Computer enhanced modelling of tidal velocities and circulation patterns, Buttermilk Bay, Massachusetts: a model study and development of methods for general application
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Abstract
Buttermilk Bay is a tidal embayment located at the northern end of Buzzards Bay, Massachusetts, and is approximately 1.5 km long, 1.0 km wide, with an average depth of 1.0 m. Little Buttermilk Bay, located to the northeast of, and connected to Buttermilk Bay via Gibbs Narrows, is approximately 1.0 km long 0.33 km wide with an average depth of 0.6 km. Pleistocene glacial outwash surrounds the Bay providing minimal (0.34 m3 /s) freshwater input.
In order to determine the behavior of pollutants in Buttermilk Bay, a circulation model was developed using three methods: 1. analyzing hydrologic field measurements, 2. implementing a computer program which predicts flow velocities within the Bay, and 3. characterizing flow from geomorphic evidence.
The first method utilized hydrographic data to model bay circulation. Current measurements were recorded at six stations at the entrances to Buttermilk and Little Buttermilk Bay over four tidal cycles. These measurements corroborate that both Cohasset Narrows and Gibbs Narrows are flood dominated. Maximum flood currents for these channels were 20 cm/sec and 16 cm/sec greater than maximum ebb currents. Ebb durations for Cohasset Narrows (6:00) averaged 11 minutes longer than flood durations (5:49). The mean tidal prism for Buttermilk and Little Buttermilk Bay is 2.6 x 10' m3. The tidal prism equals half the volume of the bay at mean low water. The tidal prism and bay volume were used to calculate an average flushing rate of 4 days for Buttermilk Bay and 6 days for Little Buttermilk Bay.
The TEA 2-dimensional, finite element program was used as a second method to model the Bay. The program yielded reasonable depth-averaged current velocity magnitudes and directions for 145 selected locations for mean tidal conditions. From the computer calculated current data, water parcel trajectory maps were generated to show the circulation of a parcel of water over time.
In the third method, geomorphic evidence,such as the orientation of spits, was examined to determine predominant near shore and tidal currents. This information indicates that flow moves in a counter-clockwise direction along the shore of Buttermilk Bay. Additionally, the presence of large flood-tidal deltas in the entrances to Buttermilk and Little Buttermilk Bay, at Cohasset Narrows and Gibbs Narrows, respectively indicates the presence of flood dominated currents in these channels.
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Thesis (M.A.)--Boston University
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