current-induced vorticity field and its impact on the formation of the residual circulation in the Delaware estuary and adjacent inner shelf waters. The model domain encompasses the entire Delaware Bay, its associated Delaware River head, the Chesapeake-Delaware canal, and neighboring coastal waters. The model grid consists of 28,831 triangular elements and 15,726 nodes. High resolution elements below 30-40 m are used to resolve the channels/shoals and highly irregular near-cape shorelines.

The numerical model was forced with tides on the open ocean boundary and river discharge at the upstream river head. The simulated subtidal circulation field exhibits multiple pronounced tidal-induced jet- and eddy-like flow structures not reported previously, largely owing to high model grid and bathymetry resolutions. It was found that intense flows appeared in areas of large bathymetric gradients (e.g., channels and shoals) or highly irregular shorelines (e.g, capes). The jet flow direction aligns well with local isobaths. Interestingly, this study revealed that except in the narrow upper-bay area profound flows tend to be coincident with shoal-channel ridges, rather than being confined within central channels. The results demonstrate favorable agreements with hydrographic and current observations and are consistent with theoretical and numerical results reported for many other estuarine regions.

Mechanisms responsible for various prominent flow features were explored in the perspective of vorticity dynamics. Time oscillating tidal vorticity is generated through either a nonfrictional potential vorticity mechanism or a bottom frictional torque mechanism, or both, due to inhomogeneous bathymetry close to the channel/shoals ridges or in the vicinity of capes. The associated vortices are advected by flooding/ebbing currents during courses of tidal excursions and are integrated into residual vorticity over subtidal periods. In areas of intense residual vorticity, the associated strong velocity shears cause the jet/eddy flows. In addition, the impact of river discharge on the subtidal circulation in DB is also addressed.