Coupled Hydrodynamic–Near-Field–Far-Field Modelling for Locational Optimisation of a Submarine Sewage Outfall in a Tropical Coastal Environment

Submarine sewage outfalls are widely used to mitigate the impacts of wastewater discharges on coastal recreational waters. However, an outfall’s environmental performance depends not only on its location but also on the complex interaction among local hydrodynamics, near-field dilution, far-field transport, and bacterial decay processes. This study presents an integrated numerical modelling framework to evaluate alternative locations for a submarine sewage outfall on a tropical coast in southeastern Brazil. The methodology couples hydrodynamic circulation, near-field plume dynamics, far-field transport, and water-quality simulations within the SisBaHiA® modelling system. Model performance was assessed by comparing it with field observations of current velocities, increasing confidence in the representation of the local hydrodynamic regime. The validated framework was subsequently applied to compare four alternative outfall alignments under representative environmental conditions. Results demonstrated substantial differences in plume dispersion, dilution, bacterial decay, and the frequency of bathing-water standard exceedances among the alternatives, highlighting the dominant influence of coastal circulation patterns on wastewater transport. Among the evaluated scenarios, the offshore alignment achieved the greatest dilution and the lowest predicted impacts on recreational water quality. Beyond identifying the most suitable outfall configuration for the study area, the proposed modelling framework provides a transferable methodology to support the environmental design and optimisation of submarine outfalls in coastal regions subject to complex hydrodynamic conditions. Four outfall alignments showed distinct impacts on nearby bathing waters. Coastal contamination risk was not determined solely by offshore distance. One alignment consistently reduced the risk of E. coli exposure.