Waikato River Pollutant Flushing Rates

A dye dispersion experiment conducted in the Waikato River, in New Zealand showed persistent lateral inhomogeneity of concentration even well below the advective zone. The Fickian model of longitudinal dispersion adequately described the shape of dye profiles for concentrations near the peak but consistently predicted that pollutants arrived earlier and left earlier than was observed. The tracer data exhibited the characteristics of a self-similar process in which average concentrations at any point can be calculated from the peak concentration, arrival time, and duration of concentrations above half the peak, using a single scaling curve. A hybrid approach is advocated for predicting the arrival and flushing times of pollutants in a nonuniform channel in which the one-dimensional Fickian model is used to predict the three independent variables of the self-similarity transformation and then the complete concentration versus time profile is reconstructed from the single scaling curve.