Application of Three-Dimensional Boundary-Fitted Circulation Model to Providence River

A three-dimensional boundary-fitted circulation model (Muin and Spaulding 1996b) was applied to simulate tidal and wind driven flow in Upper Narragansett Bay and the Providence and Seekonk Rivers. The model employs a split mode solution technique. The exterior mode (vertically averaged) was solved using a semiimplicit method. The interior mode (vertical structure) was solved explicitly, except for the vertical diffusion term that is implicit. The vertical eddy viscosity was calculated from a one-equation turbulence model. Model predicted tidal currents for the principal constituents were in good agreement with field observations. The M₂ dominated the tidal currents accounting for 80% of the total tidal energy. A combination of bottom friction coefficient of 0.0025 and constant γ= 0.3 in the mixing length formulation of the turbulence model provided the best fit to the available observations. The model predicted a cooscillating wave pattern with the M₂ water elevation about three hours out of phase with the current. An approximate one-hour phase difference between the currents in the deep dredged channel and the shallow water on either side was predicted. Wind-driven flow was simulated under constant wind forcing of 0.01 N/m² (0.1 dyne/cm²) along and across the river axis. The steady-state flow patterns are consistent with observations, showing flows against the wind direction in the dredged channel. The model predicted steady state currents in the dredged channel have maximum values of 3–4 cm/s for winds along the channel axis, and 1 cm/s for winds across the channel axis.