A Numerical Study of the Mid-field River Plume

Idealized and realistic simulations of the Merrimack River plume on the east coast of the U.S. are performed using the Regional Ocean Modeling System (ROMS). The effect of discharge, tides and rotation on the evolution of the tidal plume are examined. Experiments investigating the deceleration of the plume body through mixing and the relaxation of the tidal plume front are performed. Three primary findings result from this research. First, more ambient water interacts with the tidal plume front than source water. Because it takes several hours for source water to translate the plume and it is strongly diluted in the plume interior, only a small fraction of source water reaches the front. Therefore, the front is responsible for a small portion of mixing of the total ebb discharge. Second, the mouth and the tidal plume front communicate on an advective time scale. When the ebb discharge is stopped at the estuary mouth, the inertia of the discharge is enough to keep previously released source water necessary to sustain frontal propagation moving frontward. The front begins to slow when the withheld estuarine discharge is not supplied to the front. Third, the net plume mixing, defined as the total mixing of a parcel of source water before it enters the far-field, is altered by rotation. As discharge increases, an irrotational plume will exhibit an increasing trend in net mixing, while a rotational plume will exhibit a decreasing trend. These experiments bridge engineering and geophysical scale plume studies and provide a framework for understanding results reported in literature