Near- and far-field model coupling methodology for wastewater discharges T. Bleninger & G.H. Jirka Institute for Hydromechanics, University Karlsruhe, Karlsruhe, Germany ABSTRACT: The prediction of pollutant concentrations in the coastal waters is essential for water quality control. The description of physical processes and their implementation in com- puter models shows that there is neither a “complete model” including all important spatial and temporal scales of the dominant processes nor a standard procedure for the coupling of models. This is explained by deficiencies in either implementing dominant physical near-field processes into a far-field model and/or coupling the models to each other. Capabilities and limitations of modeling techniques applied to waste plume mixing and dispersion from submerged multiport discharges are discussed for environmental impact analysis. The recommended procedure com- bines the relative strengths of the near-field mixing zone model CORMIX and the Eulerian far- field flow and water quality models within Delft3D. 1 INTRODUCTION There is worldwide increasing utilization of controlled treatment facilities in combination with submarine outfalls to achieve better coastal water quality. Outfalls transport the effluent off- shore through a pipe or tunnel and discharge commonly through a multiport diffuser. The latter is a linear structure with a (usually large) number of ports mounted on, or connected through risers to a submerged pipe laid on the ocean floor. Multiport diffusers are effective mixing de- vices which by the choice of discharge point and hydraulic design can influence the short-term impact. Long-term influence of the discharge needs to be addressed in terms of levels of bacte- ria or persistent substances in the effluent. There are several diagnostic and predictive method- ologies for examining the mixing from point sources and showing compliance with environ- mental quality standards. Beside costly and difficult field measurements and hydraulic model studies, which both are usually limited to certain ambient conditions computer models became the most important tool. Existing scientific approaches and techniques for predicting environ- mental impacts for large regions and long time periods are still very complex and do not allow standard engineering design solutions. Therefore capabilities and limitations of modeling tech- niques applied to waste plume mixing and dispersion from submerged multiport discharges are discussed in terms of model selection and the linking of mixing zone models (near-field) and general water quality models (far-field). Mixing Processes for multiport diffuser plume predictions The hydrodynamics of an effluent continuously discharging into a receiving water body can be conceptualized as a mixing process occurring in two separate regions. In the first region, the ini- tial jet characteristics of momentum flux, buoyancy flux (due to density differences), and outfall geometry influence the effluent trajectory and degree of mixing. This region, the "near-field", encompasses the buoyant jet flow and any surface, bottom or terminal layer interaction (fig. 1). Outfall designers can usually affect these initial mixing characteristics through appropriate ma- nipulation of design variables. A review of these processes has been given by Fischer et al.