MODELING OUTFALL PLUME BEHAVIOR USING FAR-FIELD CIRCULATION MODEL

The near field behavior of the new Boston sewage outfall plume, as sim ulated by a far field circulation model of Massachusetts Bay, has been compared with similar predictions from a well-known and often-used ne ar field plume model (ULLNE). The three-dimensional circulation model applied to Massachusetts Bay is based upon far field physical processe s that may or may not apply to the near held plume physics. An accurat e characterization of initial mixing is important for reliable predict ion of the environmental impact of the new outfall's discharge. Two pa rameters predicted by both models, plume rise (or trap) height and dil ution, have been compared to determine how well the circulation model represents near field behavior. The comparison indicates that the Mass achusetts Bay far field model predicts trap heights and initial diluti ons that are surprisingly similar to those generated by the near field model. The parameterization of the horizontal and vertical mixing pro cesses in the far field circulation model, together with appropriate n umerical grid resolution and a good prediction of three-dimensional cu rrents 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.