Advances in the Art of Discharge Plume Monitoring--the Critical Validation for Realistic Numerical Modeling

This paper updates an IAHR presentation (McDonald, Hansen, 2009) where we reviewed field measurement techniques and guiding strategies to characterize effluent discharge plumes in receiving waters. We typically perform these studies to collect quantitative data for use in assessing and calibrating mathematical hydrodynamic plume mixing models, for example, NRFIELD an initial dilution model specifically designed for buoyant discharges from multiport diffusers into stratified oceanic waters. NRFIELD is included within the most recent release of the Visual Plumes, a United States Environmental Protection Agency (USEPA)-supported interface to a suite of numerical plume models.

Engineers and regulators frequently use hydrodynamic mixing models to estimate dilution of wastewater effluent discharges in receiving waters within regulatory mixing zones, and to support assimilative capacity analyses. Models serve as useful tools for predicting mixing and dilution under a range of scenarios allowing the comparison of the impact of current and future conditions, and evaluation of alternative scenarios. Field verification is often warranted or required to demonstrate model dilution predictions reflects actual mixing performance, or to account for unique site conditions.

In the field, one objective is to delineate the discharge plume distribution under different tide and current conditions including its location in the water column, vertical thickness, and horizontal spread. We use the field results to quantify actual initial dilution, plume trapping depth, and far-field dispersion rates. We then compare the results of these studies to hydrodynamic model predictions using effluent and receiving water conditions documented during the field tests as input to the models. We present the findings as they pertain to regulatory mixing zones defined under discharge permits. We often project performance for other seasons and for future growth, with much great confidence on the validity of such projections.

In this paper we discuss advances in the instruments and techniques used to characterize discharge plumes accurately and efficiently for a wide variety of conditions. These advances include custom data collection and instrument deployment systems that we developed specifically for conducting mixing zone studies. We present example results from recent studies in Guam and Hawaii, plus results from studies performed in coastal waters, estuaries, and rivers over the last 35 years by the author.

1. McDonald, R.M., A.C. Hansen. Discharge Plume Monitoring Techniques in Support of Numerical Modeling. 33rd IAHR Congress (International Association of Hydraulic Engineering and Research). 2009. Vancouver, British Columbia, Canada.