SSM Marine Pollution: Outfall Plume Model

Developing FVCOM-plume Model: A Three-Dimensional Outfall Plume Model for Dynamic Tidal Environment

Although numerous effluent plume dilution models have been developed for outfall design, water quality compliance and exposure assessment purposes, and have been in use for almost four decades, accurate predictions of plume characteristics, remains a challenge for tidally influenced estuarine environment. The reason is the calculations of plume dilution and mixing are inherently difficult. This is true for both experimental and modeling approaches due to the complex three-dimensional (3D) hydrodynamic processes associated with turbulent jet and buoyancy-induced mixing, and the unsteady and stratified nature of receiving waters in coastal regions. To address the limitations of current modeling tools, FVCOM-plume model was developed with the collaboration of US EPA and Ecology by adding extended capabilities to simulate outfall plume characteristics under tidal conditions.

Figure 1. Setup of diffuser configuration and modeling approaches used in SSM and FVCOM-plume. (a) Effluent release location node within SSM grid for the conventional Eulerian approach, (b) Overview of the full SSM grid, (c) diffuser setup for the Lagrangian approach in FVCOM-plume, and (d) detailed plume structure of the effluent release simulated by FVCOM-plume.

FVCOM-plume can capture the near-field plume characteristics, while simulating far-field transport using detailed hydrodynamic information provided by a 3D hydrodynamic model within a common computational framework. To achieve this, the initial dilution component of FVCOM-plume was developed using the Lagrangian Control Volume (LCV) method, as in the UM3 model, including the entrainment and integrated features of plume merging. A Lagrangian Particle-Tracking Method (LPTM) was implemented as an alternative modeling paradigm to simulate the far-field plume transport (Figure 1), deviating from the traditional scaling-law based approach. The LPTM allows for the capture of complex far-field plume features (i.e., tidal mixing and re-entrainment) under reversing tidal currents without excessive numerical diffusion (Figure 2). A particular benefit of the LPTM approach over the conventional Eulerian simulation is the ability to capture and sharply demarcate plume boundaries. Another important feature of FVCOM-plume is its capability of simulating multiple effluent releases from the diffusers that have staggered port configurations. Simulation of effluent plumes from staggered ports is feasible in FVCOM-plume by using the approach of co-located diffusers simultaneously.

Figure 2. FVCOM-plume can capture sharp plume boundaries effectively and cam simulate plume re-entrainment which is not possible in conventional models.

Model Features

FVCOM-plume uses FVCOM 3D hydrodynamics accounting for bathymetry and shoreline effects which enables to captures near-field/far-field plume dynamics plus re-entrainment
Ability to simulate multiport diffusers using U.S. EPA – UM3 model style treatment with a seamless transition to far-field and subsequent dispersion using particle tracking

Publications

Premathilake L, T Khangaonkar. 2019. FVCOM-plume – A three-dimensional Lagrangian outfall plume dilution and transport model for dynamic tidal environments: Model development. Marine Pollution Bulletin, 149: 110554. doi: 10.1016/j.marpolbul.2019.110554