The Exchange of Water through Multiple Entrances to the Mount Hope Bay Estuary

Abstract

Results are presented from a set of hydrographic surveys conducted within Mount Hope Bay, RI, during the summer of August, 1996. This sub-system of Narragansett Bay is interesting because it has two connections to the ocean and it has a source of thermal energy from the Brayton Point Power Plant. Data was collected on water velocity, salinity and temperature on days with relatively high (≈ 2 m range) and relatively low (≈ 1 m range) tidal forcing. Velocity data were collected along fixed transect lines defining the boundaries of the estuary and at fixed stations. Results show that flow through each of the oceanward entrances has significant horizontal and vertical structure. The source of fresh water is the Taunton River to the north, and at times, exchange through this interface exhibits vertically sheared flow. Exchange is dominated by flow through the interface with Narragansett Bay, where transports reach 3000 m3/s and 6000 m3/s under conditions of low and high amplitude tidal forcing, respectively. Peak velocities exceed 100 cm/s. Values for transport though the smaller of the two salt water connections, with the Sakonnet River, and the fresh water entrance, at the interface with the Taunton River, were ≈ 10– 20% of those through the interface with Narragansett Bay. Velocities are relatively sluggish in the shallow northern shelf region of the estuary, peaking at < 10 cm/s and ≈ 20 cm/s for the low and high tidal amplitude sampling periods, respectively. Temperature and salinity data reveal significant levels of stratification and suggest three end-member water sources including a deep Narragansett Bay source (cold, salty), a shallow river source (warm, fresh) and a source of water from the Brayton Point region (hot, intermediate salinity). A plug of warm water that evolves on the northern shelf over the ebb cycle of the tide is advected to the east–northeast into the shipping channel during the flood. Phase differences in total instantaneous transport through the two mouths of the system suggest that interactions with the Sakonnet River are dominated by the greater volume and efficiency of exchange with the East Passage of Narragansett Bay. Lateral variations in residual transport show East Passage water entering Mount Hope Bay through the deep central portion of the cross-section and exiting through confined regions along the edges of the interface. The pattern in residual exchange with the Sakonnet River shows water exiting and entering Mount Hope Bay through the western and eastern portions of the cross section, respectively. A conceptual model is suggested in which these lateral flow patterns combine with strong vertical mixing in the Sakonnet River Narrows to pump thermal energy downward in the water column and back northward into the bottom waters of Mount Hope Bay.