Role of mixing in the structure and evolution of a buoyant discharge plume

[1] A field study to examine the near-field structure of a buoyant discharge plume, and the turbulent mixing associated with its evolution, was conducted in Mount Hope Bay near Somerset, Massachusetts. The study focused on a 50 m/s thermal discharge, approximately 5 C warmer than ambient waters, emanating from an electrical generating facility. The discharge enters the Bay through a 10 m wide surface canal creating a plume that has many attributes in common with larger scale river plumes. At a distance of 200 m, the plume was characterized by a core with high velocity and 2 3 C higher temperature than ambient water. At this location the core of the plume was observed with a width of approximately 150 m during ebb tide, remaining bottom attached along the plume centerline. Analysis of the temperature budget with respect to specific isotherms yielded values of Reynolds temperature flux , on the order of 10 2 C m/s, suggesting buoyancy flux values (B = ga) of order 10 5 m/s. These values are consistent with the turbulent energy expected to enter the system as a result of bottom stress, implying that mixing across the first 200 m is driven by bottom friction and is most active along the edges of the plume where thermal gradients are strong. The industrial plume studied here is dynamically similar to larger geophysical plumes, such as river plumes. However, the aspect ratio is critical in determining whether mixing driven by bottom friction is important in the overall evolution of the plume.