Modeling Mixed Sediment Suspended Load Profiles

Abstract

Potential errors in calculated vertical suspended sediment concentration profiles in river flows carrying mixed sediments are investigated. These profiles are usually obtained through application of a standard ‘diffusion‐dispersion’ model; however, such models are normally formulated on the basis of a unique grain size for the sediment. Accordingly, in the case of sediment mixtures, the grain size distribution is usually divided into several size ranges (‘fractions’), which are then characterized by their respective geometric means. Numerical errors induced through this procedure are quantified in a parametric study. While depthaveraged concentrations can be over‐esimated by a factor of 1.2 when the concentration gradient is weak, they can be under‐estimated by a factor of 15 when the gradient is large. Estimates are found to be particularly sensitive when the chosen reference level is small and the fraction width is large. Fall velocity coefficients are introduced to correct the discrepancy between the actual representative fall velocity of a fraction and that of the geometric mean size of the fraction.