Modelling dilution and transport processes from marine outfall discharges.

Municipal, industrial and desalination wastewater is commonly discharged into coastal waters. Marine outfalls are a common wastewater disposal system used in coastal areas, consisting on a pipe or tunnel, or the combination of both, that ends in a diffuser. The aim of the diffuser is to mix the effluent in the receiving waters. Accurate prediction of mixing and dispersion in the near field of marine outfall discharges is important to prevent pollutants present in wastewater from affecting the water quality of the receiving environment. Traditionally, marine discharges have been modelled using empirical or entrainment models. Computational Fluid Dynamics (CFD) approaches have rarely been used to model this situation, despite their great potential. This thesis aimed to assess how coupling CFD near field models with far field circulation models can be implemented to improve predictive capability of dilution and pollutant fate and transport in the near and far field of marine outfalls with multiport diffusers. The present work includes the description and validation of a CFD modelling methodology for the simulation of the near field of single and multiple ports discharges at laboratory scale. A mesh adaption approach was used to capture geometric and flow detail around the discharge port while reducing computational resource requirements. Predictions of flow trajectory, velocity and dilution generally show good agreement with available experimental data. This approach demonstrates improved predictive capability for velocity and dilution when compared against conventional buoyant jet entrainment models. The dynamics of merging between plumes were successfully captured by RANS based k-ε and k-σ models. Further studies were conducted focusing on the simulation of a real-life marine outfall located in the city of Cartagena, Colombia. The sensitivity of the outfall performance to a variety of parameters, including port alignment and other discharge and ambient conditions was assessed. The level of dilution obtained by the outfall showed great sensitivity to port alignment and Roberts Froude number. A framework for the implementation of the CFD methodology implemented here to other marine outfalls is presented. The CFD near field model results obtained for the Cartagena marine outfall were successfully coupled to the far field circulation model Delft3D by inputting a near field discharge flow vertical profile into the far field model. Time-dependent and other complex processes such as tidal forcing, density stratification and wind effects were ignored to isolate the individual parameters of interest for their analysis. The far field model showed little sensitivity to the coupling model used under the conditions studied. The overall aim of this thesis was achieved and a framework for the application of CFD near field models and its subsequent coupling with far field circulation models was proposed. RANS turbulence models produce reliable predictions of single and multiple port buoyant jet discharges from marine outfalls. The computational cost of this modelling method was reduced by implementing solution-based mesh adaption. Time-dependent processes and variations in the discharge and ambient conditions would be the next step in understanding the behaviour of near and far field models of marine outfalls.