Field Observations of Ipanema Beach Outfall

Field observations of the Ipanema Beach, Rio de Janeiro, Brazil, ocean sewage outfall were obtained by adding dye tracers to the effluent and simultaneously measuring oceanographic conditions. Four experiments were performed, two during unstratified conditions, and two during stratified conditions. When stratified, the plume was trapped below the thermocline with low dilution, around 35 to 1, when unstratified, the plume surfaced and the dilution increased to more than 100 to 1. The results were compared with predictions of some commonly used near field plume mathematical models: UM3, RSB, and CORMIX. With suitable assumptions, all the models reasonably predict near field dilution. RSB predicts near field results directly; for UM3 and CORMIX, it was assumed that the end of the near field occurs when the plume reaches its terminal rise height or impacts the free surface. Different assumptions about the shape of the density profiles in CORMIX resulted in widely differing predictions of dilution. While the gross properties of the plume can be reasonably predicted by plume models, there remain many aspects which cannot be, particularly the patchy nature of the wastefield that has been observed here and in other field tests.