DETERMINING THE SUB-ESTUARINE FLUSHING DYNAMICS OF NARRAGANSETT BAY

Greenwich Bay and the Providence River represent two sub-estuaries of Narragansett Bay (RI) with chronic water quality problems. It is common for areas within these subsystems to become hypoxic, mainly as a result of excessive anthropogenic nutrient loading in combination with hydrodynamics and biological processes. One such area is the Edgewood Shoal in the Providence River. Previous underway Acoustic Doppler Current Profiler observations suggested that subtidal exchange between the shallow shoal and the adjacent dredged shipping channel were restricted, due to a large-scale circulation gyre on the shoal. Early numerical models indicated that prevailing summer sea breeze conditions were favorable to the evolution of stable gyres in Greenwich Bay, which led to high residence times. In this study the response of the sub-estuaries to environmental forcing conditions is investigated using field observations and numerical modeling. An important goal is to understand and quantify the flushing dynamics of these systems. Results from spatially and temporally detailed current meter deployments (using SeaHorse Tilt Current Meters) are reported. In order to systematically investigate the effect of specific forcing conditions on residence times, an idealized hydrodynamic model of Narragansett Bay, developed using the Regional Ocean Modeling System (ROMS), is employed. The model background forcing varies at relatively low frequencies, which reduces the complexity of the hydrodynamics and facilitates comparisons of residence times calculated for distinct forcing events. Observations confirm the existence of a robust, clockwise subtidal gyre on the Edgewood Shoal, but persistent gyres are not observed in Greenwich Bay. Model results reveal, for example, high residence times in Greenwich Bay under specific sea breeze conditions. However, the high residence times are due to laterally overturning flow, as opposed to horizontal gyres.