Case Study: Intermediate Field Mixing for a Bank Discharge in a Natural River

The intermediate field mixing characteristics of the Gold Bar Wastewater Treatment Plant effluent into the North Saskatchewan River at Edmonton were evaluated. This region may be considered to be the early part of the transverse mixing region where local channel characteristics are important. An extensive field study was conducted to delineate the bathymetry of the study area and evaluate the mixing characteristics by means of a steady state dye test. The topographic and limited velocity results of the field study were used to create and validate a depth-averaged hydrodynamic model of the study reach in order to extract streamtube information. The results from the hydrodynamic model were used to interpret the mixing characteristics of the study reach as well as extract channel characteristics. From the analysis it is evident that the distribution of effective transverse mixing coefficient is highly dependent on local river conditions. The use of the hydrodynamic model to extract channel characteristics provided a reasonable estimate of mixing characteristics without requiring detailed field velocity data. The trade-off is more detailed bathymetry data is required to have a realistic model. Plume averaged channel characteristics rather than cross sectional averaged were shown to produce more realistic transverse mixing coefficients. Assumed Gaussian profile distributions were successfully applied suggesting that for a bank discharge if the maximum bank concentration and mass flux are known this technique could be applied.