Plume dispersion in a stratified, near-coastal flow: measurements and modeling

Dispersion of a passive scalar released from a near-bed source is examined in coastal waters, (O(10 m) deep), just off San Clemente Island, California. Rhodamine WT dye was released continuously for several hours from a bottom source on May 8, 1997. Surveys of the hydrodynamic fields (currents and density) are combined with dye concentration measurements made from a towed fluorometer array to characterize the plume. The plume developed along the 12 m isobath (the depth of the source), to the southeast of the release location. It was largely confined to the near-bed region by a thermocline located at a depth of about 9 m. Therefore, the plume can be loosely characterized as advecting along the isobaths and dispersing throughout the layer below the thermocline (layer thickness of about 3 m). An analytic expression for plume concentration as a function of radial distance from the source was developed for a scale-dependent dispersion coefficient. This model agrees with the data and indicates that the dispersion coefficient obeys a “4/3-law”. This approach is contrasted by a finite difference model in an Eulerian reference frame, illustrating the challenges associated with the modeling of a meandering plume.