Modeling Outfall Plume Behavior Using Far Field Circulation Model

Abstract

The near field behavior of the new Boston sewage outfall plume, as simulated by a far field circulation model of Massachusetts Bay, has been compared with similar predictions from a well-known and often-used near field plume model (ULINE). The three-dimensional circulation model applied to Massachusetts Bay is based upon far field physical processes that may or may not apply to the near field plume physics. An accurate characterization of initial mixing is important for reliable prediction of the environmental impact of the new outfall's discharge. Two parameters predicted by both models, plume rise (or trap) height and dilution, have been compared to determine how well the circulation model represents near field behavior. The comparison indicates that the Massachusetts Bay far field model predicts trap heights and initial dilutions that are surprisingly similar to those generated by the near field model. The parameterization of the horizontal and vertical mixing processes in the far field circulation model, together with appropriate numerical grid resolution and a good prediction of three-dimensional currents and vertical mixing near the new outfall, apparently simulates the small scale and very near field entrainment processes sufficiently well to provide confidence that far field results are not biased due to poorly simulated initial dilution and trap height.