Determination of Advective or Diffusive/Dispersive Water Quality Flux Domain of the Lower Portion of the James River in Richmond, Virginia

The Peclet number is a flux characteristic parameter which represents the ratio of advective transport to diffusive/dispersive transport. A higher Peclet number indicates a bulk mass flow-driven advection dominated body of water that behaves similar to a Plug Flow Reactor, while a lower Peclet number indicates a diffusive/dispersive dominated body of water which is more sensitive to the substance concentration gradient or frictional shear velocity distribution. Study site is located on the Southern Bank of the James River in Richmond, Virginia; 1 mile downstream of the Fall Line, the area which indicates the transition from uni-directional stream/river environment to a bi-directional, weak estuarine environment. The study site also receives the combined sewer overflow (CSO) from an outfall. The purpose of this study was to determine if the water quality flux characteristic in this portion of the James River was dominated by advection or diffusion/dispersion, and subsequently, to evaluate spatiotemporal water quality response to CSO events in the James River in situ and downstream.

To determine the Peclet number the dispersion coefficient must be estimated in situ. Dispersion coefficient is highly dependent on and specific to the site characteristic and cannot be approximated from referenced values.  Estimation of dispersion coefficient is typically accomplished in two ways, dye tracer studies or salinity studies.  For this study salinity study was selected by reflecting the bi-direction estuarine nature of the site.  Salinity measurements were taken in the study site during ebb and flow cycles at selected locations longitudinal to the centerline of the James River. These salinity measurements were plotted vs distance for each sampling locations on a semi log plot, average velocity of each segment is then divided by the slope of the plotted values in order to determine the dispersion coefficient.

With dispersion coefficients for each segment determined, the Peclet number of each segment in the study area were calculated.  Results of the Peclet number indicated that this portion of the James River is dominated by diffusive/diffusive transport flux.

With estimated dispersion coefficients, a 1-dimensional finite difference water quality model is proposed to characterize longitudinal dispersion and to predict spatiotemporal concentrations of dissolved oxygen (DO) receiving CSO influxes in the estuarine portion of James River in Richmond, Virginia.