| dcterms.abstract | Great South Bay (GSB) is the central part of a multi-inlet lagoon system along the south shore of Long Island. In 2012, Superstorm Sandy caused three breaches in the barrier island. One breach was allowed to remain open due to its location within the National Seashore Fire Island. As of 2018, the lagoon-system has five inlets including this new inlet. The finite volume coastal ocean model (FVCOM) is applied to study the effects of the breach on the back-barrier bays and to quantify the changes regarding residual circulation, residence time, offshore water level transmission and salt and heat exchange with the ocean, with a focus on Great South Bay. The model is run on two model grids: one represents the lagoon system before the breach opened in 2012; the other includes the breach but is otherwise unaltered. Simulations on the two grids were forced with identical forcing conditions so that any differences in the resulting parameters could be attributed to the breach. Comparison of the model results with observations shows that the model has good skill in predicting water levels and salinity and temperature fields in Great South Bay as well as tidal discharge in the breach. The residual circulation between the inlets was quantified for the respective pre- and post-breach inlet configurations under just tidal forcing. The breach, due to its small cross-sectional size and expansive flood delta, is flood-dominant in terms of net transport. Both the Eulerian mean flow and the Stokes flow are directed inward. The average net inflow of 61 m3 s-1 contributes further to the already existing superelevation in eastern GSB and the steepening of the along-bay pressure gradient, which leads to an increase in the tidal residual east-to-west mean flow from 16 m3 s-1 to 67 m3 s-1 as well as to an increase in net outflow through Fire Island Inlet from 46 m3 s-1 to 83 m3 s-1. The increased residual circulation in GSB has led to a decrease in flushing time and local residence time. The bulk flushing time of GSB has decreased by 35% under summer conditions and by 17% under winter conditions. The biggest impact of the breach on flushing time in relative terms is observed in Bellport Bay, where flushing time in summer decreased from 19 days to 5 days. There, as well as in western Moriches Bay, a noticeable decrease in mean summer nitrogen and chlorophyll concentrations has been observed which can likely be attributed to the breach. Initially, it was feared that the breach could increase the risk of flooding, but the model results show that peak water levels during an average winter storm are increased by only -0.8 to 4.6 cm, depending on the exact location in the bay. At tidal frequencies, depending on location, there is either no statistically significant increase in tidal amplitude or a statistically significant increase in the order of 1 to 2 cm. A similar change in tidal amplitude was observed in relation to dredging at Fire Island Inlet and Jones Inlet. In terms of salt and heat exchange, the opening of the breach has led to an increase in salinity in what used to be the freshest part of the bay. The biggest increase in mean salinity of + 5.1 psu was observed in Bellport. While the increase in mean salinity in eastern GSB has resulted in a higher overall mean salinity in GSB, the pattern of the salt imports and export pathways generally did not change. The salt content in GSB is driven by the residual circulation patterns. The additional salt import through the breach is compensated by increased export through Fire Island Inlet as well as decreased import from South Oyster Bay. The same applies to the advective component of heat exchange in GSB pre- and post-breach. Overall, the most salient effect this new breach has had on GSB is increased salinity and reduced flushing time in the eastern part of GSB. Furthermore, the positive effect the breach has had on the along-bay residual circulation is the result of the asymmetric inward net transport caused by its narrow width and expansive flood delta. A differently shaped, more open breach would possibly allow an outwardly directed Eulerian flow, which would result in a smaller total inflow or outflow and ultimately in a more sluggish tidal residual circulation and flushing. | |
