Initial spread of an effluent and the overturning length scale near an underwater source in the northern Adriatic

The rise of an effluent emerging from the diffusers of the submarine sewage outfall of a small town (15000 PE) is numerically simulated with a model of initial dilution. Results were evaluated using the vertical distribution of faecal coliforms that were analyzed for two field surveys at the central position. In addition to that, the overturning length scale was calculated from vertical profiles of temperature, salinity and density. These were obtained from two autumn surveys using fine-scale CTD casts in the sewage near-field around the diffusers which lie on a flat seabed in a shallow sea. Indications that a turbulent effluent could manifest itself with a local maximum of the overturning length scale are discussed. The depth of the subsurface local peak of the overturning length scale, calculated from the density profile in the core of the sewage near-field, matches the position of the peak of the vertical distribution of faecal coliforms and the height of the simulated plume rise. The 3D space distribution of the overturning length scale indicates a structure with enhanced overturning activity below the surface mixed layer over an area of 1 km2 which could be attributed to an effluent that erupts turbulently from diffusers.