A Coupled Hydrodynamic–Bottom Boundary Layer Model of Storm and Tidal Flow in the Middle Atlantic Bight of North America

Abstract

The effects of increased friction and tides on circulation in the Middle Atlantic Bight (MAB) during the SWADE storm of 25?28 October 1990 have been investigated using a three-dimensional hydrodynamic model coupled to a bottom boundary layer model that calculates combined wave?current bottom drag coefficients. Winds were initially parallel to the coast (downwelling favorable) throughout the MAB, first shifting to offshore within the central MAB and then in the northern MAB, while remaining parallel to the coast within the southern MAB. The wind-driven circulation was approximately alongshore, with an onshore component at the surface and an offshore component at depth associated with downwelling. Compared to model runs with a pure current bottom friction formulation, the additional bottom friction in the coupled model decreased currents uniformly in shallow water and caused slight offshore rotation during downwelling circulation, but the effects were limited because of the persistent stratification and the variable wind field during the storm. The effect of tides was much more pronounced, since across-shelf tidal currents were of similar or greater magnitude than the wind-driven currents. The combination of downwelling offshore flow and tidal flow during the storm resulted in weaker bottom currents directed nearly alongshore during flood and stronger currents directed nearly offshore during ebb. Bottom shear stresses were initially highest when storm currents were largest and again later during ebb tides when tidal and storm bottom flows were in the same direction. These results suggest that sedimentation during the storm was directly related to the tidal flow.