Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2000
Publication Date: N/A
Interpretive Summary: In rivers and streams, the rate at which horizontal mixing occurs, defined as the transverse mixing coefficient, is important in predicting the transport and dispersion of water, suspended solids, and contaminants from one side of the channel to the other. This is particularly important in point source contamination and sediment deposition on floodplains. Traditionally, this mixing coefficient has been calculated by examining the concentration of dye injected at an upstream location and related to simple flow parameters like the mean velocity of the stream. In the present study, measurements were made of the turbulent fluctuations in velocity that occur naturally in the transverse direction, and these data are used to derive a new expression for this mixing coefficient. It was found that the calculated values using this new technique were comparable to those measured in natural rivers and laboratory studies using the dye injection technique. This new technique will aid federal agencies to predict the rate of transverse mixing where velocity data are available.
Technical Abstract: The rate of transverse exchange of momentum and entrained contaminants in a fluid by turbulent eddies, as defined by the transverse diffusion coefficient, is important in modeling contaminant dispersion and lateral suspended sediment fluxes. Traditionally, the transverse diffusion coefficient has been calculated by examining the rate of change of the variance of concentration profiles taken downstream of a dye release point. Using this method, transverse eddy diffusivities have been calculated in sediment-laden flows in natural rivers and in clear water flows in laboratory flumes. In the present study, turbulent velocity fluctuations and transverse velocity gradients derived from velocity profiles at several locations over half the width of a laboratory flume were used to calculate the transverse diffusion coefficient in both clear water and sediment-laden flows. It was found that values for the transverse dispersion coefficient were comparable to those measured in natural rivers and in previous studies using the dye injection technique. The technique presented here provides an alternative method, using transverse Reynolds stresses and velocity gradients, to determine the transverse diffusion coefficient. The agreement between the two methods shows that it may be valid to use an approach based on turbulence quantities.