The coastal ocean model FVCOM is applied to quantify the changes in circulation, flushing and exposure time in Great South Bay, New York after Superstorm Sandy breached the barrier island in 2012. Since then, the lagoon system is connected to the Atlantic via five instead of four inlets. The model simulations are run on two high-resolution unstructured grids, one for the pre-breach configuration, one including the new inlet, with tidal-only forcing, and summer and winter forcing conditions. Despite its small cross-sectional size, the breach has a relatively large net inflow, that leads to a strengthening of the along-bay through-flow in GSB; the tidally driven volume transport in central GSB quadrupled. The seasonal forcing scenarios show that the southwesterly sea-breeze in summer slows the tidally-driven flow down, while the forcing conditions in winter are highly variable and the circulation is dependent on wind direction and offshore sea level. Changes in flushing and exposure time associated with the modified transport patterns are evaluated using a Eulerian passive tracer technique. Results show that the new inlet produced a significant decrease in the flushing time (approximately 35% reduction under summer wind conditions and 20% reduction under winter wind conditions). Maps of exposure time reflect the changes in circulation and flushing.