| The objective of the present study was to evaluate the limits and performance of the advection-diffusion equation as applied to suspended-sediment transport in a free-surface flow of water.; Series of flume experiment were performed, and a large amount of digital-bitmap image data was obtained. The data were analyzed by means of particle tacking velocimetry (PTV) so as to produce a detailed further set of data on flow and sediment-particle distributions of use in determining the characteristics of free-surface-flow transport of suspended-sediment. Those data, together with the data from prior studies, enable the accuracy limits of the advection-diffusion equation to be delineated.; The flume experiments, and a review of data from the prior flume studies, show that, even for dilute concentrations of suspended-sediment, the Karman coefficient is reduced significantly. Moreover, the PTV analysis reveals a significant velocity-lag of sediment particles from water tracers for heavy suspended particles (natural sand). The velocity-lag reaches a maximum value of around 5∼6% of the mean streamwise flow velocity. Sediment particles move slower than do the water tracers in the outer region, while they move faster than the water tracers in the region very close to the bed, because they are not subject to the no-slip condition pertaining to water flow. The vertical velocity component of the sediment particles does not follow the assumption used for the advection-diffusion equation.; Measurements of turbulence intensities in sediment-laden flows show that the presence of suspended-sediment particles changes the profiles of turbulence intensities rather than overall enhancing or dampening the intensities. Diagnostic parameters suggested by the prior studies do not adequately explain the data trends found in the present study. The theoretical distributions of turbulent diffusion coefficient do not follow the measured trends, especially for the outer region of flow. This result arises because no velocity formula adequately describes flow in the inner region and the outer region simultaneously. Certain modifications to the governing equation of water-sediment transport models are suggested, based on the findings of the present study. |
