Estimation of the dilution field near a marine outfall by using effluent turbidity as an environmental tracer and comparison with dye tracer data

The alternative use of effluent turbidity to determine the dilution field of a domestic marine outfall located off the city of Rio de Janeiro was evaluated through field work comprising fluorescent dye tracer injection and tracking with simultaneous monitoring of sea water turbidity. A preliminary laboratory assessment was carried out with a sample of the outfall effluent whose turbidity was measured by the nephelometric method before and during a serial dilution process. During the field campaign, the dye tracer was monitored with field fluorometers and the turbidity was observed with an optical backscattering sensor interfaced to an OEM data acquisition system. About 4,000 samples were gathered, covering an area of 3 km × 3 km near the outfall diffusers. At the far field – where a drift towards the coastline was observed – the effluent plume was adequately labeled by the dye tracer. The turbidity plume was biased due to the high and variable background turbidity of sea water. After processing the turbidity dataset with a baseline detrending method, the plume presented high correlation with the dye tracer plume drawn on the near dilution field. However, dye tracer remains more robust than effluent turbidity.

Convection from a line-source into a two-layer stratified ambient fluid

We experimentally investigate the behaviour of a line-source plume falling through a finite two-layer stratified ambient where the depth of the upper ambient layer increases in time. Laboratory observations suggest one of two possible flow regimes depending on the value of $\unicode[STIX]{x1D706}$, which represents the relative loss of buoyancy experienced by the plume upon crossing the ambient interface. When $\unicode[STIX]{x1D706}>1$, a classical filling-box-type flow is realized and plume fluid always reaches the bottom boundary. By contrast, when $\unicode[STIX]{x1D706}<1$, we observe a transition by which an increasing fraction of plume fluid discharges along the interface. The approximate start time, $t_{v}$, and end time, $t_{t}$, of the transition process are well determined by $\unicode[STIX]{x1D706}$. After transition, the ambient density evolves to form a three-layer fluid with an intermediate layer that grows in time. Measured densities of the intermediate layer in experiments with $\unicode[STIX]{x1D706}<1$ are well predicted using plume theory. We further characterize the horizontal speed of the intrusion that forms along the ambient interface, the mass of solute present in the intermediate layer at time $t_{t}$ and the rate of descent of the intrusion level for $t>t_{t}$. The significance of our findings is discussed in the context of the ventilation of natural and hybrid ventilated buildings and of effluent discharge through marine outfall diffusers.