A COUPLED HYDRODYNAMIC BOTTOM BOUNDARY-LAYER MODEL OF STORM AND TIDALFLOW IN THE MIDDLE ATLANTIC BIGHT OF NORTH-AMERICA

The effects of increased friction and tides on circulation in the Midd le Atlantic Eight (MAB) during the SWADE storm of 25-28 October 1990 h ave been investigated using a three-dimensional hydrodynamic model cou pled to a bottom boundary layer model that calculates combined wave-cu rrent bottom drag coefficients. Winds were initially parallel to the c oast (downwelling favorable) throughout the MAB, first shifting to off shore within the central MAE and then in the northern MAB, while remai ning parallel to the coast within the southern MAB. The wind-driven ci rculation was approximately alongshore, with an onshore component at t he surface and an offshore component at depth associated with downwell ing. Compared to model runs with a pure current bottom friction formul ation, the additional bottom friction in the coupled model decreased c urrents 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 th e storm. The effect of tides was much more pronounced, since across-sh elf tidal currents were of similar or greater magnitude than the wind- driven currents. The combination of downwelling offshore flow and tida l flow during the storm resulted in weaker bottom currents directed ne arly alongshore during flood and stronger currents directed nearly off shore during ebb. Bottom shear stresses were initially highest when st orm currents were largest and again later during ebb tides when tidal and storm bottom flows were in the same direction. These results sugge st that sedimentation during the storm was directly related to the tid al flow.