Date of Award
Master of Science in Oceanography
The complexities of estuarine circulation control the exchange of physical, chemical and biological quantities between oceanic and riverine systems. This exchange is predominantly driven by three distinct forcing mechanisms: tides, winds and density. The influence of each mechanism must be understood in order to interpret and predict the currents. This study investigates the circulation in the upper Sakonnet River (SR) and Mount Hope Bay (MHB), with respect to each forcing mechanism.
This study also investigates how exchange between the lower SR and MHB is affected by the complex geomorphology of the upper SR. Circulation in the upper SR, or the Sakonnet River Narrows (SRN), is complicated by two factors. MHB is connected to the ocean through the SR Passage and through the East Passage (EP), both parts of greater Narragansett Bay (NB). MHB is therefore exposed to two distinct sources of tidal forcing. Furthermore, the SRN has a series of breakwaters and natural coves which have unforeseen effects on the flow. NB in general has highly variable bottom topography, and this is compounded in the SRN by the man-made structures.
The methods utilized in this study of the SRN were tide gauges, deployed for 41 days, and ADCP/CTD surveys, conducted over two spring-tide tidal cycles and two neap tide tidal cycles. These data were analyzed in conjunction with local wind data to investigate both localized and regional aspects of the circulation of MHB. To compliment the field study, a linear admittance model was developed for the region.
In general, the tides dominate upper SR volume transport. Maximum observed spring tide current velocity values reach 1.5 m/s, corresponding to a volume transport of about 1000 m3/s. The tidal prism for the SRN ranges from 4.02xl06 to 1.39x10 7 m3 for spring and neap conditions, respectively. The tidal currents between the SR and MHB are predominantly semidiurnal, but the currents exhibit a double-peaked flood and a single-peaked ebb indicative of a significant M4 component. Peak ebb current occurs shortly after high water.
The cross channel structure of the velocity field was observed to be consistently and dramatically inhomogeneous. Furthermore, flow in the vicinity of the breakwaters was often turbulent, and regularly mixed the water passing through the SRN. The mixing is believed to have a detrimental effect on the flushing of MHB heat pollution.
An empirical barotropic volume transport model was developed from sea surface height and volume transport data. Model predicted transport agrees with observed values within approximately 25%. Low frequency transport variability was examined by comparing predicted transport data with wind data. Storm events are believed to explain large anomalies in the predicted low frequency transport record; winds out of the SE/NW were found to be the most influential. Low frequency transport anomalies at times exceed tide-induced transport, suggesting that the wind-driven flow can at times exceed that due to the tides.
Overall results from the study suggest that upper SR geomorphology has a significant influence on the exchange of water between MHB and the SR. In particular, the breakwaters are believed to induce anomalous tidal current phase, and allow for significant wind induced low frequency transport anomalies. Therefore, the influence of the breakwaters should be considered for future physical oceanographic studies conducted in the region.
Deleo, William, "Investigation of the Physical Mechanisms Controlling Exchange Between Mount Hope Bay and the Sakonnet River" (2001). Open Access Master's Theses. Paper 1491.