Lateral spreading of a near-field river plume: Observations and numerical simulations

Data collected from the near-field region of the Merrimack River (Massachusetts) plume were analyzed to examine plume spreading. Estimates of the plume spreading rate were derived from a direct assessment of the spreading of clustered surface drifters and from density and velocity data along a cross-plume arc. Additionally, plume spreading rates were also derived from highly resolved numerical model output (ROMS). These three distinct observational and numerical approaches have reasonable agreement and are compared favorably with observations using a control volume approach in a previous study. It is demonstrated that these three methods are valid to estimate the plume spreading rate. The observations and numerical simulations also allowed estimation of terms in the lateral momentum balance along streamline normals, perpendicular to surface layer streamlines. One principal finding of this study is that the dominant terms in the lateral momentum balance are the centrifugal force term associated with streamline curvature, the buoyancy term, the Coriolis term, and the interfacial stress term. Furthermore, the lateral momentum balance shows that the lateral spreading process is significantly affected by interfacial stress within the first 1 to 2 km but, thereafter, is locally inviscid. However, turbulent mixing continues to play an important role in modifying the spreading rate by adjusting the internal wave speed.