Buoyant plume dispersion in a coastal environment: evolving plume structure and dynamics

A field program has been devised to study the dispersion of a buoyant plume formed by the discharge of treated wastewater from the Whites Point outfall into the coastal ocean off the Palos Verdes peninsula off southern California. The program combines high resolution towed surveys of physical and bio-optical variables with continuous time series observations of current velocity. The observations show that after discharge the wastewater forms a submerged buoyant plume of relatively fresh, turbid effluent. Identification of the plume within the water column is complicated by the ambient thermohaline and particle fields which can exhibit natural variability comparable to that introduced by the plume. We present a simple method based on threshold criteria of three measured variables to identify effluent-containing waters under general conditions. The complexity of the effluent field is found to depend strongly on the temporal variability of currents over a period of several hours preceding the observation time and on the ambient density structure. Measurements of small-scale turbulent mixing activity show that high mixing rates occur over the diffusers due to buoyant convection in the rising plume. We use the distribution of turbulent activity to separate the actively mixing region in the rising plume from the passive effluent layer away from the diffusers. In background waters, turbulent patches are occasionally found with horizontal extents of about 1.5 km and vertical extents of about 5 m. Effective eddy diffusivities within these patches are of order K_z ∼ 10^-4 m² s^-1.