TIDES AND TIDAL POWER IN PASSAMAQUODDY BAY - A NUMERICAL-SIMULATION
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A three-dimensional sigma-coordinate numerical model was used to simulate the circulation in Passamaquoddy Bay, located in the eastern Gulf of Maine at the mouth of the Bay of Fundy where the mean tidal range is 5.6 m. The model was forced by tidal height variations at the oceanic boundary, fresh water runoff from rivers, and parameterized fluxes of heat and momentum at the sea surface and sea bottom. Simulations were run for cases approximating the natural system, and also for cases modified to include the gates, dams and powerhouse structures of a tidal power project that was previously under consideration for the region. In the natural system, the results show strong tidal currents (2 m s-1) in the channels and passages, with weaker currents (0.2 m s-1) and essentially zero residual flow in the bay, in good agreement with available observations. In the modified system, the volume of each tidal flood is reduced, but a significant tidal-residual flow passes from Passamaquoddy Bay through the powerhouse into the adjacent Cobscook Bay. During a severe and sudden winter cold event, the extent of surface freezing in the region is increased when the tidal-power structures are present, but this occurs mostly because of reduced heat flux from the bottom rather than from an increase of surface stratification in the presence of reduced vertical tidal mixing. When the cold event follows a long period of near-freezing conditions, so that the heat contributions from the bottom and from rivers are smaller, the tidal structures have a moderating influence in Passamaquoddy Bay and the St Croix river estuary, where surface freezing is slightly reduced compared to the unmodified system because the persistent tidal-residual flow brings warmer ocean water into the bay. However, the areal extent of freezing in the model is much more sensitive to surface wind speed than to the presence of the tidal structures, which suggests that some of the environmental impacts of the tidal-power project might be less severe than previously thought. 1992.
