Spreading in the near-field Merrimack River plume

A numerical simulation of the near-field Merrimack River plume is used to investigate how buoyant plumes spread within a few kilometers of an estuary mouth. There are three principal findings of this study. First, many properties of the plume are primarily a function radial distance from the estuary mouth, and are uniform across the arc of the plume. Second, the percent change of plume width (in the cross-flow direction) is proportional to the percent change in radial distance from the mouth; the percent change in width is shown to be about 1.2 times the percent change in radial distance, resulting in a plume with splaying streamlines within the core of the plume. Third, plume spreading is related to the local internal gravity wave speed. These finding allow the width of the plume to be estimated as a function of distance from the estuary mouth. The principal findings are compared favorably to observations.