Distributed Entrainment Sink Approach for Modeling Mixing and Transport in the Intermediate Field

Abstract

In densely populated coastal cities in Asia, wastewater outfalls are often located not far from sensitive areas such as beaches or shellfisheries. The impact and risk assessment of effluent discharges poses particular technical challenges, as pollutant concentration needs to be accurately predicted both in the near field and intermediate field. The active mixing close to the discharge can be modeled by proven plume models, while the fate and transport far beyond the mixing zone can be well-predicted by three-dimensional ͑3D͒ circulation models based on the hydrostatic pressure approximation. These models are usually applied separately with essentially one-way coupling; the action of the plume mixing on the external flow is neglected. Important phenomena such as surface buoyant spread or source-induced changes in ambient stratification cannot be satisfactorily addressed by such an approach. A Distributed Entrainment Sink Approach is proposed to model effluent mixing and transport in the intermediate field by dynamic coupling of a 3D far field shallow water circulation model with a Lagrangian near-field plume model. The action of the plume on the surrounding flow is modeled by a distribution of sinks along the plume trajectory and an equivalent diluted source flow at the predicted terminal height of rise. In this way, a two-way dynamic link can be established at grid cell level between the near and far-field models. The method is demonstrated for a number of complex flows including the interaction of a confined rising plume with ambient stratification, and the mixing of a line plume in cross flow. Numerical predictions are in excellent agreement with basic laboratory data. The general method can be readily incorporated in existing circulation models to yield accurate predictions of mixing and transport in the intermediate/far field.

Evaluation of Offshore Wastewater Outfall and Diffuser for Onondaga Lake, NY

“…Various approaches to linkage of near and far field models have been described [2,8,10,11]. The approach taken here follows that used by Owens and Effler [27] and Choi and Lee [11] and utilizes a two-way linkage of the submodels.…”

“…The approach taken here follows that used by Owens and Effler [27] and Choi and Lee [11] and utilizes a two-way linkage of the submodels. This approach is appropriate for the Metro discharge to Onondaga Lake, where the detention time of this discharge in the stratified layers is short (~3 months), and thereby has the potential to affect lake stratification.…”

“…Simulation of far field water quality requires realistic linkage of the submodels. In large water bodies such as the coastal ocean or the Laurentian Great Lakes, a one-way linkage, which neglects the influence of the effluent on ambient (far field) stratification, has been adopted [9,10], while more general strategies for two-way linkage have been used for smaller systems where there is potential for system-wide impacts [2,11]. This paper evaluates the potential benefits and impacts of an offshore outfall and diffuser for municipal waste-water discharges to a small urban lake (Onondaga Lake, NY) where shoreline outfalls continuously release treated effluent, and intermittently release partially treated wastewater during major runoff events (bypass flow).…”

Development and application of 3D numerical model THREETOX to the prediction of cooling water transport and mixing in the inland and coastal waters

“…The non-hydrostatic sub-model (Kanarska and Maderich, 2003) can provide representation of transport processes in the near-field, but it is too laborious to be coupled with THREE-TOX. Therefore the procedure of dynamic coupling of the integral Lagrangian model JETLAG (Lee and Chu, 2003) and THREETOX code was employed, following Choi and Lee (2007). The Lagrangian model predicts the buoyant jet trajectory, jet velocity, radius, and entrainment rate at given ambient velocity and stratification.…”

“…Such linkage has been developed by various authors for the modelling of effluent discharges. Zhang and Adams (1999); Choi and Lee (2007) created the system 'integral jet model-3D hydrostatic model ', Suh (2001) linked CORMIX with a 2D hydrostatic model, Kim et al (2002) linked an integral jet model with a particle tracking model, Bleninger and Jirka (2004) linked CORMIX with a 3D hydrostatic model. However, except the recent approach by Choi and Lee (2007), this linkage is never a two-way dynamic coupling because the feedback of the jet to the ambient flow is not described.…”