Predicting Concentration Profiles in Open Channels

A steady state two-dimensional conservation of mass equation is solved numerically in order to describe the process of vertical mass transfer for both neutrally buoyant liquid and suspended sediment particles in an open channel flow. The relevancy of the equation is illustrated by comparing its solution with laboratory data obtained from experiments conducted in a large flume. The solution is then used to graphically illustrate the effects of various hydraulic parameters on mass transfer. Parameters investigated include the velocity distribution, the magnitude and distribution of the turbulent mass transfer coefficient, the particle fall velocity and the boundary conditions at the channel bed. The solutions indicate that the fall velocity controls the rate of descent of the dispersant mass but has little effect on the rate of spread of the dispersant. Conversely, the transfer coefficient controls the rate of spread of the dispersant but has little effect on its rate of descent. The most striking conclusion drawn from this study is that the dispersant distribution is not very sensitive to the distribution of the turbulent mass transfer coefficient.