Abstract
Using a validated coupled hydrodynamic-tracer transport model, this study quantified the mean residence time in Jamaica Bay, a highly eutrophic lagoonal estuary in New York City. The Bay is a well-mixed to partially-stratified estuary with heavily-dredged bathymetry and substantial wastewater treatment plant effluent inputs that lead to seasonal hypoxia in some poorly-flushed deep-water basins. Residence time was computed for Jamaica Bay and its largest isolated deep basin, Grassy Bay. The response of residence time to freshwater discharge and wind forcing during summer 2015 was also investigated. The model results showed that the mean residence time, which represents the time required to flush out 63% of tracers released into the region of interest, was 17.9 days in Jamaica Bay and 10.7 days in Grassy Bay. The results also showed that some regions in Jamaica Bay retained their tracers much longer than the calculated residence time and, thus, are potentially prone to water quality problems. Model experiments demonstrated that summertime wind forcing caused a small increase in residence time, whereas freshwater discharge substantially reduced residence time. Freshwater inputs were shown to strongly enhance the two-layer estuarine gravitational circulation and vertical shear, which likely reduced residence time by enhancing shear dispersion. Due to the Bay’s small, highly-urbanized watershed, freshwater inputs are largely derived from the municipal water supply, which is fairly uniform year-round. This water helps to promote bay flushing, yet also carries a high nitrogen load from wastewater treatment. Lastly, the tidal prism method was used to create a simple calibrated model of residence time using the geometry of the study area and the tidal range and period.
Highlights
In estuaries and coastal waters, hydrodynamic processes greatly impact the transport of living biomasses, nutrients, contaminants, dissolved gasses, and suspended particles
By setting the return flow factor, b, to a default value of 0.5 and the tidal period to 12.4 h, the calculated residence time was 2.89 days for Jamaica Bay and 3.64 days for Grassy Bay, which were significantly smaller than the values calculated by sECOM (17.94 and 10.65 days)
Based on the model application of a 40-day simulation period in July and August 2015, the model results showed that the mean residence time was 17.94 days in Jamaica Bay and 10.65 days in Grassy Bay
Summary
In estuaries and coastal waters, hydrodynamic processes greatly impact the transport of living biomasses, nutrients, contaminants, dissolved gasses, and suspended particles. [14] examined the influence of different forcing mechanisms on the residence time in Barnegat Bay, Little Egg Harbor, located in New Jersey, USA Their results showed that meteorological events and remote coastal forcing were stronger flushing mechanisms than river discharge. [15] showed that freshwater discharge and baroclinic forcing (i.e., density-induced circulation) affect the residence time in the Danshuei River estuarine system in northern Taiwan. They found that the residence time decreased from about 4.5 days to 2.5 days as the upstream freshwater input increased from 9 m3 s−1 to 300 m3 s−1. Their results revealed that the baroclinic forcing significantly reduced the residence time in their study area (a reduction between about 8% and 47%, depending on the freshwater input)
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