Dilution and Transport Predictions for Ocean Outfalls

Dilution and transport predictions for an ocean outfall proposed for the Barra da Tijuca, Rio de Janeiro are summarized. The initial characteristics of the wastefield are predicted using the results of recent experiments on submerged wastefield characteristics in stratified flows. Using measured currents and stratifications, it is predicted that the proposed design will result in a wastefield which is always submerged, whose thickness varies from 8.5 to 23.1 m and whose dilution varies from about 200 to well over 2000. The wastefield will often stay near the ocean bottom. Far field transport due to currents is calculated and expressed in terms of spatial “visitation frequency,” the probability of the wastefield visiting any location. It is shown that no effluent reaches shore with a travel time less than 5.5 hours, and computations including oceanic diffusion and bacterial mortality suggest maximum shoreline coliform concentrations of 500 per 100 mℓ, and 98% of the time they will be less than this. For reasons discussed these estimates of shoreline bacteria are conservatively high, and much lower than computed by methods assuming steady onshore currents.